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Reversible Swelling-Shrinking Behavior of Hydrogen-Bonded Free-Standing Thin Film Stabilized by Catechol Reaction Jiaxing Sun, Chao Su, Xuejian Zhang, Wenjing Yin, Shuguang Yang, and Jian Xu Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5048479 • Publication Date (Web): 22 Apr 2015 Downloaded from http://pubs.acs.org on April 27, 2015
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Reversible Swelling-Shrinking Behavior of Hydrogen-Bonded Free-Standing Thin Film Stabilized by Catechol Reaction
Jiaxing Sun1, Chao Su1, Xuejian Zhang1, Wenjing Yin1, Jian Xu2, Shuguang Yang1*
1
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,
College of Material Science and Engineering, Donghua University, Shanghai 201620 2
Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190 *To whom the correspondence should be addressed. Email: [email protected]
ABSTRACT Dopamine-modified poly(acrylic acid) (PAA-dopa) and poly(vinylpyrrolidone) (PVPON) was layer-by-layer (LbL) assembled to prepare thin film based on hydrogen bonding. Carboxylic group of acrylic acid and phenolic hydroxyl group of dopamine can both act as hydrogen bond donors. The critical assembly and the critical disintegration pH values of PVPON/PAA-dopa film are enhanced compared with PVPON/PAA film. The hydrogen-bonded PVPON/PAA-dopa thin film can be cross-linked via catechol chemistry of dopamine. After cross-linking, the film can be exfoliated from the substrate in alkaline solution to get a free-standing film. Moreover, by tuning pH value, deprotonation and protonation of PAA will make hydrogen bond in the film break and re-construct, which induces that the free-standing film has a reversible swelling-shrinking behavior.
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INTRODUCTION Layer-by-layer assembly (LbL) technique, first introduced by Decher in early 1990s, has been drawing considerable attentions due to its simplicity and versatility to fabricate structurally tunable materials with various compositions.1-5 Free-standing LbL materials, with well-controlled mechanical properties and architectures compared with the surface-attached type, are of great interest for their potential applications as drug carriers, tissue engineering scaffolds and wound healing materials.6-9 The most popular approach to prepare free-standing LbL materials is to alternately deposit the components on a sacrificial precursor layer or substrate. 10-12 Different free-standing LbL structures, like planar film,10 microcapsule,13, 14 microtubule,15 and microcube,16 have been successfully fabricated. Hydrogen-bonded LbL materials, first demonstrated by Rubner
17
and Zhang
18
individually, have excellent biocompatibility and sensitive stimuli-responsivity, showing a promising potential for biomedical and sensor applications.19-23 Lots of efforts have been made to construct hydrogen-bonded LbL free-standing structures.24-27 Hammond et al. fabricated an 8 µm free-standing hydrogen-bonded planar film by using substrates with neutral, hydrophobic surfaces such as Teflon and polypropylene.24 Zhang et al. first reported a hydrogen-bonded microcapsule fabricated
by
LbL
assembly
of
poly(vinylpyrrolidone)
(PVPON)
and
m-methylphenol-formaldehyde resin (MPR) on sacrificial SiO2 or PS nanoparticle templates.25 However, the weak mechanical strength and chemical stability limit the applications of the hydrogen-bonded free-standing materials. Chemical cross-linking has been utilized to overcome the restriction.28-37 Carbodiimide chemistry was introduced to cross-link hydrogen-bonded polyacid/nonionic systems and thus the robust capsules were prepared. 28-30 Caruso et al. utilized disulfide bonds to cross-link hydrogen-bonded systems.
31
Rubner et al.
applied thermal annealing to enhance the stability of the hydrogen-bonded PVA/PAA and PAAM/PAA films.32,
33
The hydrogen-bonded LbL film of diazo resin and
phenol-formaldehyde resin was cross-linked by photochemical reaction.35 And click chemistry reactions, such as thiol-ene and alkyne-azide, were also used to stabilize 2
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hydrogen-bonded LbL materials.36, 37 Dopamine, an analogue of the mussel adhesive proteins (MAPs), have the ability to form an adhesive polydopamine coating on almost all kinds of substrates.38 The ortho-dihydroxyphenyl (catechol) moiety of dopamine can form strong affinity with solid surfaces by series of reactions, including the dismutation, Michael addition, Schiff base reaction, and strong coordination with many metal oxides or ions.39, 40 Through introducing dopamine to polymer chain, the reactivity of catechol can be utilized for further chemical modification, like crosslinking or adjusting hydrophobicity.41-43 In this work, we fabricated a hydrogen-bonded film by alternately depositing PVPON and dopamine-modified PAA (PAA-dopa) on silicon or quartz substrate. Catechol chemistry was utilized to cross-link the film, which increased the stability of the film. The free-standing film was formed by exfoliation in the alkaline solution directly, not like general way required a sacrificial layer or template. Furthermore, the free-standing film exhibited a reversible swelling-shrinking behavior as the solution pH value changed.
EXPERIMENTAL SECTION Materials. Poly(acrylic acid) (PAA, Mw = 450 000) and poly(vinylpyrrolidone) (PVPON,
Mw
=
40
000)
were
purchased
from
Sigma-Aldrich.
N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC•HCl) was obtained from TCI. Dopamine hydrochloride was bought from J&K. Other chemicals were analytical grade and used as received. Synthesis of PAA-dopa. PAA powder (3 mmol) was dissolved in 20 mL 0.1 M phosphate buffer solution (PBS, pH 6.0). EDC•HCl (0.6 mmol) and dopamine hydrochloride (0.6 mmol) were dissolved in 10 mL 0.1 M PBS, and then added into PAA solution. The solution was continuous stirred for 24 h under N2 gas protection. The reaction mixture was dialyzed against deionized water using a regenerated cellulose dialysis tube with a molecular weight cutoff of 8 000 until dopamine in the 3
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washing solution was undetectable by UV-visible spectroscopy. The dialysate was diluted with a 0.1 M PBS to a certain volume and stored in dark for further use. Preparation of Hydrogen-Bonded Thin Film. The thin film of PAA-dopa and PVPON was deposited on quartz or silicon substrate. Before the deposition, the substrate needed to be clean, which was immersed in boiling piranha solution (30% H2O2/98% H2SO4 at 3:7 of volume ratio, extremely corrosive) for half an hour, then rinsed with abundant amounts of deionized water, and finally dried with N2 stream. The LbL procedure was performed through an automated device (Shenyang Kejing). The substrate was dipped into PVPON solution and PAA-dopa solution alternately, with three rinses after each depositing step to remove the excess polymers. The concentration of PVPON and PAA-dopa solution was 2.0 and 1.0 mg/mL, respectively. The deposition and rinse time was set as 4 and 1 min, respectively. The pH value of the rinse solution kept the same with the assembly solution, which was adjusted with HCl and NaOH solution under the monitor of a pH meter (Mettler S40K). Till the desired layer number was reached, the resultant film was immersed in the rinse solution for 24 h and finally dried with N2. For simplicity, (PVPON/PAA-dopa)n is used to mark the film prepared with n cycle number. Stabilization of Hydrogen-Bonded Thin Film. The PVPON/PAA-dopa film prepared at pH 3.0 on silicon or quartz slide was immersed in 1 mM NaIO4 aqueous solution for cross-linking via oxidization induced dismutation. The total immersion time for 40-bilayer-film was 24 h. After cross-linking, the film was rinsed thoroughly with deionized water and dried with N2 flow. Preparation of Free-Standing Thin Film. To obtain a free-standing film with regular shapes, the film was scratched near the edges of the substrate using a razor blade and then immersed in NaOH solution (pH 12.0). The planar film would detach from the substrate in several minutes. The exfoliated film was transferred from the pH 12.0 solution to the acetone bath to remove water, and then took out and spread on a PTFE plate. After acetone evaporated, the dry free-standing film was prepared.
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Scheme 1 Procedure to prepare the free-standing PVPON/PAA-dopa thin film.
Characterization. PAA-dopa was dissolved in D2O sealing in NMR tube, and 1H NMR measurement was carried out on a Bruker DMX-400 NMR spectrometer. The grafting degree of dopamine to PAA was determined by calculating the integral area ratio of H atoms in the aromatic rings of grafted catechol groups to H atoms in the polymeric backbone on the NMR spectrum. FT-IR spectra were collected on a Nicolet 8700 FT-IR spectrometer. The polymer compound was ground with KBr uniformly and then pressed into a standard disk for FT-IR measurement. With pure silicon substrate as background, the thin film deposited on the silicon substrate did IR spectroscopic characterization directly. UV-visible spectra were recorded on a Shimadzu UV-2550 spectrophotometer. The polymer solution was pour into the standard quartz cuvette (1 cm × 1 cm) for measurement. The thin film deposited on the quartz slide (12 mm × 45 mm) was measured with UV-visible spectrometer directly. SEM images were obtained on a Hitachi SU 8010 field emission scanning electron microscope with an acceleration voltage of 5.0 kV. For SEM characterization, 5
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the thin film was deposited on a silicon substrate The thickness of the PVPON/PAA-dopa film was determined with Fabry-Pérot fringes on UV-visible spectra, 44 or by the section image of SEM. AFM images were recorded on an Agilent 5500 atomic force microscope in the tapping mode. A commercial silicon probe (model TESP-100) with a typical resonant frequency of approximately 300 kHz was used to scan the film. Water contact angle measurements were performed on an OCA40 Micro Optical Contact Angle Measuring Device. A droplet of 5 µL water was produced with a micro syringe.
RESULTS AND DISCUSSION Poly(carboxylic acid)s, such as poly(acrylic acid) and poly(methacrylic acid), can be LbL assembled with nonionic polymers, such as PVPON, PEO, and PVME to prepare thin films based on hydrogen bonding.45-48 Polymers containing phenol groups can also be utilized as hydrogen bond donors to fabricate thin films by LbL assembly.49 Dopamine, contained two phenolic hydroxyl groups, was partially grafted on PAA. The resultant PAA-dopa was alternately deposited with PVPON to build up a thin film, in which carboxylic groups and phenolic hydroxyl groups both acted as hydrogen bond donors. Dopamine-modified PAA was synthesized via carbodiimide chemistry, as shown in Scheme 1.41 1H NMR spectrum (SI Figure S1) and FT-IR spectrum (SI Figure S2) prove that dopamine is successfully linked onto PAA chain. The photographs and UV-visible spectra of PAA-dopa solutions at different pH values are shown in Figure 1. In wide pH region, the PAA solution keeps transparent. As dopamine is introduced into PAA chain, the solution behavior of PAA-dopa is different to PAA. Only in the pH region from 4.0 to 7.0, the solution is transparent, where the black line behind the solution can be seen. As pH value decreases to 3.0, PAA solution becomes cloudy and the black line behind the solution is very hard to observe. At pH value below 3.0, PAA-dopa will precipitate out from the solution. At the low pH value, carboxylic acid groups are protonated, hydrogen bonds among carboxylic acids form, and thus PAA 6
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chains take compact conformation.50 As the dopamine grafted to PAA, the presence of the phenolic hydroxyl groups further strengthen the hydrogen bonding. PAA-dopa shows the stronger tendency to aggregate at low pH values than PAA.
2.0
1.5
Absorbance
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1.0
pH 3.0
0.5
pH 10.0 0.0
pH 5.0 300
400
500
600
700
800
λ (nm)
Figure 1 Photographs and UV-vis spectra of PAA-dopa solutions (0.5 mg/mL) at different pH values. At pH value above 7.0, the PAA-dopa solution gradually changes color to brown. In the basic solution, catechol is oxidized to form o-quinone.51, 52 As shown in Figure 1b, in the acidic environment, the PAA-dopa solution exhibits an absorbance peak at 280 nm. Under the basic condition, the characteristic peak shifts to 285 nm and a new broad peak at 435 nm appears, attributed to the oxidization of catechol to o-quinone. PAA-dopa will precipitate out of the solution at pH values below 3.0. At pH 3.0, the PAA-dopa solution is turbid, indicating PAA-dopa has aggregated. But the aggregated PAA-dopa still homogeneously distributes in the solution. So the pH values of the assembly solutions of PAA-dopa and PVPON were chosen to start at 3.0 and then gradually increased. FT-IR spectra of the films prepared at different pH value are shown in Figure 2.
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(a)
3.0 3.5 4.0 4.5 5.0
Absorbance
0.3
0.2
0.1
0.0 1800
1750
1700
1650
1600
1550
Wavenumber (cm-1)
(b) 1.0
A1720/A1640
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0.8
0.6
3.0
3.5
4.0
4.5
5.0
pH
Figure 2 (a) FT-IR spectra of (PVPON/PAA-dopa)20 assembled at different pH values; (b) Absorbance ratio of the carboxyl of PAA to the carboxyl of PVPON.
The absorbance band centered at 1720 cm-1 and 1640 cm-1 corresponds to the hydrogen-bonded carbonyl stretch vibrations of PAA and PVPON, respectively.53, 54 The absorbance of free carbonyl stretch vibration of PAA is located at 1742 cm-1,55 while the free carbonyl stretch vibration of PVPON is located at 1680 cm-1.53 In bulk state, PAA does not exhibit carbonyl stretching peak at 1742 cm-1, but at 1711 cm-1, which combines carbonyl stretching of different hydrogen bonding association forms of carboxylic acid groups.53, 55 Generally PVPON presents carbonyl stretching peak at 1660 cm-1 due to hydrogen-bonding with water, and after completely removing water, the carbonyl stretching peak of PVPON will shift back to 1680 cm-1.25,
53
After
hydrogen bonding LbL assembly, the carbonyl stretching peaks of PAA and PVPON both shift to the low frequencies compared with free carbonyl groups of them.53, 54 The growth of the hydrogen-bonded PVPON/PAA-dopa film shows a strong dependence on pH value. As pH value increases, the film thickness growth will 8
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become slow. As shown in Figure 2a, as the pH value increased to 5.0, the characteristic vibration peaks of PAA-dopa and PVPON was hardly observed. The UV-visible spectra also demonstrated that when pH value elevated to 5.0, PVPON and PAA-dopa almost could not deposit (SI Figure S3). At pH value above 5.0, the ionization of PAA-dopa exceeds a certain degree, and charge repulsion among polymer chains interrupts hydrogen bonding, resulting in no film. Polymer chain conformation and aggregation state in the assembly solution affect the thickness growth and morphology of LbL film.56 PVPON is a neutron polymer, and its chain conformation does not show a dependence on pH value. However, PAA-dopa is a weak polyelectrolyte, and its chain conformation has a strong dependence on pH value.50 Figure 2b shows that the absorbance ratio of the carbonyl stretching of PAA-dopa to PVPON in the film declined with the pH increasing. It indicated the PAA-dopa chain was assembled with different conformation state as the solution pH value changed. At pH 3.0, PAA-dopa is in the aggregate state, but the aggregated particles still homogeneously disperse in the solution. As shown in Figure 2 and 3, at pH 3.0 the film thickness growth is quick but the film is very rough. As the pH value elevates, the ionization degree of PAA-dopa increases. Charge repulsion strengthens and hydrogen bonding among the repeating units weakens, so the polymer chain expands. When the polymer chains are assembled at the expanded state, generally the smooth but thin film will be produced. As the pH value increased, PVPON/PAA-dopa film becomes thin and smooth. But when pH value exceeds 5.0, the PVPON and PAA-dopa cannot be LbL assembled any more. At
pH
3.5
or
pH
4.0,
the
UV-visible
spectra
monitoring
shows
PVPON/PAA-dopa film thickness growth is linear (SI Figure S4). For PVPON/PAA system, before the 10 dipping cycle the film exhibites exponential growth mode but after 10 dipping cycle, the film transferres to linear growth. 44 Hydrogen bonding LbL assembly is very hard to produce well-ordered lamellar structure, existing diffusion and interpenetration.
17
Linear and exponential growth represent different ends of a
continuous spectrum of diffusion mobility. 57 9
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At pH 3.0, as the assembling cycle increases, the film becomes very cloudy, and it is hard to monitor the accurate absorbance increment due to strong scattering effect. The SEM section image (Figure 3b) shows that the (PVPON/PAA-dopa)40 film fabricated at pH 3.0 is about 1.37 µm, the average bilayer thickness is estimated to be 34.3 nm. The (PVPON/PAA-dopa)40 films prepared at pH 3.5 and pH 4.0 are smooth, and the films exhibit the Fabry-Pérot fringes on UV-visible spectra (SI Figure S3), which can be used to calculate the film thickness.
44
The (PVPON/PAA-dopa)40 film
prepared at pH 3.5 and 4.0 is about 550 nm and 330 nm, respectively. The average thickness growth per assembling cycle at pH 3.5 is 13.8 nm while at pH 4.0 is 8.3 nm. (a)
(b)
Figure 3 SEM images of (PVPON/PAA-dopa)40 prepared at pH 3.0: (a) surface; (b) cross-section.
Hydrogen-bonded thin films are not robust enough and will disintegrate when the environmental pH value exceeds a certain value.45 Kwon et al. reported the discrepancy of critical pH value between the construction and the disintegration of the hydrogen-bonded
complex,
poly(ethyloxzaoline)/poly(methacrylic acid).58 Our
previous work found that PVPON and PAA can be LbL assembled at pH value only below 4.0, while the disintegration of the preassembled PVPON/PAA thin film occurs at pH value above 5.5.50 Phenol has higher pKa value than carboxylic acid, thus the hydrogen-bonded
film
containing
polyphenol
should
be
stable
than
the
hydrogen-bonded thin film containing poly(carboxylic acid).49 Tannic acid is a kind of polyphenol. The hydrogen-bonded PVPON/tannic acid film has a critical assembly pH value of 7.5 and a critical disintegration pH value of 9.0, which are higher than that of the PVPON/PAA film. 49 Dopamine contains two phenol groups, and its pKa is 9.39. When dopamine is introduced into PAA chain, the film should endure the high pH value compared with the PVPON/PAA film. 10
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The PVPON/PAA-dopa thin film prepared at pH 3.0 is immersed into the solution of pH value above 5.0, where no thin film deposited. As shown in Figure 4, only at pH value higher than 7.0, we observed the disintegration of the film. The PVPON/PAA-dopa thin film is prepared at pH below 5.0 and the film disintegration happens at pH value above 7.0. The critical assembly and the critical disintegration pH value of PVPON/PAA-dopa film are higher than those of PVPON/PAA film.
1.0
Fraction Retained
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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0.8 0.6 0.4 0.2 0.0 5
6
7
8
9
10
pH
Figure 4 The (PVPON/PAA-dopa)40 film prepared at 3.0 immersed into the aqueous solutions with different pH values for 48 h.
Due to dopamine is introduced to PAA chain, catechol chemistry can be applied to
cross-link
PVPON/PAA-dopa
film.
In
the
presence
of
oxidant,
the
oxidization-induced dismutation process happens between two catechol groups, and hence the film is cross-linked (Scheme 1). The (PVPON/PAA-dopa)40 film assembled at pH 3.0 was immersed in NaIO4 aqueous solution (1 mM), and the reaction process was monitored by UV-visible spectroscopy (SI Figure S5). With the time elapsing, the film gradually turns dark orange (Figure 5a). As only hydrogen bonding interaction but no covalent interaction between the substrate and the bottommost layer of the cross-linked PVPON/PAA-dopa, the film will liberate from the substrate when these hydrogen bonds break. In the alkaline solution (pH 12.0), the film exfoliation process is observed, as shown in Figure 5b. Through LbL assembly, the thin film is deposited on the all faces of the substrate. The substrate is rectangular plate, about 1 mm thick. To make the film easily remove from 11
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the substrate, the edge was scratched with a sharp blade and then immersed into the alkaline solution. The films with rectangular shape were present, as shown in Figure 5c-d. In aqueous solutions, the exfoliated film is very flexible. When nipped out with a tweezer, it likes a silk scarf and cannot fix shape. But using acetone remove water, we can get a shape-fixed film, without breaking or tearing, as shown in Figure 5c.
(a)
(b)
(c)
(d)
(f) Film Area (cm2)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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(e)
pH 12.0
8
6
4
2
pH 2.0 0
2
4
6
8
10
Cycle Number
Figure 5 The photographs of (PVPON/PAA-dopa)40 film (a) the cross-linked film attached on the substrate, (b) the film immersed in e pH 12.0 solution, (c) the dry free-standing film, (d) the swollen-state film in pH 12.0 solution, (e) the shrunk-state film in pH 2.0 solution and (f) the film area when the film alternatively immersed into pH 2.0 and pH 12.0 solution.
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In addition, it is found that in the alkaline solution, the size of the exfoliated film is distinctly larger than the surface-attached film (Figure 5d). The grafting degree of dopamine to PAA affects the swelling ratio. The (PVPON/PAA-dopa)40 film made from PAA-dopa with the 5% grafting degree shows a 2.5-fold swelling, while for the 13% grafting degree one, the swelling ratio is 2.2, compared with the film attached on the substrate. Transferring from the solution of pH 12.0 into the pH 2.0, the film quickly shrinks to a smaller size (Figure 5e). When the film alternatively immersed into the acidic solution (pH 2.0) and the basic solution (pH 12.0), a reversible swelling-shrinking behavior is present.
1720
Absorbance
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1640
a
1574 1660
b
1720
1640
c 1800
1750
1700
1650
1600
1550
1500
Wavenumber (cm-1)
Figure 6 FT-IR spectra of the (PVPON/PAA-dopa)40 films: (a) as-prepared at pH 3.0, (b) the cross-linked film immersed in the pH 12.0 solution; and (c) transferring from the pH 12.0 solution to the pH 2.0 solution.
FT-IR spectroscopy was utilized to study the swelling-shrinking behavior of the free-standing film (Figure 6). In the swollen state, the film exhibits absorbance peaks at 1660 cm-1 and 1574 cm-1. The absorbance peak at 1574 cm-1 is attributed to asymmetric vibration of carboxylate, while 1660 cm-1 is the carbonyl stretching vibration of PVPON. 50, 59, 60 In the shrunk state, the film shows absorbance peaks at 1640 cm-1 and 1720 cm-1, which is almost same to the as-prepared hydrogen-bonded thin film at pH 3.0. 53, 54 As previous discussion, the peak at 1720 cm-1 and 1640 cm-1 is assigned to the hydrogen-bonded carboxylic acid group of PAA and the carbonyl of 13
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PVPON, respectively. The FT-IR spectra indicate that in the basic environment, PVPON chains do not dissolve out and still retain in the film, and carboxylic acid groups lose proton and hydrogen bonds break, inducing the film expands. Transferring the free-standing thin film from the alkaline solution to the acidic solution, hydrogen bonds between the PVPON and PAA reconstruct and hence the film shrinks. The wettability of the free-standing film in the swollen state and in the shrunk state was investigated. As shown in Figure 7, when dripped on the surface of the swollen film, the water droplet is quickly absorbed. However, on the surface of the shrunk film, the droplet is not absorbed and the contact angle keeps for relatively long time. In the swollen state, hydrogen bonds break and hydrophilic groups, carboxylate and amide, are present, so water drop can be quickly absorbed. While in the shrunk state, carboxylic acid groups of PAA-dopa are hydrogen-bonded with PVPON. The film does not have much hydrophilic groups available, and thus water adsorption ability becomes weak.
5s
10s
5s
10s
Figure 7 Photographs of water drop profiles versus time on (up) the swollen-state and (down) the shrunk-state (PVPON/PAA-dopa)40 film built at pH 3.0.
SEM and AFM images in Figure 8 show that the surface of the swollen film is lumpy, while the shrunk film is dense and flat. When the free-standing thin film is transferred from the alkaline solution to the acidic solution, the hydrogen bonds rebuild. However, the reconstructed hydrogen-bonded film shows different morphology compared with the as-prepared film at pH 3.0. The hydrogen-bonded PVPON/PAA-dopa thin film prepared at pH 3.0 is rough due to the aggregation of PAA-dopa. After cross-linking, the PVPON/PAA-dopa film remains rough (SI Figure 14
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S6), whereas the reconstructed hydrogen-bonding thin film shows relatively smooth topology. When the cross-linked film exposes to the alkaline solution, the hydrogen bonds in the thin film completely break. All the aggregated structure of PAA-dopa disappears, and PAA-dopa chains expand. When the swollen film is put into the acidic solution, the hydrogen bonds between PVPON and PAA reconstruct, and the aggregation of PAA-dopa would not happen anymore. So the film shows relatively smooth morphology. (a)
(b)
(c)
(d)
Figure 8 SEM images (a and b) and 3D AFM images (c and d, L × W × H, 5 µm × 5 µm × 1 µm) : the swollen state (a and c); the shrunk state (b and d).
CONCLUSION Dopamine modified PAA and PVPON can be alternative deposited to prepare thin film based on hydrogen bonding. Carboxylic acid and phenol group of PAA-dopa both act as hydrogen bonding donors, which made the critical assembly pH and the critical disintegration pH value of PVPON/PAA-dopa film enhance, compared with PVPON/PAA film. Under oxidizing condition, PVPON/PAA-dopa thin film can be cross-linked by dismutation reaction. The cross-linked film is robust enough that can be exfoliated from the substrate to produce an intact free-standing film by the alkaline solution. Hydrogen bond formation and breakage can be tuned by adjusting the pH value of the solution, which makes the cross-linked free-standing PVPON/PAA-dopa film endowed with a reversible swelling-shrinking behavior. 15
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ACKNOWLEDGMENTS S. Y. gratefully acknowledges the support from National Natural Science Foundation of China (NSFC, Grant No. 51373032), Innovation Program of Shanghai Municipal Education Commission, Fundamental Research Funds for the Central University and DHU Distinguished Young Professor Program.
Supporting Information Available H-NMR and FT-IR of PAA-dopa, UV-Visible spectra of (PVPON/PAA-dopa)20, the film growth mode, the film cross-linking process monitored by UV-Visible, and SEM image of the cross-linked film are present in Supporting Information file. This information is available free of charge via the Internet at http://pubs.acs.org/.
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hydrogen-bonded polymer capsules. Macromolecules 2006, 39, 5569–5572. (30) Kozlovskaya, V.; Alexander, J. F.; Wang, Y.; Kuncewicz, T.; Liu, X.; Godin, B.; Kharlampieva, E. Internalization of red blood cell-mimicking hydrogel capsules with pH-triggered shape responses. ACS nano 2014, 8, 5725–5737. (31) Zelikin, A. N.; Quinn, J. F.; Caruso, F. Disulfide cross-linked polymer capsules: En route to biodeconstructible systems. Biomacromolecules 2006, 7, 27–30. (32) Yang, S. Y.; Rubner, M. F. Micropatterning of polymer thin films with pH-sensitive and cross-linkable hydrogen-bonded polyelectrolyte multilayers. J. Am. Chem. Soc. 2002, 124, 2100–2101. (33) Lee, H.; Mensire, R.; Cohen, R. E.; Rubner, M. F. Strategies for Hydrogen Bonding Based Layer-by-Layer Assembly of Poly(vinyl alcohol) with Weak Polyacids. Macromolecules 2012, 45, 347–355. (34) Wang, B. L.; Ren, K. F.; Chang, H.; Wang, J. L.; Ji, J. Construction of degradable multilayer films for enhanced antibacterial properties. ACS Appl. Mater. Interfaces 2013, 5, 4136–4143. (35) Chen, J.; Cao, W. Fabrication of a covalently attached self-assembly multilayer film via H-bonding attraction and subsequent UV-irradiation. Chem. Commun. 1999, 1711–1712. (36) Connal, L. A.; Kinnane, C. R.; Zelikin, A. N.; Caruso, F. Stabilization and Functionalization of Polymer Multilayers and Capsules via Thiol-Ene Click Chemistry. Chem. Mater. 2009, 21, 576–578. (37) Kinnane, C. R.; Such, G. K.; Antequera-Garcia, G.; Yan, Y.; Dodds, S. J.; Liz-Marzan, L. M.; Caruso, F. Low-fouling poly(N-vinyl pyrrolidone) capsules with engineered degradable properties. Biomacromolecules 2009, 10, 2839–2846. (38) Lee, H.; Dellatore, S. M.; Miller, W. M.; Messersmith, P. B. Mussel-inspired surface chemistry for multifunctional coatings. Science 2007, 318, 426–430. (39) Ye, Q.; Zhou, F.; Liu, W. Bioinspired catecholic chemistry for surface modification. Chem. Soc. Rev. 2011, 40, 4244–4258. (40) Lee, H.; Lee, Y.; Statz, A. R.; Rho, J.; Park, T. G.; Messersmith, P. B. Substrate-independent layer-by-layer assembly by using mussel-adhesive-inspired 19
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polymers. Adv. Mater. 2008, 20, 1619–1623. (41) Wu, J.; Zhang, L.; Wang, Y.; Long, Y.; Gao, H.; Zhang, X.; Zhao, N.; Cai, Y.; Xu, J. Mussel-inspired chemistry for robust and surface-modifiable multilayer films. Langmuir 2011, 27, 13684–13691. (42) Lee, H.; Lee, Y.; Statz, A. R.; Rho, J.; Park, T. G.; Messersmith, P. B. Substrate-independent layer-by-layer assembly by using mussel-adhesive-inspired polymers. Adv. Mater. 2008, 20, 1619–1623. (43) Lee, Y.; Chung, H. J.; Yeo, S.; Ahn, C.-H.; Lee, H.; Messersmith, P. B.; Park, T. G. Thermo-sensitive, injectable, and tissue adhesive sol–gel transition hyaluronic acid/pluronic composite hydrogels prepared from bio-inspired catechol-thiol reaction. Soft Matter 2010, 6, 977–983. (44) Yang, S.; Tan, S.; Zhang, Y.; Xu, J.; Zhang, X. Fabry–Pérot fringes of hydrogen-bonded assembly films. Thin Solid Films 2008, 516, 4018–4024. (45) Sukhishvili, S. A.; Granick, S. Layered, erasable polymer multilayers formed by hydrogen-bonded sequential self-assembly. Macromolecules 2002, 35, 301–310. (46) Kharlampieva, E.; Kozlovskaya, V.; Tyutina, J.; Sukhishvili, S. A. Hydrogen-bonded multilayers of thermoresponsive polymers. Macromolecules 2005, 38, 10523–10531. (47) Ma, J.; Yang, S.; Li, Y.; Xu, X.; Xu, J. Effect of temperature on the build-up and post hydrothermal processing of hydrogen-bonded PVPON/PAA film. Soft Matter 2011, 7, 9435–9443. (48) Cao, Y.; Wang, C.; Yang, S.; Li, Y.; Yang, X.; Zhang, C.; Ma, J.; Xu, J. Fluorescence
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with Model Decapeptides. Biochemistry 2000, 39, 11147–11153. (52) Yu, M. E.; Hwang, J. Y.; Deming, T. J. Role of L-3,4-dihydroxyphenylalanine in mussel adhesive proteins. J. Am. Chem. Soc. 1999, 121, 5825–5826. (53) Yang, S.; Zhang, Y.; Guan, Y.; Tan, S.; Xu, J.; Cheng, S.; Zhang, X. Water uptake behavior of hydrogen-bonded PVPON-PAA LBL film. Soft Matter 2006, 2, 699–704. (54) Yang, S.; Zhang, Y.; Wang, L.; Hong, S.; Xu, J.; Chen, Y.; Li, C. Composite thin film by hydrogen-bonding assembly of polymer brush and poly(vinylpyrrolidone). Langmuir 2006, 22, 338–343. (55) Dong, J.; Ozaki, Y.; Nakashima, K. Infrared, Raman, and near-infrared spectroscopic evidence for the coexistence of various hydrogen-bond forms in poly(acrylic acid). Macromolecules 1997, 30, 1111–1117. (56) Dubas, S. T.; Schlenoff, J. B. Factors Controlling the Growth of Polyelectrolyte Multilayers. Macromolecules 1999, 32, 8153–8160. (57) Schlenoff, J. B. Retrospective on the Future of Polyelectrolyte Multilayers. Langmuir 2009, 25, 14007–14010. (58) Kwon, I. C.; Bae, Y. H.; Kim, S. W. Electrically Erodible Polymer Gel for Controlled Release of Drugs. Nature 1991, 354, 291–293. (59) Ma, S.; Qi, X.; Cao, Y.; Yang, S.; Xu, J. Hydrogen bond detachment in polymer complexes. Polymer 2013, 54, 5382–5390. (60) Choi, J.; Rubner, M. F. Influence of the degree of ionization on weak polyelectrolyte multilayer assembly. Macromolecules 2005, 38, 116–124.
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For Table of Contents Only
Reversible Swelling-Shrinking Behavior of Hydrogen-Bonded Free-Standing Thin Film Stabilized by Catechol Reaction
Jiaxing Sun1, Chao Su1, Xuejian Zhang1, Wenjing Yin1, Jian Xu2, Shuguang Yang1*
1
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,
College of Material Science and Engineering, Donghua University, Shanghai 201620 2
Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190 *To whom the correspondence should be addressed. Email: [email protected]
Hydrogen bond formation-breakage is utilized to tune the film shrinking-swelling.
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Reversible Swelling-Shrinking Behavior of Hydrogen-Bonded Free-Standing Thin Film Stabilized by Catechol Reaction Jiaxing Sun, Chao Su, Xuejian Zhang, Wenjing Yin, Shuguang Yang, and Jian Xu Langmuir, Just Accepted Manuscript • DOI: 10.1021/la5048479 • Publication Date (Web): 22 Apr 2015 Downloaded from http://pubs.acs.org on April 27, 2015
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Reversible Swelling-Shrinking Behavior of Hydrogen-Bonded Free-Standing Thin Film Stabilized by Catechol Reaction
Jiaxing Sun1, Chao Su1, Xuejian Zhang1, Wenjing Yin1, Jian Xu2, Shuguang Yang1*
1
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,
College of Material Science and Engineering, Donghua University, Shanghai 201620 2
Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190 *To whom the correspondence should be addressed. Email: [email protected]
ABSTRACT Dopamine-modified poly(acrylic acid) (PAA-dopa) and poly(vinylpyrrolidone) (PVPON) was layer-by-layer (LbL) assembled to prepare thin film based on hydrogen bonding. Carboxylic group of acrylic acid and phenolic hydroxyl group of dopamine can both act as hydrogen bond donors. The critical assembly and the critical disintegration pH values of PVPON/PAA-dopa film are enhanced compared with PVPON/PAA film. The hydrogen-bonded PVPON/PAA-dopa thin film can be cross-linked via catechol chemistry of dopamine. After cross-linking, the film can be exfoliated from the substrate in alkaline solution to get a free-standing film. Moreover, by tuning pH value, deprotonation and protonation of PAA will make hydrogen bond in the film break and re-construct, which induces that the free-standing film has a reversible swelling-shrinking behavior.
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INTRODUCTION Layer-by-layer assembly (LbL) technique, first introduced by Decher in early 1990s, has been drawing considerable attentions due to its simplicity and versatility to fabricate structurally tunable materials with various compositions.1-5 Free-standing LbL materials, with well-controlled mechanical properties and architectures compared with the surface-attached type, are of great interest for their potential applications as drug carriers, tissue engineering scaffolds and wound healing materials.6-9 The most popular approach to prepare free-standing LbL materials is to alternately deposit the components on a sacrificial precursor layer or substrate. 10-12 Different free-standing LbL structures, like planar film,10 microcapsule,13, 14 microtubule,15 and microcube,16 have been successfully fabricated. Hydrogen-bonded LbL materials, first demonstrated by Rubner
17
and Zhang
18
individually, have excellent biocompatibility and sensitive stimuli-responsivity, showing a promising potential for biomedical and sensor applications.19-23 Lots of efforts have been made to construct hydrogen-bonded LbL free-standing structures.24-27 Hammond et al. fabricated an 8 µm free-standing hydrogen-bonded planar film by using substrates with neutral, hydrophobic surfaces such as Teflon and polypropylene.24 Zhang et al. first reported a hydrogen-bonded microcapsule fabricated
by
LbL
assembly
of
poly(vinylpyrrolidone)
(PVPON)
and
m-methylphenol-formaldehyde resin (MPR) on sacrificial SiO2 or PS nanoparticle templates.25 However, the weak mechanical strength and chemical stability limit the applications of the hydrogen-bonded free-standing materials. Chemical cross-linking has been utilized to overcome the restriction.28-37 Carbodiimide chemistry was introduced to cross-link hydrogen-bonded polyacid/nonionic systems and thus the robust capsules were prepared. 28-30 Caruso et al. utilized disulfide bonds to cross-link hydrogen-bonded systems.
31
Rubner et al.
applied thermal annealing to enhance the stability of the hydrogen-bonded PVA/PAA and PAAM/PAA films.32,
33
The hydrogen-bonded LbL film of diazo resin and
phenol-formaldehyde resin was cross-linked by photochemical reaction.35 And click chemistry reactions, such as thiol-ene and alkyne-azide, were also used to stabilize 2
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hydrogen-bonded LbL materials.36, 37 Dopamine, an analogue of the mussel adhesive proteins (MAPs), have the ability to form an adhesive polydopamine coating on almost all kinds of substrates.38 The ortho-dihydroxyphenyl (catechol) moiety of dopamine can form strong affinity with solid surfaces by series of reactions, including the dismutation, Michael addition, Schiff base reaction, and strong coordination with many metal oxides or ions.39, 40 Through introducing dopamine to polymer chain, the reactivity of catechol can be utilized for further chemical modification, like crosslinking or adjusting hydrophobicity.41-43 In this work, we fabricated a hydrogen-bonded film by alternately depositing PVPON and dopamine-modified PAA (PAA-dopa) on silicon or quartz substrate. Catechol chemistry was utilized to cross-link the film, which increased the stability of the film. The free-standing film was formed by exfoliation in the alkaline solution directly, not like general way required a sacrificial layer or template. Furthermore, the free-standing film exhibited a reversible swelling-shrinking behavior as the solution pH value changed.
EXPERIMENTAL SECTION Materials. Poly(acrylic acid) (PAA, Mw = 450 000) and poly(vinylpyrrolidone) (PVPON,
Mw
=
40
000)
were
purchased
from
Sigma-Aldrich.
N-(3-dimethylaminopropyl)-N’-ethylcarbodiimide hydrochloride (EDC•HCl) was obtained from TCI. Dopamine hydrochloride was bought from J&K. Other chemicals were analytical grade and used as received. Synthesis of PAA-dopa. PAA powder (3 mmol) was dissolved in 20 mL 0.1 M phosphate buffer solution (PBS, pH 6.0). EDC•HCl (0.6 mmol) and dopamine hydrochloride (0.6 mmol) were dissolved in 10 mL 0.1 M PBS, and then added into PAA solution. The solution was continuous stirred for 24 h under N2 gas protection. The reaction mixture was dialyzed against deionized water using a regenerated cellulose dialysis tube with a molecular weight cutoff of 8 000 until dopamine in the 3
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washing solution was undetectable by UV-visible spectroscopy. The dialysate was diluted with a 0.1 M PBS to a certain volume and stored in dark for further use. Preparation of Hydrogen-Bonded Thin Film. The thin film of PAA-dopa and PVPON was deposited on quartz or silicon substrate. Before the deposition, the substrate needed to be clean, which was immersed in boiling piranha solution (30% H2O2/98% H2SO4 at 3:7 of volume ratio, extremely corrosive) for half an hour, then rinsed with abundant amounts of deionized water, and finally dried with N2 stream. The LbL procedure was performed through an automated device (Shenyang Kejing). The substrate was dipped into PVPON solution and PAA-dopa solution alternately, with three rinses after each depositing step to remove the excess polymers. The concentration of PVPON and PAA-dopa solution was 2.0 and 1.0 mg/mL, respectively. The deposition and rinse time was set as 4 and 1 min, respectively. The pH value of the rinse solution kept the same with the assembly solution, which was adjusted with HCl and NaOH solution under the monitor of a pH meter (Mettler S40K). Till the desired layer number was reached, the resultant film was immersed in the rinse solution for 24 h and finally dried with N2. For simplicity, (PVPON/PAA-dopa)n is used to mark the film prepared with n cycle number. Stabilization of Hydrogen-Bonded Thin Film. The PVPON/PAA-dopa film prepared at pH 3.0 on silicon or quartz slide was immersed in 1 mM NaIO4 aqueous solution for cross-linking via oxidization induced dismutation. The total immersion time for 40-bilayer-film was 24 h. After cross-linking, the film was rinsed thoroughly with deionized water and dried with N2 flow. Preparation of Free-Standing Thin Film. To obtain a free-standing film with regular shapes, the film was scratched near the edges of the substrate using a razor blade and then immersed in NaOH solution (pH 12.0). The planar film would detach from the substrate in several minutes. The exfoliated film was transferred from the pH 12.0 solution to the acetone bath to remove water, and then took out and spread on a PTFE plate. After acetone evaporated, the dry free-standing film was prepared.
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Scheme 1 Procedure to prepare the free-standing PVPON/PAA-dopa thin film.
Characterization. PAA-dopa was dissolved in D2O sealing in NMR tube, and 1H NMR measurement was carried out on a Bruker DMX-400 NMR spectrometer. The grafting degree of dopamine to PAA was determined by calculating the integral area ratio of H atoms in the aromatic rings of grafted catechol groups to H atoms in the polymeric backbone on the NMR spectrum. FT-IR spectra were collected on a Nicolet 8700 FT-IR spectrometer. The polymer compound was ground with KBr uniformly and then pressed into a standard disk for FT-IR measurement. With pure silicon substrate as background, the thin film deposited on the silicon substrate did IR spectroscopic characterization directly. UV-visible spectra were recorded on a Shimadzu UV-2550 spectrophotometer. The polymer solution was pour into the standard quartz cuvette (1 cm × 1 cm) for measurement. The thin film deposited on the quartz slide (12 mm × 45 mm) was measured with UV-visible spectrometer directly. SEM images were obtained on a Hitachi SU 8010 field emission scanning electron microscope with an acceleration voltage of 5.0 kV. For SEM characterization, 5
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the thin film was deposited on a silicon substrate The thickness of the PVPON/PAA-dopa film was determined with Fabry-Pérot fringes on UV-visible spectra, 44 or by the section image of SEM. AFM images were recorded on an Agilent 5500 atomic force microscope in the tapping mode. A commercial silicon probe (model TESP-100) with a typical resonant frequency of approximately 300 kHz was used to scan the film. Water contact angle measurements were performed on an OCA40 Micro Optical Contact Angle Measuring Device. A droplet of 5 µL water was produced with a micro syringe.
RESULTS AND DISCUSSION Poly(carboxylic acid)s, such as poly(acrylic acid) and poly(methacrylic acid), can be LbL assembled with nonionic polymers, such as PVPON, PEO, and PVME to prepare thin films based on hydrogen bonding.45-48 Polymers containing phenol groups can also be utilized as hydrogen bond donors to fabricate thin films by LbL assembly.49 Dopamine, contained two phenolic hydroxyl groups, was partially grafted on PAA. The resultant PAA-dopa was alternately deposited with PVPON to build up a thin film, in which carboxylic groups and phenolic hydroxyl groups both acted as hydrogen bond donors. Dopamine-modified PAA was synthesized via carbodiimide chemistry, as shown in Scheme 1.41 1H NMR spectrum (SI Figure S1) and FT-IR spectrum (SI Figure S2) prove that dopamine is successfully linked onto PAA chain. The photographs and UV-visible spectra of PAA-dopa solutions at different pH values are shown in Figure 1. In wide pH region, the PAA solution keeps transparent. As dopamine is introduced into PAA chain, the solution behavior of PAA-dopa is different to PAA. Only in the pH region from 4.0 to 7.0, the solution is transparent, where the black line behind the solution can be seen. As pH value decreases to 3.0, PAA solution becomes cloudy and the black line behind the solution is very hard to observe. At pH value below 3.0, PAA-dopa will precipitate out from the solution. At the low pH value, carboxylic acid groups are protonated, hydrogen bonds among carboxylic acids form, and thus PAA 6
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chains take compact conformation.50 As the dopamine grafted to PAA, the presence of the phenolic hydroxyl groups further strengthen the hydrogen bonding. PAA-dopa shows the stronger tendency to aggregate at low pH values than PAA.
2.0
1.5
Absorbance
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1.0
pH 3.0
0.5
pH 10.0 0.0
pH 5.0 300
400
500
600
700
800
λ (nm)
Figure 1 Photographs and UV-vis spectra of PAA-dopa solutions (0.5 mg/mL) at different pH values. At pH value above 7.0, the PAA-dopa solution gradually changes color to brown. In the basic solution, catechol is oxidized to form o-quinone.51, 52 As shown in Figure 1b, in the acidic environment, the PAA-dopa solution exhibits an absorbance peak at 280 nm. Under the basic condition, the characteristic peak shifts to 285 nm and a new broad peak at 435 nm appears, attributed to the oxidization of catechol to o-quinone. PAA-dopa will precipitate out of the solution at pH values below 3.0. At pH 3.0, the PAA-dopa solution is turbid, indicating PAA-dopa has aggregated. But the aggregated PAA-dopa still homogeneously distributes in the solution. So the pH values of the assembly solutions of PAA-dopa and PVPON were chosen to start at 3.0 and then gradually increased. FT-IR spectra of the films prepared at different pH value are shown in Figure 2.
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0.3
0.2
0.1
0.0 1800
1750
1700
1650
1600
1550
Wavenumber (cm-1)
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A1720/A1640
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0.8
0.6
3.0
3.5
4.0
4.5
5.0
pH
Figure 2 (a) FT-IR spectra of (PVPON/PAA-dopa)20 assembled at different pH values; (b) Absorbance ratio of the carboxyl of PAA to the carboxyl of PVPON.
The absorbance band centered at 1720 cm-1 and 1640 cm-1 corresponds to the hydrogen-bonded carbonyl stretch vibrations of PAA and PVPON, respectively.53, 54 The absorbance of free carbonyl stretch vibration of PAA is located at 1742 cm-1,55 while the free carbonyl stretch vibration of PVPON is located at 1680 cm-1.53 In bulk state, PAA does not exhibit carbonyl stretching peak at 1742 cm-1, but at 1711 cm-1, which combines carbonyl stretching of different hydrogen bonding association forms of carboxylic acid groups.53, 55 Generally PVPON presents carbonyl stretching peak at 1660 cm-1 due to hydrogen-bonding with water, and after completely removing water, the carbonyl stretching peak of PVPON will shift back to 1680 cm-1.25,
53
After
hydrogen bonding LbL assembly, the carbonyl stretching peaks of PAA and PVPON both shift to the low frequencies compared with free carbonyl groups of them.53, 54 The growth of the hydrogen-bonded PVPON/PAA-dopa film shows a strong dependence on pH value. As pH value increases, the film thickness growth will 8
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become slow. As shown in Figure 2a, as the pH value increased to 5.0, the characteristic vibration peaks of PAA-dopa and PVPON was hardly observed. The UV-visible spectra also demonstrated that when pH value elevated to 5.0, PVPON and PAA-dopa almost could not deposit (SI Figure S3). At pH value above 5.0, the ionization of PAA-dopa exceeds a certain degree, and charge repulsion among polymer chains interrupts hydrogen bonding, resulting in no film. Polymer chain conformation and aggregation state in the assembly solution affect the thickness growth and morphology of LbL film.56 PVPON is a neutron polymer, and its chain conformation does not show a dependence on pH value. However, PAA-dopa is a weak polyelectrolyte, and its chain conformation has a strong dependence on pH value.50 Figure 2b shows that the absorbance ratio of the carbonyl stretching of PAA-dopa to PVPON in the film declined with the pH increasing. It indicated the PAA-dopa chain was assembled with different conformation state as the solution pH value changed. At pH 3.0, PAA-dopa is in the aggregate state, but the aggregated particles still homogeneously disperse in the solution. As shown in Figure 2 and 3, at pH 3.0 the film thickness growth is quick but the film is very rough. As the pH value elevates, the ionization degree of PAA-dopa increases. Charge repulsion strengthens and hydrogen bonding among the repeating units weakens, so the polymer chain expands. When the polymer chains are assembled at the expanded state, generally the smooth but thin film will be produced. As the pH value increased, PVPON/PAA-dopa film becomes thin and smooth. But when pH value exceeds 5.0, the PVPON and PAA-dopa cannot be LbL assembled any more. At
pH
3.5
or
pH
4.0,
the
UV-visible
spectra
monitoring
shows
PVPON/PAA-dopa film thickness growth is linear (SI Figure S4). For PVPON/PAA system, before the 10 dipping cycle the film exhibites exponential growth mode but after 10 dipping cycle, the film transferres to linear growth. 44 Hydrogen bonding LbL assembly is very hard to produce well-ordered lamellar structure, existing diffusion and interpenetration.
17
Linear and exponential growth represent different ends of a
continuous spectrum of diffusion mobility. 57 9
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At pH 3.0, as the assembling cycle increases, the film becomes very cloudy, and it is hard to monitor the accurate absorbance increment due to strong scattering effect. The SEM section image (Figure 3b) shows that the (PVPON/PAA-dopa)40 film fabricated at pH 3.0 is about 1.37 µm, the average bilayer thickness is estimated to be 34.3 nm. The (PVPON/PAA-dopa)40 films prepared at pH 3.5 and pH 4.0 are smooth, and the films exhibit the Fabry-Pérot fringes on UV-visible spectra (SI Figure S3), which can be used to calculate the film thickness.
44
The (PVPON/PAA-dopa)40 film
prepared at pH 3.5 and 4.0 is about 550 nm and 330 nm, respectively. The average thickness growth per assembling cycle at pH 3.5 is 13.8 nm while at pH 4.0 is 8.3 nm. (a)
(b)
Figure 3 SEM images of (PVPON/PAA-dopa)40 prepared at pH 3.0: (a) surface; (b) cross-section.
Hydrogen-bonded thin films are not robust enough and will disintegrate when the environmental pH value exceeds a certain value.45 Kwon et al. reported the discrepancy of critical pH value between the construction and the disintegration of the hydrogen-bonded
complex,
poly(ethyloxzaoline)/poly(methacrylic acid).58 Our
previous work found that PVPON and PAA can be LbL assembled at pH value only below 4.0, while the disintegration of the preassembled PVPON/PAA thin film occurs at pH value above 5.5.50 Phenol has higher pKa value than carboxylic acid, thus the hydrogen-bonded
film
containing
polyphenol
should
be
stable
than
the
hydrogen-bonded thin film containing poly(carboxylic acid).49 Tannic acid is a kind of polyphenol. The hydrogen-bonded PVPON/tannic acid film has a critical assembly pH value of 7.5 and a critical disintegration pH value of 9.0, which are higher than that of the PVPON/PAA film. 49 Dopamine contains two phenol groups, and its pKa is 9.39. When dopamine is introduced into PAA chain, the film should endure the high pH value compared with the PVPON/PAA film. 10
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The PVPON/PAA-dopa thin film prepared at pH 3.0 is immersed into the solution of pH value above 5.0, where no thin film deposited. As shown in Figure 4, only at pH value higher than 7.0, we observed the disintegration of the film. The PVPON/PAA-dopa thin film is prepared at pH below 5.0 and the film disintegration happens at pH value above 7.0. The critical assembly and the critical disintegration pH value of PVPON/PAA-dopa film are higher than those of PVPON/PAA film.
1.0
Fraction Retained
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0.8 0.6 0.4 0.2 0.0 5
6
7
8
9
10
pH
Figure 4 The (PVPON/PAA-dopa)40 film prepared at 3.0 immersed into the aqueous solutions with different pH values for 48 h.
Due to dopamine is introduced to PAA chain, catechol chemistry can be applied to
cross-link
PVPON/PAA-dopa
film.
In
the
presence
of
oxidant,
the
oxidization-induced dismutation process happens between two catechol groups, and hence the film is cross-linked (Scheme 1). The (PVPON/PAA-dopa)40 film assembled at pH 3.0 was immersed in NaIO4 aqueous solution (1 mM), and the reaction process was monitored by UV-visible spectroscopy (SI Figure S5). With the time elapsing, the film gradually turns dark orange (Figure 5a). As only hydrogen bonding interaction but no covalent interaction between the substrate and the bottommost layer of the cross-linked PVPON/PAA-dopa, the film will liberate from the substrate when these hydrogen bonds break. In the alkaline solution (pH 12.0), the film exfoliation process is observed, as shown in Figure 5b. Through LbL assembly, the thin film is deposited on the all faces of the substrate. The substrate is rectangular plate, about 1 mm thick. To make the film easily remove from 11
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the substrate, the edge was scratched with a sharp blade and then immersed into the alkaline solution. The films with rectangular shape were present, as shown in Figure 5c-d. In aqueous solutions, the exfoliated film is very flexible. When nipped out with a tweezer, it likes a silk scarf and cannot fix shape. But using acetone remove water, we can get a shape-fixed film, without breaking or tearing, as shown in Figure 5c.
(a)
(b)
(c)
(d)
(f) Film Area (cm2)
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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(e)
pH 12.0
8
6
4
2
pH 2.0 0
2
4
6
8
10
Cycle Number
Figure 5 The photographs of (PVPON/PAA-dopa)40 film (a) the cross-linked film attached on the substrate, (b) the film immersed in e pH 12.0 solution, (c) the dry free-standing film, (d) the swollen-state film in pH 12.0 solution, (e) the shrunk-state film in pH 2.0 solution and (f) the film area when the film alternatively immersed into pH 2.0 and pH 12.0 solution.
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In addition, it is found that in the alkaline solution, the size of the exfoliated film is distinctly larger than the surface-attached film (Figure 5d). The grafting degree of dopamine to PAA affects the swelling ratio. The (PVPON/PAA-dopa)40 film made from PAA-dopa with the 5% grafting degree shows a 2.5-fold swelling, while for the 13% grafting degree one, the swelling ratio is 2.2, compared with the film attached on the substrate. Transferring from the solution of pH 12.0 into the pH 2.0, the film quickly shrinks to a smaller size (Figure 5e). When the film alternatively immersed into the acidic solution (pH 2.0) and the basic solution (pH 12.0), a reversible swelling-shrinking behavior is present.
1720
Absorbance
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60
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1640
a
1574 1660
b
1720
1640
c 1800
1750
1700
1650
1600
1550
1500
Wavenumber (cm-1)
Figure 6 FT-IR spectra of the (PVPON/PAA-dopa)40 films: (a) as-prepared at pH 3.0, (b) the cross-linked film immersed in the pH 12.0 solution; and (c) transferring from the pH 12.0 solution to the pH 2.0 solution.
FT-IR spectroscopy was utilized to study the swelling-shrinking behavior of the free-standing film (Figure 6). In the swollen state, the film exhibits absorbance peaks at 1660 cm-1 and 1574 cm-1. The absorbance peak at 1574 cm-1 is attributed to asymmetric vibration of carboxylate, while 1660 cm-1 is the carbonyl stretching vibration of PVPON. 50, 59, 60 In the shrunk state, the film shows absorbance peaks at 1640 cm-1 and 1720 cm-1, which is almost same to the as-prepared hydrogen-bonded thin film at pH 3.0. 53, 54 As previous discussion, the peak at 1720 cm-1 and 1640 cm-1 is assigned to the hydrogen-bonded carboxylic acid group of PAA and the carbonyl of 13
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PVPON, respectively. The FT-IR spectra indicate that in the basic environment, PVPON chains do not dissolve out and still retain in the film, and carboxylic acid groups lose proton and hydrogen bonds break, inducing the film expands. Transferring the free-standing thin film from the alkaline solution to the acidic solution, hydrogen bonds between the PVPON and PAA reconstruct and hence the film shrinks. The wettability of the free-standing film in the swollen state and in the shrunk state was investigated. As shown in Figure 7, when dripped on the surface of the swollen film, the water droplet is quickly absorbed. However, on the surface of the shrunk film, the droplet is not absorbed and the contact angle keeps for relatively long time. In the swollen state, hydrogen bonds break and hydrophilic groups, carboxylate and amide, are present, so water drop can be quickly absorbed. While in the shrunk state, carboxylic acid groups of PAA-dopa are hydrogen-bonded with PVPON. The film does not have much hydrophilic groups available, and thus water adsorption ability becomes weak.
5s
10s
5s
10s
Figure 7 Photographs of water drop profiles versus time on (up) the swollen-state and (down) the shrunk-state (PVPON/PAA-dopa)40 film built at pH 3.0.
SEM and AFM images in Figure 8 show that the surface of the swollen film is lumpy, while the shrunk film is dense and flat. When the free-standing thin film is transferred from the alkaline solution to the acidic solution, the hydrogen bonds rebuild. However, the reconstructed hydrogen-bonded film shows different morphology compared with the as-prepared film at pH 3.0. The hydrogen-bonded PVPON/PAA-dopa thin film prepared at pH 3.0 is rough due to the aggregation of PAA-dopa. After cross-linking, the PVPON/PAA-dopa film remains rough (SI Figure 14
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S6), whereas the reconstructed hydrogen-bonding thin film shows relatively smooth topology. When the cross-linked film exposes to the alkaline solution, the hydrogen bonds in the thin film completely break. All the aggregated structure of PAA-dopa disappears, and PAA-dopa chains expand. When the swollen film is put into the acidic solution, the hydrogen bonds between PVPON and PAA reconstruct, and the aggregation of PAA-dopa would not happen anymore. So the film shows relatively smooth morphology. (a)
(b)
(c)
(d)
Figure 8 SEM images (a and b) and 3D AFM images (c and d, L × W × H, 5 µm × 5 µm × 1 µm) : the swollen state (a and c); the shrunk state (b and d).
CONCLUSION Dopamine modified PAA and PVPON can be alternative deposited to prepare thin film based on hydrogen bonding. Carboxylic acid and phenol group of PAA-dopa both act as hydrogen bonding donors, which made the critical assembly pH and the critical disintegration pH value of PVPON/PAA-dopa film enhance, compared with PVPON/PAA film. Under oxidizing condition, PVPON/PAA-dopa thin film can be cross-linked by dismutation reaction. The cross-linked film is robust enough that can be exfoliated from the substrate to produce an intact free-standing film by the alkaline solution. Hydrogen bond formation and breakage can be tuned by adjusting the pH value of the solution, which makes the cross-linked free-standing PVPON/PAA-dopa film endowed with a reversible swelling-shrinking behavior. 15
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ACKNOWLEDGMENTS S. Y. gratefully acknowledges the support from National Natural Science Foundation of China (NSFC, Grant No. 51373032), Innovation Program of Shanghai Municipal Education Commission, Fundamental Research Funds for the Central University and DHU Distinguished Young Professor Program.
Supporting Information Available H-NMR and FT-IR of PAA-dopa, UV-Visible spectra of (PVPON/PAA-dopa)20, the film growth mode, the film cross-linking process monitored by UV-Visible, and SEM image of the cross-linked film are present in Supporting Information file. This information is available free of charge via the Internet at http://pubs.acs.org/.
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For Table of Contents Only
Reversible Swelling-Shrinking Behavior of Hydrogen-Bonded Free-Standing Thin Film Stabilized by Catechol Reaction
Jiaxing Sun1, Chao Su1, Xuejian Zhang1, Wenjing Yin1, Jian Xu2, Shuguang Yang1*
1
State Key Laboratory for Modification of Chemical Fibers and Polymer Materials,
College of Material Science and Engineering, Donghua University, Shanghai 201620 2
Laboratory of Polymer Physics and Chemistry, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190 *To whom the correspondence should be addressed. Email: [email protected]
Hydrogen bond formation-breakage is utilized to tune the film shrinking-swelling.
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