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Langmuir 1996, 12, 1296-1302
Effect of Acetate Distribution on Surface Segregation in Poly(vinyl alcohol-co-vinyl acetate) Copolymer Films Martin A. Helfand,* John B. Mazzanti, Matilda Fone, and Robert H. Reamey* Raychem Corporation, 300 Constitution Drive, Menlo Park, California 94025
Patricia M. Lindley Charles Evans and Associates, 301 Chesapeake Drive, Redwood City, California 94063 Received June 19, 1995. In Final Form: November 22, 1995X The level of acetate-group surface segregation in poly(vinyl alcohol-co-vinyl acetate) (PVA-PVAc) films was found to depend markedly on the functional group distribution along the backbone (blockiness). PVAPVAc polymers with both random and blocky distributions were prepared at levels between 2 and 12 mol % acetate and cast into films from aqueous solution. Films from polymers with blocky distributions showed significantly higher levels of acetate at the surface than in the bulk, while polymers with random distributions of acetate functionality exhibited little or no surface segregation. The level of acetate surface segregation was determined by both X-ray photoelectron spectroscopy (XPS) and time-of-flight secondary ion mass spectroscopy (ToF-SIMS). Blocky PVA-PVAc films containing 4 mol % were seen to have approximately 40 mol % PVAc at the outermost surface by XPS at low take-off angle. Bulk acetate levels and acetate group distributions were determined by NMR. Variations in the weight average molecular weight (Mw) between 40k and 155k, modifying the casting solvent, or annealing the films above Tg did not effect the level of surface segregation, suggesting that the segregation is not simply a kinetic phenomenon.
I. Introduction This study utilizes surface and bulk characterization techniques in order to quantitate the level of surface segregation in PVA-PVAc films. Poly(vinyl alcohol), often referred to as PVA, is widely used as a protective colloid, as a thickening agent, and in coatings and adhesives.1 It is made commercially via the alcoholysis2 of poly(vinyl acetate) and usually contains residual acetate functional groups. The properties of poly(vinyl alcohol) are greatly effected by the presence of these acetate groups,3 which can be present in relatively high amounts. One of the most commonly used PVA types (referred to as “partially hydrolyzed” PVA) contains approximately 12 mol % residual acetate functionality. These polymers are often loosely referred to as “poly(vinyl alcohol)” or PVA, but for many of their properties, it is important to explicitly acknowledge that they are copolymers. In this paper we refer to PVA as poly(vinyl alcohol-co-vinyl acetate) or PVA-PVAc. The PVA-PVAc polymers prepared by alcoholysis of PVAc are known to have a nonrandom distribution of acetate groups. It has been shown that for PVA-PVAc copolymers formed in this manner the acetate groups exist in blocks,4 presumably due to the participation of neighboring hydroxyl groups in the alcoholysis reaction.5 The “blockiness” of the acetate distribution can be seen qualitatively with infrared spectrometry (IR)6 and more quantitatively with nuclear magnetic resonance spectroscopy (NMR).7 PVA-PVAc with a random distribution of acetate groups can be prepared by acetylation of the X Abstract published in Advance ACS Abstracts, February 1, 1996.
(1) Pritchard, J. G. Poly(Vinyl Alcohol) Basic Properties and Uses; Gordon and Breach: New York, 1970; pp 120-123. Sakurada, I. Polyvinyl Alcohol Fibers; Dekker: New York, 1985. (2) The process of acetate removal is sometimes referred to as hydrolysis or saponification. For acetate removal in alcoholic media alcoholysis is a more correct term. (3) Toyoshima, K. In Polyvinyl Alcohol, Properties and Applications; Finch, C. A., Ed.; Wiley: Chichester, 1973; pp 17-64. (4) Tubbs, R. K. J. Polym. Sci., 1966, A-1 (4), 623-629. (5) Fujii, K. J. Polym. Sci. 1971, D (5), 496. (6) Nagai, E.; Sagane, N. Kobunshi Kagaku 1955, 12, 195.
Figure 1. Schematic representation of block and random poly(vinyl alcohol-co-vinyl acetate).
PVA homopolymer.4 Block and random PVA-PVAc copolymers are depicted schematically in Figure 1. A number of papers have dealt with the surface properties of aqueous solutions containing dissolved PVAPVAc,8 but relatively little work has been done on the surfaces of solid films. We have previously investigated the surfaces of a series of PVA-PVAc films which were produced by alcoholysis by XPS and ToF-SIMS and showed that in these relatively “blocky” copolymers there was significant surface segregation of the acetate functional groups that was independent of bulk acetate levels examined.9 It was shown that the C(1s) spectrum for the PVA and PVAc units could be well resolved, providing a quantitative measure of the relative proportion of PVA and PVAc at the polymer surface. In addition, the negative SIMS spectra also provided a semiquantitative measure of the surface composition, as the m/z ) 59 (C2H3O2-) contribution was attributed to the acetate unit. Akhter (7) Moritani, T.; Fujiwara, Y. Macromolecules 1977, 10 (3), 532535. Bugada, B. C.; Rudin, A. Polymer 1984, 25, 1759. (8) Hayashi, S.; Nakano, C.; Motoyama, T. Kobunshi Kagaku 1964, 21, 300. Beihn, G. F.; Erusberger, M. L. Ind. Eng. Chem. 1951, 43, 1108. (9) Mazzanti, J. B.; Reamey, R. H.; Helfand, M. A.; Lindley, P. M. J. Vac. Sci. Technol. 1992, A10 (4), 2419.
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et al.10 have also observed the preferential enhancement of acetate groups at the surface of these polymers by XPS but attributed the segregation to isolated pendent groups. The general phenomena of surface segregation and dynamics in polymeric systems is the subject of a recent chapter.11 In this study we report on the surface composition of a series of PVA-PVAc copolymers with relatively random distributions, produced by the acetylation of PVA, and directly compare the results with data from analogous copolymers produced by alcoholysis. We utilize 13C NMR to characterize the distribution of acetate groups along the polymer chain for copolymers produced by the two different synthesis routes. The characterization of the surface composition of a series of polymers with the same bulk composition, but with different sequence distributions, provides a rare opportunity to assess the effect of the sequence distribution of a particular chemical species on the extent of surface segregation. II. Experimental Section a. Polymer Preparation. Random PVA-PVAc copolymers were obtained via acetylation of 100% hydrolyzed PVA. The 100% hydrolyzed PVA was prepared by hydrolysis of the residual acetate groups on Airvol 205 PVA-PVAc copolymer (13% acetate, blocky distribution, weight average molecular weight 3100050000, Air Products). The hydrolysis was carried out by adding 84 g of 5 N NaOH to 500 g of a 19.4% aqueous Airvol 205 solution and stirring for 4 h. The solution was then poured into 1 L of acetone in an explosion-proof Waring blender to precipitate the polymer. The polymer was isolated by filtration, extracted for 18 h with methanol in a Soxhlet extractor, and dried in a 60 °C vacuum oven. The 100% hydrolyzed PVA was converted to randomly acetylated PVA-PVAc copolymers in the following manner. To 4 g of 100% hydrolyzed PVA (0.091 mol) dissolved in 100 mL of 1-methyl-2-pyrrolidinone was added 1.3 mL (0.018 mol) of acetyl chloride. The solution was stirred 20 h at 20-25 °C and then poured into 500 mL of acetone in an explosion-proof Waring blender to precipitate the polymer. The polymer was isolated by filtration, extracted for 18 h with methanol in a Soxlet extractor, and dried in a 60 °C vacuum oven. The resulting polymer had an acetate content of 6.8% as determined by proton NMR spectroscopy. Polymers with other acetate levels were prepared by varying the amount of acetyl chloride. Block PVAPVAc copolymers were prepared as described in ref 9. b. Nuclear Magnetic Resonance (NMR). 13C{1H} solutionstate NMR measurements were conducted on a Varian XL-300 spectrometer operating at 75.4 MHz. Specimens were prepared by dissolving 300 mg of the copolymer in 3 mL of D2O at 90 °C. A total of 13 144 transients were collected in the experiment with the NOE (nuclear Overhauser effect) applied to obtain increased signal-to-noise. A delay time of 15 s was utilized to ensure complete relaxation of the C nuclei. The chemical composition and relative blockiness were determined from the data by well-established methods.12,13 c. Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS). Positive and negative ion ToF-SIMS mass spectra were acquired on a Charles Evans & Associates TFS surface analyzer using a pulsed 9 keV Cs+ primary ion source. The Cs gun was operated in an ion microscope mode a pulse repetition rate of 10 kHz, which corresponds to a mass range from 0 to 1100 Da. The mass resolution for this analysis was approximately 3000 fwhm (full-width half maximum) at m/z 41. Primary ion doses were kept below 1012 ions/cm2, so that all spectra were obtained under static SIMS conditions. Since the films are insulating, a pulsed, low-energy (15 eV) electron gun was used (10) Akhter, S.; Cannon, K. C.; White, J. M. Appl. Surf. Sci. 1990, 44, 49. (11) Andrade, J. D.; Gregonis, D. E.; Smith, L. M. In Surface and Interfacial Aspects of Biomedical Polymers; Andrade, J. D., Ed.; Plenum Press: New York, 1987; Chapter 2. (12) Miller, R. L.; Nelson, L. E. J. Polym. Sci. 1960, 46, 303; 1962, 56, 357. (13) Ito, K.; Yamashita, Y. J. Polym. Sci., Part A 1965, 3, 2165.
Langmuir, Vol. 12, No. 5, 1996 1297 Table 1.
13C
NMR Determination of Block Character of PVA-PVAc Copolymers
method of preparation
acetate level (%)
LOH
LOAc
η
alcoholysis alcoholysis alcoholysis reacetylation reacetylation reacetylation reacetylation
12.7 4.2 3.0 16.1 10.4 8.8 5.0
17.6 54.7 77.6 7.3 12.9 16.5 35.1
2.6 2.4 2.4 1.4 1.5 1.6 1.9
0.45 0.43 0.43 0.82 0.74 0.69 0.59
for charge compensation.14 For each polymer film, spectra were obtained from four 250 µm diameter areas with acquisition times of 5-15 min. d. X-ray Photoelectron Spectroscopy (XPS). The variable angle XPS spectra were acquired with a VG Scientific ESCALab Mk II spectrometer utilizing an achromatic Mg KR1,2 (1253.6 eV) X-ray source at 200 W of power. The hemispherical analyzer was operated in the constant analyzer energy (CAE) mode. The data collection procedure consisted of first obtaining survey scans from 0 to 1000 eV with 0.75 eV steps at a 100 eV pass energy to assess specimen integrity followed by narrow scans of the C(1s) and O(1s) regions with 0.1 eV steps at a 20 eV pass energy. The base pressure during data acquisition ranged between 2 × 10-8 and 2 × 10-9 Torr. Binding energies are referenced to methylene carbon (CH2) at 285.0 eV. Nonlinear least-squares curve fitting of the C(1s) region was performed utilizing the VGS 5000 data system. Since curve fitting can provide many solutions to the same spectrum, chemical knowledge of the specimens and the variable angle data in addition to subtraction and derivative techniques15 were utilized to determine the peak assignments. Further detail regarding the peak fitting procedure is provided in the results section. PVA-PVAc is known to degrade during Mg KR X-ray irradiation.16 In this investigation degradation was minimized by limiting the photon flux and data acquisition time. A full set of spectra were acquired in under 30 min, and the angles were staggered to minimize any possible time-dependent effects. In addition, the photodesorption products were monitored during the XPS experiments with a quadrupole mass spectrometer. Background scans from m/z 1 to 50 with the X-ray source running, but not exposed to the sample, showed primary peaks at m/z 2, 18, 28, and 44, corresponding to H2, H2O, CO, and CO2. Subsequent exposure of the sample to the X-ray source resulted in an increase in the intensity of the m/z 2 contribution.
III. Results a. Composition and Block Character by NMR. Bulk acetate levels of PVA-PVAc copolymers were determined by both proton and 13C NMR, while 13C NMR was used to determine the block character. The methylene resonances of PVA-PVAc copolymers show three wellresolved lines due to dyads from the three polymer sequences denoted (OH, OH), (OH, OAc), and (OAc, OAc). Integration of the three spectral regions yielded the block character (or co-monomer sequence distribution). The results are summarized in Table 1. The values LOH and LOAc represent the number average length of the respective monomer units, and η represents the block character, where η ) 0 would indicate complete block character and η ) 1 indicates complete randomness. The results show the expected trend that the blocky polymers made from saponification have lower values of (η) than those made by reacetylation, which would be expected to be completely random in sequence distribution. Although the trend in measured block character is as (14) Hagenhoff, B.; Van Leyen, D.; Niehuis, E.; Benninghoven, A., Proceedings of the Sixth International Conference on Secondary Ion Mass Spectrm. (SIMS VI); Wiley: Chichester, 1994; p 235. (15) Sherwood, P. M. A. In Practical Surface Analysis by Auger and Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; Wiley: Chichester, 1990; Appendix 3. (16) Akhter, S.; Allan, K.; Buchanan, D.; Cook, J. A.; Campion, A.; White, J. M. Appl. Surf. Sci. 1988-89, 35, 241.
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Figure 2. Negative secondary ion mass spectra obtained from block PVA-PVAc copolymers containing 12, 6.9, and 2.2% bulk PVAc and random PVA-PVAc copolymers containing 13, 6.8, and 2.7% bulk PVAc.
expected, it is striking that the magnitude of the difference in block character as measured by NMR is relatively small in comparison to the differences seen in the surface compositions of these materials presented in sections IIIb and IIIc. Note, however, that the block character determined by NMR represents a number average, and therefore says nothing about the distribution of block length within the sample which could significantly effect properties such as the level of acetate segregation. b. Copolymer Surfaces As Revealed by ToF-SIMS. The negative ion SIMS spectra obtained from three block and three random copolymers containing nominally 3, 7, and 12% bulk PVAc over the m/z 20-70 range are presented in Figure 2. The two peaks of primary interest appear at m/z 25 and 59 and are assigned to C2H- arising predominantly from the polymer backbone and C2H3O2arising predominantly from the acetate unit, as established in earlier work by Briggs and Munro.17 For the block copolymers the relative intensities of the m/z 25 and 59 peaks remain approximately equivalent over the 3-13% range of bulk PVAc concentrations. This reveals that the block copolymer surfaces consist of approximately equivalent concentrations of acetate regardless of bulk concentration. In contrast, for the random copolymers, the intensity of the m/z 25 peak is notably higher with respect to the m/z 59 peak and varies with bulk PVAc concentration. This indicates that the acetate concentration of the random copolymer surfaces decreases with decreasing bulk PVAc concentration. In Figure 3 the normalized intensity (to total counts) of the m/z 59 ion (C2H3O2-) obtained from the block and random copolymers examined is plotted as a function of bulk PVAc concentration as determined by proton NMR. Block copolymer surfaces are seen to be highly enriched in acetate in comparison with the random copolymers. In the random copolymer systems the C2H3O2- intensity varies more as a function of bulk acetate than that for block copolymer systems over the examined range; however more scatter is present in the data obtained from the block copolymers than from the random copolymers. Imaging experiments were also conducted to determine if lateral segregation of acetate could be observed. No (17) Briggs, D.; Munro, H. S. Polym. Commun. 1987, 28, 307.
Figure 3. Normalized intensity of m/z ) 59.044 peak (C2H3O2-) as a function of bulk PVAc concentration in block and random PVA-PVAc copolymers.
significant or regular variations in the distribution of secondary ions were observed; hence if bulk segregation or micelle formation is occurring in these copolymers the scale is smaller than the 0.15 µm lateral resolution of the experiment. c. Copolymer Surfaces As Revealed by XPS. Results from XPS experiments corroborate those from ToFSIMS. In Figure 4 the angle-resolved C(1s) photoelectron spectra obtained from the surfaces of random and block copolymers containing nominally 3% bulk PVAc are presented with curve synthesis results superimposed over the raw spectra. As the photoelectron take-off angle (measured with respect to the surface horizontal) is decreased from 70° to 10°, the information (or probe) depth of the experiment decreases from roughly 75 to 15 Å. Curve fitting of the C(1s) region was accomplished by accounting for the individual C contributions from the PVA and PVAc units separately. Seven peaks were
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The angle-resolved C(1s) spectra in Figure 4 illustrate two main points regarding the segregation of PVAc units to the surface. First, the surface region contains more PVAc units in films where the groups are distributed in blocks than when they are distributed randomly. Second, a gradation of PVAc units within the probe depth is observed only for films with PVAc units distributed in blocks. One advantage of XPS over SIMS is the ease of quantification, especially when the necessary information is contained in a single core level. The concentration of PVAc in the copolymers within the XPS probe depth can be calculated from the C(1s) curve fitting results per mol % on a monomer unit basis with the following relationship (1);
CPVAc ) (IPVAc1 + IPVAc2 + IPVAc3 + IPVAc4)/2 IPVA1 + IPVA2 + (IPVAc1 + IPVAc2 + IPVAc3 + IPVAc4)/2 (1) where
CPVAc ) PVAc concentration (mol %, monomer unit basis) Figure 4. Angle-resolved C(1s) photoelectron spectra obtained from random and block PVA-PVAc copolymers containing 3.1 and 3.0% bulk PVAc, respectively. Table 2. C(1s) Binding Energies ((0.1 eV) and Peak Fitting Parametersa species
binding energy (eV)
shift
methylene (CH2) methyl (CH3) hydroxyl (CHOH) carbonyl (CdO) carboxyl (O-CdO)
285.0b 285.7 286.6 287.8 289.3
0.0 0.7 1.6 2.8 4.3
IPVA1 ) peak intensity, methylene from PVA IPVA2 ) peak intensity, alcoholic methine from PVA IPVAc1 ) peak intensity, methylene from PVAc IPVAc2 ) peak intensity, methyl from PVAc IPVAc3 ) peak intensity, oxygen bearing from PVAc JPVAc4 ) peak intensity, carboxyl from PVAc
a
b
fwhm ) 1.50 ( 0.05 eV, Gaussian/Lorentzian ratio ) 50%. Reference.
utilized for each curve fit. Each peak is specified by four parameters (position, height, width, and Gaussian/ Lorentzian ratio), and two parameters (slope and intercept) are utilized to specify the linear background. Salient peak parameters are given in Table 2 and compare well with the work of others.18 Fortunately, the carboxyl carbon of the PVAc unit exhibits a large chemical shift and its contribution is well separated from the main envelope. Hence the carboxyl peak provided an unambiguous and convenient means to determine the concentrations of PVA and PVAc directly from the C(1s) photoelectron spectra. Fitting therefore proceeded by first considering the carboxyl carbon at 289.3 eV. Second, the methylene (285.0 eV), methyl (285.7 eV), and alcoholic (286.6 eV) contributions from the PVAc units were added in their expected 1:1:1:1 intensity ratios based upon PVAc stoichiometry. Third, the methylene (285.0 eV) and alcoholic methine (286.6 eV) contributions from the PVA units were added in their expected 1:1 intensity ratios. Finally, a minor carbonyl contribution at 287.8 eV was added. Carbonyl impurities are common in PVA/PVAc,19 resulting from manufacturing, handling, and degradation of the polymer, and are also produced during X-ray irradiation. Peak heights were then varied in order to find the best fit. Shakeup peaks were not included in the fitting process, as their effect on the results was determined to be insignificant. (18) Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers; Wiley: Chichester, 1992. (19) Dunn, A. S. Chem. Ind. 1980, 801.
In Figure 5 the surface PVAc concentration as determined by XPS from eq 1 at three photoelectron take-off angles is plotted as a function of bulk PVAc concentration as determined by proton NMR for the block and random copolymers examined. Again PVAc units are observed to be surface segregated in the block copolymer systems, but not in the random copolymer systems. The outermost surfaces of the block copolymers are found to be composed of roughly 40% PVAc units despite the variations in bulk PVAc concentration, while the random copolymer surface compositions closely match their respective bulk values. Further evidence for the PVAc enrichment in the block copolymers is observed as the probe depth is increased. The block copolymers show a gradation of PVAc units that increases with increasing bulk PVAc concentrations. The random copolymers show little to no gradation of PVAc concentration, indicating little to no surface segregation. The uncertainties associated with each of these quantitative determinations are estimated to be on the order of several percent. Powell and Seah20 have recently reviewed the various salient factors that introduce uncertainty in quantitative measurements by XPS. d. Annealing Studies. A series of experiments were conducted to determine if the observed segregation was simply a kinetic phenomenon related to the solvent casting process. Films were annealed at 80 °C and at 110 °C for 18 h at atmospheric pressure in a separate oven and then transferred to the spectrometer for analysis. Particular attention was paid to the possibility of contamination due (20) Powell, C. J.; Seah, M. P. J. Vac. Sci. Technol. 1990, A8 (2), 735.
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Figure 7. Overlay of C(1s) photoelectron spectra obtained from block PVA-12% PVAc copolymers of varying molecular weight: Mw ) 40 k, 115k, and 155k; 30° and 70° photoelectron take-off angles.
Figure 5. Surface concentration of PVAc in random and block PVA/PVAc copolymers as a function of the bulk concentration as determined by XPS from three photoelectron take-off angles.
obtained from block copolymers containing 2% bulk acetate. Hence, variations in Mw have little or no effect on the propensity of PVAc to segregate over the range of molecular weights and acetate levels examined. IV. Discussion
Figure 6. Overlay of C(1s) photoelectron spectra obtained from random PVA-3.1% PVAc copolymer before and after annealing at 80 and 110 °C, 30° photoelectron take-off angle.
to sample transfer; therefore both survey and the narrow scans were obtained before and after annealing, negligible change was observed in either. The two temperatures were chosen because they are approximately at and above the glass transition temperature (Tg ∼ 80 °C) for PVAPVAc and therefore should provide enough energy for the surface of the polymer to rearrange to a more stable configuration. In Figure 6 an overlay of the C(1s) spectra from a random PVA/3.1% PVAc copolymer obtained before and after annealing is presented. In addition, films cast from a 25 wt % methanol/75 wt % water solution were also indistinguishable from controls cast from water alone. e. Effect of Molecular Weight on Segregation. A series of block copolymers with differing weight average molecular weights (Mw) were also examined. An overlay of C(1s) spectra obtained from the surfaces of block copolymers containing 12% bulk acetate of varying Mw is shown in Figure 7. No variations in the angular dependent photoelectron distributions were observed from polymers of differing molecular weight. Analogous results were
a. Driving Force for Segregation. Generally the dominant driving force for surface segregation is the minimization of surface free energy. In block copolymer systems, the block with the lowest surface energy (gs) would be expected to preferentially segregate to the polymer surface.21 The literature data on the surface energies of PVAc and PVA are consistent with the view that the preferential surface segregation of the PVAc that we have observed is due to the lower surface energy of these groups relative to the PVA units. The surface energy (gs) of pure poly(vinyl acetate) (PVAc) measured on molten polymer by the pendant drop method was reported to be 36.5 dyn/cm by Wu22 and 38 dyn/cm by Roe23 when extrapolated to 20 °C. These values are in reasonable agreement with the value for the critical surface tension of wetting of 36 dyn/cm obtained on a solid polymer film via contact angle measurements.24 The surface energy of poly(vinyl alcohol) (PVA) was reported by Shiomi et al. to be 44 and 46 dyn/cm for the surface energy obtained by contact angle measurements with ethylene glycol and formamide, respectively, on the surface of PVA under a series of hydrocarbons.25 The measurements of Matsunaga and Ikada26 are consistent with these results, but these authors point out that the polar component of the surface tension for PVA is due largely to hydrogen bonding, in which case the use of the geometric mean equation in order to extract a surface (21) Patel, N. M.; Dwight, D. W.; Hedrick, J. L.; Webster, D. C.; McGrath, J. E. Macromolecules 1988, 21 (9), 2689. Gaines, G. L.; Bender, G. W. Macromolecules 1972, 5 (1), 82. (22) Wu, S. J. Colloid Interface Sci. 1969, 31 (2), 153. (23) Roe, R.-Y. J. Colloid Interface Sci. 1969, 31 (2), 228. (24) Wu, S. J. Phys. Chem. 1968, 72, 3332. (25) Shiomi, T.; Nishioka, S.; Tezuka, Y.; Imai, L. Polymer 1985, 26, 429. (26) Matsunaga, T.; Ikada, Y. J. Colloid Interface Sci., 1981, 84 (1), 8.
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energy value is questionable. The critical surface tension of wetting (gc) of 37 dyn/cm measured by Ray et al.27 is not in agreement with the values quoted above. The reason for this disagreement may be due to the fact that these authors did not explicitly include the dispersive and polar components of the surface tension. The occurrence of discrepancies of this type have been explained by Kaelble.28 It is also possible that part of the discrepancy is due to the fact that the “PVA” used by Ray et al. contained approximately 3% PVAc which, according to this work, could result in >35% PVAc at the surface of the film. The arguments presented above for segregation of the acetate groups are based strictly on thermodynamic considerations. The observed segregation could also, however, result from a kinetic hindrance during the solvent casting process. Results from the annealing experiments and casting from different solvents indicate that the observed concentrations of acetate at the polymer surface are due not to kinetic but to thermodynamic driving forces. b. Acetate Distribution and Surface Segregation. The experimental data clearly show that acetate units in the block copolymers segregate to the surface while those in the random copolymers do not, indicating that it is critical to have groups of acetate units together in order for segregation to occur. When segregation occurs, knowledge of the extent and depth of segregation is of great importance in furthering our understanding of such phenomena. Recently much progress has been made in the area of quantitative surface analysis of layered structures.29-31 Unfortunately, unambiguous determination of the concentration profile from angle-resolved XPS data is an ill-posed mathematical problem further complicated by experimental effects such as surface roughness, channeling, etc. Previously, Ahkter et al.10 has modeled XPS intensities from a PVA/PVAc copolymer containing 3% acetate (carbon basis) with standard XPS attenuation equations32 and concluded the observed segregation was confined to the topmost surface region (∼3 Å). Ahkter et al. did not report on the synthetic route or the acetate sequence distribution but concluded that the pendent groups were isolated. Data presented here suggest the PVA-PVAc copolymer investigated by Ahkter was blockly. Ahkter applied eq 2 to model the dependence of the carboxyl functionality on the XPS take-off angle. Equation 2 assumes the segregation to be a single step function of uniform concentration (although such an assumption may not correspond to the true situation, it does reduce the number of variables to a manageable level)
R ) χs - (χs - χb) exp[-t/(λ sin θ)]
(2)
where R is the ratio of carboxylate to the total C(1s) intensity, χs and χb are the concentrations of carboxylate in the surface segregated zone and bulk, respectively, t is the thickness of the surface-segregated zone, λ is the photoelectron mean free path of the C(1s) photoelectrons in the solid (27 Å), and θ is the photoelectron take-off angle. Although Ahkter et al. did not find a unique solution to eq 2 for values of χs, χb, and t, subsequent (27) Ray, B. R.; Anderson, J. R.; Sholz, J. J. J. Phys. Chem. 1958, 62, 1220. (28) Kaelble, D. H. Physical Chemistry of Adhesion; Wiley: New York, 1971; pp 149-170. (29) Holloway, P. H.; Bussing, T. D. Surf. Interface Anal. 1992, 18, 251. (30) Nefedov, V. I. Surf. Interface Anal. 1991, 17, 825. (31) Tyler, B. J.; Castner, D. G.; Ratner, B. D. Surf. Interface Anal. 1989, 14, 443. (32) Fadley, C. S. Prog. Surf. Sci. 1984, 16 (3), 275.
Figure 8. Angular dependence of the C(1s) carboxylate to total C(1s) intensity ratio for block copolymers containing low (2%) and high (12%) PVAc bulk concentrations.
polymer metalization studies led these workers to conclude that the segregation was limited to the top monolayer and 6-fold over the bulk. If we plot R as a function of θ for the block copolymers investigated in this study containing low (2%) and high (12%) bulk acetate concentrations, we observe that either the surface concentration or the thickness of the segregated zone or both are varying. The data are presented in Figure 8. Recalling that the quantitative XPS and the semiquantitative SIMS results found the composition of the topmost surface only varies slightly over the range of bulk acetate levels studied in the block copolymers, we believe that the segregation is not confined to the top monolayer of the polymer film. We conclude that the distribution of acetate in the surface region which we have observed cannot be explained by a simple conformational rearrangement (such as a rotation of the acetate pendent groups about the polymer backbone) of the topmost layer of polymer. A mechanism more consistent with these data is one in which there is a balance between the driving force of lowered surface energy due to moving an acetate group to the surface and the entropic and enthalpic cost associated with accommodating the polymeric chain to which the acetate unit is attached. Thus, the lowest surface energy would be obtained if all of the acetate units were at the surface (displacing the higher energy hydroxyl groups). An acetate group present as an isolated pendent surrounded by alcohol groups along the chain must, however, drag with it a number of alcohol groups into the surface region. In order to bring a high concentration of acetate groups to the surface in this manner, a high entropic and enthalpic penalty is paid for accommodating the associated polymer chains. If the acetate groups are present in blocks of even a few units along the chain, surface energy can be lowered without paying as great an energy penalty for accommodating the associated chains. Thus the block length and distribution of block lengths are critical in dictating both the level and depth of segregation. This type of mechanism is based on thermodynamic considerations and would be expected to result in a surface segregation which extends over several molecular layers into the material as seen in our angle-resolved XPS results. c. Molecular Orientation at the Top Monolayer of the Surface. The XPS data consistently exhibited increased intensity about the methylene peak at low take-
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off angle. Although similar observations regarding the methyl contribution are more difficult to make because of overlap of the methyl peak with the methylene contribution at low binding energy and alcoholic methine at high binding energy, we note that more accurate curve fits were achieved by allowing the height of the methyl peak to increase by several percent at low take-off angles, especially for the copolymers containing higher PVAc concentrations. Such results suggest that the functional groups at the topmost surface layer orient so that the lower energy groups are presented. Thus, both the methyl and methylene groups may be oriented outward at the surface of these polymers. This observation is also consistent with minimization of surface free energy, as both these components are the lowest energy species. V. Conclusions XPS and ToF-SIMS have been used to quantitate the amount of surface segregation of acetate units within a series of PVA-PVAc copolymers with different distributions of acetate groups along the polymer chain. We found that relatively small differences in blockiness as revealed
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by 13C NMR lead to large differences in the amount of acetate segregated to the surface. The results are consistent with a segregation mechanism which is thermodynamically driven by a minimization of the surface free energy. The results are not consistent with previously proposed mechanisms involving simple surface reorientation of acetate groups by rotation about the polymer chains. The molecular weight of the polymer was found to have no detectable effect on the amount of segregation. In PVA-PVAc copolymers with random acetate group distributions the surface composition of acetate was indistinguishable to that of the bulk, indicating no surface segregation. Although molecular reorientation is not responsible for the large acetate levels at the surface of blocky PVA-PVAc copolymers, low-angle XPS measurements indicate that molecular reorientation may occur in the top-most surface layer to preferentially expose the methyl and methylene groups. Acknowledgment. We gratefully acknowledge the assistance of Dr. David Duff with the NMR analysis. LA950481D
