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Langmuir 1997, 13, 2988-2992
Selective Accumulation of Functional Groups at the Film Surfaces of Stereoregular Poly(methyl methacrylate)s Oleg N. Tretinnikov† B. I. Stepanov Institute of Physics, Academy of Science of Belarus, 70 Prospekt F. Skariny, Minsk 220072, Belarus Received January 8, 1997. In Final Form: March 24, 1997X Contact angles of water on stereoregular poly(methyl methacrylate)s (PMMA) solvent-cast in the form of films against a glass substrate were measured by using the sessile drop method. The syndiotactic films exhibited contact angle values indicative of relative dominance of the nonpolar R-methyl and methylene groups of polymer chains at the air-side surface and the polar ester groups at the glass-side surface of the films. The selectivity in the surface exposure of functional groups increased rapidly with syndiotacticity. On the contrary, the wetting results for the isotactic films showed no effect of the polarity of contacting medium on the surface chemical composition and the wettability of the films represented an average for R-methyl, methylene, and ester groups both at the air-side and at the glass-side surface. The observed influence of tacticity on the surface activity of stereoregular PMMAs was interpreted in terms of the surface conformational structure of these materials.
Introduction Given sufficient mobility, the macromolecules at the surface of a polymer tend to reorient or restructure in order to rise the surface concentration of polar or nonpolar moieties depending on the polarity of the surrounding phase.1 This phenomenon is attributed to the thermodynamic driving force to minimize the interfacial free energy. In 1975, Holly and Refojo2 studied the water wettability of poly(2-hydroxyethyl methacrylate) hydrogels and found that surface rearrangement took place with the change of contacting medium from air to water. Since then, a number of investigations of surface reorganizations in the presence of water have been reported for various polymers.3-13 The selective accumulation of functional groups is considered to take place also during the surface formation by casting the polymer solutions: the polymer chains adopt configurations or orientations which favor maximizing the surface exposure of polar or nonpolar moieties, and as the solvent evaporates the macromolecules are frozen into this structure.14,15 Consequently, polymer films cast onto polar substrates exhibit an excess of polar groups at † Current address: Research Center for Biomedical Engineering, Kyoto University, 53 Kawahara-cho, Shogoin, Sakyo-ku, Kyoto606, Japan. Fax: +81(75)751-4144. E-mail:
[email protected]. X Abstract published in Advance ACS Abstracts, May 1, 1997.
(1) Andrade, J. D., Ed. Polymer Surface Dynamics; Plenum Press: New York, 1988. (2) Holly, F. J.; Refojo, M. F. J. Biomed. Mater. Res. 1975, 9, 315. (3) Yasuda, H.; Sharma, A. K.; Yasuda, T. J. Polym. Sci., Polym. Phys. Ed. 1981, 19, 1285. (4) Ikada, Y.; Matsunaga, T.; Suzuki, M. J. Chem. Soc. Jpn., Chem. Ind. Chem. 1985, 6, 1079. (5) Lavielle, L.; Schultz, J. J. Colloid Interface Sci. 1985, 106, 438. (6) Hogt, A. H.; Gregonis, D. E.; Andrade, J. D.; Kim, S. W.; Dankert, J.; Feijen, J. J. Colloid Interface Sci. 1985, 106, 289. (7) Van Damme, H. S.; Hogt, A. H.; Feijen, J. J. Colloid Interface Sci. 1986, 114, 167. (8) Yasuda, H.; Charlson, E. J.; Charlson, E. M.; Yasuda, T.; Miyama, M.; Okuno, T. Langmuir 1991, 7, 2394. (9) Morra, M.; Occhiello, E.; Garbassi, F. J. Colloid Interface Sci. 1992, 149, 167. (10) Tretinnikov, O. N.; Ikada, Y. Langmuir 1994, 10, 1606. (11) Tezuka, Y.; Araki, A. Langmuir 1994, 10, 1865. (12) Kasemura, T.; Takahashi, S.; Nakane, N.; Maegava, T. Polymer 1996, 37, 3659. (13) Pike, J. K.; Ho, T.; Wynne, K. J. Chem. Mater. 1996, 8, 856. (14) Schreiber, H. P.; Croucher, M. D. J. Appl. Polym. Sci. 1980, 25, 1961.
S0743-7463(97)00027-9 CCC: $14.00
the substrate-facing surface, while the air-facing surface is enriched in nonpolar groups.15-19 The control of the environmental response of polymer surfaces has become progressively important in a broad spectrum of areas, including biomaterials,20 adhesives,21 nonstick (low surface energy) coatings,22,23 plasma-treated polymers,24,25 and other polymer-based materials.26,27 In many of these applications, the successful material development is associated with the design of the polymer surface which is dominated by the specific functional groups and permanently retains them in various contacting mediums. An understanding of the microstructural factors that control the selective surface accumulation of functional groups could be extremely useful in tailoring the desired surface structure. The extent to which a macromolecule may expose polar or nonpolar groups toward the surrounding phase depends on the chemical structure of its repeating unit. This is clearly evident in the water contact angle behavior on the poly(vinyl alcohol) (PVA) and cellulose films.4,10 PVA, which has a very asymmetric repeat unit structure -CH2CH(OH)-, shows rather hydrophobic surface characteristics if contacted with air and becomes very hydrophilic when brought into water. In cellulose, the chemical structure of the repeating unit consists of hydrophilic groups located symmetrically on both sides of the sugar ring. Consequently, cellulose shows high surface hydrophilicity, in contact with both air and water. (15) Carre, A.; Gamet, D.; Schultz, J.; Schreiber, H. P. J. Macromol. Sci., Chem. 1986, A 23, 1. (16) Zhbankov, R. G.; Tretinnikov, O. N.; Tretinnikova, G. K. Vysokomol. Soedin. 1984, B26, 104. (17) Haq, Z.; Mingins, J. Polym. Commun. 1984, 25, 269. (18) Tretinnikov, O. N.; Zhbankov, R. G. Polymer-Solid Interfaces; Pireaux, J. J., Ed.; IOP Publishing: Bristol, 1992; p 361. (19) Zhu, L.; Gunnarsson, O.; Wesslen, B. J. Polym. Sci., Part A: Polym. Chem. 1995, 33, 1257. (20) Tingey, K. T.; Andrade, J. D. Langmuir 1991, 7, 2471. (21) Xu, M. X.; Zhang, W. X.; Xue, P. X.; Gao, W.; Yao, K. D. J. Appl. Polym. Sci. 1995, 58, 1047. (22) Shmidt, D. L.; DeKoven, B. M.; Coburn, C. E.; Potter, G. E.; Meyers, G. F.; Fischer, D. A. Langmuir 1996, 12, 518. (23) Chapman, T. M.; Marra, K. G. Macromolecules 1995, 28, 2081. (24) Xie, X.; Gengenbach, T. R.; Griesser, H. J. J. Adhes. Sci. Technol. 1992, 6, 1411. (25) Chatelier, R. C.; Xie, X.; Gengenbach, T. R.; Griesser, H. J. Langmuir 1995, 11, 2576. (26) Noda, I. Polymer Solutions, Blends, and Interfaces; Noda, I., Rubingh, D. N., Eds.; Elsevier: Amsterdam, 1992; p 1. (27) Lin, Y.; Yasuda, H.; Miyama, M.; Yasuda, T. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 1843.
© 1997 American Chemical Society
Selective Accumulation of Functional Groups
Next to the chemical and spatial constitution of polymer repeat unit, the polymer backbone conformation must be taken into consideration. The importance of this issue has been addressed by several authors.8,15,28 However, very little work has been done to relate the surface restructuration behavior and the resultant physicochemical structure of the surface to the conformational characteristics of polymer chains. Baier and Zisman29 first reported the influence of polymer conformation on the nature and packing of the exposed surface atoms of the solid. This was demonstrated by comparing critical surface tensions obtained on the films of poly(γ-methylL-glutamate) at different conformational states of the polymer; R-helix and random conformations had wettability characteristics of the polyamide backbone, while the extended-chain β-conformation showed wettability indicative of only the methyl ester side chains in the outermost layer. Vergelati et al.30 employed molecular modeling to describe the surface restructuring process of the poly(ethylene terephthalate) (PET) film when going from air to water. They found that, in air, the macromolecule adopts a coil conformation in order to favor the PET-PET interactions. On the other hand, in water, the macromolecule “spreads out” in order to expose its ester groups to water and maximize the possibility of hydrogenbond formation. We have previously investigated the surfaces of poly(methyl methacrylate) (PMMA) films cast on glass substrates by attenuated total reflection infrared (IR-ATR) spectroscopy combined with the contact angle method.16,18,31 The results of contact angle measurement were indicative of different concentrations of polar (ester) and nonpolar (R-methyl) functional groups at the air-side and the glassside surface of the films. The IR-ATR results revealed that, in the near-surface region, PMMA adopts a conformation with the methyl and the ester groups on the different sides of the polymer backbone, resulting in an amphiphilic character of the macromolecule. These findings suggested that the specific surface conformation of polymer chain is responsible for the selective surface exposure of functional groups. In the cited work on PMMA surfaces, commercially available methyl methacrylate polymer was used which is known to be predominantly syndiotactic (53-65% rr triads). The matter of the tacticity of the polymer has not been addressed. In this paper we would like to contribute to this topic, reporting the differences in the surface activity of syndiotactic and isotactic PMMA (s-PMMA and i-PMMA) macromolecules observed by contact angle measurements made on solvent-cast films of these materials. The observed influence of tacticity on the surface activity of polymer chains is interpreted in terms of the conformational difference of stereoregular polymers. Experimental Section Characteristics of the samples used are listed in Table 1. The molecular weights and triad contents were determined by gel permeation chromatography and proton NMR, respectively. s-PMMA-1 is commercially available, s-PMMA-2 was obtained from Polymer Laboratories (U.K.), and s-PMMA-3 was kindly provided by Dr. H. Yasuda (Hiroshima University, Hiroshima, Japan). i-PMMA-1 was kindly provided by Dr. J. Spevacek (Institute of Macromolecular Chemistry, Prague, the Czech Republic). i-PMMA-2 was prepared by anionic polymerization (28) Bernett, M. K. Macromolecules 1974, 7, 917. (29) Baier, R. E.; Zisman, W. A. Macromolecules 1970, 3, 70. (30) Vergelati, C.; Perwuelz, A.; Vovelle, L.; Romero, M. A.; Holl, Y. Polymer 1994, 35, 262. (31) Tretinnikov, O. N.; Zhbankov, R. G. J. Mater. Sci. Lett. 1991, 10, 1032.
Langmuir, Vol. 13, No. 11, 1997 2989 Table 1. Characteristics of the PMMA Samples tacticity (%) polymer
Mn × 10-3
Mw/Mn
mm
mr
rr
s-PMMA-1 s-PMMA-2 s-PMMA-3 i-PMMA-1 i-PMMA-2
234 62 880 160 47
2.30 3.37 1.03 2.05 4.24
3 4 1 74 91
32 21 6 15 6
65 75 93 11 3
initiated by phenylmagnesium bromide in toluene at 0 °C. The polymers were purified by precipitation prior to use. Each sample of the polymers was first dissolved in benzene at a concentration of 2% (w/v). The films studied were prepared in a dust-free chamber by casting the solutions on chromic acid cleaned glass substrates (Pyrex glass Petry dishes). The residual solvent was removed from the films formed at room temperature by drying films at 70 °C for about 20 h in a clean oven. They then were separated from the substrate by doubly distilled water and stored over desiccant under reduced pressure until required. Static contact angles of PMMA films against water were measured by the sessile drop method using a telescopic goniometer (M2010A-6II type, Erma Inc., Tokyo, Japan) at 25 °C and at about 65% relative humidity. These were determined 20-40 s after application of the drop. The volume of the water drops used was always 3 µL. In all cases, no appreciable changes in contact angle were observed in 2 min, suggesting that underwater surface restructuration did not play a role in the time frame of the measurements described here.32 All reported values are average of at least eight measurements taken at different locations of the film surface and have a typical error of the mean of (1°.
Results and Discussion Contact Angles. The objective of this work is to test the surface activity of several PMMAs of various tacticity. It is expected that for the polymer film cast on a highly polar glass substrate the selective surface accumulation of functional groups will result in an observable difference between the water contact angles on the air-side surface and the glass-side surface of the film, and the higher the surface activity of the polymer, the larger the difference in the contact angles. Within this approach, it is important to dismiss other factors responsible for contact angle changes, such as surface roughness33,34 and surface crystallinity.35,36 All the polymer films were optically smooth and showed no differences in the surface morphology examined on one and then on the other side of the films by a light microscope. The s-PMMA-1, i-PMMA-1, and i-PMMA-2 films appeared totally amorphous in wideangle X-ray diffraction measurements.37 The bulk crystallinity of the s-PMMA-2 and s-PMMA-3 films was estimated to be 9 and 31%, respectively. No substantial differences in crystallinity between the air-side surface and the glass-side surface were detected when the s-PMMA-2 and s-PMMA-3 samples were examined by (32) Restructuration of solid polymer surfaces may occur on extended contact with water, resulting in the time dependence of advancing and receding contact angles (refs 5, 22, and 30). Advancing and receding contact angles on the surfaces of stereoregular PMMAs, as well as their time-dependent dynamics, will be examined in a future report. (33) Johnson, R. E., Jr.; Dettre, R. H. Adv. Chem. Ser. 1964, No. 43, 112. (34) Johnson, R. E., Jr.; Dettre, R. H. J. Phys. Chem. 1964, 68, 1744. (35) Schonhorn, H.; Ryan, F. W. J. Phys. Chem. 1966, 70, 3811. (36) Schonhorn, H. Macromolecules 1968, 1, 145. (37) Wide-angle X-ray diffraction was measured by means of a Rigaku automated diffractometer using Cu KR radiation, with a scan speed of 4°/min. (38) The FTIR-ATR spectra were recorded on a Nicolet 7199 FTIR spectrometer equipped with a mercury-cadmium-telluride (MCT) detector at a resolution of 2 cm-1 using a Wilks Model 50 ATR attachment and a Ge internal reflection element with an angle of incidence of 60°. The relative intensity of of the “crystalline” band at 860 cm-1 to the “amorphous” band at 841 cm-1 was used as an index of the crystallineamorphous content at the film surfaces.
2990 Langmuir, Vol. 13, No. 11, 1997
Tretinnikov
Figure 1. Water contact angles on PMMA films as a function of the percentage of syndiotactic triads in the polymer chain. Table 2. Contact Angles for Water on the Air-Side (θair) and Glass-Side Surfaces (θglass) of PMMA Films and the Difference between θair and θglass (∆θ) polymer
θair
θglass
∆θ
s-PMMA-1 s-PMMA-2 s-PMMA-3 i-PMMA-1 i-PMMA-2
69.0 72.0 79.5 67.0 66.5
62.0 60.0 55.0 65.5 65.0
7.0 12.0 24.5 1.5 1.5
FTIR-ATR,38 suggesting that these polymers are incapable of forming trans-crystalline states or their involvement is precluded under the film formation conditions employed. The water contact angles (θ) on PMMA films are listed in Table 2. The angles quoted are the mean angles rounded off to the nearest 0.5°. θair and θglass refer to the contact angles on the surfaces contacted with air and glass substrate during the film formation, respectively. As is seen, the syndiotactic PMMAs showed substantially greater contact angles at the air-side surfaces as compared to the glass-side surfaces. The difference between θair and θglass, ∆θ, increased rapidly with triad syndiotacticity (rr) from 7° for s-PMMA-1 (rr 65%) to 24.5° for highly syndiotactic s-PMMA-3 (rr 93%). In stark contrast to the syndiotactic PMMAs, the value of ∆θ observed with the isotactic surfaces was only 1.5°, i.e., just outside experimental error. In Figure 1, the measured contact angles, θair and θglass, are plotted versus percentage of syndiotactic triads, rr. Two interesting observations can be made when considering these contact angle data. First, θair decreases and θglass increases with decrease in rr, and the shape of the two curves seems to point to an overall asymptotic approach to θ ) 66.0 ( 0.5°. Secondly, tacticity has a large effect on wetting when rr g 60% but becomes less significant when rr e 60%. Thus, for example, when rr changes from 93 to 3%, θair is lowered from 79.5 to 66.5° with the larger decrease of 10.5° when 65 e rr e 93% and the smaller decrease of 2.5° when 3 e rr e 65%. As stated above, all the film surfaces were free from any effects of roughness or trans-crystallinity that might be responsible for the contact angle changes. It is also unlikely that the differences in wettability are due to surface contamination. All five polymer samples were subjected to identical experiments, so contamination might introduce a systematic error in the measured contact angle, but it would not result in the differences between the samples. An accepted model for explaining the effect of
the contacting medium on the water wettability of solvent cast PMMA films is the selective surface accumulation of polar (COOCH3) or nonpolar (CH2, R-CH3) functional groups.16,18 During the film formation, ester groups adsorb at the glass substrate and as the solvent evaporates the solid surface enriched in the polar groups is formed. On the contrary, with the air phase the energetics favor a predominance of methylene and methyl groups in the surface with consequent increase in the water contact angle. As judged by the magnitude of ∆θ (Table 2), the selectivity in the surface exposure of functional groups is substantially high in the syndiotactic films but becomes negligibly low in the isotactic ones. Furthermore, as is evident from a fast decrease (increase) in θair (θglass) in the range 100-60% rr (Figure 1), the orienting capability of the polymer chain segments goes down rapidly with decrease in the syndiotactic content. We conclude from this behavior that only syndiotactic sequences in the polymer chain orient at the interface, exposing energetically preferable functionality, whereas isotactic sequences adopt isotropic surface configuration possessing nonselective, random distribution of functional groups. The fast change of the contact angle in the range 100-60% rr appears to be followed by a rather slow decay at the syndiotactic content below 60%. This suggests that the surface activity of the PMMA macromolecule is controlled not only by the degree of syndiotacticity but also by the length of the syndiotactic sequences in the polymer chain. If our conclusion on the relationship between the syndiotactic content and the surface activity of PMMA macromolecule is valid, a decrease in the syndiotactic content must eventually result in a fully isotropic PMMA surface possessing nonselective, random distribution of functional groups, so that θair ) θglass ) θid, where θid denotes the contact angle on such an “ideal” PMMA surface. Indeed, both θair and θglass revealed an asymptotic approach to a limiting value of 66° at rr ) 0% (Figure 1). Then, it is logical to assign the asymptotic contact angle to the “ideal” PMMA surface, i.e., θid ) 66°. Obviously, the wettability of the “ideal” PMMA surface must be tacticity independent because of invariant chemical structure. Accordingly, the water contact angle of 66° should be considered as an intrinsic characteristic of the PMMA polymers. The surfaces of both isotactic PMMAs seem to approximate closely to this isotropic surface configuration, as corresponding θair and θglass showed negligibly low deviations from 66° (Figure 1). As to the s-PMMA films, the substantial positive deviation of θair from 66° reflects an excess of the nonpolar methylene and R-methyl groups at the air-side surface, whereas the negative shift in θglass is indicative of relative dominance of the polar ester groups at the glass-side surface. Functional Group Composition. At this point it is interesting to estimate quantitatively the fraction of polar and nonpolar groups at the surfaces of the PMMA films. This can be done by using the Cassie equation39
cos θ ) f1 cos θ1 + f2 cos θ2
(f1 + f2 ) 1)
(1)
or the Israelachvili-Gee equation40
(1 + cos θ)2 ) f1 (1 + cos θ1)2 + f2(1 + cos θ2)2 (2) (f1 + f2 ) 1) which give the equilibrium contact angle θ of a liquid on a heterogeneous surface composed of a fraction f1 of chemical groups type 1 and f2 of groups type 2, where θ1 and θ2 are the equilibrium contact angles on the pure (39) Cassie, A. B. D. Discuss. Faraday Soc. 1952, 75, 5041.
Selective Accumulation of Functional Groups
homogeneous surfaces of 1 and 2, respectively. The Cassie approach assumes that the surface is composed of distinct macroscopic patches or domains of either type 1 or 2. The Israelachvili-Gee equation replaces the Cassie equation whenever the chemical heterogeneity approaches atomic or molecular dimensions. Despite the simple form of eqs 1 and 2, little is known about the actual quantitative applicability of either equation to real systems.40,41 The two wetting treatments do not account for the dependence of the wetting interaction potentials on the nature of the functional group involved. Furthermore, appreciable contact angle hysteresis observed in all cases on complex, heterogeneous surfaces indicates that the contact angle is not measuring a system at thermodynamic equilibrium. Consequently, the actual f1 and f2 values derived from experiment do not necessarily represent true surface coverages. Nevertheless, the models have been shown to be useful in providing a semiquantitative appreciation of the structural and chemical characteristics of complex, multifunctional surfaces.25,42-45 In the case of the PMMA surface, eqs 1 and 2 can be written
cos θ ) fCH2,CH3 cos θCH2,CH3 + fCOOCH3 cos θCOOCH3 (3) and
(1 + cos θ)2 ) fCH2,CH3(1 + cos θCH2,CH3)2 + fCOOCH3(1 + cos θCOOCH3)2 (4) (fCH2,CH3 + fCOOCH3 ) 1) Generally, a surface composed of CH3 groups has a θ of 110°, and one of CH2 groups, θ ) 94°.46,47 We estimate from these values that θCH2,CH3 ) 101°. Using θ ) 66° and fCH2,CH3 ) fCOOCH3 ) 0.5 for the “ideal” PMMA surface and θCH2,CH3 ) 101°, we find θCOOCH3 ) 0 and 35° from eqs 3 and 4, respectively. Quantitative discrepancies between the two wetting treatments are expected to be the greatest in the case of highly specific, short-range interactions (e.g., hydrogen bonding) at the liquid/solid interface. Since COOCH3 groups are capable of strong hydrogen bonding interactions with water molecules, the difference between θCOOCH3 values derived from eqs 3 and 4 is not surprising. The Israelachvili-Gee approach might seem preferable, as the literature suggest a θCOOCH3 value in the range 3040°.6 However, in the absence of independent information on the chemical heterogeneity scale of the homopolymer surfaces, the applicability of the Cassie equation cannot be denied. Therefore, we used both approaches to calculate the fractional coverage of the functional groups on the surfaces of PMMA films. The results of these calculations are presented in Table 3. Both equations give similar estimates for fractional coverages. The values of fCH2,CH3 and fCOOCH3 show that the surfaces of isotactic films consist of comparable fractions of nonpolar and polar groups, both at the air-facing side and at the glass-facing side of the films. In other words, the isotactic surfaces show no selectivity in the surface exposure of functional groups. (40) Israelachvili, J. N.; Gee, M. L. Langmuir 1989, 5, 288. (41) Johnson, R. E.; Dettre, R. H. Wettability; Berg, J. S.; Ed.; Marcel Dekker Inc.: New York, 1993. (42) Holmes-Farley, S. R.; Reamey, R. H.; Nuzzo, R.; McCarthy, T. J.; Whitesides, G. M. Langmuir 1987, 3, 799. (43) Atre, S. V.; Liedberg, B.; Allara, D. L. Langmuir 1995, 11, 3882. (44) Schaub, T. F.; Kellog, G. J.; Mayes, A. M.; Kulasekere, R.; Ankner, J. F.; Kaiser, H. Macromolecules 1996, 29, 3982. (45) Peach, S.; Polak, R. D.; Frank, C. Langmuir 1996, 12, 6053. (46) Zisman, W. A. Adv. Chem. Ser. 1964, No. 43, 1. (47) Shafrin, E. G.; Zisman, W. A. Adv. Chem. Ser. 1964, No. 43, 145.
Langmuir, Vol. 13, No. 11, 1997 2991 Table 3. CH2, CH3, and COOCH3 Surface Coverage Derived from Contact Angle Results for PMMA Films air side
glass side
polymer
eq
fCH2,CH3
fCOOCH3
fCH2,CH3
fCOOCH3
s-PMMA-1
3 4 3 4 3 4 3 4 3 4
0.54 0.55 0.58 0.60 0.68 0.72 0.51 0.52 0.50 0.51
0.46 0.45 0.42 0.40 0.32 0.28 0.49 0.48 0.50 0.49
0.44 0.43 0.42 0.40 0.35 0.31 0.49 0.49 0.48 0.48
0.56 0.57 0.58 0.60 0.65 0.69 0.51 0.51 0.52 0.52
s-PMMA-2 s-PMMA-3 i-PMMA-1 i-PMMA-2
On the contrary, the syndiotactic surfaces are found to be preferentially covered with nonpolar groups at the air side and with polar ones at the glass side of the films. The surface enrichment increased with triad syndiotacticity and reached the highest observed value for the s-PMMA-3 film, with 68-72% of the surface area consisting of nonpolar (CH2, CH3) groups on the air-side surface and 65-69% covered with polar (COOCH3) groups on the glassside surface of the film.48 Structural Interpretation. The surface composition of the stereoregular poly(methyl methacrylate)s was greatly affected by the tacticity of the polymer backbone as described above. Since the monomer unit is the same in all the polymers under investigation, conformational explanations can be used to interpret the relation between the surface characteristics and the stereochemical configuration of these materials. The molecular structure of stereoregular poly(methyl methacrylate)s has been intensively studied over last 30 years and a large amount of information concerning the conformations of PMMA macromolecules (in the solid state and in solution) is currently available from the results of conformational energy calculations,49 IR spectroscopy,50 X-ray diffraction,51 and NMR spectroscopy.52 For sPMMA, theoretical and experimental results indicate that the macromolecule may have a planar zigzag and glideplane structure formed by the nearly trans-trans (tt) and trans-gauche (tg) backbone conformation, respectively. The tt conformation is energetically the most favorable and predominates both in crystalline and amorphous s-PMMA; however, amorphous s-PMMA also contains some fraction of tg conformational sequences. In solution, the relative content of tt and tg conformers is dependent (48) One might have expected the difference between advancing and receding contact angles (i.e., contact angle hysteresis) to be more sensitive to the variations in the chemical heterogeneity, as compared to the “classic”, static contact angle. However, it must be realized that other factors, such as surface roughness, swelling, penetration of wetting liquid, and surface reconstruction, also contribute to the contact angle hysteresis. Because so many factors are involved, general quantitative models of the phenomenon have not been developed so far (ref 41). Consequently, wetting hysteresis cannot, at present, be used to reliably diagnose the surface chemical compositions (ref 45). (49) (a) Grigoreva, F. P.; Birshtein, T. M.; Gotlib, Y. Y. Polum. Sci. USSR 1967, 9, 650. (b) Tadokoro, H.; Chatani, Y.; Kusanagi, H.; Yokoyama, M. Macromolecules 1970, 3, 441. (c) Sundararajan, P. R.; Flory, P. J. J. Am. Chem. Soc. 1974, 96, 5025. (d) Vacatello, M.; Flory, P. J. Macromolecules 1986, 19, 405. (e) Sundararajan, P. R. Macromolecules 1986, 19, 415. (50) (a) Havriliak, S.; Roman, N. Polymer 1966, 7, 387. (b) Schneider, B.; Stokr, J.; Dirlikov, S.; Mihailov, M. Macromolecules 1971, 4, 715. (c) O’Reilly, J. M.; Mosher, R. A. Macromolecules 1981, 14, 602. (d) Dybal, J.; Stokr, J.; Schneider, B. Polymer 1983, 24, 971. (51) (a) Liquori, A. M., Anzuino, G., Coiro, V. M.; D’Alagni, M.; De Santis, P.; Savino, M. Nature (London) 1965, 206, 358. (b) Kusanagi, H.; Tadokoro, H.; Chatani, Y. Macromolecules 1976, 9, 531. (c) Lovell, R.; Windle, A. H. Polymer 1981, 22, 175. (d) Kusanagi, H; Chatani, Y.; Tadokoro, H. Polymer 1994, 35, 2028 and references cited therein. (52) (a) Amiya, S.; Ando, I.; Watanabe, S.; Chujo, R. Polym. J. 1974, 6, 194. (b) Inoue, Y.; Konno, T. Makromol. Chem. 1978, 179, 1311. (c) Spevacek, J.; Schneider, B.; Straka, J. Macromolecules 1990, 23, 3042.
2992 Langmuir, Vol. 13, No. 11, 1997
Figure 2. 10/1 (A) and 5/1 helix (B) models of isotactic PMMA viewed along the helix axis.
on the solvent species. On the other hand, according to theoretical and experimental studies, the most probable conformational models for i-PMMA are (10/1) and (5/1) helices. The (10/1) helix has the lowest potential energy and forms the crystalline structure of i-PMMA. The (5/1) conformational sequences are present in a small amount in amorphous i-PMMA and in solution of the polymer. The conformational forms of stereoregular PMMAs can be represented schematically by the molecular models shown in Figures 2 and 3. Figure 2 contains the (10/1) and (5/1) helix models of i-PMMA viewed along the helix axis.51d It can be seen that in the two models the polar and nonpolar parts of the chain radiate from the helix, forming an array of spatially mixed methylene, methyl, and ester groups. Obviously, this spatial arrangement of the polymer chain does not allow for selective exposure of polar or nonpolar groups at the polymer surface. Thus, one can understand the wetting results for i-PMMA films which showed that there was no effect of the polarity of contacting medium on the surface chemical composition and that the wettability of the films represented an average for CH2, CH3, and COOCH3 groups. Figure 3 contains the schematic drawing of s-PMMA in the tt and tg backbone conformation, showing crosssectional and side views. It should be apparent from this drawing that the tt conformation does not allow for selective exposure of the functional groups. The R-methyl groups are sandwiched between the ester groups along the chain axis. Consequently, the macromolecule may expose both the nonpolar and polar functionality but not
Tretinnikov
Figure 3. Planar zigzag (A) and glide-plane (B) structures of syndiotactic PMMA formed by the trans-trans and transgauche backbone conformation, respectively. Open and filled circles represent oxygen and carbon atoms, respectively. Note the amphiphilic character of the glide-plane structure.
one or the other. In contrast, the tg conformation has a clear amphiphilic character: the polar and nonpolar groups lie on the different sides of the plane passing through the chain axis. This allows the tg sequences of the polymer chain by a simple rotation to orient themselves at the interface so that, depending on the polarity of adjacent phase, only ester groups or only R-methyl and methylene groups are exposed. Thus, the model analysis reveals that the tg conformational sequences of s-PMMA are responsible for the selective exposure of functional groups at the surfaces of s-PMMA films. At this point, it is to be noted again that, in the bulk, s-PMMA shows a very strong preference for the tt backbone conformation. As determined from conformational analysis,49d the fraction of the chain units occurring in the tg conformation is only about 0.2. Obviously, the surface concentration of tg conformers must substantially exceed this bulk value in order to account for the observed high surface activity of the polymer. Indeed, the increased content of tg conformers in the surface regions of s-PMMA films has been observed in our previous work using attenuated total reflection infrared spectroscopy.18,31,53 LA9700275 (53) Tretinnikov, O. N.; Nakao, K.; Ohta, K. Polym. Prepr. Jpn. 1994, 43, 1570.
