Behavior of Syndiotactic Poly (methyl methacrylate) Monolayers at the

J. Phys. Chem. C 2009, 113, 17455–17463

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Behavior of Syndiotactic Poly(methyl methacrylate) Monolayers at the Air/Water Interface: Influence of Temperature and Molecular Weight on the Surface Pressure-Area Isotherms and Brewster Angle Microscopy Images Jose´ Min˜ones, Jr.,*,† Mercedes Min˜ones Conde,‡ Eva Yebra-Pimentel,‡ and Jose´ M. Trillo† Department of Physical Chemistry, Faculty of Pharmacy, UniVersity of Santiago de Compostela, Campus Sur, 15706-Santiago de Compostela, Spain, and Department of Optometry, School of Optics and Optometry, UniVersity of Santiago de Compostela, Campus Sur, 15706-Santiago de Compostela, Spain ReceiVed: May 15, 2009; ReVised Manuscript ReceiVed: July 15, 2009

On aqueous subphases of pH 6, the behavior of syndiotactic poly(methyl methacrylate) (synd-PMMA) (average molecular weight, Mw )120 000 g/mol) monolayers at the air/water interface was investigated in the range of temperatures between 15 and 50 °C. The monolayer characteristics of synd-PMMA stereoisomer were studied and compared in terms of surface pressure-area per residue (π-A) isotherms, surface compressional modulus-surface pressure (Cs-1- π) curves, hysteresis phenomena, film thickness, and the phase images observed from Brewster angle microscopy (BAM). The results show that synd-PMMA (120 000) monolayer exhibits a phase transition LE-L′E (from a liquid-expanded to another liquid-expanded state) at surface pressures of ca. 15-17 mN/m when the temperature is raised from 25 to 50 °C, which is attributed to the formation of reversible loops and tails in the monolayer. On the other hand, the study of the effect of the molecular weight on the PMMA monolayer behavior showed that the polymer of Mw ) 15 000 g/mol exhibits a limiting area of 19.7 Å2 per monomer repeating unity at 30 °C, which is close to the value for the trans conformation of PMMA at the air-water interface. Nevertheless, the limiting area of PMMA (120 000) was lower (14.1 Å2/monomer), suggesting that not all monomers are located at the interface. The morphology and thickness of monolayers confirm at microscopic level the structural characteristics deduced from the π-A isotherms. 1. Introduction Poly(methyl methacrylate) (PMMA) is a hard, resistant, transparent, acrylic material, with excellent optical characteristics and a high refractive index, which makes it suitable for the manufacture of rigid contact lenses. MMA monomer is also a main constituent of gas-permeable rigid contact lenses and of nonionic hydrophilic soft lenses of low and high hydration. To our knowledge, no report on molecular interactions between a contact lens and tear film has been presented, and therefore, given its importance, we should carry out a thorough study of possible interactions between these components, thereby using the Langmuir film balance technique. With this purpose, mixed monolayers of PMMA (as a major component of contact lenses) and sterols, lipids, phospholipids or proteins (as major constituents of the tear film) will be prepared to reproduce in vitro the conditions of the optical lens in contact with the eye. With this simple model, the analysis of surface pressure-area (π-A) isotherms obtained by compression of the mixed monolayers will allows us to determine the behavior (interactions) of the mixed system components according to its composition. To achieve this goal, we have to know a priori the behavior of PMMA monolayers at the air/water interface. Therefore, in this paper we study the influence of the spreading conditions and the influence of the polymer molecular weight on the characteristics of these monolayers; in subsequent publications * To whom correspondence should be addressed. Phone: +34 981 563100, ext. 14917. Fax: +34 981 594912. E-mail: [email protected]. † School of Optics and Optometry. ‡ Faculty of Pharmacy.

we will deal with showing PMMA interactions with tear components. Monolayers of PMMA have been reviewed by Crisp1 in 1946. Since then, much effort has been devoted to the investigation of the monolayer behavior of other kinds of poly (methacrylates), with the ester side chains ranging up from methyl to octadecyl groups,2,3 including aromatic4,5 and branched6,7 substituents. The stereoregularity of polymers has also been subject of much attention in the literature. Beredjick and Ries8-10 reported the existence of significant differences in the behavior of isotactic, atactic, and sindiotactic PMMA monolayers, showing that iso-PMMA isotherms exhibit three segments as surface pressure is increased, while synd-PMMA and atac-PMMA isotherms exhibit only two segments that actually coincided with each other in the region of surface low pressure. Jaffe´ et al.11 showed that the behavior of such monolayers could serve as a criterion for measuring the polymer tacticity. Later, Henderson and Richards12 published isotherms of isoPMMA and Brinkhuis and Schouten13 extensively discussed the monolayer behavior of this material. These authors had also investigated the effect of the tacticity on the behavior of poly(methacrylates) with short ester side chains (methyl, ethyl, and isobuthyl).14 In all cases the iso-polymers form expanded monolayers, whereas the synd-materials yield to condensed films. These observations are attributed to differences in the lateral cohesive interactions of the segments in the monolayer, which, again, can be correlated with the difference in spatial orientation of the ester groups with respect to the backbone and the air-water interface. Recently, the monolayer characteristics

10.1021/jp904848c CCC: $40.75  2009 American Chemical Society Published on Web 09/14/2009

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of the three PMMA stereoisomers were studied by Hsu et al.15 and the results obtained showed that iso-PMMA monolayer has a more extended and compressible property and exhibit pronounced hysteresis and relaxation characteristics among the three tactic polymers. On the other hand, synd-PMMA exhibits the most condensed monolayer with the opposite properties with regard to iso-PMMA. The BAM images show that PMMA molecules are not well extended on the subphase, and thus, aggregative phases were observed even in a state corresponding to the gas phase. The aggregative structures are especially significant on synd-PMMA monolayer. In the literature, the results on the value of collapse pressure of PMMA monolayer are very contradictory, ranging from 15 mN/m in some cases16,17 to 17-20 mN/m in others.18,19 Some authors reported considerably higher values, about 26,8 45,20 or even 54 mN/m.21 However, in many of these works, the reported value as “collapse pressure” is actually the pressure corresponding to a phase transition of the monolayer from a liquid state to another, shown in the π-A isotherm as a plateau at a surface pressure of about 16 mN/m, which is wrongly regarded as the beginning of the monolayer collapse. Moreover, the fact that in many cases not enough data were available on the stereoregularity of polymers (isotactic, sindiotactic, or atactic), their molecular weight, the temperature at which experiments were performed, the compression speed of the films or substrate conditions, etc., explains the diversity of the results obtained, since all these factors affect significantly to the collapse pressure of these films. The discrepancies noted in the literature, regarding to the collapse pressure values, are observed again for the limiting area occupied by a residue of the polymer (area extrapolated to zero pressure corresponding to the initial linear portion of the pressure-area isotherm). Thus, while Crisp1 provides the value of 14 Å2, Kawaguchi et al.22 point to 15.8 Å2 for the limiting area of the monomer, and Rogers and Mandelkern23 obtain 16.4 Å2. On the other hand, Schick24 assigns a slightly higher value, 17.6 Å2, slightly above the 16.4 Å2 recorded by Wu and Huntsberger.6 Values ranging between 17 and 18.5 Å2 also appear in the literature.18,21,25,26 Consequently, as a result of these discrepancies, this work is aimed at investigating the behavior of the PMMA monolayers at the air/water interface, studying the influence of different factors on its spreading (mainly the subphase temperature and the polymer molecular weight) and interpreting this behavior taking into consideration the analysis of π-A isotherms and the compressibility, hysteresis, and thickness of the monolayers, as well as the BAM images obtained. 2. Experimental Details Syndiotactic PMMAs of average Mw molecular weights 15 000 and 120 000 g/mol were purchased from Aldrich (purity 95%) and used without further purification. The polydispersities (Mw/Mn) of these PMMAs were not measured because, according to other authors,1,8,15,27 the molecular weight distribution effect is believed to be minimal on the monolayers behavior. The storage of these materials was made according to the supplier information. Spreading solutions of concentration 0.1-0.3 mg/mL were prepared using chloroform as a solvent. Adequate volumes of these solutions were spread to attain an initial polymer surface concentration of 2.35 × 1014 monomer/cm2, which corresponds to an initial area of 42.5 Å2/ monomer. For these calculations, an average number of 150 monomers for PMMA (15 000) and 1200 for PMMA (120 000) was assumed.

Min˜ones, Jr. et al. Pure water, which was purified by means of a Milli-Q plus water purification system, with a resistivity of 18.2 MΩ · cm was used in all experiments. π-A isotherms were recorded with a KSV (Finland) Langmuir trough (total area ) 850 cm2) placed on an isolated vibration-free table and enclosed in a glass chamber to avoid contaminants from the air. Regulation of the trough temperature was controlled by circulating constant temperature water from a Haake thermostat through the tubes attached to the aluminum-based plate of the trough. The subphase temperature was measured by a thermocouple located just below the air/water interface. Two barriers confining the monolayer at the interface were driven symmetrically at a constant speed of 36 cm2/min (1.8 Å2/monomer · min) during the film compression. This is the highest value for which isotherms have been found to be reproducible in preliminary experiments. Surface pressure was measured with the accuracy of (0.1 mN/m using a Wilhelmy plate made from platinum foil as a pressure sensor. After spreading, monolayers were left for 10 min to ensure the solvent evaporation. To observe the morphological characteristics of the monolayer at the air/water interface by Brewster angle microscopy (BAM), a large trough constructed by Nima Technology Ltd., UK (model 601 BAM) was used. The trough has a working area of 500 cm2 (71.5 cm long and 7 cm wide), which is large enough to mount the BAM directly on it. BAM images and ellipsometric measurements were performed with BAM 2 Plus (NFT, Go¨ttingen, Germany) equipped with a 30 mW laser emitting p-polarized light with a wavelength of 532 nm which was reflected off the air/water interface at ∼53.1° (incident Brewster angle). Under such condition, the reflectivity of the beam was almost zero on the clean water surface. The reflected beam passes through a focal lens into a analyzer at a known angle of incident polarization and finally to a CCD camera. To measure the relative thickness of the film, a camera calibration was necessary previously in order to determine the relationship between the gray level (GL) (intensity unit) and the relative reflectivity (I), according to the procedure described by Rodrı´guez Patino et al.28 The light intensity at each point in the BAM image depends on the local thickness and film optical properties. These parameters can be measured by determining the light intensity at the camera and analyzing the polarization state of the reflected light by the method based on Fresnel equations. At the Brewster angle:

I ) |Rp2 |)C × d2 where I is the relative reflectivity (defined as the ratio of the reflected intensity (Ir) and the incident intensity (I0), I ) Ir/I0), Rp is the p component of the light, C is a constant, and d is the film thickness. The lateral resolution of the microscope was 2 µm, the shutter speed used was 1/50 s and the images were digitalized and processed to optimize image quality; those shown below correspond to 768 × 572 pixels. 3. Results 3.1. π-A Isotherms of synd-PMMA (M ) 120 000 g/mol) Monolayers. 3.1.1. Effect of the Compression Rate and Polymer Surface Concentration. Figure 1 shows the π-A isotherms obtained when PMMA (120 000) was spread on the water contained in the trough of the KSV surface balance at 30 °C and compressed at different rates ranging from 6 (0.3 Å2/ monomer · min) to 90 cm2/min (4.5 Å2/monomer · min). Under

Behavior of PMMA Monolayers at the Air/Water Interface

Figure 1. π-A isotherms of synd-PMMA (120 000) monolayers spread on water (pH 6) Ta ) 30 °C) at different rates of compression: -g- 6 cm2/min; -(- 18 cm2/min; -2- 36 cm2/min; -O- 90 cm2/ min. (Inset) Influence of polymer surface concentration on π-A isotherms: -b- 2.35 × 1014 monomer/cm2; -0- 1.75 × 1014 monomer/cm2; -g- 1.2 × 1014 monomer/cm2; -\-1.0 × 1014 monomer/cm2; -1- 0.5 × 1014 monomer/cm2.

these conditions, the compression speed has hardly influence on the monolayer behavior. However, the higher is the surface concentration of PMMA in the air/water interface, the lower is their extension (inset of Figure 1), although the values of the surface pressures corresponding to the discontinuities (plateaux) observed in the π-A isotherms remain practically unchanged. This behavior evidence a poor extension of the polymer when the surface concentration is elevated, indicating that not all the monomer units in PMMA (120 000) take up a site at the interface. However, the monolayer structural characteristics seem to be unaltered. 3.1.2. Effect of the Subphase Temperature. The change in subphase temperature involves two effects on the characteristics of synd-PMMA (120 000) monolayers, (Figure 2): first, the film compressibility increases with temperature, so the isotherms obtained at low temperatures exhibit no plateau, suggesting the existence of a condensed monolayer, whereas at temperatures above 25 °C isotherms show the presence of a plateau at a surface pressure of about 15-17 mN/m, similar to that obtained by Brinkhuis and Schouten14 for the syndiotactic polymer with a molecular weight of 46 000 and by Hsu et al.15 for syndPMMA (M ) 100 000). This plateau corresponds to a phase transition of the monolayer, as will be discussed further on. Second, the monolayer limiting area corresponding to the region of low surface pressures (A0), increases as the substrate temperature rises from about 12.6 Å2/monomer at a temperature of 15 °C, up to 22.7 Å2/ monomer at 50 °C. In contrast, the limiting area for the region of high surface pressures (A′0) decreases as the temperature rises. The physical state of the monolayers and their collapse surface pressure values can be determined in a more precise way (compared to the π-A isotherms) with the plot of the compressional modulus (Cs-1), as a function of surface pressure (π) (Figure 3). This parameter, defined by Davies and Rideal29 as the inverse of the two-dimensional compressibility, is given by the following relation:

Cs-1 ) -A

∂π ∂A

(1)

J. Phys. Chem. C, Vol. 113, No. 40, 2009 17457 Cs-1 values were obtained by numerical calculation of the first derivative from the isotherm data points according to eq 1. Cs-1-π plots show a sharp minimum which corresponds to the plateau observed in the π-A isotherms. However, this minimum is only evidenced (at a surface pressure of about 15 mN/m) in the corresponding curves at 25 °C and higher temperatures (Figure 3A). Below 25 °C, Cs-1-π curves exhibit, instead of a clear minimum, a region where the compressional modulus remains constant throughout a wide range of surface pressures (Figure. 3B). Therefore, in these terms, we cannot consider the existence of a phase transition, confirming these results the absence of the plateau in the corresponding compression curves. Table 1 shows the results for the most significant parameters of synd-PMMA monolayers at the temperatures at which the study was performed: limiting areas at low (A0) and high (A′0) surface pressures; compressional modulus (Cs-1) at different pressures; surface pressure values corresponding to the phase transition (πtrans) and at the monolayer collapse (πcoll). At pressures below the phase transition (π 0 and ∆A< 0, ∆H and ∆S negative values were obtained for the transition. It is, therefore, an exothermic transition, in which the molecular order is increased. However, the ∆H and ∆S values are so low (around 75 J/mol and 0.24 J/K · mol, respectively), that the transition can be seen as an “apparent” phase transition, according to the terminology used by Horn and Gershfeld.33 The existence of a plateau in the π-A isotherms of polymer and polypeptide monolayers has been explained in different ways. Malcolm34-36 has suggested that the plateau observed in the isotherms of methyl and ethyl polyglutamate monolayers arises as a result of the formation of

SCHEME 2: Schematic Representation of the Loops Formation along the LE-L′E Phase Transition

a bilayer, a suggestion which was supported by Takenaka et al.37 and Takeda et al.,38,39 who showed that the films transferred to germanium plates at surface pressures above the plateau were almost twice as thick as those transferred to lower surface pressures. However, our results showed that the monolayer thickness was not doubled during the transition region (Figure 10A, obtained on water) as the formation of a bilayer would be expected, on the contrary, this thickness remains constant when the substrate consists of 3 M NaCl (Figure 10B). A more plausible explanation may be the following. The plateau could be the result of the strong folding of the polymer segments, which are directed outward from the interface, forming loops and tails, and hence reducing the number of contact points (trains) with subphase water (Scheme 2). The formation of these loops, resulting from the monolayer compression, leads to a decrease in the area occupied by the film, but without the corresponding increase in pressure in the π-A isotherm during this process, resulting in the appearance of a constant surface pressure plateau. Once the loops are formed, the monolayer is in a new L′E surface phase, and despite continuing in the same liquid-expanded state as before, it has different physical properties, as Labbauf and Zack22 showed. The existence of monolayer phase transitions whithin the same state, i.e., in two-dimensional solid states40 or two liquidcondensed states,41-45 was described in the literature. However, to our knowledge, no report on LE-L′E phase transitions has been presented. The “bands” seen in the BAM images corresponding to the phase transition (Figures 7 and 9) seem to confirm the formation of these loops in this region. Moreover, the chain folding is much easier as the more expanded is the monolayer, which explains why the compression curve of the polymer with a

Behavior of PMMA Monolayers at the Air/Water Interface molecular weight of 15 000 exhibits a sharper plateau than the 120 000 PMMA, which is more viscous and less compressible at surface pressures below the transition, as revealed by data in Table 2. Furthermore, the fact that this LE-L′E phase transition only occurs at temperatures above 25 °C (Figure 2, Table 1) confirms that the formation of loops is facilitated when the monolayer is expanded, which occurs at high temperatures. Finally, the formation of loops seems to be a reversible process, since the compression curves coincide with the decompression ones, without any hysteresis, when the compressionexpansion cycles are performed before reaching the collapse. (Figure 4A and B). Only when the monolayer is decompressed after reaching the collapse, an irreversible behavior occurs (Figures 5 and 6), as a consequence, partly, of the film-breakage during the collapse, as well as of the fact that when the collapse is reached the folded polymer chains, closely packed, remain in this state during the monolayer expansion. Consequently, the decompression π-A curve is characterized by a remarkable hysteresis relative to the compression one and the recompression curve does not match with the initial one. Acknowledgment. The authors acknowledge the support of the Xunta de Galicia (Spain) through Project No. PGIDIT07PXIB203133PR. References and Notes (1) Crisp, D. J. J. Colloid Sci 1946, 1, 49. ibid 1946, 1, 146. (2) Nakahara, T.; Motomura, K.; Matuura, R. J. Polym. Sci. Part A-2 1966, 4, 649. Bull. Chem. Soc. Jpn. 1967, 40, 495. (3) Duda, G.; Schouten, A. J.; Arndt, T. A.; Lieser, G.; Schmidt, G. F.; Bubeck, C.; Wegner, G. Thin Solid Films 1988, 159, 221. (4) Caminati, G.; Gabrielli, G.; Ferroni, E. Colloid Polym. Sci. 1988, 266, 775. (5) Caminati, G.; Gabrielli, G.; Pugelli, M.; Ferroni, E. Colloid Polym. Sci. 1989, 267, 237. (6) Wu, S.; Huntsberger, J. J. Colloid Interface Sci. 1969, 29, 138. (7) Naito, K. J. Colloid Interface Sci. 1989, 131, 218. (8) Beredjick, N.; Ahlbeck, R. A.; Kwei, T. K.; Ries, H. E. J. Polym. Sci. 1960, 46, 268. (9) Beredjick, N.; Ries, H. E., Jr. J. Polym. Sci. 1962, 62, 864. (10) Hwa, J. C. H.; Ries, H. E., Jr J. Polym. Sci. Polym. Lett 1964, 2, 389. (11) Jaffe´, J.; Berliner, C.; Lambert, M. J. Chim. Phys 1967, 63, 498. (12) Henderson, J. A.; Richards, R. W. Polym. Prepr. 1990, 31 (2), 83. (13) Brinkhuis, R. H. G.; Schouten, A. J. Macromolecules 1991, 24, 1487. ibid 1991, 24, 1487.

J. Phys. Chem. C, Vol. 113, No. 40, 2009 17463 (14) Brinkhuis, R. H. G.; Schouten, A. J. Langmuir. 1992, 8, 2247. (15) Hsu, W. P.; Lee, Y. L.; Liou, S. H. Appl. Surf. Sci. 2006, 252, 4312. (16) Crisp, D. J. Surface Phenomena in Chemistry and Biology; Danielli, J. F., Pankhurst, K. G. A., Riddiford, A. C., Eds.; Pergamon Press: London, 1958; pp 23-54. (17) Gabrielli, G.; Madii, A. J. Colloid Interface Sci. 1978, 64, 19. (18) Peng, J. B.; Barnes, G. T. Langmuir 1991, 7, 1749. ibid 1991, 7, 3090. (19) Kawaguchi, M.; Nagata, K. Langmuir. 1991, 57, 1478. (20) Kim, E.; Cho, S. J.; Suh, H. R.; Shin, D.-M. Thin Solid Films. 1998, 42, 327. (21) Labbauf, A.; Zack, J. R. J. Colloid Interface Sci. 1971, 35, 569. (22) Kawaguchi, M.; Tohyama, M.; Mutoh, Y.; Takahashi, A. Langmuir 1988, 4, 407. (23) Rogers, S. S.; Mandelkern, L. J. Phys. Chem. 1957, 61, 985. (24) Schick, M. J. J. Polym. Sci. 1957, 25, 465. (25) Beredjick, N. New Methods of Polymer Characterization; Interscience: New York, 1964; pp 677-688. (26) Blumstein, A.; Ries, H. E., Jr. J. Polym. Sci. 1965, 3, 927. (27) Gaines, G. L., Jr. Insoluble Monolayers at Liquid-Gas Interface; Prigogine, I., Ed.; Interscience: New York, 1966; pp 269-270. (28) Rodriguz Patino, J. M.; Sa´nchez, C. C.; Rodrı´guez Nin˜o, M. R. Langmuir 1999, 15, 2484. (29) Davies, J. T.; Rideal, E. K. Interfacial Phenomena; Academic Press: New York, 1961; p 265. (30) Gaines, G. L., Jr. Langmuir 1991, 7, 834. (31) Kawaguchi, M.; Tohyama, M.; Takahashi, A. Langmuir 1988, 4, 411. (32) Freeman, R. Kinetics of Nonhomogeneous Process; John Wiley and Sons: New York, 1987. (33) Horn, L. W.; Gersffeld, N. L. Biophys. J. 1977, 18, 302. (34) Malcolm, B. R. Proc. Roy. Soc. London, Ser. A 1968, 305, 363. (35) Malcolm, B. R. Progr. Surface Membrane Sci. 1973, 7, 183. (36) Malcolm, B. R. J. Colloid Interface Sci. 1985, 104, 520. (37) Takenaka, T.; Harada, K.; Matsumoto, M. J. Colloid Inteface Sci. 1980, 73, 569. (38) Takeda, F.; Matsumoto, M.; Takenaka, T.; Fujiyoshi, Y. J. Colloid Inteface Sci. 1981, 84, 220. (39) Takeda, F.; Matsumoto, M.; Takenaka, T.; Fujiyoshi, Y.; Uyeda, N. J. Colloid Interface Sci. 1983, 91, 267. (40) Imura, K. I.; Kato, T. Mol. Cryst. Liq. Cryst. 1997, 294, 87. (41) Overbeck, G. A.; Mo¨bius, D. J. Phys. Chem. 1993, 97, 7999. (42) Deschenaux, R.; Mergert, S.; Zumbrunn, C.; Ketterer, J.; Steiger, R. Langmuir 1997, 13, 2363. (43) Durbin, M. K.; Malik, A. G.; Ghaskadvi, R.; Gog, T.; Dutta, P. J. Chem. Phys. 1997, 106, 8216. (44) Bolan˜os-Garcı´a, V. M.; Mas-Oliva, J.; Ramos, S.; Castillo, R. J. Phys. Chem. B 1999, 103, 6236. (45) Rey Go´mez-Serranillos, I.; Min˜ones, J., Jr.; Dynarowizck’Latka, P.; Min˜ones, J.; Conde, O. Langmuir 2004, 20, 11414.

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