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Adsorption of Poly(Vinyl Alcohol) from Water to a Hydrophobic Surface: Effects of Molecular Weight, Degree of Hydrolysis, Salt, and Temperature Mikhail Kozlov and Thomas J. McCarthy* Polymer Science and Engineering Department, University of Massachusetts, Amherst, Massachusetts 01003 Received March 24, 2004. In Final Form: July 24, 2004 The adsorption of poly(vinyl alcohol) (PVOH) from aqueous solutions to a silicon-supported fluoroalkyl monolayer is described. Thickness, wettability, and roughness of adsorbed films are studied as a function of polymer molecular weight, degree of hydrolysis (from the precursor, poly(vinyl acetate)), polymer concentration, salt type and concentration, and temperature. The data suggest a two-stage process for adsorption of the polymer: physisorption due to a hydrophobic effect (decrease in interfacial free energy) and subsequent stabilization of the adsorbed layer due to crystallization of the polymer. Adsorption of lower-molecular-weight polymers results in thicker films than those prepared with a higher molecular weight; this is ascribed to better crystallization of more mobile short chains. Higher contents of unhydrolyzed acetate groups on the poly(vinyl alcohol) chain lead to thicker adsorbed films. Residual acetate groups partition to the outermost surface of the films and determine wettability. Salts, including sodium chloride and sodium sulfate, promote adsorption, which results in thicker films; at the same time, their presence over a wide concentration range leads to formation of rough coatings. Sodium thiocyanate has little effect on PVOH adsorption, only slightly reducing the thickness in a 2 M salt solution. Increased temperature promotes adsorption in the presence of salt, but has little effect on salt-free solutions. Evidently, higher temperatures favor adsorption but cause crystallization to be less thermodynamically favorable. These competing effects result in the smoothest coatings being formed in an intermediate temperature range.
Introduction For many applications of hydrophobic polymers, such as polyethylene and poly(tetrafluoroethylene), their surfaces need to be modified in order to improve wettability, spreading, adhesion, and biocompatibility. We have recently reported1 a new approach for surface modification of hydrophobic materials. We showed that poly(vinyl alcohol) (PVOH) spontaneously adsorbs to various hydrophobic surfaces, producing continuous, stable coatings with thickness in the range of 10-50 Å. Crystallization of the polymer within the thin film was shown by electron diffraction and infrared spectroscopy and is implicated as a major driving force for the formation of stable films. This crystallization sets PVOH apart from other synthetic water-soluble polymers in terms of adsorption behavior. PVOH is an unusual polymer in that it is atactic (prepared by hydrolysis of radically polymerized poly(vinyl acetate)) yet crystalline. This adsorption behavior may prove useful in various applications of hydrophobic materials, where a hydrophilic surface is desired. The initial report1 demonstrated the universal character of PVOH adsorption, which occurs on all hydrophobic substrates that we have tried. The solubility of the adsorbed thin films was also studied: the films do not dissolve in room-temperature water but do dissolve in hot water (this is identical to the behavior of bulk PVOH). Cross-linking the film with glutaraldehyde renders the thin films stable to water at all temperatures. This report addresses various factors that affect PVOH adsorption on a single hydrophobic surface: a fluoroalkyl monolayer covalently attached to a silicon wafer (Figure 1). The * To whom correspondence should be addressed. E-mail:
[email protected]. (1) Kozlov, M.; Quarmyne, M.; Chen, W.; McCarthy, T. J. Macromolecules 2003, 36, 6054-6059.
Figure 1. Monolayer of tridecafluoro-1,1,2,2-tetrahydrooctyldimethylchlorosilane covalently attached to the surface of a silicon wafer.
factors reported here are molecular weight of the polymer, degree of hydrolysis (percentage of remaining acetate groups in the polymer chain), polymer concentration, ionic strength and type of salt present in the PVOH solution, and temperature. Our research was initiated on the basis of the report by Akashi et al., who demonstrated the assembly of multiple PVOH layers on a gold substrate2 by a repetitive adsorption/drying process. It was shown that the thickness of each adsorbed layer can be increased by increasing one of the following parameters: polymer concentration in solution, molecular weight, concentration of NaCl, and temperature. No information is provided regarding PVOH adsorption to a bare hydrophobic surface. A recent report discusses PVOH adsorption to polystyrene,3 including adsorption kinetics, effects of molecular (2) Serizawa, T.; Hashiguchi, S.; Akashi, M. Langmuir 1999, 15, 5363-5368. (3) Barrett, D. A.; Hartshorne, M. S.; Hussain, M. A.; Shaw, P. N.; Davies, M. C. Anal. Chem. 2001, 73, 5232-5239.
10.1021/la0492299 CCC: $27.50 © 2004 American Chemical Society Published on Web 09/15/2004
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weight, concentration, and degree of hydrolysis. The authors used an in situ method (surface plasmon resonance) to monitor adsorption. This differs significantly from the techniques we have used that exclusively study dry films. Also, most effects are reported3 for a polymer with a degree of hydrolysis of 87-89%, so the contributions by the residual acetate groups in this case need to be accounted for. Despite the important input that these reports make to our knowledge of PVOH adsorption, they do not address the characterization of the dry adsorbed films (in particular, their wettability), which will be crucial for many applications. Our approach to study PVOH adsorption employs hydrophobic monolayers supported on silicon wafers as substrates. Since our substrates are extremely smooth and reflective, ellipsometry can be conveniently used to measure adsorbed film thickness, and contact angle measurements and AFM can be taken full advantage of. We are not aware of another study of dry PVOH films adsorbed on a hydrophobic surface. Experimental Section Materials. The following grades of poly(vinyl alcohol) were purchased from Aldrich and used as received: 99%+ hydrolyzed with molecular weights 14 000, 89 000-98 000, 86 000-146 000, and 124 000-186 000, partially hydrolyzed 86 000-146 000 polymer, with degrees of hydrolysis of 87-89%, 96%, and 9899%. Silicon wafers were obtained from International Wafer Service (100 orientation; P/B doped; resistivity, 20-40 Ω cm; thickness, 450-575 µm). The thickness of the native oxide on these wafers was determined to be ∼20 Å by ellipsometry. Tridecafluoro-1,1,2,2-tetrahydrooctyldimethylchlorosilane was obtained from Gelest and purified by vacuum distillation. Housepurified water (reverse osmosis) was further purified using a Millipore Milli-Q system that involves reverse osmosis, ionexchange, and filtration steps (1018 Ω/cm). Other reagents and solvents were obtained from Aldrich or Fisher and used as received. Characterization. X-ray photoelectron spectra (XPS) were recorded with a Perkin-Elmer Physical Electronics 5100 with Mg KR excitation (15 kV, 400 W). Spectra were obtained at a takeoff angle of 15° (between the plane of the surface and the entrance lens of the detector optics). Contact angle measurements were made with a Rame`-Hart telescopic goniometer and a Gilmont syringe with a 24-gauge flat-tipped needle. The probe fluid was water, purified as described above. Dynamic advancing (θA) and receding angles (θR) were recorded while the probe fluid was added to and withdrawn from the drop, respectively. The values reported are averages of 3-5 measurements made on different areas of the sample surfaces. Ellipsometric measurements were made with a Rudolph Research model SL-II automatic ellipsometer with an angle of incidence of 70° from the normal. The light source was a He-Ne laser with λ ) 632.8 nm. Measurements were performed for 3-5 different locations on each sample. The thicknesses of the layers were calculated from the ellipsometric parameters (∆ and ψ) using DafIBM software. Calculations were performed for the transparent double-layer model (silicon substrate/silicon oxide + alkylsilane layer/PVOH/ air) with the following parameters: air, no ) 1; PVOH, n1 ) 1.5; silicon oxide + fluoroalkyl monolayer, n2 ) 1.462, the thickness of this layer was 29 Å and varied negligibly from batch to batch; silicon substrate, ns ) 3.858, ks ) 0.018 (imaginary part of refractive index). AFM images were obtained with a Digital Instruments Dimension 3100 scanning probe microscope with a NanoScope III controller operated in tapping mode. Preparation of Silicon-Supported Substrates. This procedure was an adaptation of one previously reported.4 Silicon wafers were cut into 1.5 cm × 1.5 cm samples and cleaned by being submerged (overnight) in a freshly prepared mixture of 7 parts of concentrated sulfuric acid containing ∼3 wt % of sodium (4) Fadeev, A. F.; McCarthy, T. J. Langmuir 1999, 15, 3759-3766.
Figure 2. Ellipsometric thickness (a) and advancing watercontact angle (b) of PVOH thin films adsorbed on the fluoroalkyl monolayers at room temperature. Molecular weights of PVOH: 14 000 (O), 89 000-98 000 (4), 124 000-186 000 (9). The degree of hydrolysis is 99+% for all grades. The lines are to guide the eye. dichromate and 3 parts of 30% hydrogen peroxide. Upon preparation, the solution warms to ∼90 °C and foams extensively due to the formation of oxygen and ozone. Wafers were then removed and rinsed with copious amounts of water and dried in a clean oven (in air) at 120 °C for 1 h. Silanization reactions were carried out immediately after treating the wafers in this fashion. Silanization with tridecafluoro-1,1,2,2-tetrahydrooctyldimethylchlorosilane was performed in the vapor phase at 70 °C for 3 days using ∼1 mL of the silane. There was no direct contact between the liquid silanes and the wafer. After silanization, the wafers were soaked in perfluoro-2-butyltetrahydrofuran for 30 min then rinsed with acetone, ethanol, and water (in this order). PVOH Solutions. Stock solutions of PVOH containing 10% w/v polymer were prepared by heating (85-100 °C) the polymerwater suspension for ∼1 h and allowing it to cool. Solutions of lower concentrations, as well as those containing salt, were prepared shortly thereafter by diluting the stock solutions. All solutions were allowed to equilibrate for at least 3 days before the adsorption studies. Adsorption Experiments. The procedure published in our previous report1 was followed. Modified silicon wafers were submerged in polymer solutions for 2 h, removed, and rinsed with copious amounts of water and dried under a stream of nitrogen for 1 h. For temperature dependence studies, polymer solutions were preheated (or pre-cooled) to the desired temperature before adsorption experiments, and the adsorption vessels were placed in a thermostated water bath. To eliminate any possibility that some polymer would transfer to samples at the solution-air interface during removal of the samples, the adsorption solutions were diluted repetitively after adsorption and the samples were removed from pure water.
Results and Discussion We have reported1 that PVOH readily adsorbs to hydrophobic surfaces and that the properties of the obtained coatings depend on time and PVOH concentration. A fluoroalkyl monolayer covalently attached to a
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Figure 3. Tapping mode AFM images of PVOH (fully hydrolyzed; molecular weight, 89 000-98 000) adsorbed on fluoroalkyl monolayers from salt-free solutions. Image size, 3 µm × 3 µm; data scale, 10 nm.
silicon wafer (Figure 1) was the most hydrophobic substrate we used in our earlier work. This covalently attached monolayer (CAM) has a lower coverage density than conventional self-assembled monolayers (SAMs), which leads to greater surface exposure of CF2 and CH2. Its choice for the detailed study presented here was dictated by the unsurpassed reproducibility we were able to achieve both in the preparation of this particular monolayer and in the subsequent PVOH adsorption to it. Kinetics of PVOH adsorption to this fluoroalkyl monolayer indicate that nearly 90% of the final thickness is formed within the first few hours, and after several hours in solution, some surface reconstruction takes place. The mechanism of this reconstruction is still not completely clear to us. To minimize these reconstruction processes and perform all studies in uniform conditions, the time of adsorption was fixed at 2 h. Adsorption under these conditions is highly reproducible. For example, over 50 ellipsometric measurements were made on 17 different samples (molecular weight 89 000-98 000, degree of hydrolysis 99+%, solution concentration 0.023 M, room temperature, no salt), and all thickness values were 26 ( 2 Å. We have briefly reported1 the effect of polymer concentration on the properties of adsorbed layers. Here, we
present a more detailed study including three different molecular weights of PVOH: 14 000, 89 000-98 000, and 124 000-186 000. Figure 2a shows the dependence of net ellipsometric thickness (does not include oxide layer or fluoroalkyl monolayer) of adsorbed PVOH coatings upon polymer concentration in solution (concentrations are based on repeat units). Two regions can be distinguished on these curves: (1) the PVOH thickness gradually increases with concentration and levels at intermediate concentration and (2) the adsorbed amount rises further at higher concentrations. We ascribe this second sharp growth to increased viscosity of PVOH solutions. Instead of single molecules adsorbing from dilute solutions, at high concentrations, entangled molecular agglomerates contribute to adsorption. This effect is more pronounced with the highest molecular weight of PVOH sample, which supports the increased viscosity hypothesis. The reported value5 of the entanglement concentration C* for aqueous PVOH solutions (molecular weight 155 000, 30 °C, 0.99 g/dL (0.22 M)) approximately corresponds to leveling of this dependence in the region of high concentration. As we have clearly demonstrated in our previous publication1, PVOH adsorbed from more dilute solutions produces (5) Hong, P. D.; Chou, C. M.; He, C. H. Polymer 2001, 42, 6105-6112.
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Figure 4. Ellipsometric thickness (a) and advancing watercontact angle (b) of PVOH thin films adsorbed on the fluoroalkyl monolayers at room temperature. Degrees of hydrolysis of PVOH: 99+% (O), 98-99% (4), 96% (9), 87-89% (×). The molecular weight of all grades are 86 000-146 000. The lines are to guide the eye.
coatings with higher degrees of crystallinity than those adsorbed from viscous, concentrated solutions. Apparently, the entangled macromolecules adsorbing from concentrated solutions are forced to assume conformations that reflect their solution structure and their crystallization is inhibited. The AFM images of adsorbed PVOH demonstrate increased adsorption of molecular aggregates on the size scale of 100 nm with increased concentration (Figure 3). Apparently, the surfaces composed of these aggregates have increased roughness, which also contributes to lower water-contact-angle values (see below). This importance of chain freedom at the moment of adsorption is illustrated in Figure 2a by the fact that PVOH of the lowest molecular weight, 14 000, produces thicker adsorbed layers in the region of dilute solutions. This is readily explained by the greater mobility of short polymer chains and their ability to adjust easily to their adsorbed neighbors forming coatings of higher crystallinity. This behavior may also indicate that in a dilute solution of a polydisperse PVOH sample, the fraction of lowest molecular weight is likely to form stable coatings preferentially. The present results are in contrast to those reported by Barrett et al.3, who observed that higher-molecularweight PVOH produces thicker layers. This group reported the results of an in situ study, a very different situation. The results reported here were obtained from carefully rinsed and dried films and actually may reveal the importance of better crystallization of low-molecularweight polymer. Nevertheless, our results agree well with the data on surface activity of different molecular weights of PVOH.6 (6) Glass, J. E. J. Phys. Chem. 1968, 72, 4450-4458.
Figure 5. Ellipsometric thickness (a), advancing water-contact angle (b), and AFM surface roughness (root-mean-square) (c) of fully hydrolyzed PVOH (molecular weight, 89 000-98 000; concentration, 0.023 M) adsorbed on fluoroalkyl monolayers at room temperature, in the presence of NaCl (O), Na2SO4 (9), and NaSCN (4). The lines are to guide the eye. The straight horizontal line indicates PVOH thickness normally obtained without salt.
Advancing water-contact angles as a function of adsorption solution concentration for three molecular weights are shown in Figure 2b. The major trend noticeable from this graph is that water-contact angles drop as the solution concentration increases. The small initial drops are most likely due to incompletecoverage of the surface at low solution concentration (a small fraction of hydrophobic substrate remains exposed). After an intermediate plateau, the contact angles drop sharply for the two highermolecular-weight PVOH samples. As we have shown1, adsorption from concentrated solutions produces coatings of lower crystallinity. There are two reasons for them to have a lower water-contact angle. First, since crystalline PVOH is characterized by the presence of doubly hydrogenbonded hydroxyl groups,7 less-crystalline surfaces have more free OH groups, which make them more hydrophilic.
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Figure 6. Tapping mode AFM images of PVOH (fully hydrolyzed; molecular weight, 89 000-98 000; concentration, 0.023 M) adsorbed on fluoroalkyl monolayers. Image size, 3 µm × 3 µm; data scale, 10 nm.
Second, the contact angle is lowered by absorbed water, which is always present in the amorphous regions of PVOH. The molecular weight of the polymer has a dramatic effect on the position and magnitude of the drop in the contact angle-concentration curve, which can also be explained by higher viscosity and multiple entanglement of larger polymer chains. The study of four grades of PVOH with the same molecular weight, 86 000-146 000, but different degrees of hydrolysis reveals that a higher content of residual polyvinyl acetate (PVAc) units in the polymer leads to thicker coatings, particularly in the region of low concentrations (Figure 4a). This can be ascribed to better adsorption of a more-hydrophobic polymer to a hydrophobic substrate. Water-contact-angle data of these same coatings are shown in Figure 4b. It is apparent that even a small presence of vinyl acetate repeat units in the polymer totally removes the effect discussed in the previous section: there is no sharp drop of contact angle in the region of high concentration. Vinyl acetate units do not participate in the formation of the crystalline phase of the polymer, and being more hydrophobic, they partition to the outermost surface of PVOH coatings. PVOH adsorption was studied as a function of salt concentration in solution. We chose sodium chloride, sodium sulfate (PVOH coagulators), and sodium thiocyanate (PVOH stabilizer)8 for these studies. A dilute solution (0.023 M) of PVOH (99%+ hydrolyzed, 89 00098 000) was used, and the salt concentration was varied from 0.001 to 2 M. PVOH precipitated only from 1 M and 2 M solutions of Na2SO4 (corresponding data points were omitted), whereas in other salt solutions, PVOH appeared to be soluble. The ellipsometric thickness of adsorbed layers vs salt concentration is shown in Figure 5a. Apparently, chloride and sulfate promote PVOH adsorption at the fluoroalkyl monolayer-solution interface at high concentration. Thiocyanate has little effect until high concentration, at which point it causes a slightly reduced thickness. This behavior agrees well with the Hoffmeister series of anions arranged in the order of their “salting out” effect.9 Sodium sulfate is one of the strongest kosmotropes (water structure makers, which exclude hydrophobic substances from aqueous media), sodium chloride is a relatively weak kosmotrope, and sodium thiocyanate is a potent chaotrope (water structure breaker, which increases solubility of hydrophobes in water). Kosmotropes enhance hydrophobic interactions10 thus promoting PVOH adsorption, which is seen in Figure 5a. (7) Bunn, C. W. Nature 1948, 161, 929-930. (8) Toyoshima, K. In Polyvinyl Alcohol; properties and applications; Finch, C. A., Ed.; John Wiley and Sons: London, 1973; p 39. (9) Collins, K. D.; Washabuagh, M. W. Quart. Rev. Biophys. 1985, 18, 323-422. (10) Mancera, R. L. J. Phys. Chem. B 1999, 103, 3774-3777.
Figure 7. Ellipsometric thickness (a) and advancing watercontact angle (b) of fully hydrolyzed PVOH (molecular weight, 89 000-98 000; concentration, 0.023 M) adsorbed on the fluoroalkyl monolayers from salt-free solution (b) and from 2 M solution of NaCl (O). The lines are to guide the eye.
Contact-angle data shown in Figure 5b and surface roughness values (RMS) assessed by AFM in Figure 5c show that, in the case of NaCl and Na2SO4, smoother PVOH films are obtained at either high salt concentration or at relatively low concentrations. Peaks corresponding to 1 M NaCl and 0.01-0.1 M Na2SO4, both in contactangle and roughness plots, represent intermediate regions of rough coatings. AFM images of PVOH coatings obtained without salt, at 1 M and at 2 M NaCl, are shown in Figure 6 to illustrate this trend. This interesting effect of salt concentration on roughness can be explained if we take into consideration that PVOH adsorption is a two-stage process. First, PVOH adsorbs at a hydrophobic-hydrophilic interface; second, the molecules in the adsorbed layer develop a network of intraand intermolecular hydrogen bonds, which stabilize the coating and make it insoluble in cold water. It should be emphasized that all measurements were performed on the coatings that were removed from the polymer solution, rinsed and dried. Thus, we are only able to detect stable coatings that successfully pass both steps.
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Figure 8. O:F ratio (XPS at 15° takeoff angle) (a) and AFM surface roughness (root-mean-square) (b) of fully hydrolyzed PVOH (molecular weight, 89 000-98 000; concentration, 0.023 M) adsorbed on the fluoroalkyl monolayers from salt-free solution.
Obviously, salts such as NaCl and Na2SO4 favor the step of adsorption. At the same time, these salts are likely to impede formation of a developed network of hydrogen bonds among PVOH molecules. There is both experimental11 and theoretical12 evidence that kosmotropes disrupt hydrogen bonding in water, significantly reducing the average number of water-water hydrogen bonds. Thus, the observed dependence may be a superposition of these two effects: salt at low concentration has a greater effect on hydrogen bonding, while the same salt at high concentration is responsible for a strong “salting out” effect, which drives the polymer adsorption. Our studies of the effect of temperature on film thickness, wettability, surface composition, and surface roughness of adsorbed PVOH coatings further illustrate the dual character of PVOH adsorption. Figure 6a shows temperature dependence of ellipsometric film thickness, both in the presence of 2 M NaCl and without salt. Higher temperatures increase the “salting-out” power of a concentrated salt solution, which results in thicker films. On the other hand, temperature has little effect on adsorption from a salt-free solution below 80 °C, at which temperature water starts breaking interpolymer hydrogen bonds. Figure 7b shows the temperature dependence of advancing water-contact angle. It is seen that for coatings made in the presence of 2 M NaCl, contact angles drop significantly with temperature, which may be a consequence of greater surface roughness and lower crystallinity (as proven by infrared spectroscopy, data not shown). It is also seen in Figure 7b that the contact angle of PVOH adsorbed without salt practically does not change until 50 °C. (11) Narten, A. H. J. Phys. Chem. 1970, 74, 765-768. (12) Hribar, B.; Southall, N. T.; Vlachy, V.; Dill, K. A. J. Am. Chem. Soc. 2002, 124, 12302-12311.
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Figure 9. Tapping mode AFM images of PVOH (fully hydrolyzed; molecular weight, 89 000-98 000; concentration, 0.023 M) adsorbed on fluoroalkyl monolayers from salt-free solutions at four temperatures. Image size, 3 µm × 3 µm; data scale, 10 nm for 10-50 °C and 20 nm for 80 °C.
Further studies of the films adsorbed from salt-free solutions were performed. Figure 8a shows the adsorption temperature dependence of the O:F XPS atomic ratio at a 15° takeoff angle. At this angle, the depth of analysis is ∼15 Å. Since the ellipsometric thickness of films in this temperature region is around 27 Å and does not vary much, the XPS data can only be explained by studying the physical structure of these films. Indeed, AFM analysis performed on these surfaces has shown that films prepared in the intermediate temperature range (40-50 °C) are smoother than those adsorbed at either low (10-20 °C) or high (60-70 °C) temperature (Figure 8b). This is fully consistent with the XPS data in Figure 8a: the higher atomic concentration of fluorine at low and high temperatures simply indicates the presence of defects in PVOH films, exposing the fluoroalkyl substrate. AFM images of PVOH surfaces prepared at different temperatures are shown in Figure 9 for further illustration. This critical dependence may be readily explained again by considering the two-stage mechanism of PVOH adsorption. The physical adsorption is driven by hydrophobic interactions that become stronger as temperature increases. On the other hand, at higher temperatures, water molecules start competing with hydroxyls for hydrogen bonding, breaking intermolecular association in PVOH thin film, which eventually (∼80 °C) leads to dissolution of the film. There is an intermediate temperature region where the sum of these two factors produces homogeneous, smooth coatings. Considering the kinetics of these two phenomena, we speculate that the formation of a network of intra- and intermolecular hydrogen bonding stabilizing the adsorbed PVOH layer is the rate-limiting (slow) stage of the overall process, while the adsorption occurs very fast, at least initially. An in situ time-resolved infrared investigation would test this assumption. Barrett et al.3 showed that the rate of PVOH adsorption on polystyrene increases significantly with concentration. In our previous report,1
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we demonstrate that the crystallinity of adsorbed PVOH decreases with increasing concentration of adsorption solutions. Both of these facts argue that crystallization lags behind adsorption. Conclusions The study of five factors (molecular weight, concentration, degree of hydrolysis, salt, and temperature) affecting the adsorption of poly(vinyl alcohol) to a model hydrophobic surface have elucidated the mechanism of this process. PVOH samples with lower molecular weight or a higher percentage of unhydrolyzed acetate groups adsorb better, especially from dilute solutions. Residual acetate groups partition to the outermost surface of the film and determine its wettability. The formation of stable coatings
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is driven by two major forces: polymer adsorption on the hydrophobic surface and crystallization within the adsorbed layer. Evidently, salts with pronounced “salting out” power (sodium sulfate and sodium chloride), as well as temperature, modulate these forces in opposite directions, which leads, for example, to formation of smooth coatings only under certain conditions. Acknowledgment. We thank the Office of Naval Research and Presstek, Inc. for financial support. Use of central facilities of the NSF-sponsored Materials Research Science and Engineering Center at the University of Massachusetts is also acknowledged. LA0492299
