Surface Properties of Poly(vinyl alcohol) Films Dominated by

Article pubs.acs.org/JPCC

Surface Properties of Poly(vinyl alcohol) Films Dominated by Spontaneous Adsorption of Ethanol and Governed by Hydrogen Bonding Biao Zuo, Yanyan Hu, Xiaolin Lu, Shanxiu Zhang, Hao Fan, and Xinping Wang* Department of Chemistry, Key Laboratory of Advanced Textile Materials and Manufacturing Technology of Education Ministry, Zhejiang Sci-Tech University, Hangzhou 310018, China S Supporting Information *

ABSTRACT: The surface structures of poly(vinyl alcohol) (PVA) films with four different degrees of hydrolysis after immersion in ethanol were investigated using sum frequency generation (SFG) vibrational spectroscopy and contact angle (CA) goniometry. The result showed that the surface chemical structure of the PVA films was strongly dependent on the degree of hydrolysis. The vinyl acetate (VAc) units in the PVA chains resulting from incomplete hydrolysis segregate to the film surface and strongly affect the adsorption behavior of ethanol molecules on their surfaces. The surface hydrophilicity decreased greatly for PVA films with relatively high hydrolysis degrees (i.e., 99% and 97.7%), in which the water contact angle increased by 20°, and increased for PVA with relatively low hydrolysis degrees (95.1% and 84%) after immersion in ethanol. It was found that ethanol molecules adsorb from solution onto a PVA film surface in an ordered and cooperative way governed by hydrogen bonding when the hydrolysis degrees of PVA were higher than 98%. When the hydrolysis degree of PVA was lower than 96%, the surface structure obtained by surface reconstruction dominated after immersion in ethanol, with fewer ethanol molecules adsorbed on the surface, resulting in a decrease of its water contact angle. developed,4,15−17 mainly due to the lack of effective surfacesensitive probes for the polymer surface. It is therefore necessary to probe the change of the surface structures by adsorption in detail and correlate such changes to the resulting surface properties. For this purpose, certain surface-sensitive techniques are required for a comprehensive characterization of the surface molecular structure variations induced by adsorption. Over the last 20 years, sum frequency generation (SFG) vibrational spectroscopy has been developed into a very powerful nonlinear optical technique for probing polymer surfaces and interfacial molecular structures.18−21 Many heuristic studies have been undertaken aimed at probing adsorption-induced polymer surface structural changes at the molecular level, including the ordered alignment of liquid crystal molecules on rubbed polymer substrate surfaces,22 formation of surface hydrogen bonds of poly(2-methoxyethyl acrylate) with water and bisphenol A,5,23 and adsorption-induced orientational order changes of the phenyl groups at phenolic resin surfaces.24 Because of their excellent biocompatibility, biodegradability, and water solubility properties, polyvinyl alcohol (PVA)-based materials have received considerable attention for application in

1. INTRODUCTION The spontaneous adsorption from solution onto a polymer surface has been extensively studied for decades because of its significant impact on many applications involving polymer interfaces, such as biocompatible devices, nonfouling materials, separation and purification sciences, and nanotechnology.1−8 Upon adsorption, polymer surface structure may change significantly, leading to unexpected properties, which may be of great importance. A distinct example is the effect of a layer of interfacial water on the fouling properties of polymer surfaces; with subsequent further understanding of this effect, many different antifouling or foul-releasing polymer materials were developed.7,8 It was found that in the application of poly(methyl methacrylate) (PMMA) microfluidic chips, adsorption of various analytes such as organic dyes, DNA, and proteins reduced the performance of PMMA chips.9,10 The adsorption of surfactants on polymer surfaces can alter the wetting and spreading behavior of films and is the physical origin of the well-known autophilic and autophobic effect.11−14 These examples suggest that adsorption can play a key role in determining the consequent polymer surface properties. It is generally assumed that spontaneous adsorption on a solid surface is governed by noncovalent interactions. However, our understanding of the chemistry of such interfacial and surface phenomena at the molecular level, such as the governing forces and the molecular arrangements on polymer surfaces, is still poorly © 2013 American Chemical Society

Received: November 16, 2012 Revised: January 24, 2013 Published: January 28, 2013 3396

dx.doi.org/10.1021/jp3113304 | J. Phys. Chem. C 2013, 117, 3396−3406

The Journal of Physical Chemistry C

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many areas, such as environmentally safe products,25 protein purification,26 enzyme immobilization,27 membrane separation,28,29 and biomechanical applications.30 In most of the applications above, spontaneous adsorption of various molecules is common on PVA surfaces, resulting in poor performance of the PVA materials. At the same time, short-chain aliphatic alcohols are of great interest, from both fundamental and applied points of view, and have been widely used as industrial solvents and chemical regents in numerous processes.31,32 In applications such as cleaning, etching, and electrochemical reactions, the interfacial behavior of alcohol solutions at different solid surfaces often plays a key role.33,34 Shultz et al.33 found that methanol molecules can stably adsorb on TiO2 surfaces by formation of a monolayer coverage. Shen35 reported that alcohol molecules adsorb preferentially at the interface in the form of dimers when C1−C4 1-alcohols in aqueous solution contacted with a fused silica substrate. However, very few studies have considered the adsorption of alcohols on soft polymeric surfaces. Unlike inorganic rigid surfaces, polymer surfaces may experience a thermodynamic reconstruction by contacting with an adsorbate media, which perplex the adsorption behavior of alcohols on a soft surface. In this Article, polyvinyl alcohol (PVA) films with different degrees of hydrolysis were chosen to investigate the interaction between the PVA surface and ethanol molecules. Using SFG spectroscopy and contact angle (CA) goniometry, we demonstrate that even minor bulk structural differences for a polymer material can induce totally different surface structures and corresponding adsorption behaviors. The detailed molecular-level surface structures of PVAs after adsorption were revealed by SFG and correlated to the measured macroscopic surface properties by contact angle measurement.

Table 1. Chemical Structures of PVAs and Their Surface Properties Used in This Study surface free energy (mN/m)d polymer film

degree of hydrolysis (mol %)a

Mw (kg/mol)b

contact angle (deg)c

γsD

γsP

γs

PVA-99 PVA-98 PVA-95 PVA-84 PVAc

99.0 97.7 95.1 84.0 0.0

85−124 103 85−124 85−124 100

61 68 74 79 80

38.6 39.6 39.4 39.9 39.4

12.3 8.1 5.6 3.5 3.5

50.9 47.8 44.9 43.4 42.8

a

Determined by 1H NMR. bProvided by suppliers. cContact angle of water. dCalculated according Owens and Wendt’s theory.36

and hydrogen peroxide for 30 min to remove possible surface contamination. The substrates were then rinsed with deionized water and dried in a nitrogen flux. The PVA and PVAc films were prepared by casting the solutions onto the glass substrates at 25 °C for 24 h and then put in a vacuum oven for another 24 h at 50 °C. The thickness of the PVA and PVAc cast films is about 5 μm. Spin-coated PVA films with thickness of approximately 180 nm were prepared by spin-coating the solutions at 2500 rpm for 30 s on the glass plates, drying at 25 °C for 24 h, and then in a vacuum at 50 °C for another 24 h. 2.3. Film Characterization. The SFG spectra were collected using a custom-designed Ekspla SFG spectrometer (EKSPLA, Lithuania) by overlapping a visible and a tunable IR beam on the sample film surface with incident angles of 60° and 55°, respectively. The 532 nm wavelength visible beam was generated by frequency-doubling the fundamental output pulses of ∼30 ps pulse width with wavelength of 1064 nm from an EKSPLA Nd:YAG laser. The tunable IR beam was generated from an optical parametric generation/amplification and difference frequency generation system based on BBO and AgGaS2 crystals. Both beams were focused on the sample surface with diameters of ∼0.5 mm. Photodiodes were used to monitor the visible beam and IR beam powers by detecting parts of reflections from focus lenses. The sum frequency signal was collected by a monochromatic spectrograph. The SFG spectra as a function of the input IR frequency (or wavenumber cm−1) were thus normalized by the powers of the input laser beams. In this study, the SFG spectra were taken in the ssp (spolarized sum frequency output, s-polarized visible input, and ppolarized IR input) and ppp polarization combinations. X-ray photoelectron spectroscopy (XPS, PHI5000C ESCA System) with a Mg Kα X-ray source (1253.6 eV) was employed to characterize the surface of PVA films. The X-ray gun was operated at a power of 250 W, and the high voltage was kept at 140 kV with a detection angle of 45°. Each sample was directly pressed to a self-supported disk (10 × 10 mm) and mounted on a sample holder and then transferred into the analyzer chamber. All survey and high-resolution spectra were referenced to the C1s hydrocarbon peak at 284.6 eV. The data analysis was carried out using the PHI-MATLAB software provided by PHI Corp. Study on environmentally dependent change in surface property requires appropriate tools. Contact angle measurement is one of the most effective and sensitive methods to characterize polymer surface structure, by which subtle changes in polymer surface properties can be detected.37,38 Variations in surface properties of the films during exposure to ethanol were measured following a reported method.28,29,39 Because ethanol was a nonsolvent for PVA,40 the swelling or dissolution of PVA

2. EXPERIMENTAL SECTION 2.1. Materials. Poly(vinyl alcohol) (PVA) is a polymer generally prepared by partial hydrolysis of polyvinyl acetate (PVAc) to substitute acetate groups with hydroxyl groups, as shown in Figure 1. In this study, PVAs with hydrolysis degrees

Figure 1. Schematic representation of the chemical structure and formation of PVA.

of 84.0% (PVA-84), 95.1% (PVA-95), and 99.0% (PVA-99) and polyvinyl acetate (PVAc) were purchased from SigmaAldrich Inc. PVA with a hydrolysis degree of 97.7% (PVA-98) was purchased from Sinopharm Chemical Reagent Co., Ltd. The hydrolysis degrees of PVAs were further measured by 1H NMR spectroscopy (Figure S1, Supporting Information), and their characterizations are fully described in Table 1. Deuterated ethanol (CD3CD2OD) was purchased from Cambridge Isotope Laboratories, Inc. 2.2. Film Formation. The 2 wt % PVA solutions were prepared in a water bath at 90 °C and filtered using a polytetrafluoroethylene (PTFE) filter with a pore diameter of 0.25 μm. PVAc was dissolved in cyclohexanone to prepare the 2 wt % solutions. The glass substrates (Fisher Scientific Co., USA) were washed with acetone and then soaked in a mixture of sulfuric acid 3397



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Figure 2. The surface SFG spectra (left, ssp; right, ppp) and fitting results (solid lines) of PVA with different hydrolysis degrees. The spectra have been offset for clarity.

There are four discernible peaks in the ssp spectrum of PVA-99, located at 2942, 2910, 2875, and 2850 cm−1 (Figure 2). The peak at 2942 cm−1 in the ssp spectrum may originate from the as mode of the backbone methylene22 or the Fermi mode of the methyl groups in the VAc units.41−43 However, the apparent peak at 2965 cm−1 in the ppp panel from the as mode of methyl groups in the VAc units suggests that the 2942 cm−1 peak in the ssp spectrum is the corresponding ss or Fermi mode of methyl groups. Additionally, if the peak at 2942 cm−1 in the ssp spectrum originates from the methylene as mode, this mode should appear as a much stronger peak in the ppp spectrum, according to the polarization selection rule discussed by Wang et al.44 Because of the fact that the ss mode of methyl is unlikely to be observed in this high frequency region, the peak at 2942 cm−1 in the ssp spectrum of PVA-99 is thus mainly attributed to the Fermi mode of the methyl groups in the VAc units. The strongest peak at 2910 cm−1 and the middle-intensity peak at 2875 cm−1 are assigned to the ss mode of the VA backbone methylene groups and methenyl stretching mode, respectively.22 The peak at 2850 cm−1 may be attributed to a combination mode involving the methylene groups bridging the VA units and VAc units, because this peak does not exist for PVAc. In the ppp spectrum, the peak at 2965 cm−1 is assigned to the as mode of methyl groups in the VAc units.43 It is thus evident that the surface of PVA-99 is dominated by the VA backbone methylene groups pointing outward, with few pendant acetoxyl methyl groups. It can be seen from Figure 2 that the surface SFG spectra of PVA-98 show features similar to those of the PVA-99 film, in which the 2910 cm−1 peak from the VA backbone is the dominating peak in the spectra. However, the spectral features of PVA-95 and PVA-84 are very close to those of the PVAc film, in which the surface spectra are dominated by the peaks from the methyl group in VAc. This similarity indicates that the VAc units are more likely to segregate to the surface than the VA units, due to their lower critical surface tension. The peaks at both 2942 cm−1 in the ssp spectra and 2965 cm−1 in the ppp spectra from the methyl groups of VAc units gradually decreased with increasing degree of hydrolysis of PVA. This was also confirmed by contact angle and surface free energy measurements. Table 1 shows water contact angles and surface free energies of various PVA films. The water contact angle increased from ∼61° to ∼80° and the surface free energies decreased from 50.9 to 42.8 mN/m with hydrolysis degree decreasing from 99% to 84%. At the same time, the polar part of the surface free energy showed a significant decrease from 12.3 to 3.5 mN/m with decreasing degree of hydrolysis. When the hydrolysis degree is 84% and

by ethanol during immersion treatment is prohibited. The PVA films coated on glass slides were immersed in ethanol at various temperatures, and the water contact angle of the film was measured at certain time intervals. Each time a sample was removed from the feed, it was immediately cooled in roomtemperature feed and then dried quickly with a stream of nitrogen prior to contact angle analysis. The contact angle of water on the sample was measured with a drop shape analysis system (KRÜ SS BmbH Co., Germany) based on the Sessile droplet method, in a temperature and humidity controlled room (25 °C, 60% relative humidity). Each contact angle reported in this work is the average of the values obtained from at least 10 different points on the sample surface. The error of measurement was less than ±2°. The surface free energy was calculated according Owens and Wendt’s theory36 from the measured contact angles of water and diiodomethane on the samples. Surface morphologies of the sample films were obtained by atomic force microscopy (AFM XE-100, PASI Co., Korea) in the tapping mode. A commercial V-shaped silicon nitride integrated cantilever/tip (Parks Scientific Instruments) with force constant of 0.45−5.0 N/m was used for the AFM measurements.

3. RESULTS AND DISCUSSION 3.1. Surface Structures of Poly(vinyl alcohol) Films with Various Hydrolysis Degrees. Figure 2 shows the SFG spectra of poly(vinyl alcohol) (PVA) films with different hydrolysis degrees in the infrared frequency range of 2800− 3000 cm−1 corresponding to the C−H stretching vibrations. The spectra of poly(vinyl acetate) (PVAc) were also collected for a comparison. The ssp spectrum of PVAc is dominated by a strong peak at 2942 cm−1 and a weak shoulder peak at 2910 cm−1, which are assigned to the Fermi modes of the methyl groups from acetoxy groups and the backbone methylene groups, respectively.41−43 The ppp spectrum of PVAc is dominated by a strong peak at 2965 cm−1 and a weak peak at 2875 cm−1, which are assigned to the antisymmetric stretching (as) mode of the methyl groups and the stretching mode of the methenyl groups, respectively.22,42,43 The appearance of the dominating peaks of the methyl Fermi (ssp spectrum) and as modes (ppp spectrum) on PVAc surface indicates that side hydrophobic methyl groups of the VAc units preferentially protrude toward the air, with few backbone methylene groups pointing outward. 3398



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Figure 3. High-resolution XPS spectra in the C1s region for the surfaces of PVA and PVAc films.

section, we will see that these surface structural characteristics have a conclusive effect on the adsorption behavior of the films surface. 3.2. Change of the Surface Wettability of PVA Films upon Immersion in Ethanol. The contact angle measurements were conducted on PVA surfaces after the PVA films were immersed in ethanol for various times and temperatures. Figure 4a depicts the effect of immersion time in ethanol on water contact angle of PVA-99 and PVA-84 films. It is evident that there are opposite trends of water contact angle change for the PVA-99 and PVA-84 films, as a function of immersion time. For PVA with high hydrolysis degree (PVA-99), the contact angle increased from 61° to about 81° and became stable at 82.3° after exposure in alcohol for 10 min, while that of the PVA-84 film decreased from 81° to around 72°. To compare the changes in surface properties of the various samples, the variation in contact angle (Δθ) was employed, which can be estimated from the difference between the starting contact angle (θ0) and contact angle attained at complete equilibrium (θc). A positive value of Δθ represents an increase of contact angle, and a negative value of Δθ means a decrease of contact angle after immersion in ethanol. The effect of hydrolysis degree of PVA on Δθ of corresponding PVA films is presented in Figure 4b. These results show that the value of Δθ increases with increasing degree of hydrolysis. However, there is one notable result, in that there exists a critical degree of hydrolysis of 96%, below which Δθ is a negative value, indicting decreasing contact angle of the PVA film after immersion treatment. When the hydrolysis degree exceeds 96%, Δθ changes to a positive value, which means an increase of water contact angle upon immersion in ethanol. AFM images in Figure S2 (Supporting Information) show that the RMS roughness of PVA99 film changes from 1.1 to 0.78 nm after immersion in ethanol, which is well below the lower limits of the surface roughness of 100 nm necessary to affect the wettability.46 Accordingly, the effect of surface roughness on contact angle appears to be negligible in our experiments. In a controlled experiment, a surface wettability study of PVA-99, PVA-98, and PVA-95 films after immersion in ethanol at various temperatures was carried out. The corresponding

95.1%, the total surface free energy of PVA (43.4 and 44.9 mN/m) approaches that of PVAc (42.8 mN/m), indicating that these surface structures are similar. Further characterization of the surface of various PVA films was conducted by X-ray photoelectron spectroscopy (XPS). Figure 3 presents the high-resolution XPS spectra of the C1s region for the PVA films with various hydrolysis degrees. In each case, three spectral components, at 284.6, 286.2, and 289.0 eV, were observed, which correspond to C−H, C−O, and O−CO groups, respectively.28,45 It should be pointed out that the peak assigned to the O−CO group arises from the VAc part due to the incomplete hydrolysis of PVA. The compositions of the PVA films with various hydrolysis degrees and their corresponding binding energies are summarized in Table 2. It is obvious that the Table 2. Surface Chemical Compositions for the Five Films Obtained from XPS Spectra surface chemical compositions measured by XPS (%) chemical groups

binding energy (eV)

OC−O C−O C−H

289.0 286.2 284.6

PVA-99 PVA-98 PVA-96 PVA-84 PVAc 2.4 24.8 72.8

3.7 35.4 60.9

9.0 34.5 56.5

15.5 34.4 50.2

22.9 23.0 54.1

composition of the O−CO groups on the PVA-99 and PVA-98 surfaces is only 2.4% and 3.7%, which is much lower than that on the PVA-95 and PVA-84 surfaces (9.0% and 15.5%). The composition of the O−CO groups on the PVA film surfaces increases with decreasing degree of hydrolysis. This is consistent with the results of the SFG spectra and contact angle measurements. According to the above experimental results, it is clear that the VAc units in partially hydrolyzed PVA chains are more likely to segregate to the PVA/air interface. For PVA-84 and PVA-95 films, the surface is covered with the pendant acetoxyl methyl groups (VAc units) as shown in Scheme 1. For PVA-99 and PVA-98 films, the surface is mostly covered with methylene groups (VA units) with few acetoxyl methyl groups. In the next 3399



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Scheme 1. Schematic of Surface Structures of PVA Films with High (a) and Relative Low (b) Hydrolysis Degrees Induced by Contacting with Ethanol

Figure 4. (a) Evolution of water contact angles of PVA-99 (●) and PVA-84 (◇) films as a function of immersion time in ethanol. Inset shows the contact angle variance of PVA-99 spin-coated film with immersion time in ethanol. (b) Effect of hydrolysis degree of PVA on value of Δθ after immersion in ethanol for 30 min. Ethanol temperature: 20 °C.

Figure 5. Change of water contact angle of PVA-98 film after immersion in ethanol at various temperatures (a) and ethanol temperature dependence of Δθ (b) for PVA-99 (■), PVA-98 (●), and PVA-95 (◆) films.

results for the PVA-98 film are shown in Figure 5a. It is observed that when the ethanol temperature is raised to 50 °C, the water contact angle of PVA-98 film experiences a reduction from 69° to about 62° after immersion in ethanol, which is opposite to the trend observed for PVA-98 immersed in ethanol at 20 and 40 °C. Figure 5b summarizes the change of Δθ with ethanol temperature for PVA-99, PVA-98, and PVA-95 films. The results show that Δθ decreases with increasing ethanol temperature for these films.

Polymer surface reconstruction, as a result of the thermodynamic drive to attain the lowest free energy state, is heavily dependent on the polarity of the contacting medium.38,39,47−49 As reported previously,28,29 the decrease in water contact angle on the surface of a PVA membrane after immersion in aqueous ethanol solution is attributed to the reorientation of hydroxyl groups at the surface of the membrane. When the surface contacts the ethanol/water feed mixture, polar −OH groups reorient at the surface, creating a more hydrophilic conformation. Tretinnikov50 demonstrated that in the 3400



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Figure 6. The ssp and ppp spectra of PVA-99 (a), PVA-98 (b), PVA-95 (c), and PVA-84 (d) before and after immersion in ethanol for 30 min. Dashed lines mark the characteristic peak positions of ethanol, and the arrowheads mark the peak change of the PVA samples relevant to the characteristic ethanol peaks.

polystyrene (PS) film casting process, the surface segregation of benzene groups was related greatly to the polarity of the castsubstrate. Therefore, when the polymer surface contacts with polar

media (i.e., water, alcohols), some groups having large dipole moments will migrate to the interface, driven by the minimization of interface energy. Therefore, it is reasonable for the PVA film 3401



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Figure 7. (a) SFG spectra for air/PVA film interfaces after immersion in deuterated ethanol for 30 min (20 °C) and air/deuterated ethanol interface. (b) SFG spectra for air/PVA-99 interface after immersion in deuterated ethanol at various temperatures.

decreasing trend with decreasing degree of hydrolysis of the PVA films. No obvious characteristic peaks of d-ethanol were found when the hydrolysis degree of PVA was 84%. Thus, it is suggested that the d-ethanol molecules have adsorbed on the almost completely hydrolyzed PVA surfaces (PVA-99, PVA-98) in an ordered fashion, as evidenced by the appearance of the CD3 ss and Fermi peaks in the SFG spectra of PVA films after immersion in d-ethanol. However, for the films with relatively low hydrolysis degree of PVA (PVA-95, PVA-84), few d-ethanol molecules can adsorb on the film surface. Meanwhile, the temperature-dependent experiments showed that the d-ethanol molecules cannot adsorb on the PVA-99 film surface after immersing the film in d-ethanol at temperatures above 60 °C, as shown in Figure 7b. This result suggests that adsorption of ethanol molecules on a PVA film surface depends greatly on both its hydrolysis degree and ethanol temperature. When a PVA film with high hydrolysis degree is soaked in ethanol at low temperature, the adsorption of ethanol molecules on the PVA film surface readily occurs. Hydrogen bonding is a universal force, which induces adsorption of organic compounds with polar groups, such as OH, NH2, or CO, on a solid surface.4,5,23,54 The −OH groups in ethanol have a strong ability to form H-bonds with the polar groups on a surface, resulting in adsorption of ethanol.5,55 Here, the SFG technique was employed to detect the interfacial H-bond between ethanol and PVA surfaces, by detecting the frequency shift of the interfacial O−H group. Figure 8 shows the SFG spectra in the O−H stretching region of PVA film with 99% hydrolysis degree before and after immersion in ethanol. Before immersion in ethanol, a very broad peak covering the range from 3000 to 3700 cm−1 was observed (Figure 8), including a main peak at 3605 cm−1, and two shoulders at 3240 and 3445 cm−1. On the basis of a VSF report on short-chain alcohols and nonionic surfactant with −OH group,56,57 the bands centered at around 3240 and 3445 cm−1 were assigned primarily to the −OH stretching of the stronger hydrogen-bonded and weaker hydrogen-bonded hydroxyl groups on PVA surface, respectively. At the same time, the peak at 3605 cm−1 was attributed to non-hydrogen-bonded O−H stretching mode in the PVA chains. After immersion in ethanol, the SFG peak at 3240 cm−1 is enhanced greatly, while the resonant peak at 3445 and 3605 cm−1 displayed an apparent decrease in intensity. This spectral change suggests that the more-ordered H-bonding structure was enhanced greatly after the surface of the PVA film contacted with ethanol molecules. This increase of more-ordered H-bonds on the PVA-99 surface further supports the fact that ethanol molecules have adsorbed on the film surface through H-bonding between the −OH group on a PVA surface and the −OH of ethanol.

surface to become more hydrophilic after immersion in ethanol. Nevertheless, in this experiment, more hydrophobic surfaces were produced when 99.0% and 97.7% hydrolysis degree films of PVA were immersed in ethanol at lower temperature, contrary to that expected based on the tendency of systems to minimize interfacial free energy. 3.3. The Mechanism of Wettability Variation in PVA Film Surfaces after Immersion in Ethanol. Further characterizations of the surface structure of PVA surfaces after immersing in ethanol are discussed in the following section, mainly based on SFG results. The structural changes of PVA surfaces with hydrolysis degrees of 84.0%, 95.1%, 97.7%, and 99.0% before and after immersion in ethanol for 30 min were monitored by the SFG technique in the 2800−3000 cm−1 region, with ssp and ppp polarization combination, as shown in Figure 6. The SFG spectra at the air/ethanol interface are also shown in Figure 6a with the Fermi mode of methyl groups at 2940 cm−1 in the ssp spectrum and the as mode of methyl groups at 2965 cm−1 in the ppp spectrum.51 As compared to the PVA surfaces before immersion, the surface spectra of PVA-84 and PVA-95 did not show significant ethanol peaks, but the surface spectra of PVA-99 and PVA-98 did exhibit ethanol peaks. For PVA-84 and PVA-95, the methylene ss mode near 2910 cm−1 for ssp and ppp polarization combination increased significantly. This observation suggests that the surface restructuring involving the backbone occurred during immersion for PVA84 and PVA-95. For PVA-99 and PVA-98 samples, the intensities of the characteristic ethanol peaks at 2940 cm−1 in the ssp spectra and at 2965 cm−1 in the ppp spectra show an observable increase, especially for the ppp spectra. This observation by SFG suggests that the ethanol molecules were merely adsorbed onto the surfaces of PVA with higher hydrolysis degrees (PVA-99 and PVA-98). This kind of surface adsorption cannot be detected by the attenuated total reflectance infrared spectroscopy (ATR-FTIR), which shows no detectable change of the IR spectra of PVA-99 films after immersion in ethanol for 30 min (Figure S3, Supporting Information). To further confirm this result of enhanced ethanol adsorption with PVA surfaces of higher degrees of hydrolysis, deuterated ethanol (d-ethanol) was used for adsorption studies, and the resultant SFG ssp spectra are shown in Figure 7a. There are four resonant peaks at the air/d-ethanol interface, located at 2070, 2095, 2150, and 2227 cm−1, which are sequentially assigned to the ss mode of CD3, ss mode of CD2, Fermi resonance of CD3, and as mode of CD3, respectively.52,53 It is obvious in Figure 7a that the intensities of the characteristic peaks of d-ethanol show a 3402



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further confirm the currently proposed mechanism responsible for the contact angle increase for PVA-98 and PVA-99 films. A second question worth being addressed is the origin of the observed surface hydrophobicity reduction upon immersion in ethanol for PVA surfaces with relatively lower hydrolysis degrees. This can be explained by considering the surface structure of the polymer films after reconstruction. It was shown in section 1 that the PVA-95 and PVA-84 surfaces were dominated by the VAc part of the structure, rather than the VA part, as was the case for the PVA-98 and PVA-99 surfaces (shown in Scheme 1).When the PVA films with low hydrolysis degrees were immersed in ethanol, the polar CO and C−O groups would migrate to the liquid/solid interface to minimize the interface energy, as shown in Scheme 1. The pendant acetoxyl groups of PVAc are shielded by the pendant methyl group, resulting in less ordering of the CO groups and creating steric hindrance which inhibits ordered adsorption. Because this interfacial structure is not favorable for forming well-ordered and high density interfacial H-bonding with ethanol molecules by cooperative effects, ethanol molecules cannot effectively form a well-ordered adsorption structure on this kind of reconstructed surface. Thus, the surface structures of the PVA-95 and PVA-84 films are formed primarily by surface reconstruction, which dominates after immersion in ethanol, with few ethanol molecules adsorbed on the surfaces, resulting in a decrease of its water contact angle (shown in Scheme 1). This was confirmed by the SFG spectra in Figure 6, where the peak at 2910 cm−1 assigned to CH2 in the PVA backbone was greatly enhanced after contacting with ethanol. Similar surface restructuring behaviors have been reported for polyacrylates in contact with water by Wang et al.59,60 and Tateishi et al.61 It has been pointed out that the ester methyl groups oriented reversely when in contact with water and the hydrophilic carbonyl groups may be prone to exposure toward the surface. We believe that the surface restructuring involving the backbone can favor this polar side-group reorientation, thereby inducing surface hydrophilicity. The proposed mechanism of surface hydrophobicity reduction was also confirmed by immersion treatment of a PVA spin-coated film. The spinning torque occurring during spin coating can induce a large perturbation of the orientation of the side phenyl groups of poly(styrene) (PS) at the interface.62 Thus, spin-coating method for PVA film formation was selected to prepare PVA-99 films with less ordered structure on the surface. Because of very rapid evaporation of solvent and the high spinning rate of the substrate, the chain conformations of the PVA spin-coated film were frozen in a nonequilibrated and less aligned conformation; in addition, the −OH groups have a disordered orientation near the surface. However, the cast film was obtained by natural evaporation of water, thus exhibiting better alignment of the PVA chains and more ordered −OH groups oriented on the surface with high density. Figure 4 (inset) displays the change of contact angle of the PVA-99 spin-coated film after immersion in ethanol at 20 °C. It is apparent that the contact angle of the spincoated PVA-99 film only increases by 10°, which is much lower than that of the corresponding cast film. This fact supports our speculation that ethanol molecules adsorb from solution onto a PVA film surface in an ordered and cooperative way governed by H-bonding when the hydrolysis degree of PVA is higher than 96%. This study promotes our understanding of the mechanism of adsorption of organic components on a soft matter surface driven by interfacial H-bonding, and the consequent modification of its surface wettability by the adsorbed ethanol.

Figure 8. SFG spectra (ssp) in the OH stretching region of PVA film with 99% hydrolysis degree before and after immersion in ethanol.

Accordingly, the adsorption of ethanol on PVA surfaces through H-bonding is a reasonable explanation for the hydrophobicity increase of the PVA-99 and PVA-98 surfaces, after immersion in ethanol. Unlike traditional solid materials such as metals, glasses, and ceramics, whose surfaces are often considered to be rigid and unchangeable, polymer surfaces display dynamic behavior with time and environmental conditions. When a polymer is in contact with different media, the side chains, segments, pendant groups, or end groups of polymer chains can reorient or reconstruct themselves at the surfaces, in accordance with the nature of their surrounding environment.47−49 The driving force for surface reconstruction is the tendency of the minimization of interfacial free energy between the polymer surface and its environment; thus, the characteristics of the surrounding environment, such as polarity, composition, and special interactions with polymer surfaces, play an important role in such reconstruction processes. When the PVA films with high hydrolysis degree were immersed in ethanol, the polar OH groups would migrate to the liquid/solid interface to minimize the interfacial energy. The wellordered and high density interfacial −OH group is favorable for forming the more “ice-like” H-bond with ethanol molecules. The adsorption of ethanol on PVA surfaces gives rise to the hydrophobic methyl group of ethanol in ordered arrangement on the PVA surface (Scheme 1), as evidenced by the SFG spectra of PVA-99 and PVA-98 films after immersion in d-ethanol, as shown in Figure 7a. As a result, in our case, it can be seen that the tightly arranged methyl group of adsorbed ethanol on the surface modifies the PVA surface in a more hydrophobic fashion (Scheme 1). The ultimate surface properties of PVA-99 and PVA-98 were manipulated by the adsorption of ethanol on the surface governed by H-bonding. At the same time, a higher temperature will increase the distance between the H-bond acceptor and donor because of the temperature sensitivity of H-bonds, resulting in the reduction in the intensity of the H-bond and a decrement of the number of molecules that form H-bonds.58 Therefore, no d-ethanol molecules were detected by SFG on the PVA-99 film surface when the film was immersed in d-ethanol at temperatures above 60 °C (Figure 7b). As a consequence, hydrophobicity was not enhanced for the PVA-99 and PVA-98 films, after immersion in ethanol at higher temperatures (Figure 5). The temperaturedependent adsorption and the consequent wettability changes for PVA-99 and PVA-98 films subjected to immersion treatment 3403



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4. CONCLUSIONS Spontaneous adsorption from solution onto a solid surface has been extensively studied for decades because of its significant impact in many areas. However, our understanding of the chemistry of such interfacial phenomena at the molecular level, such as the governing forces and the molecular arrangement on a soft polymeric surface, is still poorly developed. In this Article, surface-sensitive sum frequency generation (SFG) vibrational spectroscopy and contact angle (CA) goniometry were employed to investigate the surface structures of poly(vinyl alcohol)s (PVAs) with different degrees of hydrolysis before and after immersion in ethanol. It was found that the VAc units from incomplete hydrolysis were prone to segregate to the surface. The surfaces of PVAs with low hydrolysis degrees (PVA-84 and PVA-95) were almost completely covered by methyl groups in VAc units with low surface free-energy. The surfaces of PVAs with high hydrolysis degrees (PVA-99 and PVA-98) were mainly dominated by methylene groups in the PVA backbone. After these PVA film surfaces were contacted with ethanol, their resulting surface structures and properties greatly depended on the corresponding hydrolysis degrees of PVA. The water contact angle of PVA films with relatively low hydrolysis degrees decreased after immersion in ethanol; however, those of PVA films with relatively high hydrolysis degrees instead were found to increase. The SFG spectra showed that variation in surface properties of PVA films after contacting ethanol is related to the adsorption of ethanol molecules. When the hydrolysis degree of PVA is higher than 96%, ethanol molecules adsorb from solution onto a PVA film surface in an ordered and cooperative way governed by H-bonding, resulting in a more hydrophobic surface. However, when the hydrolysis degree of PVA was lower than 96%, the surface structure obtained by surface reconstruction dominated after immersion in ethanol, with few ethanol molecules adsorbed on the surface, resulting in a decrease of its water contact angle. The difference in the adsorption of ethanol on PVA surfaces with high and low hydrolysis degree is related to the ordering and density of H-bond acceptors and donors on the surface. This work provides a new mechanism for the change of surface properties of a soft material by contacting with a hydrogen-bonding medium, and also provides a deeper understanding of the mechanism for the adsorption from solution onto a soft surface governed by interfacial H-bonding interactions.

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and the Natural Science Foundation of Zhejiang Province (Grant no. Z4100463).

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ASSOCIATED CONTENT

S Supporting Information *

1 H NMR spectroscopy of PVAs with various hydrolysis degrees; and surface morphologies of PVAs films by AFM and ATR-FITR spectra of PVA-99 films before and after immersion in ethanol. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel./fax: +86-571-8684-3600. E-mail: [email protected], [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are thankful for support from the National Natural Science Foundation of China (NSFC, nos. 21174134 and 20904048) 3404



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Surface Properties of Poly(vinyl alcohol) Films Dominated by

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