Carbohydrate Polymers 129 (2015) 208–215

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Reinforcement and nucleation of acetylated cellulose nanocrystals in foamed polyester composites Fei Hu a,1 , Ning Lin a,b,1 , Peter R. Chang c , Jin Huang a,∗ a b c

School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, Luoshi Road 122, Wuhan, China Université Grenoble Alpes, Laboratoire Génie des Procédés Papetiers (LGP2), F-38000 Saint Martin d’Hères Cedex, France Bioproducts and Bioprocesses National Science Program, Agriculture and Agri-Food Canada, 107 Science Place, Saskatoon, SK S7N 0X2, Canada

a r t i c l e

i n f o

Article history: Received 16 February 2015 Received in revised form 28 April 2015 Accepted 30 April 2015 Available online 8 May 2015 Keywords: Cellulose nanocrystals Surface acetylation Nucleation Nanoreinforcement Foamed composites

a b s t r a c t The biodegradable foamed nanocomposites were developed from the reinforcement of surface acetylated cellulose nanocrystals (ACNC) as bionanofillers and the poly(butylene succinate) (PBS) as polymeric matrix. The surface modification of high-efficiency acetylation on the cellulose nanocrystals converted the hydrophilic hydroxyl groups to hydrophobic acetyl groups, which improved the compatibility between rigid nanoparticles and polyester matrix through the similar ester groups of two components. With the introduction of 5 wt% ACNC, the specific flexural strength (/f ) and the specific flexural modulus (E/f ) of the foamed composites significantly increased by 75.7% and 57.2% in comparison with those of the neat PBS foamed material. Meanwhile, with the change of the ACNC concentrations, the cell size and cell density of the foamed composites can be regulated and achieved the high cell density of 1.95 × 105 cells/cm3 bearing the small average cell size of 178.84 ␮m (5 wt% ACNC). The microstructure observation of the foamed composites indicated the moderate loading levels of rigid ACNC can serve as the reinforcing phase for the stress transfer and promote the crystallinity advancement of the foamed composites. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Foamed plastic composites represent a group of lightweight materials full of pores in the microstructure that have attracted a large attention in a variety of fields, such as insulation, cushion, absorbents and weight-bearing structures materials (Klempner & Frisch, 1991; Srivastava & Rajeev, 2014). Furthermore, attributed to their high porosity with interconnected pores, the foamed nanocomposites are expected to develop the biomedical materials, involving the tissue engineering scaffolds for cell attachment and growth (Mikos & Temenoff, 2000). Generally, depending on the physical performance and the matrix type, the foamed composites can be divided as rigid or flexible foams. Rigid foamed composites are commonly developed by the synthesized polymers as the matrices, which have the promising potential in building insulation, appliances, transportation, packaging, furniture, flotation and cushion, and food and drink containers (Lee et al., 2005). Traditional rigid foamed composites are produced from the petrochemical-based polymers, such as propylene (PP)

∗ Corresponding author. Tel.: +86 27 87749300; fax: +86 27 87749300. E-mail addresses: [email protected], [email protected] (J. Huang). 1 Fei Hu and Ning Lin are the co-first authors, who contribute equally to this work. http://dx.doi.org/10.1016/j.carbpol.2015.04.061 0144-8617/© 2015 Elsevier Ltd. All rights reserved.

(Wong, Wijnands, Kuboki, & Park, 2013), polystyrene (PS) (Shen, Zeng, & Lee, 2005), polyvinyl chloride (PVC) (Matuana, Park, & Balatinecz, 1998), etc. Recently, in order to develop the “green” and environmental-friendly materials, the recyclable and biodegradable polymers are used to produce of rigid foamed composites for the replacement of petrochemical-based foams. Poly(butylene succinate) (PBS) is one of the most promising synthetic biodegradable aliphatic polyester, which exhibits the outstanding degradability, melt processability and chemical resistance (Lin, Yu, Chang, Li, & Huang, 2011). Poly(butylene succinate) was attempted to prepare the neat PBS foamed materials (Li et al., 2013; Zhang et al., 2012; Zhou, Wang, Du, Li, & Yu, 2014), or used as the polymeric matrix to develop the foamed composites enhancing by montmorillonite (Zhou, Yao, Zhou, Wei, & Li, 2014), clay (Lim, Lee, Jang, Lee, Lee, et al., 2011), carbon nanofiber (Lim et al., 2010) or carbon nanotube (Lim, Lee, Jang, Lee, Choi, et al., 2011). However, due to the limitations of PBS on the linear molecular structure, low-melting strength and much higher price than petrochemical polymers, its practical application as the foamed materials is restricted. It was reported that the introduction of nanofiller as reinforcing agents can significantly improve the strength and modulus of polymeric foams (Lobos & Velankar, 2014). A small amount of welldispersed nanoparticles in the foamed composites may serve as the nucleation sites to facilitate the bubble nucleation process.

F. Hu et al. / Carbohydrate Polymers 129 (2015) 208–215

Therefore, the high aspect ratio and large surface area of nanoparticles can offer the potential for high reinforcing efficiency, good barrier properties, and improved dimensional and thermal stability. The nanometer dimension is especially beneficial for reinforcing foam materials, considering the thickness of cell walls in the micron and submicron regime (Chen, Rende, Schadler, & Ozisik, 2013; Harikrishnan, Singh, Kiesel, & Macosko, 2010). Different from inorganic fillers, cellulose nanocrystal (CNC) is a kind of rigid and highly-crystalline biomass nanoparticles extracted from natural cellulose, which has attracted much attention in the fields of material science because of its numerous advantages including the renewability and degradability, nontoxicity, high surface area, low density, rod-like morphology, high mechanical modulus, etc. (Lee, Aitomäki, Berglund, Oksman, & Bismarck, 2014; Lin, Huang, & Dufresne, 2012). Particularly, cellulose nanocrystal possesses the promisingly high specific modulus of about 87 GPa/(g/cm3 ), which is even 3–4 times potentially stronger than that of the steel (Dufresne, 2013). Therefore, during last ten years cellulose nanocrystal was widely studied as the rigid nanoreinforcing fillers to improve the mechanical and thermal properties of polymeric composites. As the pioneering studies, cellulose fibers have been reported to reinforce the polymers of polylactide (PLA) (Boissard, Bourban, Plummer, Neagu, & Månson, 2012), polypropylene (PP) (Kuboki, 2014) and poly(vinyl alcohol) (PVA) (Avella, Cocca, Errico, & Gentile, 2012) for the preparation of the foamed composites. However, resulting from the incompatibility between hydrophilic cellulose and hydrophobic polymers, it was inefficient for the improvement of the structural stability and mechanical property of the foamed composites from the reinforcement of cellulose fibers. The purpose of this study is the attempt of overcoming the weakness of poly(butylene succinate) as the foamed material through the enhancement of rod-like and rigid cellulose nanocrystals. Acetylation treatment was performed on the CNC via the chemical reaction between the surface hydroxyl groups of nanocrystals and acetic anhydride, which will promote the compatibility of acetylated nanofillers (ACNC) and polymeric matrix (PBS) from the ester groups of two components. The analysis of the structure and properties of the PBS/ACNC foamed nanocomposites was divided as three sections, which included the mechanical property of the foamed composites (involving the flexural strength and modulus), thermal property and crystallinity of the foamed composites, and the observation of cell morphology and microstructure, together with the nucleation discussion of the foamed composites. To the best of our knowledge, this is the first study on the use of acetylated cellulose nanocrystals for the enhancement of poly(butylene succinate)-based foamed materials, which will broaden the application fields of the biomass nanoparticles and promote the development of the rigid polymer foamed nanocomposites.

2. Experimental 2.1. Materials Commercial poly(butylene succinate) (PBS) pellets with the number average molar weight of 1.8 × 105 Da and relative density of 1.26 were purchased from Anqing Hexing Chemical Co., Ltd. (Anhui, China). The PBS pellets were dried at 60 ◦ C for 24 h before the use. Azodicarbonamide (AC) as the chemical blowing agent was purchased from Shanghai Heritage Group Co., Ltd. (Shanghai, China). The cotton linter was supplied by Hubei Chemical Fiber Group Co., Ltd. (Xiangfan, China). Acetic anhydride was purchased from Xilong Chemical Industry Inc. Co., Ltd. (Shantou, China). Ammonia (NH3 ·H2 O), zincoxide (ZnO), sulfuric acid (H2 SO4 )

209

and other analytical-grade reagents were purchased from Shenshi Chemicals and Instrument Co., Ltd. (Wuhan, china). 2.2. Extraction and surface acetylation of cellulose nanocrystals Cellulose nanocrystals (CNC) were extracted by H2 SO4 hydrolysis of native cotton linter according to our previous report (Lin, Bruzzese, & Dufresne, 2012). Briefly, after the removal of lignin and hemicellulose with the alkaline solution (2 wt%), the cotton cellulose fiber hydrolyzed by 65 wt% aqueous H2 SO4 at 45 ◦ C for 45 min. After the acid hydrolysis, a small amount of ammonia (0.5 wt%) was dropped into the suspension to partially remove the sulfate groups of the nanocrystals. The aqueous suspension of CNC was dialyzed overnight against the distilled water and released the powders with the freeze-drying treatment. Surface acetylation of cellulose nanocrystals were performed according to our previous study with a little modification (Lin, Huang, Chang, Feng, & Yu, 2011). Dried CNC was added into the flask with the anhydrous pyridine as the solvent and dispersed for 15 min by ultrasonic treatment. Acetic anhydride was dissolved in the anhydrous pyridine, and added into the CNC suspension. The acetylation of CNC was performed with constant magnetic stirring under a nitrogen atmosphere at 80 ◦ C for 5 h. After the reaction, the product was precipitated in 1.0 L of distilled water, and then washed with a solution of acetone/water and the distilled water to remove unreacted compounds and by-products. The powders of acetylated CNC (and ACNC) were obtained after the freeze-drying treatment. 2.3. Preparation of the PBS/ACNC foamed composites The PBS pellets, ACNC, blowing agent (AC) and blowing promoter (ZnO, 1 wt%) were added into an internal mixer instrument (Changzhou Suyan Science and Technology Co., Jiangsu, China), and melting-compounded at 120 ◦ C for 15 min with the rotor speed of 72 rpm. The resultant mixture was kept in a desiccator with the silica gel at room temperature for 24 h, and then pressed with an R-302 hot-press machine (Qien Development Technology Co., Wuhan, China) at 165 ◦ C and 10 MPa for 10 min. The foamed composite was obtained with the size of 60 mm × 13 mm × 2 mm after the cooling treatment. A series of foamed nanocomposites were prepared according to the contents of the blowing agents (AC: 3, 4, 5, 6 wt%) and the nanoreinforcing fillers (ACNC: 0, 1, 2, 3, 5, 7, 10 wt%). All the foamed composites were coded as PBS/AC(x)/ACNCy. The x and y represent the contents of the components in the foamed composites, such as the code of PBS/AC(5)/ACNC-1 shows this sample containing 5 wt% AC (as the blowing agent) and 1 wt% ACNC (as the reinforcing nanofillers). 2.4. Characterization 2.4.1. Fourier transforms infrared spectroscopy (FTIR) To prove the presence of surface acetylation, FTIR spectra of CNC and ACNC were recorded on an FTIR 5700 spectrometer (Nicolet, Madison, WI). The powders were scanned using a KBr-pellet method in the range of 4000–400 cm−1 . 2.4.2. Elemental analysis The contents of carbon (C%), hydrogen (H%), nitrogen (N%) and sulfur (S%) were investigated by elemental analysis (Elementar Vario EL cube, Germany). The precision of measurements is 0.01% for C and 0.001% for H. Degree of acetylated substitution (DSsurface acetyl ) for ACNC was calculated according to Eq. (1): DSsurface acetyl =

nsurface acetyl nsurface−OH

=

(C%/12)/2 nsurface−OH

(1)

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where nsurface acetyl represents the amount of surface acetyl groups on ACNC; nsurface−OH is the content of active hydroxyl groups on the surface of CNC; C% is the increment of carbon after surface acetylation (C% = C%ACNC − C%CNC ). 2.4.3. X-ray diffraction analysis (XRD) In order to check the crystalline integrity of nanocrystals after the surface acetylation, XRD measurements were performed on CNC and ACNC on a D/max-2500 X-ray diffractometer (RigakuDenku, Tokyo, Japan) with Cu K␣1 radiation ( = 0.154 nm) in a range of 2 = 5–50◦ using a fixed time mode with a step interval of 0.02◦ . The crystallinity index (Ic ) of CNC and ACNC were calculated by the Segal equation (Segal, Creely, Martin, & Conrad, 1959): Ic =

I0

02

I0

− Iam 02

× 100%

(2)

where I0 0 2 is the intensity of the peak associated with the crystalline region of cellulose (2 = 22.6◦ ), and Iam represents the intensity of the baseline at 2 = 18.0◦ . 2.4.4. Transmission electron microscopy (TEM) The morphological change of cellulose nanocrystals after surface acetylation was observed by TEM, which was performed on an H7000FA electron microscope (Hitachi, Tokyo, Japan) at 75 kV. Before the observation, the CNC or ACNC was dispersed in the distilled water, and then negatively stained with a 2% (w/v) ethanol solution of uranyl acetate. 2.4.5. Mechanical measurement (flexural tests) The mechanical properties of foamed nanocomposites, involving flexural strength () and flexural modulus (E), were measured on a CMT6503 universal testing machine (SANS, Shenzhen, China) with a flexural rate of 1 mm/min following the procedure of GB/8812-88. The maximum flexural strain of the foamed materials was controlled as 3.51%. Before the measurements, all the samples were kept at 35% humidity for 7 days. A mean value of five replicates from each sample was taken.

The cell density (N0 ) defined as the number of cells per unit volume of the original unfoamed polymer can be calculated by Eq. (5):

 N0 =

nM 2 A

1.5 

1 1 − Vf

 (5)

where n is the number of cells from SEM images, M and A are the magnification factor of the micrograph and the area of the micrograph, respectively (Matuana, Faruk, & Diaz, 2009). 3. Results and discussion 3.1. Structure and properties of acetylated cellulose nanocrystals Chemical acetylation of cellulose nanocrystals was investigated by FTIR spectroscopy, as shown in Fig. 1. With the comparison of chemical structures of CNC and ACNC, two additional covalent bonds, –C O and carbonyl –C–O–, appeared after the surface acetylation, which were associated to the characteristic peaks located at 1746 cm−1 and 1240 cm−1 on the FTIR spectra. The presence of carbonyl and carbonyl C–O stretching vibration proved the chemical acetylation on cellulose nanocrystals. According to the change of carbon contents from elemental analysis, the DS value of surface acetyl groups to surface hydroxyl groups of acetylated cellulose nanocrystals can be calculated (Lin et al., 2014). Table 1 summarized the results of elemental analysis and estimated DSsurface acetyl according to Eq. (1). After the acetylation reaction, the sulfur element assigned to the sulfate groups on the surface of nanocrystals was removed, which indicated the possible solvolytic desulfation during this treatment (Jiang, Esker, & Roman, 2010). The total content of hydroxyl groups on the surface of CNC (nsurface−OH ) was calculated as 1.09 mmol/g according to the previous report (Lin & Dufresne, 2014). It was proved that most of active hydroxyl groups on the surface of nanocrystals were

2.4.6. Differential scanning calorimetry (DSC) DSC analysis was performed on a DSC-Q 200 instrument (TAInstruments, USA) under nitrogen atmosphere at a heating or cooling rate of 20 ◦ C/min. All the foamed nanocomposites were put in the desiccator for three days and then scanned in the range of −70 to 150 ◦ C, after a pretreatment of heating from 20 ◦ C to 150 ◦ C and then cooling to −70 ◦ C to eliminate thermal history. 2.4.7. Scanning electron microscopy (SEM) The microstructure and cellular morphology of the foamed composites were observed by X-650 SEM instrument (Hitachi, Tokyo, Japan). All the samples were treated in liquid nitrogen and immediately snapped, and the fracture surfaces were sputtered with gold for the observation. The cell size (Di ) of the foamed composites was measured by the software image-pro 8.0 from SEM images. The ¯ is calculated by Eq. (3) average cell diameter (D) 1 Di n

Fig. 1. The FTIR spectra of pristine cellulose nanocrystals (CNC) and acetylated cellulose nanocrystals (ACNC).

n

¯ = D

(3)

i

The void fraction (Vf ) of foamed composites was determined by Eq. (4): Vf = 1 −

f 

(4)

where f and  are the densities of foamed composites and PBS materials.

Table 1 The elemental contents, and calculated surface hydroxyl groups content (nsurface–OH ) and degree of acetyl substitution (DSsurface acetyl ) of CNC and ACNC from elemental analysis. Samples

C %

H %

N %

S %

nsurface–OH mmol/g

DSsurface acetyl %

CNC ACNC

43.08 44.94

6.12 6.28

0.12 0.04

0.15 0.00

1.09 –

– 71.1

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211

Fig. 2. The XRD patterns and TEM images of cellulose nanocrystals before and after surface acetylation.

substituted by acetyl groups from the high efficiency of DSsurface acetyl value (71.1%). The main challenge for the chemical modification of CNC is to conduct the reaction only on the surface of nanocrystals, while preserving the original morphology and crystalline integrity to avoid any polymorphic conversion (Habibi, Lucia, & Rojas, 2010). The crystalline property and morphological observation of CNC and ACNC were investigated by XRD and TEM, as shown in Fig. 2. In comparison with the diffraction pattern of pristine CNC, the diffraction peaks of the 2 angles at about 14.2◦ , 16.4◦ , 22.6◦ and 33.4◦

were preserved on that of ACNC, which were assigned to the typ¯ (0 0 2) and (0 4 0) ical reflection planes of cellulose I (1 0 1), (1 0 1), respectively (Liu, Zhong, Chang, Li, & Wu, 2010). The crystallinity index (Ic ) of CNC and ACNC were calculated as 86.3% and 81.1%, which proved the maintenance of crystallinity and rigidity of cellulose nanocrystals after the chemical acetylation. The morphology of CNC and ACNC was further observed by TEM as shown in Fig. 2. The pristine CNC displayed a rod-like morphology with the length and diameter of 150–250 nm and 10–20 nm, while ACNC exhibited the same rod-like shape and similar dimensions as CNC.

Fig. 3. Effects of the ACNC contents on the flexural strength and modulus of PBS/ACNC foamed composites (with the addition of 5 wt% AC as blowing agent); pictures of PBS/AC(5)/ACNC-0 and PBS/AC(5)/ACNC-5 foamed composites.

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3.2. Mechanical property of foamed nanocomposites The surface and cross-section appearance of PBS/AC(5)/ACNC-5 foamed nanocomposite was similar as those of PBS/AC(5)/ACNC-0 (as shown in Fig. 3), which indicated the homogeneous dispersion and stability of 5 wt% acetylated cellulose nanoparticles during the melt-compounding and hot-compression process. The enhancement of ACNC to the mechanical properties of foamed composites was analyzed by the flexural strength () and flexural modulus (E) of the materials, as shown in Fig. 3. With the increase of ACNC concentrations, the flexural strength and modulus of foamed composites gradually increased, which reached to the maximum values of 3.0 MPa () and 287.1 MPa (E) for the foamed composite PBS/AC(5)/ACNC-5. Compared with the neat foamed material PBS/AC(5)/ACNC-0 ( = 2.0 MPa, E = 214.0 MPa), the presence of 5 wt% ACNC induced the 50.0% and 34.1% increases of flexural strength and modulus for the foamed composite PBS/AC(5)/ACNC-5. The improvement of mechanical properties of foamed composites can be attributed to the homogeneous dispersion of nanofillers and interfacial compatibility between modified nanofillers and polyester matrix (similar ester groups of acetylated nanocrystals and PBS matrix), which facilitated the nanoreinforcement and stress transferring of rigid ACNC in the foamed composites. The introduction of superfluous nanofillers (such as 7 wt% and 10 wt% concentrations) may cause the self-aggregation of ACNC and possible microphase separation in the composites, which will induce the decrease of mechanical properties of foamed composites.

Table 2 DSC data of the foamed composites including the glass transition temperature at midpoint (Tg,mid ), melting temperature (Tm ), heat enthalpy (Hm ) and the calculated crystallinity index (c ) (with the addition of 5 wt% AC). Samples PBS/AC(5)/ACNC-0 PBS/AC(5)/ACNC-1 PBS/AC(5)/ACNC-2 PBS/AC(5)/ACNC-3 PBS/AC(5)/ACNC-5 PBS/AC(5)/ACNC-7 PBS/AC(5)/ACNC-10



Tg,mid C

Tm1 ◦ C

Tm2 ◦ C

Hm J/g

c %

−35.8 −35.0 −34.7 −34.2 −32.4 −32.9 −34.0

94.1 93.9 93.7 94.3 95.4 94.3 94.0

104.1 104.1 104.6 104.2 105.0 104.2 104.4

34.9 37.1 36.5 35.6 41.7 35.6 36.9

31.6 34.0 33.8 33.3 39.8 34.7 37.2

crystalline property of the foamed composites. The crystallinity index (c ) of the PBS component in the foamed composites can be calculated with the following equation: c =

Hm ∗ ωHm

(6)

∗ = 110.3 J/g is the melting enthalpy of the 100% cryswhere Hm talline PBS, and ω represents the weight fraction of PBS in the foamed composites (Lin, Yu, et al., 2011; Lin, Huang, et al., 2011). The presence of highly-crystalline and rigid ACNC in the foamed composites may restrict the free mobility of the amorphous PBS polymeric chains, which induced the crystallization and nucleation of the PBS component in the foamed composites. Therefore, especially for the high loading levels, the values c increased with the addition of ACNC in the foamed composites.

3.3. Thermal property and crystallinity of foamed nanocomposites

3.4. Microstructure, cell morphology and nucleation of foamed nanocomposites

The thermal properties influenced by the loading levels of ACNC in the foamed composites were investigated with DSC analysis. The data of the glass transition temperature at midpoint (Tg,mid ), melting temperature (Tm ) and heat enthalpy (Hm ) of the foamed composites were summarized in Table 2. All the foamed composites exhibited two melting points (Tm1 and Tm2 ) due to the recrystallization of the PBS component during the melting process (Lim, Lee, Jang, Lee, Lee, et al., 2011). In general, the incorporation of ACNC in the PBS foamed composites caused only slight changes of thermal properties on the values of Tg,mid , Tm1 and Tm2 , which was in agreement with most of reported studies on cellulose nanocrystalsreinforced composites (Dufresne, 2012). However, the values of Hm varied with the increase of ACNC contents in the foamed composites, which was associated with the transformation of

The microstructure and cell properties (involving cell size and density) will determine the performance of foamed composites. In this study, the cell properties will be significantly affected by the blowing agent AC and the rigid nanofillers ACNC. Fig. 4 showed the cross-sectional morphologies of the foamed composites containing different concentrations of the blowing agent (3, 4, 5, 6 wt%) without the ACNC or with the addition of 2 wt% ACNC. In comparison with others foamed materials, the sample of PBS/AC(5)/ACNC-0 exhibited the better dimensional homogeneity of cell sizes with the addition of 5 wt% blowing agent AC. Furthermore, from the SEM images of the foamed composites containing 0 and 2 wt% ACNC, the introduction of rigid nanoparticles can promote the cells formation (more round shapes), which may be attributed to the nucleation effects induced by ACNC.

Fig. 4. SEM images of the cross-sectional morphologies of foamed composites (with the enlargement of ×100): (a) PBS/AC(3)/ACNC-0, (b) PBS/AC(4)/ACNC-0, (c) PBS/AC(5)/ACNC-0, (d) PBS/AC(6)/ACNC-0 (without the ACNC); and (a ) PBS/AC(3)/ACNC-2, (b ) PBS/AC(4)/ACNC-2, (c ) PBS/AC(5)/ACNC-2, (d ) PBS/AC(6)/ACNC-2 (with the addition of 2 wt% ACNC).

F. Hu et al. / Carbohydrate Polymers 129 (2015) 208–215

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Fig. 5. The effects of AC contents on the cell size and cell density of foamed composites. (a) PBS/AC/ACNC-0 (without the ACNC), (b) PBS/AC/ACNC-2 (with the addition of 2 wt% ACNC).

The calculated values of cell sizes and cell densities affected by the various concentrations of blowing agent AC for the foamed composites was shown in Fig. 5. Regarding the foamed materials without the introduction of ACNC, the sample of PBS/AC(5)/ACNC-0 possessed the smallest cell sizes (203.7 ␮m) and highest cell density (1.25 × 105 cells/cm3 ), which indicated the use of 5 wt% AC as the blowing agent can provide the optimal foamed effect for the PBS matrix (Fig. 5a). Interestingly, when the low content of rigid nanoparticles (2 wt%) was added in the foamed composites, the cell size decreased and the cell density increased (Fig. 5b), such as comparing the samples of PBS/AC(5)/ACNC-0 (203.7 ␮m average cell size, 1.25 × 105 cells/cm3 cell density) and PBS/AC(5)/ACNC-2 (190.5 ␮m average cell size, 1.53 × 105 cells/cm3 cell density).

This result was in agreement with the SEM observation in Fig. 4, which can be explained by the nucleation and growth mechanism of foamed cells induced by the blowing agent and nanocrystalline cellulose. On the one hand, the cell size and density of foamed materials will depend on the amount of gas molecules dissolved in the molten polymer matrix during the foaming process (Baldwin, Park, & Suh, 1996). Once the cells are nucleated and there is sufficient gas, the cells will continue to grow, the gas molecules are to diffuse into the nucleated cells and the molten PBS can be expanded (Lee et al., 2005). Therefore, the cell density of the foamed materials will be advanced with the increase of the blowing agent (3, 4, 5 wt% AC). However, if the nucleated cells are grown completely, the cell collapse and/or coalescence may occur with the higher content of

Fig. 6. SEM images of the cross-sectional morphologies of foamed composites: (a) PBS/AC(5)/ACNC-0, (b) PBS/AC(5)/ACNC-2, (c) PBS/AC(5)/ACNC-5, (d) PBS/AC(5)/ACNC-10 (with the enlargement of ×100).

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Table 3 Morphological parameters of foamed nanocomposites affected by the loading levels of ACNC (f and Vf represent the density and void fraction of foamed composites; E and  are the flexural modulus and flexural strength). Samples

f g/cm3

/f MPa

E/f MPa

Vf %

Cell size ␮m

Cell density cell/cm3

PBS/AC(5)/ACNC-0 PBS/AC(5)/ACNC-2 PBS/AC(5)/ACNC-5 PBS/AC(5)/ACNC-10

0.75 0.71 0.64 0.81

2.67 3.24 4.69 2.84

285.3 318.7 448.6 302.2

36.7 41.5 47.3 34.7

203.7 190.5 178.8 212.6

1.25 × 105 1.53 × 105 1.95 × 105 1.08 × 105

the blowing agent (6 wt% AC) (Matuana, Park, & Balatinecz, 1997), which will cause the distortion of cells and the decrease of cell density for the foamed materials. On the other hand, the addition of rigid and highly-crystalline cellulose nanoparticles will induce the nucleation effect and advance the crystallinity of the PBS matrix (as shown in the DSC results). Meanwhile, the presence of ACNC can promote the structural integrity of cells and avoid the collapse of cells during the foaming process, which induced the increase of cell density for the foamed composites in comparison with neat foamed materials. In order to further investigate the effects of ACNC to the microstructure of foamed composites, the cross-sectional morphologies of foamed composites with the introduction of various loading levels of ACNC were observed by SEM, as shown in Fig. 6. Under the same concentration of the blowing agent (5 wt% AC), the presence of moderate ACNC can promote the shapes stability of cells (2 wt% and 5 wt% for images b and c), while the addition of superfluous ACNC will cause the shapes distortion of cells and decrease of cell density (10 wt% for image d). The influence of loading levels of ACNC on the density (f ), void fraction (Vf ), cell size and cell density of foamed composites were summarized in Table 3. Significantly, the specific flexural strength (/f ) and the specific flexural modulus (E/f ) were advanced 75.7% and 57.2% in comparison with those of the neat foamed material PBS/AC(5)/ACNC-0, which showed the promising nanoreinforcing effects of ACNC to the PBS foamed composites. In addition, the moderate increase of ACNC loading levels will induce the reduction of the cell size and the advancement of cell density, such as the samples of PBS/AC(5)/ACNC-5 with the average cell size of 178.84 ␮m and the cell density of 1.95 × 105 cells/cm3 . The presence of 5 wt% ACNC in the foamed composite can be treated as the nucleation

agents, which is favorable to the bubble heterogeneous nucleation and the formation of the cell nucleation sites during the foaming process. As discussed before, the addition of superfluous ACNC may lead to the self-aggregation of nanoparticles and microphase separation of foamed composites, which will cause the shapes distortion of cells, the decrease of cell density, and the reduction of performances. Fig. 7 showed the SEM images of the foamed composites with the introduction of 5 wt% and 10 wt% ACNC. In comparison with the homogeneous cross-sectional morphology of image (a), the aggregates of nanoparticles from the ACNC in the 10 wt% loading level of the foamed composite can be observed on image (b) (as indicated by the red arrows).

4. Conclusions Acetylated cellulose nanocrystals (ACNC) were introduced in poly(butylene succinate) (PBS) polymeric matrix for the development of the biodegradable foamed nanocomposites. Derived from their well-distribution in the composites, the highly-crystalline and rigid ACNC can be treated as the nucleation agents, and promote the bubble heterogeneous nucleation and the formation of the cell nucleation sites during the foaming process. With the nanoreinforcement of 5 wt% ACNC, the prepared foamed composites exhibited the improved mechanical properties, enhanced crystallinity, stabilized cell morphology and advanced cell density. Through the melt-compounding process, the attempt of using modified biomass nanoparticles for the enhancement of the polyester foamed materials in this study will promote the development of the PBS foamed nanocomposites and broaden the practical application of CNC as the packaging or construction materials.

Acknowledgements The research work was financially supported by the National Natural Science Foundation of China (51373131), Project of New Century Excellent Talents of Ministry of Education of China (NCET11-0686), ecoENERGY Innovation Initiative of Canada, Program of Energy Research and Development (PERD) of Canada, and Fundamental Research Funds for the Central Universities (SelfDetermined and Innovative Research Funds of WUT, 2014-II-009).

Fig. 7. SEM images of foamed composites (a) PBS/AC(5)/ACNC-5 and (b) PBS/AC(5)/ACNC-10 (with the enlargement of ×500). (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)

F. Hu et al. / Carbohydrate Polymers 129 (2015) 208–215

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Reinforcement and nucleation of acetylated cellulose nanocrystals in foamed polyester composites.

The biodegradable foamed nanocomposites were developed from the reinforcement of surface acetylated cellulose nanocrystals (ACNC) as bionanofillers an...
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