Accepted Manuscript Title: Photoresponsive Cellulose Fibers by Surface Modification with Multifunctional Cellulose Derivatives Author: Olga Grigoray Holger Wondraczek Elina Heikkil¨a Pedro Fardim Thomas Heinze PII: DOI: Reference:
S0144-8617(14)00446-9 http://dx.doi.org/doi:10.1016/j.carbpol.2014.04.089 CARP 8850
To appear in: Received date: Revised date: Accepted date:
11-8-2013 4-12-2013 22-4-2014
Please cite this article as: Grigoray, O., Wondraczek, H., Heikkil¨a, E., Fardim, P., & Heinze, T.,Photoresponsive Cellulose Fibers by Surface Modification with Multifunctional Cellulose Derivatives, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.04.089 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photoresponsive Cellulose Fibers by Surface
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Modification with Multifunctional Cellulose
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Derivatives
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Olga Grigoraya, Holger Wondraczeka, Elina Heikkiläa, Pedro Fardima,b,*, Thomas Heinzea,c
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a
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20500, Åbo, Finland; bCenter of Excellence for Advanced Materials Research (CEAMR)-King
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Abdulaziz University -Jeddah 21589, Saudi Arabia; cCenter of Excellence for Polysaccharide
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Research, Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller
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University of Jena, Humboldtstraße 10, 07743 Jena, Germany. *Corresponding author.
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Telephone: +358504096424. E-mail address: [email protected]
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ABSTRACT: Eucalyptus bleached kraft pulp fibers were modified by adsorption of novel bio-
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based multifunctional cellulose derivatives in order to generate light responsive surfaces. The
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cellulose derivatives used were decorated with both cationic groups (degree of substitution,
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DS of 0.34) and photoactive groups (DS of 0.11 and 0.37). The adsorption was studied by UV-
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Vis spectroscopy, surface plasmon resonance (SPR) and time-of-flight secondary ion mass
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spectroscopy (ToF-SIMS). The adsorption isotherms followed the Freundlich model and it
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turned out that the main driving force for the adsorption was electrostatic interaction.
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Moreover, strong indications for hydrophobic interactions between the fibers and the
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derivatives and the derivatives themselves were found. ToF-SIMS imaging revealed an even
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Laboratory of Fiber and Cellulose Technology, Åbo Akademi University, Porthansgatan 3, FI-
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distribution of the derivatives on the fiber surfaces. The modified fibers underwent fast
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photocrosslinking under UV-irradiation as demonstrated by light absorbance and fluorescence
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measurements. Thus, our results proved that the modified fibers exhibited light-responsive
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properties and can potentially be used for the manufacture of smart bio-based materials.
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KEYWORDS: eucalyptus pulp fibers, cationic cellulose derivatives, photo-activity, surface
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modification, ToF-SIMS, SPR
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1. INTRODUCTION
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Pulp fibers are intermediate products mainly used in the production of paper and paper
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boards (Freire, Silvestre, Gandini & Neto, 2011). However, pulp fibers represent also an
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attractive substrate for the manufacturing of novel materials owing to their outstanding
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characteristics. They are relatively cheap, renewable, biodegradable, and can be recycled or
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converted to energy by combustion at the end of the life cycle of the product. Additionally, the
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fibers have good mechanical properties and relatively low density (Freire, Silvestre, Gandini &
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Neto, 2011; Belgacem & Gandini, 2008; Ly, Bras, Sadocco, Belgacem, Dufresne &
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Thielemans, 2010).
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Currently more than 180 million ton per year of pulp fibers (Sixta, 2006) are produced by
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well-established technologies like mechanical, chemo-mechanical and chemical pulping
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methods. The properties and the chemical composition (bulk and surface) of pulp fibers from
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the classical pulping techniques are very well-studied and vary significantly depending on the
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raw material and the way of the manufacturing. In order to utilize pulp fibers in new value
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added products, current research is focusing on the modification of their surfaces while keeping
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bulk properties such as morphology and native fiber wall structure (Grönqvist, Rantanen, Alen, 2 Page 2 of 31
Mattinen, Buchert & Viikari, 2006; Hedenberg & Gatenholm, 1995; Pan & Ragauskas, 2012).
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Biological (e.g. by enzymes, fungi), chemical (carboxylation, grafting) or plasma treatments
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and adsorption methods can be applied for this purpose (Gandini & Pasquini, 2012; Utsel,
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2012).
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From our point of view, the adsorption method appears to be the most promising tool among
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the techniques mentioned since it is water-based and can be easily combined with existing fiber
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line stages (Maguet, Pedrazzi & Colodette, 2011). Traditionally, the research in fiber chemistry
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and technology has been mainly focusing on the improvement of mechanical, drainage and
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optical properties of pulp by adsorbing polysaccharides and polysaccharide-based
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polyelectrolytes for paper manufacture (Köhnke, Lund, Brelid & Westman, 2010; Schönberg,
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Oskanen, Suurnäkki, Kettunen & Buchert, 2001). In this regard, the application of underutilized
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biomass derived polysaccharides like hemicelluloses recently came into focus (Schwikal,
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Heinze, Saake, Puls, Kaya & Esker, 2011; Vega, Petzold-Welcke, Fardim & Heinze, 2012;
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Silva, Colodette, Lucia, de Oliveira, Oliveira, & Silva, 2011).
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Functional biopolymers with stimuli-responsive capabilities can be used for the modification
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of pulp in order to generate new fiber-based materials, whose properties could be triggered by
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an external stimulus. Multifunctional polysaccharide derivatives decorated with cationic and, in
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particular, light responsive substituents can be applied for this purpose. The light induced
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alterations, including isomerization, dissociation or crosslinking, of the photoactive substituents
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were described to cause changes in physical properties of the polymers itself or polymeric
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solutions (Irie, 1990; Wondraczek, Pfeifer & Heinze, 2011; Wondraczek, Kotiaho, Fardim &
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Heinze, 2010). Consequently, the possibility to control the properties of the interface of the
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fibers to which they are attached can be expected, and the properties of fiber-based materials
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and composites might be influenced.
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The focus of the studies presented in this paper was to gain information about the interaction
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of pulp fibers and cellulose derivatives, which were decorated on one hand with cationic (3-
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carboxypropyl)trimethylammonium chloride ester moieties and on the other with hydrophobic
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(photoactive) 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetate substituents (Wondraczek,
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Pfeifer & Heinze, 2012). In particular, the influence of the hydrophobic/hydrophilic balance of
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the derivatives on the adsorption behavior was investigated and a tentative hypothesis of the
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interaction of pulp fibers and such multifunctional cellulose derivatives was suggested.
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Moreover, the fiber surface composition and lateral distribution was evaluated by means of
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ToF-SIMS spectrometry and imaging.
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2.1. Materials
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2. EXPERIMENTAL SECTION
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Eucalyptus air-dried unrefined bleached kraft pulp (bleaching sequence: A-ZD-EOP-D-Z-P
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brightness: 86 % ISO) obtained from Botnia (Kaskö) was disintegrated according ISO
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5263:1995(E). The disintegrated pulp was dewatered by centrifugation and the dry matter
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content according to ISO 638-1978 was determined to be approximately 30 %. The total
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amount of anionic groups in the pulp was determined by the methylene blue sorption method
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(Fardim, Moreno & Holmbom, 2005) to be 99 μmol/g. The amount of anionic groups on the
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fiber surfaces was analyzed by XPS measurements of the methylene blue treated pulps (Fardim,
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Moreno & Holmbom, 2005). Deionized water was obtained from a Millipore Milli-Q system
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and possessed a conductivity of 0.05 μS. Photoactive cationic cellulose derivatives (PCCD-1
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and PCCD-2, Figure 1) were prepared as described (Wondraczek, Pfeifer & Heinze, 2012).
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Briefly, microcrystalline cellulose dissolved in N,N-dimethylacetamide (DMA)/LiCl was
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allowed to react with 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetic acid by the activation of
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the carboxylic acid with equimolar amounts of N,N-carbonyldiimidazole (CDI). Products with
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different degrees of substitution (DS) were obtained by the variation of the molar ratio of
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polymeric repeating unit to carboxylic acid and CDI. Subsequently, the photoactive cellulose
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esters dissolved in DMA were reacted with (3-carboxypropyl)trimethylammonium chloride in
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the presence of equimolar amounts of CDI. The final products were isolated by precipitation in
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acetone and washed with acetone and ethanol and further purified using dialysis against water
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through a cellulose membrane (Spectra/Porr; MWCO = 3500 g/mol) and subsequent
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lyophilisation. Characterization of the PCCDs was done using NMR and IR spectroscopy and
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described elsewhere in detail (Wondraczek, Pfeifer & Heinze, 2012). The PCCD stock
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solutions were prepared by dissolving of PCCDs in 10-5 M sodium bicarbonate solution under
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stirring. All other chemicals were purchased from Sigma Aldrich and used as obtained.
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2.2. Methods
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2.2.1 Adsorption experiments
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2.2.1.1. The design of the adsorption experiments
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The adsorption experiments were designed considering conditions during papermaking process.
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Thus, a pulp consistency of 0.6 % and neutral or weakly acid media were chosen similar to the
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properties of suspensions fed to paper machines. Sodium bicarbonate solution was used as a
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buffer to control the pH and electrolyte concentration of the suspension, which are known to
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influence the adsorption of charged polymers. The concentration of sodium bicarbonate
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solution was based on a preliminary study that showed a better adsorption of the derivatives at
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low electrolyte concentration (10-5 M). Different amounts of the PCCDs were used to study
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their adsorption behavior onto the fiber surfaces, preferably, in the range of low dosages. The
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influence of the time and temperature on the adsorption was not in the focus of the paper.
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2.2.1.2. Adsorption conditions
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Dewatered pulp corresponding to 150 mg of oven-dry pulp was dispersed in 10-5 M sodium
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bicarbonate solution. Different volumes of the PCCDs stock solutions were added
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corresponding to polymer dosages of 0.5, 1.6, 3.2, 5.0, 7.0, or 10.0 % (w/w, based on dry pulp),
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respectively. The final pulp consistency was adjusted to 0.6 % and the slurries were stirred at
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270 rpm for 17 h at room temperature. Subsequently, the pulp fibers were separated from the
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aqueous media using a self-designed sheet former, which consisted of a hollow cylinder with
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the inner diameter of 36 mm, 200 mesh screen, ceramic funnel and suction flask. The mesh was
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placed between the cylinder and the funnel in order to form even sheets. Finally, the pads
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obtained were pressed and dried according to SCAN-CM 11:95, yielding sheets with a
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grammage of about 148 g/m . All adsorption experiments were done shielded from light and the
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sheets were kept in the dark in order to avoid undesired photoreactions.
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2.2.2. Quantification of the PCCDs adsorption onto the pulp fibers
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The amount of the PCCDs in filtrates collected during the preparation of the sheets was
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quantified by UV-Vis spectroscopy. For this purpose, the UV-Vis spectra of the different
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PCCDs dissolved in aqueous sodium bicarbonate solution (10-5 M) were measured in the
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wavelength range of 200-400 nm. The concentration range applied was 0.01 to 0.07 g/l. Two
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calibration curves were obtained by plotting the absorbance at 320 nm against the
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concentration. Utilizing this calibration curves the concentrations of the PCCDs in the filtrates
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was determined from their absorption value at 320 nm. Subsequently, the amount of the 6 Page 6 of 31
cellulose derivatives adsorbed on the pulp fibers was calculated by subtracting the amount of
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the polymers in the filtrates from the amount added. A blank sample was prepared without
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addition of pulp to evaluate the amount of PCCDs adsorbed on the used glassware.
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2.2.3. Interactions of the PCCDs with model surfaces
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The adsorption of the cellulose derivatives onto the modified sensors was measured by a
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SPR-Navi 200 instrument. All solutions used for the SPR measurements were filtered, degassed
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and centrifuged to prevent introduction of impurities and air into the instrument. Basic gold
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sensor plates were purchased from Bionavis and treated with 11-mercapto-undecanoic acid, 11-
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mercapto-1-undecanol or 1-dodecanethiol in order to obtain self-assembled monolayers (SAM)
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possessing anionic (SAM-COOH), hydrophilic (SAM-OH) and hydrophobic (SAM-CH3)
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moieties (Schwikal, Heinze, Saake, Puls, Kaya & Esker, 2011; Kaya et al. 2009). The sensor
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slide containing the SAM was placed into the SPR flow cell and 10-5 M aqueous sodium
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bicarbonate solution was introduced in the instrument at 23 °C with a flow rate of 20 μl/min for
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at least 0.5 h until a stable baseline was obtained. Subsequently, the PCCD solution (0.2 g/l in
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-5 10 M aqueous sodium bicarbonate solution) was injected into the flow cell via a switch valve
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and allowed to run for 5 minutes at the same flow rate and temperature as described above.
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Finally, the system was rinsed with 10-5 M aqueous sodium bicarbonate solution to remove
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unbound polymer. Experiments were run at least four times and the reported values indicate the
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average ± standard deviation.
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2.2.4. Surface Analysis of the modified pulp fibers
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The time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements were carried
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out using a Physical Eletronics ToF-SIMS TRIFT II instrument equipped with a primary ion
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beam of 69Ga+ (liquid metal ion source) and an electron flood gun for charge compensation. The
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applied acceleration voltage was 25 kV, used in positive ion mode. A raster size of
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200x200 μm2 was scanned for 8 min in parallel measurement, and one scanning was made
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using the raster size 100x100 μm2. As a reference, the PCCD was analysed after dissolution in
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water, dropping on an ash-free filter paper and air-drying. Both positive and negative spectra of
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the reference were investigated in order to identify the characteristic fragments in the surface
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mass spectrum.
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2.2.5. Optical properties of the modified pulp fibers
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Diffuse reflectance UV-Vis spectra of the pulp sheets were measured with Shimadzu 2600
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spectrophotometer equipped with an integrating sphere ISR-2600 Plus. Fluorescence of the
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pulp sheets was studied using a Perkin-Elmer LS50B spectrofluorometer.
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2.2.6. Photocrosslinking
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Photocrosslinking reaction was performed by Bluepoint 4 UV-source (Hönle AG, Munich,
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Germany) equipped with BP4 320-390 nm filter.
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2.2.7. Mechanical properties of modified hand-sheets
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For evaluation of mechanical properties, hand-sheets were prepared according to SCAN-CM
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64:00 using a standard sheet former. Tensile strength of the hand-sheets was measured
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according to T 494 om-01 at 50 % relative humidity and 23 °C. To investigate the effect of the
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crosslinking, the samples were irradiated from both-sides for 7 min before the measurements.
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RESULTS AND DISCUSSIONS 8 Page 8 of 31
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3.1. Interaction of PCCDs with the pulp fibers Multifunctional cellulose-based polyelectrolytes decorated with photoactive 2-[(4-methyl-2-
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oxo-2H-chromen-7-yl)oxy]acetate and cationic (3-carboxypropyl)trimethylammonium chloride
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ester moieties were applied for the modification of eucalyptus unrefined bleached kraft pulp
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(Figure 1). The different photoactive cationic cellulose derivatives (PCCDs) possess a degree of
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substitution of cationic groups (DSN+) of 0.34 and photoactive groups (DSPhoto) of 0.11 (PCCD-
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1) and 0.37 (PCCD-2), respectively. Different amounts of aqueous PCCD solutions were added
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to aqueous pulp suspensions of equal concentration to study the adsorption behavior of the
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additives and to determine the feasible dosage in respect to relative adsorbed amount. The
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concentration of PCCD in solutions was determined before and after their contact with the pulp
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fibers by means of UV-Vis spectroscopy and from the difference, the amounts of PCCDs
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adsorbed were calculated. As shown in Figure 2a the sorption isotherms were obtained by
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plotting the amount of positive charge adsorbed per gram of pulp (based on dry weight) versus
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the amount of the charge in solution after the contact with the pulp fibers. The adsorption
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isotherms did not reach a saturation plateau, which means that a maximum adsorption capacity
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of the fibers was not achieved, and the amount of adsorbed cationic groups was much lower
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than the amount of the available anionic groups on the fiber surfaces (73 μmol/g).
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((Figure 1))
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((Figure2))
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The adsorption data of both PCCDs were treated in accordance with two classical adsorption
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models for liquid-solid interface, the Langmuir and Freundlich models described by Equation
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(1) and Equation (2), respectively (Fardim, Moreno & Holmbom, 2005). C/S = 1/(ns × KL) + C/ns, (1)
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Where C is the equilibrium concentration of PCCDs in solution (μmol/L), S is the amount of
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PCCDs adsorbed at equilibrium (μmol/g), KL is the Langmuir constant (L/μmol), and ns is a
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monolayer sorption capacity at equilibrium (μmol/g). KL and ns can be estimated from the
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intercept and slope of the linear fit to the plot of C/S against C.
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Log S = Log A + (1/n) × log C,
(2)
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Where A and n are the Freundlich constants related to adsorption capacity and adsorption
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intensity. A and n can be estimated in the same manner as described above for KL and ns of the
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Langmuir model.
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Figures 2c, d show plots of C/S vs C and log S vs log C with fitted linear regressions. The
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2 comparison of the linear fits showed higher correlation coefficient R for the Freundlich
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equation than for the Langmuir one for both PCCDs (Table 1). This indicates that the
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adsorption of the PCCDs is well described by the Freundlich model. The model refers to the
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adsorption onto a heterogeneous surface with different affinities of bonding sites. Also it
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assumes infinite adsorption with an increase of added dosage of the adsorbate and that the
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mechanism of adsorption involves both adsorbate-adsorbent and adsorbate-adsorbate
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interactions (Itodo, 2011).
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The adsorption intensity (n) estimated from the Freundlich equation is higher than 2 (Table
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1), indicating good adsorption characteristics of PCCDs with respect to cellulosic fibers
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(Tiwari, Gupta, Pandey, Singh &Mishra, 2009). The slope 1/n (Table 1) was lower than unity
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implying favourable adsorption of PCCDs at low concentrations (Figure 2b) (Guo, 2008).
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Moreover, it could be noticed that there was no significant difference in the adsorption capacity
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(A) of the fibers for the different PCCDs. This indicated that the adsorption was governed
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predominately by electrostatic interactions and that the hydrophobic groups had only minor
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influence.
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((Table 1))
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3.2. Interaction of PCCDs with the model surfaces For a better understanding of the adsorption phenomena described above the interaction of
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the PCCDs with different model surfaces was investigated by SPR measurements. For this
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purpose model surfaces, bearing anionic (SAM-COOH), hydrophilic (SAM-OH), or
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hydrophobic (SAM-CH3) groups, were used instead of cellulose model surfaces in order to
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distinguish between the different driving forces of the adsorption of the PCCDs. As expected, a
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fast, strong and irreversible interaction occurs between the different cationic cellulose
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derivatives and the negatively charged SAM-COOH surface (Figure 3a). In contrast, only a
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very weak interaction was found between the hydrophilic surface (SAM-OH) and the cellulose
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derivatives (Figure 3c). In addition, it could be noticed that the cellulose derivatives were
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removed almost completely from the surface by washing with aqueous solution of sodium
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bicarbonate. All studied derivatives had certain affinity to the hydrophobic surface where they
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were attached irreversibly (Figure 3b). The increase of the DSPhoto improved the adsorption,
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which could be explained by the enhanced hydrophobic interactions. It is worth to note that the
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adsorption plateau was reached much faster for the anionic surface than for the hydrophobic
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one. Summing up, it can be concluded that an addition of the photoactive groups facilitates the
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adsorption of the cationic derivatives, although the dominant driving force for adsorption is
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electrostatic interaction.
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The SPR data were in agreement with the adsorption isotherms of the PCCDs on the fibers.
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As it was shown, the adsorption of the PCCDs onto the fibers could be explained by the
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Freundlich model, which considers energetically heterogeneous surface and adsorbate-
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adsorbate interaction resulting in creation of the layers on the surface. From the SPR 11 Page 11 of 31
measurements it is suggested that the adsorption takes place predominately on the negatively
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charged groups of the fibers followed by intermolecular interactions between the hydrophobic
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groups of the cellulose derivatives. Moreover, interactions between the hydrophobic groups of
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the fibers and the PCCDs could also be involved in the adsorption mechanism. Combining the
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findings from the different analytical approaches it is possible to draw a tentative hypothesis of
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the adsorption of the PCCDs on the fibers (Figure 4).
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((Figure 4))
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The modified fibers were analysed with ToF-SIMS in order to gain information about the
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surface distribution of the fiber modifying agents and surface chemical composition of the
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modified fibers. Figure 5 shows the ToF-SIMS spectra of PCCD-1 as a reference and the fiber
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treated with this derivative. On the basis of the reference spectrum (Figure 5b), several
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characteristic signals from secondary ions with m/z = 58, 159, 189, and 235 were assigned to
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ions originating from the cationic- (fragment [C3H8N]+) and the photoactive substituent
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(fragments [C10H7O2]+, [C11H9O3]+ and [C12H11O5]+), respectively. In the surface mass spectra of
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the modified samples (Figure 5a), these peaks had high intensity, which additionally proved
267
adsorption of the cellulose derivatives onto the fiber surfaces. Accordingly, the mentioned
268
peaks were not found in the previously published spectrum of the reference fiber sample (Neto,
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Silvestre, Evtuguin, Freire, Pinto & Santiago, 2004). As shown in Figure 6 the chemical
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immaging of the total secondary ions yields qualitatively the same image as the mapping of the
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characteristic fragments. Additionally, it could be noticed, that this effect was independent from
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the dosage of the PCCD applied. This clearly indicated a homogeneous surface distribution of
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the fiber modifying agents within the resolution of ToF-SIMS.
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((Figure 5))
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((Figurer 6))
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Light absorption by modified and reference fibers were studied with UV-Vis spectroscopy
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(Figure 7). Unmodified fiber sheets showed two clear peaks at ca. 240 and 280 nm that
280
originate from hexeneuronic acid groups and residual lignin, respectively (Liitia & Tamminen,
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2007). The absorbance spectra of the modified pulps have an additional absorbance band with
282
the maximum at around 320 nm arising from the
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chromophor. A higher derivative dosage resulted in an increase of the peak intensity, thus the
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highest absorbance at 320 nm was reached at the highest dosage applied (10 % (w/w),
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Figure 7b).
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((Figure 7))
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In addition to light absorbance, coumarin-containing compounds possess natural fluorescence
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(Trenor, Shultz, Love & Long, 2004). For fluorescence measurements the pulp sheets were
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irradiated with UV light at a wavelength of 320 nm corresponding to the absorption maximum
290
of the derivatives. Reference pulp samples also show a natural fluorescence (Liukko, Tasapuro
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& Liitia, 2007) that had considerably lower emission intensity at the applied excitation
292
wavelength, compared to the samples containing the PCCDs (Figure 7c). A maximum
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fluorescence intensity of the modified sample was found to be at around 380 nm.
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3.5. Photocrosslinking, preliminary experiments
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Previous studies revealed that polysaccharide derivatives bearing such photoactive groups are
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able to reversibly crosslink under UV light irradiation (Wondraczek, Pfeifer & Heinze, 2012).
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Figure 8 shows changes in UV spectra measured from the fiber surfaces after it was exposed to
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the UV light. A decrease of absorbance at the wavelength in the vicinity of 320 nm could serve
299
as an evidence of the crosslinking reaction which occured via [2+2] cycloaddition of ethylene
300
groups. The spectra also revealed that the reaction was fast and was completed within less than
301
five minutes. Hence, the fluorescence of the modified fibers was drastically reduced (spectra
302
not shown). Moreover, the intersection point of the spectra at 357 nm, the isosbestic point,
303
indicates an uniform reaction, meaning that no degradation of the cellulose derivatives or
304
formation of side products took place (Sauer, Hofkens & Enderlein, 2011). The irradiation of
305
the unmodified pulp fibers under similar conditions caused no changes in the UV spectra,
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which prove their stability under UV light.
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As a preliminary study, a tensile index of the hand-sheets was measured. The tensile index
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refers to the strength and it is a crucial quality for the papers, which undergo direct tensile
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stress e.g. wrapping paper and paper bags (T 494 om-01). Also, printing paper grades should
310
have high tensile strength to resist web breaking during printing and converting (T 494 om-01).
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The tensile index increased by 22 % after the modification of pulp with PCCD-2 at the dosage
312
of 3.2 % (w/w, based on dry pulp). After irradiation of the modified sample, the tensile index
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increased by 39 % compared with the modified sample and by 70 % compared with the
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reference. The tensile index is directly related to the bonding ability of the fibers. Hence, the
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strength values showed that the modification and irradiation improved bonding between the
316
fibers. It can be assumed that an addition of the flexible chains of the cellulose derivative
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increased the amount of the hydrophilic and hydrophobic groups available for interactions in
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the zones of fiber contact. The subsequent photocrosslinking led to a creation of covalent bonds
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between the pendant coumarin groups and thus the irradiated samples possessed better
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mechanical properties. The photocontrol of the properties of the fiber interface (e.g. wetting) and directly related
322
properties (e.g. fiber to fiber bonding) is currently under investigation and will be published
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elsewhere.
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((Figure 8))
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326 3. CONCLUSIONS
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Novel multifunctional cellulose derivatives decorated with both cationic and photoactive
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moieties were adsorbed onto the pulp fiber surfaces. Based on the results from UV-Vis
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spectroscopy and SPR measurements, it was found that the electrostatic interaction was the
331
main driving force of the adsorption of the derivatives onto the fibers. However, hydrophobic
332
interactions between the fibers and the cellulose derivatives as well as between the cellulose
333
derivatives themselves also contributed to the adsorption. The mechanism of the adsorption was
334
well described by the Freundlich model. ToF-SIMS imaging showed that the derivatives were
335
evenly distributed on the fiber surfaces independently of the dosage of the derivatives and DS
336
of the photoactive group. UV light irradiation of the modified fibers triggered fast
337
photocrosslinking of the attached photoactive groups resulting in changes of light absorption
338
and fluorescence of the fibers. It can be suggested that the modified fibers bearing such
339
photoactive groups have a great potential for application in novel bio-based light-responsive
340
materials. Preliminary experiments on the photocontrol of the mechanical properties showed
341
remarkable increase of the tensile index. Therefore a more detailed study on the photocontrol of
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these properties at the structural levels of single fibers and fiber network is currently under
343
preparation and will be reported elsewhere.
344 ACKNOWLEDGEMENTS
346
Finnish Funding Agency for Technology and Innovation (Tekes) is acknowledged for
347
funding. The authors are grateful to Professor Peter Mattjus from Åbo Akademi University for
348
the access to the SPR instrument and to Josefin Halin for her assistance with the SPR
349
measurements. Annett Pfeifer and Cíntia Salomão Pinto Zarth are acknowledged for providing
350
PCCDs and CCDs.
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351 352
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Guo, C., & Gemeinhart, R. A. (2008). Understanding the adsorption mechanism of chitosan
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Kaya, A., Du, X., Liu, Z., Lu, J.W., Morris J.R., Glasser, W.G., Heinze, T., & Esker, A.R.
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(2009). Surface plasmon resonance studies of pullulan and pullulan cinnamate adsorption onto
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Köhnke, T., Lund, K., Brelid, H., & Westman, G. (2010). Kraft pulp hornification: A closer
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look at the preventive effect gained by glucuronoxylan adsorption. Carbohydrate Polymers, 81,
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Li, W., Lynch, V., Thompson, H., & Fox, M. A. (1997), Self-assembled monolayers of 7-(10-
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Liitia, T., & Tamminen, T. (2007). How to evaluate the kraft pulp brightness stability? In 3rd
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International Colloquium on Eucalyptus Pulp. Brazil.
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Liukko, S., Tasapuro, V., & Liitia, T. (2007). Fluorescence spectroscopy for chromophore
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studies on bleached kraft pulps. Holzforschung, 61, 509–515.
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Ly, E.h.B., Bras, J., Sadocco, P., Belgacem, M.N., Dufresne, A., & Thielemans, W. (2010).
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Surface functionalization of cellulose by grafting oligoether chains. Materials Chemistry and
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Muguet, M. C. S., Pedrazzi, C., & Colodette, J. L. (2011). Xylan deposition onto eucalypt pulp
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fibers during oxygen delignification. Holzforschung, 65, 605-612.
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Neto C. P.; Silvestre A. J. D.; Evtuguin, D. V.; Freire C. S. R, Pinto P. C. R., Santiago A. S.
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(2004). Nordic Pulp and Paper Research Journal, 19, 513-520.
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Pan, S., & Ragauskas, A. J. (2012). Preparation of superabsorbent cellulosic hydrogels.
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Carbohydrate Polymers, 87, 1410– 1418.
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Sauer, M., Hofkens, J., & Enderlein, J. (2011). Handbook of Fluorescence Spectroscopy and
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Imaging. Wiley-VCH.
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Schönberg, C., Oksanen, T., Suurnäkki, A., Kettunen, H., & Buchert, J. (2001). The importance
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of xylan for the strength properties of spruce kraft pulp fibers. Holzforschung, 55, 639-644.
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Schwikal, K., Heinze, T., Saake, B., Puls, J., Kaya, A., & Esker, A. R. (2011). Properties of
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spruce sulfite pulp and birch kraft pulp after sorption of cationic birch xylan. Cellulose, 18,
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727–737.
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Silva, T. C. F., Lucian, J. L., Lucia, L. A., de Oliveira, R. C., Oliveira, F. N., & Silva, L. H. M.
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(2011). Adsorption of chemically modified xylans on eucalyptus pulp and its effect on the pulp
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physical properties. Industrial and Engineering Chemistry Research, 50, 1138–1145.
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Sixta, H. (2006). Handbook of pulp, Wiley-VCH: Winheim, Vol. 1, p 9.
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Tiwari, S., Gupta, V. K., Pandey, P.C., Singh, H., Mishra, P.K. (2009). Adsorption chemistry of
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light harvesting to photo-cross-linkable tissue scaffolds. Chemical Reviews, 104, 3059-3077.
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Utsel, S. (2012). Surface modification of cellulose-based materials for tailoring of interfacial
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interactions. Doctoral thesis in pulp and paper chemistry and technology. Stockholm, Sweden.
417
Vega, B., Petzold-Welcke, K., Fardim, P., & Heinze, T. (2012). Studies on the fiber surfaces
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modified with xylan polyelectrolytes. Carbohydrate Polymers, 89, 768-776.
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Wondraczek, H., Kotiaho, A., Fardim, P., & Heinze, T. (2011). Photoactive polysaccharides.
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Carbohydrate Polymers, 83, 1048–1061.
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Wondraczek, H., Pfeifer, A., & Heinze, T. (2010). Synthetic photocrosslinkable polysaccharide
422
sulfates. European Polymer Journal, 46, 1688–1695.
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Wondraczek, H., Pfeifer, A., & Heinze, T. (2012). Water soluble photoactive cellulose
424
derivatives: synthesis and characterization of mixed 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]
425
acetic acid- (3-carboxypropyl)trimethylammonium chloride esters of cellulose. Cellulose, 19,
426
1327-1335.
427
Zhou, J., Ma, Y., Zhang, J., Tong, J. (2009). Influence of surface photocrosslinking on
428
properties of thermoplastic starch sheets. Journal of Applied Polymer Science, 112, 99-106.
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Zhou, J., Zhang, J., Ma, Y., & Tong, J. (2008). Surface photo-crosslinking of corn starch
430
sheets.
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Carbohydrate
Polymers,
74,
405-410.
20 Page 20 of 31
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Figure 1. Schematic presentation of the structure of the photoactive cationic cellulose derivative
433
(PCCD).
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Figure 2. (a) The photoactive cationic cellulose derivatives (PCCDs) adsorption isotherms onto
436
the fibers. (b) Amount of the PCCDs adsorbed as the percentage of added amount. Linear
437
representation of the isotherms according to Langmuir (c) and Freundlich (d) models. DS of
438
cationic group is 0.34 and DS of photoactive group is 0.11 and 0.37 for PCCD-1 and PCCD-2,
439
respectively.
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440 441
Table 1. Adsorption data of photoactive cationic cellulose derivatives (PCCDs) treated with
442
Langmuir and Freundlich equations
Langmuir
Freundlich
KL
R²
R²
PCCD-1a
36.76
0.007
0.8980
0.9606
PCCD-2a
30.30
0.011
0.9478
0.9915
443
a
444
PCCD-2, respectively.
n
1/n
A
2.86
0.35
3.0
0.39
2.4
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ns
cr
Derivative
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Adsorption model
2.56
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DS of cationic group is 0.34 and DS of photoactive group is 0.11 and 0.37 for PCCD-1 and
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Figure 3. Adsorption isotherms of cationic and photoactive cationic cellulose derivatives (CCD,
448
PCCDs) onto SAM-COOH (a), SAM-CH3 (b) and SAM-OH (c) surfaces. DS of cationic group
449
is 0.34 and DS of photoactive group is 0.11 and 0.37 for PCCD-1 and PCCD-2, respectively.
450
DS of cationic group is 0.29 for CCD.
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Figure 4. A schematic illustration of the adsorption of the photoactive cationic cellulose
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derivative
onto
the
pulp
fibers.
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(PCCD)
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25 Page 25 of 31
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Figure 5. ToF-SIMS spectrum m/z 0-250 of the pulp modified by photoactive cationic cellulose
459
derivative (a) and the reference polymer (b).
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461 462
Figure 6. ToF-SIMS images of bleached unrefined eucalyptus kraft pulp treated with
463
photoactive cationic cellulose derivative (PCCD-1, DSPhoto of 0.11, DSCat of 0.34) of applied
464
dosages 0.5 % (w/w) (a, b) and 10.0 % (w/w) (c, d). (a, c) and (b, d) total ion and characteristic
465
ion images, respectively.
466 27 Page 27 of 31
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Figure 7. (a) UV-Vis spectra of the fibers modified by photoactive cationic cellulose derivative
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(PCCD-1, DSPhoto of 0.11, DSCat of 0.34) at dosages 0.5-10 % (w/w). (b) Maximum absorbance
469
of the fibers as a function of adsorbed amount of photoactive groups of the PCCD-1. (c)
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Fluorescence spectra of the reference and modified by PCCD-1 sample.
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Figure 8. UV-Vis spectra of unmodified pulp fibers (reference) and the fibers modified by
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photoactive cationic cellulose derivative (PCCD-1, DSPhoto of 0.11, DSCat of 0.34, dosage of
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3.2 % (w/w)) recorded before and after irradiation with UV light.
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Graphical abstract
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Highlights (for review)
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Surface modification of pulp fibers with multifunctional cellulose derivatives Evaluation of the mechanism of adsorption by SPR Surface characterization by means of ToF-SIMS Preparation of photoactive fibers
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S0144-8617(14)00446-9 http://dx.doi.org/doi:10.1016/j.carbpol.2014.04.089 CARP 8850
To appear in: Received date: Revised date: Accepted date:
11-8-2013 4-12-2013 22-4-2014
Please cite this article as: Grigoray, O., Wondraczek, H., Heikkil¨a, E., Fardim, P., & Heinze, T.,Photoresponsive Cellulose Fibers by Surface Modification with Multifunctional Cellulose Derivatives, Carbohydrate Polymers (2014), http://dx.doi.org/10.1016/j.carbpol.2014.04.089 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photoresponsive Cellulose Fibers by Surface
2
Modification with Multifunctional Cellulose
3
Derivatives
4
Olga Grigoraya, Holger Wondraczeka, Elina Heikkiläa, Pedro Fardima,b,*, Thomas Heinzea,c
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a
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20500, Åbo, Finland; bCenter of Excellence for Advanced Materials Research (CEAMR)-King
7
Abdulaziz University -Jeddah 21589, Saudi Arabia; cCenter of Excellence for Polysaccharide
8
Research, Institute of Organic Chemistry and Macromolecular Chemistry, Friedrich Schiller
9
University of Jena, Humboldtstraße 10, 07743 Jena, Germany. *Corresponding author.
10
Telephone: +358504096424. E-mail address: [email protected]
11
ABSTRACT: Eucalyptus bleached kraft pulp fibers were modified by adsorption of novel bio-
12
based multifunctional cellulose derivatives in order to generate light responsive surfaces. The
13
cellulose derivatives used were decorated with both cationic groups (degree of substitution,
14
DS of 0.34) and photoactive groups (DS of 0.11 and 0.37). The adsorption was studied by UV-
15
Vis spectroscopy, surface plasmon resonance (SPR) and time-of-flight secondary ion mass
16
spectroscopy (ToF-SIMS). The adsorption isotherms followed the Freundlich model and it
17
turned out that the main driving force for the adsorption was electrostatic interaction.
18
Moreover, strong indications for hydrophobic interactions between the fibers and the
19
derivatives and the derivatives themselves were found. ToF-SIMS imaging revealed an even
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Laboratory of Fiber and Cellulose Technology, Åbo Akademi University, Porthansgatan 3, FI-
1 Page 1 of 31
distribution of the derivatives on the fiber surfaces. The modified fibers underwent fast
21
photocrosslinking under UV-irradiation as demonstrated by light absorbance and fluorescence
22
measurements. Thus, our results proved that the modified fibers exhibited light-responsive
23
properties and can potentially be used for the manufacture of smart bio-based materials.
24
KEYWORDS: eucalyptus pulp fibers, cationic cellulose derivatives, photo-activity, surface
25
modification, ToF-SIMS, SPR
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26
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1. INTRODUCTION
27
Pulp fibers are intermediate products mainly used in the production of paper and paper
29
boards (Freire, Silvestre, Gandini & Neto, 2011). However, pulp fibers represent also an
30
attractive substrate for the manufacturing of novel materials owing to their outstanding
31
characteristics. They are relatively cheap, renewable, biodegradable, and can be recycled or
32
converted to energy by combustion at the end of the life cycle of the product. Additionally, the
33
fibers have good mechanical properties and relatively low density (Freire, Silvestre, Gandini &
34
Neto, 2011; Belgacem & Gandini, 2008; Ly, Bras, Sadocco, Belgacem, Dufresne &
35
Thielemans, 2010).
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36
Currently more than 180 million ton per year of pulp fibers (Sixta, 2006) are produced by
37
well-established technologies like mechanical, chemo-mechanical and chemical pulping
38
methods. The properties and the chemical composition (bulk and surface) of pulp fibers from
39
the classical pulping techniques are very well-studied and vary significantly depending on the
40
raw material and the way of the manufacturing. In order to utilize pulp fibers in new value
41
added products, current research is focusing on the modification of their surfaces while keeping
42
bulk properties such as morphology and native fiber wall structure (Grönqvist, Rantanen, Alen, 2 Page 2 of 31
Mattinen, Buchert & Viikari, 2006; Hedenberg & Gatenholm, 1995; Pan & Ragauskas, 2012).
44
Biological (e.g. by enzymes, fungi), chemical (carboxylation, grafting) or plasma treatments
45
and adsorption methods can be applied for this purpose (Gandini & Pasquini, 2012; Utsel,
46
2012).
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From our point of view, the adsorption method appears to be the most promising tool among
48
the techniques mentioned since it is water-based and can be easily combined with existing fiber
49
line stages (Maguet, Pedrazzi & Colodette, 2011). Traditionally, the research in fiber chemistry
50
and technology has been mainly focusing on the improvement of mechanical, drainage and
51
optical properties of pulp by adsorbing polysaccharides and polysaccharide-based
52
polyelectrolytes for paper manufacture (Köhnke, Lund, Brelid & Westman, 2010; Schönberg,
53
Oskanen, Suurnäkki, Kettunen & Buchert, 2001). In this regard, the application of underutilized
54
biomass derived polysaccharides like hemicelluloses recently came into focus (Schwikal,
55
Heinze, Saake, Puls, Kaya & Esker, 2011; Vega, Petzold-Welcke, Fardim & Heinze, 2012;
56
Silva, Colodette, Lucia, de Oliveira, Oliveira, & Silva, 2011).
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Functional biopolymers with stimuli-responsive capabilities can be used for the modification
58
of pulp in order to generate new fiber-based materials, whose properties could be triggered by
59
an external stimulus. Multifunctional polysaccharide derivatives decorated with cationic and, in
60
particular, light responsive substituents can be applied for this purpose. The light induced
61
alterations, including isomerization, dissociation or crosslinking, of the photoactive substituents
62
were described to cause changes in physical properties of the polymers itself or polymeric
63
solutions (Irie, 1990; Wondraczek, Pfeifer & Heinze, 2011; Wondraczek, Kotiaho, Fardim &
64
Heinze, 2010). Consequently, the possibility to control the properties of the interface of the
65
fibers to which they are attached can be expected, and the properties of fiber-based materials
66
and composites might be influenced.
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The focus of the studies presented in this paper was to gain information about the interaction
68
of pulp fibers and cellulose derivatives, which were decorated on one hand with cationic (3-
69
carboxypropyl)trimethylammonium chloride ester moieties and on the other with hydrophobic
70
(photoactive) 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetate substituents (Wondraczek,
71
Pfeifer & Heinze, 2012). In particular, the influence of the hydrophobic/hydrophilic balance of
72
the derivatives on the adsorption behavior was investigated and a tentative hypothesis of the
73
interaction of pulp fibers and such multifunctional cellulose derivatives was suggested.
74
Moreover, the fiber surface composition and lateral distribution was evaluated by means of
75
ToF-SIMS spectrometry and imaging.
76
2.1. Materials
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2. EXPERIMENTAL SECTION
77
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Eucalyptus air-dried unrefined bleached kraft pulp (bleaching sequence: A-ZD-EOP-D-Z-P
80
brightness: 86 % ISO) obtained from Botnia (Kaskö) was disintegrated according ISO
81
5263:1995(E). The disintegrated pulp was dewatered by centrifugation and the dry matter
82
content according to ISO 638-1978 was determined to be approximately 30 %. The total
83
amount of anionic groups in the pulp was determined by the methylene blue sorption method
84
(Fardim, Moreno & Holmbom, 2005) to be 99 μmol/g. The amount of anionic groups on the
85
fiber surfaces was analyzed by XPS measurements of the methylene blue treated pulps (Fardim,
86
Moreno & Holmbom, 2005). Deionized water was obtained from a Millipore Milli-Q system
87
and possessed a conductivity of 0.05 μS. Photoactive cationic cellulose derivatives (PCCD-1
88
and PCCD-2, Figure 1) were prepared as described (Wondraczek, Pfeifer & Heinze, 2012).
89
Briefly, microcrystalline cellulose dissolved in N,N-dimethylacetamide (DMA)/LiCl was
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4 Page 4 of 31
allowed to react with 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]acetic acid by the activation of
91
the carboxylic acid with equimolar amounts of N,N-carbonyldiimidazole (CDI). Products with
92
different degrees of substitution (DS) were obtained by the variation of the molar ratio of
93
polymeric repeating unit to carboxylic acid and CDI. Subsequently, the photoactive cellulose
94
esters dissolved in DMA were reacted with (3-carboxypropyl)trimethylammonium chloride in
95
the presence of equimolar amounts of CDI. The final products were isolated by precipitation in
96
acetone and washed with acetone and ethanol and further purified using dialysis against water
97
through a cellulose membrane (Spectra/Porr; MWCO = 3500 g/mol) and subsequent
98
lyophilisation. Characterization of the PCCDs was done using NMR and IR spectroscopy and
99
described elsewhere in detail (Wondraczek, Pfeifer & Heinze, 2012). The PCCD stock
100
solutions were prepared by dissolving of PCCDs in 10-5 M sodium bicarbonate solution under
101
stirring. All other chemicals were purchased from Sigma Aldrich and used as obtained.
102
2.2. Methods
103
2.2.1 Adsorption experiments
104
2.2.1.1. The design of the adsorption experiments
105
The adsorption experiments were designed considering conditions during papermaking process.
106
Thus, a pulp consistency of 0.6 % and neutral or weakly acid media were chosen similar to the
107
properties of suspensions fed to paper machines. Sodium bicarbonate solution was used as a
108
buffer to control the pH and electrolyte concentration of the suspension, which are known to
109
influence the adsorption of charged polymers. The concentration of sodium bicarbonate
110
solution was based on a preliminary study that showed a better adsorption of the derivatives at
111
low electrolyte concentration (10-5 M). Different amounts of the PCCDs were used to study
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their adsorption behavior onto the fiber surfaces, preferably, in the range of low dosages. The
113
influence of the time and temperature on the adsorption was not in the focus of the paper.
114
2.2.1.2. Adsorption conditions
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Dewatered pulp corresponding to 150 mg of oven-dry pulp was dispersed in 10-5 M sodium
116
bicarbonate solution. Different volumes of the PCCDs stock solutions were added
117
corresponding to polymer dosages of 0.5, 1.6, 3.2, 5.0, 7.0, or 10.0 % (w/w, based on dry pulp),
118
respectively. The final pulp consistency was adjusted to 0.6 % and the slurries were stirred at
119
270 rpm for 17 h at room temperature. Subsequently, the pulp fibers were separated from the
120
aqueous media using a self-designed sheet former, which consisted of a hollow cylinder with
121
the inner diameter of 36 mm, 200 mesh screen, ceramic funnel and suction flask. The mesh was
122
placed between the cylinder and the funnel in order to form even sheets. Finally, the pads
123
obtained were pressed and dried according to SCAN-CM 11:95, yielding sheets with a
124
grammage of about 148 g/m . All adsorption experiments were done shielded from light and the
125
sheets were kept in the dark in order to avoid undesired photoreactions.
126
2.2.2. Quantification of the PCCDs adsorption onto the pulp fibers
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The amount of the PCCDs in filtrates collected during the preparation of the sheets was
128
quantified by UV-Vis spectroscopy. For this purpose, the UV-Vis spectra of the different
129
PCCDs dissolved in aqueous sodium bicarbonate solution (10-5 M) were measured in the
130
wavelength range of 200-400 nm. The concentration range applied was 0.01 to 0.07 g/l. Two
131
calibration curves were obtained by plotting the absorbance at 320 nm against the
132
concentration. Utilizing this calibration curves the concentrations of the PCCDs in the filtrates
133
was determined from their absorption value at 320 nm. Subsequently, the amount of the 6 Page 6 of 31
cellulose derivatives adsorbed on the pulp fibers was calculated by subtracting the amount of
135
the polymers in the filtrates from the amount added. A blank sample was prepared without
136
addition of pulp to evaluate the amount of PCCDs adsorbed on the used glassware.
137
2.2.3. Interactions of the PCCDs with model surfaces
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The adsorption of the cellulose derivatives onto the modified sensors was measured by a
139
SPR-Navi 200 instrument. All solutions used for the SPR measurements were filtered, degassed
140
and centrifuged to prevent introduction of impurities and air into the instrument. Basic gold
141
sensor plates were purchased from Bionavis and treated with 11-mercapto-undecanoic acid, 11-
142
mercapto-1-undecanol or 1-dodecanethiol in order to obtain self-assembled monolayers (SAM)
143
possessing anionic (SAM-COOH), hydrophilic (SAM-OH) and hydrophobic (SAM-CH3)
144
moieties (Schwikal, Heinze, Saake, Puls, Kaya & Esker, 2011; Kaya et al. 2009). The sensor
145
slide containing the SAM was placed into the SPR flow cell and 10-5 M aqueous sodium
146
bicarbonate solution was introduced in the instrument at 23 °C with a flow rate of 20 μl/min for
147
at least 0.5 h until a stable baseline was obtained. Subsequently, the PCCD solution (0.2 g/l in
148
-5 10 M aqueous sodium bicarbonate solution) was injected into the flow cell via a switch valve
149
and allowed to run for 5 minutes at the same flow rate and temperature as described above.
150
Finally, the system was rinsed with 10-5 M aqueous sodium bicarbonate solution to remove
151
unbound polymer. Experiments were run at least four times and the reported values indicate the
152
average ± standard deviation.
153
2.2.4. Surface Analysis of the modified pulp fibers
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154
The time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements were carried
155
out using a Physical Eletronics ToF-SIMS TRIFT II instrument equipped with a primary ion
7 Page 7 of 31
beam of 69Ga+ (liquid metal ion source) and an electron flood gun for charge compensation. The
157
applied acceleration voltage was 25 kV, used in positive ion mode. A raster size of
158
200x200 μm2 was scanned for 8 min in parallel measurement, and one scanning was made
159
using the raster size 100x100 μm2. As a reference, the PCCD was analysed after dissolution in
160
water, dropping on an ash-free filter paper and air-drying. Both positive and negative spectra of
161
the reference were investigated in order to identify the characteristic fragments in the surface
162
mass spectrum.
163
2.2.5. Optical properties of the modified pulp fibers
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Diffuse reflectance UV-Vis spectra of the pulp sheets were measured with Shimadzu 2600
165
spectrophotometer equipped with an integrating sphere ISR-2600 Plus. Fluorescence of the
166
pulp sheets was studied using a Perkin-Elmer LS50B spectrofluorometer.
167
2.2.6. Photocrosslinking
te
Photocrosslinking reaction was performed by Bluepoint 4 UV-source (Hönle AG, Munich,
Ac ce p
168 169
Germany) equipped with BP4 320-390 nm filter.
170 171
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2.2.7. Mechanical properties of modified hand-sheets
172
For evaluation of mechanical properties, hand-sheets were prepared according to SCAN-CM
173
64:00 using a standard sheet former. Tensile strength of the hand-sheets was measured
174
according to T 494 om-01 at 50 % relative humidity and 23 °C. To investigate the effect of the
175
crosslinking, the samples were irradiated from both-sides for 7 min before the measurements.
176 177
RESULTS AND DISCUSSIONS 8 Page 8 of 31
178
3.1. Interaction of PCCDs with the pulp fibers Multifunctional cellulose-based polyelectrolytes decorated with photoactive 2-[(4-methyl-2-
180
oxo-2H-chromen-7-yl)oxy]acetate and cationic (3-carboxypropyl)trimethylammonium chloride
181
ester moieties were applied for the modification of eucalyptus unrefined bleached kraft pulp
182
(Figure 1). The different photoactive cationic cellulose derivatives (PCCDs) possess a degree of
183
substitution of cationic groups (DSN+) of 0.34 and photoactive groups (DSPhoto) of 0.11 (PCCD-
184
1) and 0.37 (PCCD-2), respectively. Different amounts of aqueous PCCD solutions were added
185
to aqueous pulp suspensions of equal concentration to study the adsorption behavior of the
186
additives and to determine the feasible dosage in respect to relative adsorbed amount. The
187
concentration of PCCD in solutions was determined before and after their contact with the pulp
188
fibers by means of UV-Vis spectroscopy and from the difference, the amounts of PCCDs
189
adsorbed were calculated. As shown in Figure 2a the sorption isotherms were obtained by
190
plotting the amount of positive charge adsorbed per gram of pulp (based on dry weight) versus
191
the amount of the charge in solution after the contact with the pulp fibers. The adsorption
192
isotherms did not reach a saturation plateau, which means that a maximum adsorption capacity
193
of the fibers was not achieved, and the amount of adsorbed cationic groups was much lower
194
than the amount of the available anionic groups on the fiber surfaces (73 μmol/g).
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195
((Figure 1))
196
((Figure2))
197
The adsorption data of both PCCDs were treated in accordance with two classical adsorption
198
models for liquid-solid interface, the Langmuir and Freundlich models described by Equation
199
(1) and Equation (2), respectively (Fardim, Moreno & Holmbom, 2005). C/S = 1/(ns × KL) + C/ns, (1)
200
9 Page 9 of 31
Where C is the equilibrium concentration of PCCDs in solution (μmol/L), S is the amount of
202
PCCDs adsorbed at equilibrium (μmol/g), KL is the Langmuir constant (L/μmol), and ns is a
203
monolayer sorption capacity at equilibrium (μmol/g). KL and ns can be estimated from the
204
intercept and slope of the linear fit to the plot of C/S against C.
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201
Log S = Log A + (1/n) × log C,
(2)
206
Where A and n are the Freundlich constants related to adsorption capacity and adsorption
207
intensity. A and n can be estimated in the same manner as described above for KL and ns of the
208
Langmuir model.
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205
Figures 2c, d show plots of C/S vs C and log S vs log C with fitted linear regressions. The
210
2 comparison of the linear fits showed higher correlation coefficient R for the Freundlich
211
equation than for the Langmuir one for both PCCDs (Table 1). This indicates that the
212
adsorption of the PCCDs is well described by the Freundlich model. The model refers to the
213
adsorption onto a heterogeneous surface with different affinities of bonding sites. Also it
214
assumes infinite adsorption with an increase of added dosage of the adsorbate and that the
215
mechanism of adsorption involves both adsorbate-adsorbent and adsorbate-adsorbate
216
interactions (Itodo, 2011).
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209
217
The adsorption intensity (n) estimated from the Freundlich equation is higher than 2 (Table
218
1), indicating good adsorption characteristics of PCCDs with respect to cellulosic fibers
219
(Tiwari, Gupta, Pandey, Singh &Mishra, 2009). The slope 1/n (Table 1) was lower than unity
220
implying favourable adsorption of PCCDs at low concentrations (Figure 2b) (Guo, 2008).
221
Moreover, it could be noticed that there was no significant difference in the adsorption capacity
222
(A) of the fibers for the different PCCDs. This indicated that the adsorption was governed
223
predominately by electrostatic interactions and that the hydrophobic groups had only minor
224
influence.
10 Page 10 of 31
225
((Table 1))
226 227
3.2. Interaction of PCCDs with the model surfaces For a better understanding of the adsorption phenomena described above the interaction of
229
the PCCDs with different model surfaces was investigated by SPR measurements. For this
230
purpose model surfaces, bearing anionic (SAM-COOH), hydrophilic (SAM-OH), or
231
hydrophobic (SAM-CH3) groups, were used instead of cellulose model surfaces in order to
232
distinguish between the different driving forces of the adsorption of the PCCDs. As expected, a
233
fast, strong and irreversible interaction occurs between the different cationic cellulose
234
derivatives and the negatively charged SAM-COOH surface (Figure 3a). In contrast, only a
235
very weak interaction was found between the hydrophilic surface (SAM-OH) and the cellulose
236
derivatives (Figure 3c). In addition, it could be noticed that the cellulose derivatives were
237
removed almost completely from the surface by washing with aqueous solution of sodium
238
bicarbonate. All studied derivatives had certain affinity to the hydrophobic surface where they
239
were attached irreversibly (Figure 3b). The increase of the DSPhoto improved the adsorption,
240
which could be explained by the enhanced hydrophobic interactions. It is worth to note that the
241
adsorption plateau was reached much faster for the anionic surface than for the hydrophobic
242
one. Summing up, it can be concluded that an addition of the photoactive groups facilitates the
243
adsorption of the cationic derivatives, although the dominant driving force for adsorption is
244
electrostatic interaction.
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245
The SPR data were in agreement with the adsorption isotherms of the PCCDs on the fibers.
246
As it was shown, the adsorption of the PCCDs onto the fibers could be explained by the
247
Freundlich model, which considers energetically heterogeneous surface and adsorbate-
248
adsorbate interaction resulting in creation of the layers on the surface. From the SPR 11 Page 11 of 31
measurements it is suggested that the adsorption takes place predominately on the negatively
250
charged groups of the fibers followed by intermolecular interactions between the hydrophobic
251
groups of the cellulose derivatives. Moreover, interactions between the hydrophobic groups of
252
the fibers and the PCCDs could also be involved in the adsorption mechanism. Combining the
253
findings from the different analytical approaches it is possible to draw a tentative hypothesis of
254
the adsorption of the PCCDs on the fibers (Figure 4).
256
((Figure 4))
an
257 3.3. The distribution of the PCCDs onto the fiber surfaces
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258
cr
((Figure 3))
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255
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249
The modified fibers were analysed with ToF-SIMS in order to gain information about the
260
surface distribution of the fiber modifying agents and surface chemical composition of the
261
modified fibers. Figure 5 shows the ToF-SIMS spectra of PCCD-1 as a reference and the fiber
262
treated with this derivative. On the basis of the reference spectrum (Figure 5b), several
263
characteristic signals from secondary ions with m/z = 58, 159, 189, and 235 were assigned to
264
ions originating from the cationic- (fragment [C3H8N]+) and the photoactive substituent
265
(fragments [C10H7O2]+, [C11H9O3]+ and [C12H11O5]+), respectively. In the surface mass spectra of
266
the modified samples (Figure 5a), these peaks had high intensity, which additionally proved
267
adsorption of the cellulose derivatives onto the fiber surfaces. Accordingly, the mentioned
268
peaks were not found in the previously published spectrum of the reference fiber sample (Neto,
269
Silvestre, Evtuguin, Freire, Pinto & Santiago, 2004). As shown in Figure 6 the chemical
270
immaging of the total secondary ions yields qualitatively the same image as the mapping of the
271
characteristic fragments. Additionally, it could be noticed, that this effect was independent from
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12 Page 12 of 31
the dosage of the PCCD applied. This clearly indicated a homogeneous surface distribution of
273
the fiber modifying agents within the resolution of ToF-SIMS.
274
((Figure 5))
275
((Figurer 6))
ip t
272
276 3.4. Optical properties of the modified fibers
278
Light absorption by modified and reference fibers were studied with UV-Vis spectroscopy
279
(Figure 7). Unmodified fiber sheets showed two clear peaks at ca. 240 and 280 nm that
280
originate from hexeneuronic acid groups and residual lignin, respectively (Liitia & Tamminen,
281
2007). The absorbance spectra of the modified pulps have an additional absorbance band with
282
the maximum at around 320 nm arising from the
283
chromophor. A higher derivative dosage resulted in an increase of the peak intensity, thus the
284
highest absorbance at 320 nm was reached at the highest dosage applied (10 % (w/w),
285
Figure 7b).
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* transition of the photoactive coumarin
((Figure 7))
287
In addition to light absorbance, coumarin-containing compounds possess natural fluorescence
288
(Trenor, Shultz, Love & Long, 2004). For fluorescence measurements the pulp sheets were
289
irradiated with UV light at a wavelength of 320 nm corresponding to the absorption maximum
290
of the derivatives. Reference pulp samples also show a natural fluorescence (Liukko, Tasapuro
291
& Liitia, 2007) that had considerably lower emission intensity at the applied excitation
292
wavelength, compared to the samples containing the PCCDs (Figure 7c). A maximum
293
fluorescence intensity of the modified sample was found to be at around 380 nm.
294
3.5. Photocrosslinking, preliminary experiments
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Previous studies revealed that polysaccharide derivatives bearing such photoactive groups are
296
able to reversibly crosslink under UV light irradiation (Wondraczek, Pfeifer & Heinze, 2012).
297
Figure 8 shows changes in UV spectra measured from the fiber surfaces after it was exposed to
298
the UV light. A decrease of absorbance at the wavelength in the vicinity of 320 nm could serve
299
as an evidence of the crosslinking reaction which occured via [2+2] cycloaddition of ethylene
300
groups. The spectra also revealed that the reaction was fast and was completed within less than
301
five minutes. Hence, the fluorescence of the modified fibers was drastically reduced (spectra
302
not shown). Moreover, the intersection point of the spectra at 357 nm, the isosbestic point,
303
indicates an uniform reaction, meaning that no degradation of the cellulose derivatives or
304
formation of side products took place (Sauer, Hofkens & Enderlein, 2011). The irradiation of
305
the unmodified pulp fibers under similar conditions caused no changes in the UV spectra,
306
which prove their stability under UV light.
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As a preliminary study, a tensile index of the hand-sheets was measured. The tensile index
308
refers to the strength and it is a crucial quality for the papers, which undergo direct tensile
309
stress e.g. wrapping paper and paper bags (T 494 om-01). Also, printing paper grades should
310
have high tensile strength to resist web breaking during printing and converting (T 494 om-01).
311
The tensile index increased by 22 % after the modification of pulp with PCCD-2 at the dosage
312
of 3.2 % (w/w, based on dry pulp). After irradiation of the modified sample, the tensile index
313
increased by 39 % compared with the modified sample and by 70 % compared with the
314
reference. The tensile index is directly related to the bonding ability of the fibers. Hence, the
315
strength values showed that the modification and irradiation improved bonding between the
316
fibers. It can be assumed that an addition of the flexible chains of the cellulose derivative
317
increased the amount of the hydrophilic and hydrophobic groups available for interactions in
318
the zones of fiber contact. The subsequent photocrosslinking led to a creation of covalent bonds
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14 Page 14 of 31
319
between the pendant coumarin groups and thus the irradiated samples possessed better
320
mechanical properties. The photocontrol of the properties of the fiber interface (e.g. wetting) and directly related
322
properties (e.g. fiber to fiber bonding) is currently under investigation and will be published
323
elsewhere.
ip t
321
((Figure 8))
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cr
324
326 3. CONCLUSIONS
an
327
Novel multifunctional cellulose derivatives decorated with both cationic and photoactive
329
moieties were adsorbed onto the pulp fiber surfaces. Based on the results from UV-Vis
330
spectroscopy and SPR measurements, it was found that the electrostatic interaction was the
331
main driving force of the adsorption of the derivatives onto the fibers. However, hydrophobic
332
interactions between the fibers and the cellulose derivatives as well as between the cellulose
333
derivatives themselves also contributed to the adsorption. The mechanism of the adsorption was
334
well described by the Freundlich model. ToF-SIMS imaging showed that the derivatives were
335
evenly distributed on the fiber surfaces independently of the dosage of the derivatives and DS
336
of the photoactive group. UV light irradiation of the modified fibers triggered fast
337
photocrosslinking of the attached photoactive groups resulting in changes of light absorption
338
and fluorescence of the fibers. It can be suggested that the modified fibers bearing such
339
photoactive groups have a great potential for application in novel bio-based light-responsive
340
materials. Preliminary experiments on the photocontrol of the mechanical properties showed
341
remarkable increase of the tensile index. Therefore a more detailed study on the photocontrol of
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342
these properties at the structural levels of single fibers and fiber network is currently under
343
preparation and will be reported elsewhere.
344 ACKNOWLEDGEMENTS
346
Finnish Funding Agency for Technology and Innovation (Tekes) is acknowledged for
347
funding. The authors are grateful to Professor Peter Mattjus from Åbo Akademi University for
348
the access to the SPR instrument and to Josefin Halin for her assistance with the SPR
349
measurements. Annett Pfeifer and Cíntia Salomão Pinto Zarth are acknowledged for providing
350
PCCDs and CCDs.
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351 352
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Li, W., Lynch, V., Thompson, H., & Fox, M. A. (1997), Self-assembled monolayers of 7-(10-
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Liitia, T., & Tamminen, T. (2007). How to evaluate the kraft pulp brightness stability? In 3rd
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Liukko, S., Tasapuro, V., & Liitia, T. (2007). Fluorescence spectroscopy for chromophore
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(2004). Nordic Pulp and Paper Research Journal, 19, 513-520.
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Sauer, M., Hofkens, J., & Enderlein, J. (2011). Handbook of Fluorescence Spectroscopy and
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Schönberg, C., Oksanen, T., Suurnäkki, A., Kettunen, H., & Buchert, J. (2001). The importance
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of xylan for the strength properties of spruce kraft pulp fibers. Holzforschung, 55, 639-644.
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Schwikal, K., Heinze, T., Saake, B., Puls, J., Kaya, A., & Esker, A. R. (2011). Properties of
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Silva, T. C. F., Lucian, J. L., Lucia, L. A., de Oliveira, R. C., Oliveira, F. N., & Silva, L. H. M.
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Sixta, H. (2006). Handbook of pulp, Wiley-VCH: Winheim, Vol. 1, p 9.
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Utsel, S. (2012). Surface modification of cellulose-based materials for tailoring of interfacial
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Vega, B., Petzold-Welcke, K., Fardim, P., & Heinze, T. (2012). Studies on the fiber surfaces
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modified with xylan polyelectrolytes. Carbohydrate Polymers, 89, 768-776.
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Wondraczek, H., Kotiaho, A., Fardim, P., & Heinze, T. (2011). Photoactive polysaccharides.
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Carbohydrate Polymers, 83, 1048–1061.
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Wondraczek, H., Pfeifer, A., & Heinze, T. (2010). Synthetic photocrosslinkable polysaccharide
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sulfates. European Polymer Journal, 46, 1688–1695.
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Wondraczek, H., Pfeifer, A., & Heinze, T. (2012). Water soluble photoactive cellulose
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derivatives: synthesis and characterization of mixed 2-[(4-methyl-2-oxo-2H-chromen-7-yl)oxy]
425
acetic acid- (3-carboxypropyl)trimethylammonium chloride esters of cellulose. Cellulose, 19,
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1327-1335.
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Zhou, J., Ma, Y., Zhang, J., Tong, J. (2009). Influence of surface photocrosslinking on
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properties of thermoplastic starch sheets. Journal of Applied Polymer Science, 112, 99-106.
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Zhou, J., Zhang, J., Ma, Y., & Tong, J. (2008). Surface photo-crosslinking of corn starch
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Carbohydrate
Polymers,
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20 Page 20 of 31
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431
Figure 1. Schematic presentation of the structure of the photoactive cationic cellulose derivative
433
(PCCD).
M
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432
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21 Page 21 of 31
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434
Figure 2. (a) The photoactive cationic cellulose derivatives (PCCDs) adsorption isotherms onto
436
the fibers. (b) Amount of the PCCDs adsorbed as the percentage of added amount. Linear
437
representation of the isotherms according to Langmuir (c) and Freundlich (d) models. DS of
438
cationic group is 0.34 and DS of photoactive group is 0.11 and 0.37 for PCCD-1 and PCCD-2,
439
respectively.
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435
440
22 Page 22 of 31
440 441
Table 1. Adsorption data of photoactive cationic cellulose derivatives (PCCDs) treated with
442
Langmuir and Freundlich equations
Langmuir
Freundlich
KL
R²
R²
PCCD-1a
36.76
0.007
0.8980
0.9606
PCCD-2a
30.30
0.011
0.9478
0.9915
443
a
444
PCCD-2, respectively.
n
1/n
A
2.86
0.35
3.0
0.39
2.4
us
ns
cr
Derivative
ip t
Adsorption model
2.56
an
DS of cationic group is 0.34 and DS of photoactive group is 0.11 and 0.37 for PCCD-1 and
M
445
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446
23 Page 23 of 31
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446
Figure 3. Adsorption isotherms of cationic and photoactive cationic cellulose derivatives (CCD,
448
PCCDs) onto SAM-COOH (a), SAM-CH3 (b) and SAM-OH (c) surfaces. DS of cationic group
449
is 0.34 and DS of photoactive group is 0.11 and 0.37 for PCCD-1 and PCCD-2, respectively.
450
DS of cationic group is 0.29 for CCD.
Ac ce p
447
451 452
24 Page 24 of 31
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452
453
Figure 4. A schematic illustration of the adsorption of the photoactive cationic cellulose
455
derivative
onto
the
pulp
fibers.
Ac ce p
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(PCCD)
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454
25 Page 25 of 31
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Figure 5. ToF-SIMS spectrum m/z 0-250 of the pulp modified by photoactive cationic cellulose
459
derivative (a) and the reference polymer (b).
Ac ce p
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458
460
26 Page 26 of 31
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461 462
Figure 6. ToF-SIMS images of bleached unrefined eucalyptus kraft pulp treated with
463
photoactive cationic cellulose derivative (PCCD-1, DSPhoto of 0.11, DSCat of 0.34) of applied
464
dosages 0.5 % (w/w) (a, b) and 10.0 % (w/w) (c, d). (a, c) and (b, d) total ion and characteristic
465
ion images, respectively.
466 27 Page 27 of 31
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466
Figure 7. (a) UV-Vis spectra of the fibers modified by photoactive cationic cellulose derivative
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(PCCD-1, DSPhoto of 0.11, DSCat of 0.34) at dosages 0.5-10 % (w/w). (b) Maximum absorbance
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of the fibers as a function of adsorbed amount of photoactive groups of the PCCD-1. (c)
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Fluorescence spectra of the reference and modified by PCCD-1 sample.
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Figure 8. UV-Vis spectra of unmodified pulp fibers (reference) and the fibers modified by
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photoactive cationic cellulose derivative (PCCD-1, DSPhoto of 0.11, DSCat of 0.34, dosage of
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3.2 % (w/w)) recorded before and after irradiation with UV light.
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Graphical abstract
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Highlights (for review)
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Surface modification of pulp fibers with multifunctional cellulose derivatives Evaluation of the mechanism of adsorption by SPR Surface characterization by means of ToF-SIMS Preparation of photoactive fibers
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