surface properties of poly(vinyl alcohol) films dominated by spontaneous adsorption of ethanol and...

Surface Properties of Poly(vinyl alcohol) Films Dominatedby Spontaneous Adsorption of Ethanol and Governed byHydrogen BondingBiao 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 afterimmersion in ethanol were investigated using sum frequencygeneration (SFG) vibrational spectroscopy and contact angle(CA) goniometry. The result showed that the surface chemicalstructure of the PVA films was strongly dependent on thedegree of hydrolysis. The vinyl acetate (VAc) units in the PVAchains resulting from incomplete hydrolysis segregate to thefilm surface and strongly affect the adsorption behavior ofethanol molecules on their surfaces. The surface hydrophilicitydecreased greatly for PVA films with relatively high hydrolysisdegrees (i.e., 99% and 97.7%), in which the water contact angleincreased by 20°, and increased for PVA with relatively lowhydrolysis degrees (95.1% and 84%) after immersion in ethanol. It was found that ethanol molecules adsorb from solution onto aPVA film surface in an ordered and cooperative way governed by hydrogen bonding when the hydrolysis degrees of PVA werehigher than 98%. When the hydrolysis degree of PVA was lower than 96%, the surface structure obtained by surfacereconstruction dominated after immersion in ethanol, with fewer ethanol molecules adsorbed on the surface, resulting in adecrease of its water contact angle.

1. INTRODUCTIONThe spontaneous adsorption from solution onto a polymersurface has been extensively studied for decades because of itssignificant impact on many applications involving polymerinterfaces, such as biocompatible devices, nonfouling materials,separation and purification sciences, and nanotechnology.1−8

Upon adsorption, polymer surface structure may changesignificantly, leading to unexpected properties, which may beof great importance. A distinct example is the effect of a layer ofinterfacial water on the fouling properties of polymer surfaces;with subsequent further understanding of this effect, manydifferent antifouling or foul-releasing polymer materials weredeveloped.7,8 It was found that in the application of poly(methylmethacrylate) (PMMA) microfluidic chips, adsorption of variousanalytes such as organic dyes, DNA, and proteins reduced theperformance of PMMA chips.9,10 The adsorption of surfactants onpolymer surfaces can alter the wetting and spreading behavior offilms and is the physical origin of the well-known autophilic andautophobic effect.11−14 These examples suggest that adsorptioncan play a key role in determining the consequent polymer surfaceproperties. It is generally assumed that spontaneous adsorption ona solid surface is governed by noncovalent interactions. However,our understanding of the chemistry of such interfacial and surfacephenomena at the molecular level, such as the governing forcesand the molecular arrangements on polymer surfaces, is still poorly

developed,4,15−17 mainly due to the lack of effective surface-sensitive probes for the polymer surface. It is therefore necessaryto probe the change of the surface structures by adsorption indetail and correlate such changes to the resulting surfaceproperties. For this purpose, certain surface-sensitive techniquesare required for a comprehensive characterization of the surfacemolecular structure variations induced by adsorption.Over the last 20 years, sum frequency generation (SFG)

vibrational spectroscopy has been developed into a very powerfulnonlinear optical technique for probing polymer surfaces andinterfacial molecular structures.18−21 Many heuristic studies havebeen undertaken aimed at probing adsorption-induced polymersurface structural changes at the molecular level, including theordered alignment of liquid crystal molecules on rubbed polymersubstrate surfaces,22 formation of surface hydrogen bonds ofpoly(2-methoxyethyl acrylate) with water and bisphenol A,5,23 andadsorption-induced orientational order changes of the phenylgroups at phenolic resin surfaces.24

Because of their excellent biocompatibility, biodegradability,and water solubility properties, polyvinyl alcohol (PVA)-basedmaterials have received considerable attention for application in

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

Article

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

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many areas, such as environmentally safe products,25 proteinpurification,26 enzyme immobilization,27 membrane separa-tion,28,29 and biomechanical applications.30 In most of theapplications above, spontaneous adsorption of various moleculesis common on PVA surfaces, resulting in poor performance of thePVA materials. At the same time, short-chain aliphatic alcohols areof great interest, from both fundamental and applied points ofview, and have been widely used as industrial solvents andchemical regents in numerous processes.31,32 In applications suchas cleaning, etching, and electrochemical reactions, the interfacialbehavior of alcohol solutions at different solid surfaces often playsa key role.33,34 Shultz et al.33 found that methanol molecules canstably adsorb on TiO2 surfaces by formation of a monolayercoverage. Shen35 reported that alcohol molecules adsorbpreferentially at the interface in the form of dimers when C1−C41-alcohols in aqueous solution contacted with a fused silica sub-strate. However, very few studies have considered the adsorptionof alcohols on soft polymeric surfaces. Unlike inorganic rigidsurfaces, polymer surfaces may experience a thermodynamicreconstruction by contacting with an adsorbate media, whichperplex the adsorption behavior of alcohols on a soft surface. Inthis Article, polyvinyl alcohol (PVA) films with different degrees ofhydrolysis were chosen to investigate the interaction between thePVA surface and ethanol molecules. Using SFG spectroscopy andcontact angle (CA) goniometry, we demonstrate that even minorbulk structural differences for a polymer material can induce totallydifferent surface structures and corresponding adsorptionbehaviors. The detailed molecular-level surface structures ofPVAs after adsorption were revealed by SFG and correlated tothe measured macroscopic surface properties by contact anglemeasurement.

2. EXPERIMENTAL SECTION

2.1. Materials. Poly(vinyl alcohol) (PVA) is a polymergenerally prepared by partial hydrolysis of polyvinyl acetate(PVAc) to substitute acetate groups with hydroxyl groups, asshown in Figure 1. In this study, PVAs with hydrolysis degrees

of 84.0% (PVA-84), 95.1% (PVA-95), and 99.0% (PVA-99)and polyvinyl acetate (PVAc) were purchased from Sigma-Aldrich 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 1HNMR spectroscopy (Figure S1, Supporting Information), andtheir characterizations are fully described in Table 1.Deuterated ethanol (CD3CD2OD) was purchased fromCambridge 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 polytetra-fluoroethylene (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) werewashed with acetone and then soaked in a mixture of sulfuric acid

and hydrogen peroxide for 30 min to remove possible surfacecontamination. The substrates were then rinsed with deionizedwater and dried in a nitrogen flux. The PVA and PVAc films wereprepared by casting the solutions onto the glass substrates at 25 °Cfor 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 for30 s on the glass plates, drying at 25 °C for 24 h, and then in avacuum at 50 °C for another 24 h.

2.3. Film Characterization. The SFG spectra werecollected using a custom-designed Ekspla SFG spectrometer(EKSPLA, Lithuania) by overlapping a visible and a tunable IRbeam on the sample film surface with incident angles of 60° and55°, respectively. The 532 nm wavelength visible beam wasgenerated by frequency-doubling the fundamental outputpulses of ∼30 ps pulse width with wavelength of 1064 nmfrom an EKSPLA Nd:YAG laser. The tunable IR beam wasgenerated from an optical parametric generation/amplificationand difference frequency generation system based on BBO andAgGaS2 crystals. Both beams were focused on the samplesurface with diameters of ∼0.5 mm. Photodiodes were used tomonitor the visible beam and IR beam powers by detectingparts of reflections from focus lenses. The sum frequency signalwas collected by a monochromatic spectrograph. The SFGspectra as a function of the input IR frequency (or wavenumbercm−1) were thus normalized by the powers of the input laserbeams. In this study, the SFG spectra were taken in the ssp (s-polarized sum frequency output, s-polarized visible input, and p-polarized 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 employedto characterize the surface of PVA films. The X-ray gun wasoperated at a power of 250 W, and the high voltage was kept at140 kV with a detection angle of 45°. Each sample was directlypressed to a self-supported disk (10 × 10 mm) and mounted on asample holder and then transferred into the analyzer chamber. Allsurvey and high-resolution spectra were referenced to the C1shydrocarbon peak at 284.6 eV. The data analysis was carried outusing the PHI-MATLAB software provided by PHI Corp.Study on environmentally dependent change in surface

property requires appropriate tools. Contact angle measure-ment is one of the most effective and sensitive methods tocharacterize polymer surface structure, by which subtle changesin polymer surface properties can be detected.37,38 Variations insurface properties of the films during exposure to ethanol weremeasured following a reported method.28,29,39 Because ethanolwas a nonsolvent for PVA,40 the swelling or dissolution of PVA

Figure 1. Schematic representation of the chemical structure andformation of PVA.

Table 1. Chemical Structures of PVAs and Their SurfaceProperties Used in This Study

surface free energy(mN/m)d

polymerfilm

degree ofhydrolysis(mol %)a

Mw(kg/mol)b

contactangle(deg)c γs

D γsP γs

PVA-99 99.0 85−124 61 38.6 12.3 50.9PVA-98 97.7 103 68 39.6 8.1 47.8PVA-95 95.1 85−124 74 39.4 5.6 44.9PVA-84 84.0 85−124 79 39.9 3.5 43.4PVAc 0.0 100 80 39.4 3.5 42.8aDetermined by 1H NMR. bProvided by suppliers. cContact angle ofwater. dCalculated according Owens and Wendt’s theory.36

The Journal of Physical Chemistry C Article

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by ethanol during immersion treatment is prohibited. The PVAfilms coated on glass slides were immersed in ethanol at varioustemperatures, and the water contact angle of the film wasmeasured at certain time intervals. Each time a sample wasremoved from the feed, it was immediately cooled in room-temperature feed and then dried quickly with a stream ofnitrogen prior to contact angle analysis. The contact angle ofwater on the sample was measured with a drop shape analysissystem (KRUSS BmbH Co., Germany) based on the Sessiledroplet method, in a temperature and humidity controlledroom (25 °C, 60% relative humidity). Each contact anglereported in this work is the average of the values obtained fromat least 10 different points on the sample surface. The error ofmeasurement was less than ±2°. The surface free energy wascalculated according Owens and Wendt’s theory36 from themeasured contact angles of water and diiodomethane on thesamples.Surface morphologies of the sample films were obtained by

atomic force microscopy (AFM XE-100, PASI Co., Korea) inthe tapping mode. A commercial V-shaped silicon nitrideintegrated cantilever/tip (Parks Scientific Instruments) withforce constant of 0.45−5.0 N/m was used for the AFMmeasurements.

3. RESULTS AND DISCUSSION

3.1. Surface Structures of Poly(vinyl alcohol) Filmswith Various Hydrolysis Degrees. Figure 2 shows the SFGspectra of poly(vinyl alcohol) (PVA) films with differenthydrolysis 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 acomparison. The ssp spectrum of PVAc is dominated by a strongpeak at 2942 cm−1 and a weak shoulder peak at 2910 cm−1, whichare assigned to the Fermi modes of the methyl groups fromacetoxy groups and the backbone methylene groups, respec-tively.41−43 The ppp spectrum of PVAc is dominated by a strongpeak at 2965 cm−1 and a weak peak at 2875 cm−1, which areassigned to the antisymmetric stretching (as) mode of the methylgroups and the stretching mode of the methenyl groups,respectively.22,42,43 The appearance of the dominating peaks ofthe methyl Fermi (ssp spectrum) and as modes (ppp spectrum) onPVAc surface indicates that side hydrophobic methyl groups of theVAc units preferentially protrude toward the air, with fewbackbone methylene groups pointing outward.

There are four discernible peaks in the ssp spectrum of PVA-99,located at 2942, 2910, 2875, and 2850 cm−1 (Figure 2). Thepeak at 2942 cm−1 in the ssp spectrum may originate from theas mode of the backbone methylene22 or the Fermi mode of themethyl groups in the VAc units.41−43 However, the apparentpeak at 2965 cm−1 in the ppp panel from the as mode of methylgroups in the VAc units suggests that the 2942 cm−1 peak in thessp spectrum is the corresponding ss or Fermi mode of methylgroups. Additionally, if the peak at 2942 cm−1 in the sspspectrum originates from the methylene as mode, this modeshould appear as a much stronger peak in the ppp spectrum,according to the polarization selection rule discussed by Wanget al.44 Because of the fact that the ss mode of methyl is unlikelyto 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 theFermi mode of the methyl groups in the VAc units. Thestrongest peak at 2910 cm−1 and the middle-intensity peak at2875 cm−1 are assigned to the ss mode of the VA backbonemethylene groups and methenyl stretching mode, respectively.22

The peak at 2850 cm−1 may be attributed to a combination modeinvolving the methylene groups bridging the VA units and VAcunits, because this peak does not exist for PVAc. In the pppspectrum, the peak at 2965 cm−1 is assigned to the as mode ofmethyl groups in the VAc units.43 It is thus evident that the surfaceof PVA-99 is dominated by the VA backbone methylene groupspointing 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, inwhich the 2910 cm−1 peak from the VA backbone is thedominating peak in the spectra. However, the spectral featuresof PVA-95 and PVA-84 are very close to those of the PVAcfilm, in which the surface spectra are dominated by the peaksfrom the methyl group in VAc. This similarity indicates that theVAc units are more likely to segregate to the surface than theVA units, due to their lower critical surface tension. The peaksat both 2942 cm−1 in the ssp spectra and 2965 cm−1 in the pppspectra from the methyl groups of VAc units gradually decreasedwith increasing degree of hydrolysis of PVA. This was alsoconfirmed by contact angle and surface free energy measurements.Table 1 shows water contact angles and surface free energies ofvarious PVA films. The water contact angle increased from ∼61°to ∼80° and the surface free energies decreased from 50.9 to42.8 mN/m with hydrolysis degree decreasing from 99% to 84%.At the same time, the polar part of the surface free energy showeda significant decrease from 12.3 to 3.5 mN/m with decreasingdegree of hydrolysis. When the hydrolysis degree is 84% and

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



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 thesesurface 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 C1sregion for the PVA films with various hydrolysis degrees. In eachcase, three spectral components, at 284.6, 286.2, and 289.0 eV,were observed, which correspond to C−H, C−O, and O−COgroups, respectively.28,45 It should be pointed out that the peakassigned to the O−CO group arises from the VAc part due tothe incomplete hydrolysis of PVA. The compositions of the PVAfilms with various hydrolysis degrees and their correspondingbinding energies are summarized in Table 2. It is obvious that the

composition of the O−CO groups on the PVA-99 and PVA-98surfaces is only 2.4% and 3.7%, which is much lower than thaton the PVA-95 and PVA-84 surfaces (9.0% and 15.5%). Thecomposition of the O−CO groups on the PVA film surfacesincreases with decreasing degree of hydrolysis. This isconsistent with the results of the SFG spectra and contactangle measurements.According to the above experimental results, it is clear that

the VAc units in partially hydrolyzed PVA chains are morelikely to segregate to the PVA/air interface. For PVA-84 andPVA-95 films, the surface is covered with the pendant acetoxylmethyl groups (VAc units) as shown in Scheme 1. For PVA-99and PVA-98 films, the surface is mostly covered with methylenegroups (VA units) with few acetoxyl methyl groups. In the next

section, we will see that these surface structural characteristicshave a conclusive effect on the adsorption behavior of the filmssurface.

3.2. Change of the Surface Wettability of PVA Filmsupon Immersion in Ethanol. The contact angle measure-ments were conducted on PVA surfaces after the PVA filmswere immersed in ethanol for various times and temperatures.Figure 4a depicts the effect of immersion time in ethanol onwater contact angle of PVA-99 and PVA-84 films. It is evidentthat there are opposite trends of water contact angle change forthe PVA-99 and PVA-84 films, as a function of immersion time.For PVA with high hydrolysis degree (PVA-99), the contactangle increased from 61° to about 81° and became stable at82.3° after exposure in alcohol for 10 min, while that of thePVA-84 film decreased from 81° to around 72°. To comparethe changes in surface properties of the various samples, thevariation in contact angle (Δθ) was employed, which can beestimated from the difference between the starting contact angle(θ0) and contact angle attained at complete equilibrium (θc). Apositive value of Δθ represents an increase of contact angle, and anegative value of Δθ means a decrease of contact angle afterimmersion in ethanol. The effect of hydrolysis degree of PVA onΔθ of corresponding PVA films is presented in Figure 4b. Theseresults show that the value of Δθ increases with increasing degree ofhydrolysis. However, there is one notable result, in that there existsa critical degree of hydrolysis of 96%, below which Δθ is a negativevalue, indicting decreasing contact angle of the PVA film afterimmersion treatment. When the hydrolysis degree exceeds 96%,Δθchanges to a positive value, which means an increase of watercontact angle upon immersion in ethanol. AFM images in Figure S2(Supporting Information) show that the RMS roughness of PVA-99 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 100nm necessary to affect the wettability.46 Accordingly, the effect ofsurface roughness on contact angle appears to be negligible in ourexperiments.In a controlled experiment, a surface wettability study of

PVA-99, PVA-98, and PVA-95 films after immersion in ethanolat various temperatures was carried out. The corresponding

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

Table 2. Surface Chemical Compositions for the Five FilmsObtained from XPS Spectra

surface chemical compositions measured byXPS (%)

chemicalgroups

bindingenergy (eV) PVA-99 PVA-98 PVA-96 PVA-84 PVAc

OC−O 289.0 2.4 3.7 9.0 15.5 22.9C−O 286.2 24.8 35.4 34.5 34.4 23.0C−H 284.6 72.8 60.9 56.5 50.2 54.1



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

Polymer surface reconstruction, as a result of the thermodynamicdrive to attain the lowest free energy state, is heavily dependent onthe polarity of the contacting medium.38,39,47−49 As reportedpreviously,28,29 the decrease in water contact angle on the surface ofa PVA membrane after immersion in aqueous ethanol solution isattributed to the reorientation of hydroxyl groups at the surface ofthe membrane. When the surface contacts the ethanol/water feedmixture, polar −OH groups reorient at the surface, creating a morehydrophilic conformation. Tretinnikov50 demonstrated that in the

Scheme 1. Schematic of Surface Structures of PVA Films with High (a) and Relative Low (b) Hydrolysis Degrees Induced byContacting 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 thecontact angle variance of PVA-99 spin-coated film with immersion time in ethanol. (b) Effect of hydrolysis degree of PVA on value of Δθ afterimmersion 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 dependenceof Δθ (b) for PVA-99 (■), PVA-98 (●), and PVA-95 (◆) films.



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

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

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 thecharacteristic ethanol peaks.



surface to become more hydrophilic after immersion in ethanol.Nevertheless, in this experiment, more hydrophobic surfaces wereproduced when 99.0% and 97.7% hydrolysis degree films of PVAwere immersed in ethanol at lower temperature, contrary to thatexpected based on the tendency of systems to minimize interfacialfree energy.3.3. The Mechanism of Wettability Variation in PVA

Film Surfaces after Immersion in Ethanol. Furthercharacterizations of the surface structure of PVA surfacesafter immersing in ethanol are discussed in the followingsection, mainly based on SFG results. The structural changes ofPVA surfaces with hydrolysis degrees of 84.0%, 95.1%, 97.7%,and 99.0% before and after immersion in ethanol for 30 minwere monitored by the SFG technique in the 2800−3000 cm−1

region, with ssp and ppp polarization combination, as shown inFigure 6. The SFG spectra at the air/ethanol interface are alsoshown in Figure 6a with the Fermi mode of methyl groups at2940 cm−1 in the ssp spectrum and the as mode of methylgroups at 2965 cm−1 in the ppp spectrum.51 As compared to thePVA surfaces before immersion, the surface spectra of PVA-84and PVA-95 did not show significant ethanol peaks, but thesurface spectra of PVA-99 and PVA-98 did exhibit ethanolpeaks. For PVA-84 and PVA-95, the methylene ss mode near2910 cm−1 for ssp and ppp polarization combination increasedsignificantly. This observation suggests that the surface restructur-ing involving the backbone occurred during immersion for PVA-84 and PVA-95. For PVA-99 and PVA-98 samples, the intensitiesof the characteristic ethanol peaks at 2940 cm−1 in the ssp spectraand at 2965 cm−1 in the ppp spectra show an observable increase,especially for the ppp spectra. This observation by SFG suggeststhat the ethanol molecules were merely adsorbed onto the surfacesof PVA with higher hydrolysis degrees (PVA-99 and PVA-98).This kind of surface adsorption cannot be detected by theattenuated total reflectance infrared spectroscopy (ATR-FTIR),which shows no detectable change of the IR spectra of PVA-99films after immersion in ethanol for 30 min (Figure S3, SupportingInformation).To further confirm this result of enhanced ethanol adsorption

with PVA surfaces of higher degrees of hydrolysis, deuteratedethanol (d-ethanol) was used for adsorption studies, and theresultant SFG ssp spectra are shown in Figure 7a. There are fourresonant peaks at the air/d-ethanol interface, located at 2070,2095, 2150, and 2227 cm−1, which are sequentially assigned to thess mode of CD3, ss mode of CD2, Fermi resonance of CD3, and asmode of CD3, respectively.

52,53 It is obvious in Figure 7a that theintensities of the characteristic peaks of d-ethanol show a

decreasing trend with decreasing degree of hydrolysis of thePVA films. No obvious characteristic peaks of d-ethanol werefound when the hydrolysis degree of PVA was 84%. Thus, it issuggested that the d-ethanol molecules have adsorbed on thealmost completely hydrolyzed PVA surfaces (PVA-99, PVA-98) inan ordered fashion, as evidenced by the appearance of the CD3 ssand Fermi peaks in the SFG spectra of PVA films after immersionin d-ethanol. However, for the films with relatively low hydrolysisdegree of PVA (PVA-95, PVA-84), few d-ethanol molecules canadsorb on the film surface. Meanwhile, the temperature-dependentexperiments showed that the d-ethanol molecules cannot adsorbon the PVA-99 film surface after immersing the film in d-ethanolat temperatures above 60 °C, as shown in Figure 7b. This resultsuggests that adsorption of ethanol molecules on a PVA filmsurface depends greatly on both its hydrolysis degree and ethanoltemperature. When a PVA film with high hydrolysis degree issoaked in ethanol at low temperature, the adsorption of ethanolmolecules 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, orCO, on a solid surface.4,5,23,54 The −OH groups in ethanol havea strong ability to form H-bonds with the polar groups on a surface,resulting in adsorption of ethanol.5,55 Here, the SFG technique wasemployed to detect the interfacial H-bond between ethanol andPVA surfaces, by detecting the frequency shift of the interfacialO−H group. Figure 8 shows the SFG spectra in the O−Hstretching region of PVA film with 99% hydrolysis degree beforeand after immersion in ethanol. Before immersion in ethanol, a verybroad peak covering the range from 3000 to 3700 cm−1 wasobserved (Figure 8), including a main peak at 3605 cm−1, and twoshoulders at 3240 and 3445 cm−1. On the basis of a VSF report onshort-chain alcohols and nonionic surfactant with −OH group,56,57

the bands centered at around 3240 and 3445 cm−1 were assignedprimarily to the −OH stretching of the stronger hydrogen-bondedand weaker hydrogen-bonded hydroxyl groups on PVA surface,respectively. At the same time, the peak at 3605 cm−1 was attributedto non-hydrogen-bonded O−H stretching mode in the PVA chains.After immersion in ethanol, the SFG peak at 3240 cm−1 isenhanced greatly, while the resonant peak at 3445 and 3605 cm−1

displayed an apparent decrease in intensity. This spectral changesuggests that the more-ordered H-bonding structure was enhancedgreatly after the surface of the PVA film contacted with ethanolmolecules. This increase of more-ordered H-bonds on the PVA-99surface further supports the fact that ethanol molecules haveadsorbed on the film surface through H-bonding between the−OH group on a PVA surface and the −OH of ethanol.

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.



Accordingly, the adsorption of ethanol on PVA surfaces throughH-bonding is a reasonable explanation for the hydrophobicityincrease of the PVA-99 and PVA-98 surfaces, after immersion inethanol.Unlike traditional solid materials such as metals, glasses, and

ceramics, whose surfaces are often considered to be rigid andunchangeable, polymer surfaces display dynamic behavior withtime and environmental conditions. When a polymer is incontact with different media, the side chains, segments, pendantgroups, or end groups of polymer chains can reorient orreconstruct themselves at the surfaces, in accordance with thenature of their surrounding environment.47−49 The driving forcefor surface reconstruction is the tendency of the minimization ofinterfacial free energy between the polymer surface and itsenvironment; thus, the characteristics of the surrounding environ-ment, such as polarity, composition, and special interactions withpolymer surfaces, play an important role in such reconstructionprocesses. When the PVA films with high hydrolysis degree wereimmersed in ethanol, the polar OH groups would migrate to theliquid/solid interface to minimize the interfacial energy. The well-ordered and high density interfacial −OH group is favorable forforming the more “ice-like” H-bond with ethanol molecules. Theadsorption of ethanol on PVA surfaces gives rise to thehydrophobic methyl group of ethanol in ordered arrangementon the PVA surface (Scheme 1), as evidenced by the SFG spectraof PVA-99 and PVA-98 films after immersion in d-ethanol, asshown in Figure 7a. As a result, in our case, it can be seen thatthe tightly arranged methyl group of adsorbed ethanol on thesurface modifies the PVA surface in a more hydrophobic fashion(Scheme 1). The ultimate surface properties of PVA-99 and PVA-98were manipulated by the adsorption of ethanol on the surfacegoverned by H-bonding.At the same time, a higher temperature will increase the

distance between the H-bond acceptor and donor because ofthe temperature sensitivity of H-bonds, resulting in thereduction in the intensity of the H-bond and a decrement ofthe number of molecules that form H-bonds.58 Therefore, nod-ethanol molecules were detected by SFG on the PVA-99 filmsurface when the film was immersed in d-ethanol at temperaturesabove 60 °C (Figure 7b). As a consequence, hydrophobicity wasnot enhanced for the PVA-99 and PVA-98 films, after immersionin ethanol at higher temperatures (Figure 5). The temperature-dependent adsorption and the consequent wettability changes forPVA-99 and PVA-98 films subjected to immersion treatment

further confirm the currently proposed mechanism responsible forthe 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 inethanol for PVA surfaces with relatively lower hydrolysisdegrees. This can be explained by considering the surfacestructure of the polymer films after reconstruction. It wasshown in section 1 that the PVA-95 and PVA-84 surfaces weredominated by the VAc part of the structure, rather than the VApart, as was the case for the PVA-98 and PVA-99 surfaces(shown in Scheme 1).When the PVA films with low hydrolysisdegrees were immersed in ethanol, the polar CO and C−Ogroups would migrate to the liquid/solid interface to minimizethe interface energy, as shown in Scheme 1. The pendantacetoxyl groups of PVAc are shielded by the pendant methylgroup, resulting in less ordering of the CO groups andcreating steric hindrance which inhibits ordered adsorption.Because this interfacial structure is not favorable for formingwell-ordered and high density interfacial H-bonding withethanol molecules by cooperative effects, ethanol moleculescannot effectively form a well-ordered adsorption structure onthis kind of reconstructed surface. Thus, the surface structuresof the PVA-95 and PVA-84 films are formed primarily bysurface reconstruction, which dominates after immersion inethanol, with few ethanol molecules adsorbed on the surfaces,resulting in a decrease of its water contact angle (shown inScheme 1). This was confirmed by the SFG spectra in Figure 6,where the peak at 2910 cm−1 assigned to CH2 in the PVAbackbone was greatly enhanced after contacting with ethanol.Similar surface restructuring behaviors have been reported forpolyacrylates in contact with water by Wang et al.59,60 andTateishi et al.61 It has been pointed out that the ester methylgroups oriented reversely when in contact with water and thehydrophilic carbonyl groups may be prone to exposure towardthe surface. We believe that the surface restructuring involvingthe 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-coatedfilm. The spinning torque occurring during spin coating caninduce a large perturbation of the orientation of the side phenylgroups of poly(styrene) (PS) at the interface.62 Thus, spin-coatingmethod for PVA film formation was selected to prepare PVA-99films with less ordered structure on the surface. Because of veryrapid evaporation of solvent and the high spinning rate of thesubstrate, the chain conformations of the PVA spin-coated filmwere frozen in a nonequilibrated and less aligned conformation; inaddition, the −OH groups have a disordered orientation near thesurface. However, the cast film was obtained by naturalevaporation of water, thus exhibiting better alignment of thePVA chains and more ordered −OH groups oriented on thesurface with high density. Figure 4 (inset) displays the change ofcontact angle of the PVA-99 spin-coated film after immersion inethanol at 20 °C. It is apparent that the contact angle of the spin-coated PVA-99 film only increases by 10°, which is much lowerthan that of the corresponding cast film. This fact supports ourspeculation that ethanol molecules adsorb from solution onto aPVA film surface in an ordered and cooperative way governed byH-bonding when the hydrolysis degree of PVA is higher than 96%.This study promotes our understanding of the mechanism ofadsorption of organic components on a soft matter surface drivenby interfacial H-bonding, and the consequent modification of itssurface wettability by the adsorbed ethanol.

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



4. CONCLUSIONSSpontaneous adsorption from solution onto a solid surface hasbeen extensively studied for decades because of its significantimpact in many areas. However, our understanding of thechemistry of such interfacial phenomena at the molecular level,such as the governing forces and the molecular arrangement ona soft polymeric surface, is still poorly developed. In this Article,surface-sensitive sum frequency generation (SFG) vibrationalspectroscopy and contact angle (CA) goniometry wereemployed to investigate the surface structures of poly(vinylalcohol)s (PVAs) with different degrees of hydrolysis beforeand after immersion in ethanol. It was found that the VAc unitsfrom incomplete hydrolysis were prone to segregate to the surface.The surfaces of PVAs with low hydrolysis degrees (PVA-84 andPVA-95) were almost completely covered by methyl groups inVAc units with low surface free-energy. The surfaces of PVAs withhigh hydrolysis degrees (PVA-99 and PVA-98) were mainlydominated by methylene groups in the PVA backbone. After thesePVA film surfaces were contacted with ethanol, their resultingsurface structures and properties greatly depended on thecorresponding hydrolysis degrees of PVA. The water contactangle of PVA films with relatively low hydrolysis degrees decreasedafter immersion in ethanol; however, those of PVA films withrelatively high hydrolysis degrees instead were found to increase.The SFG spectra showed that variation in surface properties ofPVA films after contacting ethanol is related to the adsorption ofethanol molecules. When the hydrolysis degree of PVA is higherthan 96%, ethanol molecules adsorb from solution onto a PVAfilm surface in an ordered and cooperative way governed byH-bonding, resulting in a more hydrophobic surface. However,when the hydrolysis degree of PVA was lower than 96%, thesurface structure obtained by surface reconstruction dominatedafter immersion in ethanol, with few ethanol molecules adsorbedon the surface, resulting in a decrease of its water contact angle.The difference in the adsorption of ethanol on PVA surfaces withhigh and low hydrolysis degree is related to the ordering anddensity of H-bond acceptors and donors on the surface. This workprovides a new mechanism for the change of surface properties of asoft material by contacting with a hydrogen-bonding medium, andalso provides a deeper understanding of the mechanism for theadsorption from solution onto a soft surface governed by interfacialH-bonding interactions.

■ ASSOCIATED CONTENT

*S Supporting Information1H NMR spectroscopy of PVAs with various hydrolysisdegrees; and surface morphologies of PVAs films by AFMand ATR-FITR spectra of PVA-99 films before and afterimmersion in ethanol. This material is available free of chargevia the Internet at http://pubs.acs.org.

■ AUTHOR INFORMATION

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

NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTS

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

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

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