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Stereospecificity in the Adsorption of Tactic PMMA on Silica P. Carriere,† Y. Grohens,*,† J. Spevacek,‡ and J. Schultz† Institut de Chimie des Surfaces et Interfaces-CNRS, 15, rue Jean Starcky, BP 2488, 68057 Mulhouse Cedex, France, and Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, 16206 Prague 6, Czech Republic Received June 28, 1999. In Final Form: January 20, 2000 The mechanisms of the adsorption of stereoregular polymers are only poorly understood. This work is devoted to the study of the influence of PMMA tacticity and conformation on the interfacial behavior of the polymer. 1H NMR usually used to determine the tacticity of PMMA in solution in CDCl3 can also be applied to systems containing the adsorbed polymer. The strong relative lowering of the peak ascribed to isotactic sequences has provided evidence of a stereospecific adsorption of the isotactic segments. These isotactic segments of i-PMMA show the most significant preference for the adsorption, while the isotactic sequences of the a- and s-PMMA also show this preference. The selective adsorption is more pronounced at room temperature than above the conformational transition temperature, namely, 50 °C. The conformation induced by the PMMA stereoregularity may, therefore, be considered as a significant factor in the adsorption process. The driving forces assumed to induce the stereoselective adsorption are discussed.
Introduction Understanding the mechanism of adsorption of polymers is relevant from both fundamental and practical perspectives. Many works, devoted to the study of the adsorption of PMMA,1-4 have taken into account factors such as molecular weight, nature of the solvent, or chemistry of the surface. However, the effect of the stereoregularity of the polymer on its interfacial behavior has not often been considered. Thus, HPLC investigations using stereoregular PMMA as stationary phases5 have indicated that isotactic PMMA changes its conformation depending on the polarity of the eluents. Similarly, the strong expansion of i-PMMA Langmuir-Blodgett monolayers at the air/water interface has been shown,6 as compared to s-PMMA, and an amphiphilic character of the polymer chain that depends on stereoregularity has been suggested.7 Molecular simulation studies have shown, first, that tacticity influences the dynamics of an isolated adsorbed PMMA chain.8 In previous works we have shown that this parameter strongly influences the structure of a thin layer9 as well as the glass transition of the polymer in confined geometry.10 Upstream from these studies, we investigated here by 1H NMR the relation between PMMA tacticity/conformation in solution and the adsorption of the stereoregular polymer on silica. * To whom correspondence should be addressed. † Institut de Chimie des Surfaces et Interfaces. ‡ Academy of Sciences of the Czech Republic. (1) Fowkes, F.; Kaczinski., M. B.; Dwight., D. W. Langmuir, 1991, 7, 2464. (2) Konstadinis, K.; Thakkar, B.; Chakraborty, A.; Potts, L. W.; Tannenbaum, R.; Tirrell, M.; Evans, J. F. Langmuir 1992, 8, 1307. (3) Fontana, B. J.; Thomas, J. R. J. Phys. Chem. 1961, 65, 480. (4) Abel, M. L.; Camalet, J. L.; Chehimi, M. M.;, Watts, J. F.; Zhdan, P. A. Synth. Met. 1996, 81, 23. (5) Okamoto, Y.; Yanagida, M.; Hatada, K. Polym. J. 1989, 21, 795. (6) Shaffer, J. S.; Chakraborty, A. K. Macromolecules 1993, 26, 1120. (7) Tretinnikov, O. N.; Ohta, K. Langmuir 1998, 14, 915. (8) Brinkhuis, R. H. G.; Schouten, A. J. Macromolecules 1991, 24, 1487 and 1496. (9) Grohens, Y.; Brogly, M.; Labbe, C.; Schultz, J. Polymer 1997, 38 (24), 5913. (10) Grohens, Y.; Brogly, M.; Labbe, C.; David, M. O.; Schultz, J. Langmuir 1998, 14 (11), 2929-2932.
Experimental Section The stereoregular PMMA (Mn ) 30 000 g/mol, polydispersity < 1.1) are purchased from Polymer Source in Canada. The tacticity of isotactic, syndiotactic, and atactic PMMA henceforward called i-, s-, and a-PMMA, respectively, is detailed in Table 1. mm, mr, and rr triads refers to isotactic, heterotactic, and syndiotactic sequences, respectively. Pyrogenic silica is amorphous, nonporous Aerosil 130 and purchased from Degussa in Germany. The average particle size is 16 nm, the specific surface determined by BET is 105 m2/g and the average density of silanol groups at the surface is 4 OH/nm2.11 Bruker AC250 NMR spectrometer operating at 250 MHz was used in 1H NMR measurements. The 70 °C proton pulse length was typically 3 µs, the recycle delay was set to 6 s, and 32 scans were acquired. The tubes were initially filled with a 3 g/L PMMA solution in CDCl3, the reference spectra were recorded, and then, a given amount of SiO2 (of Table 1) was added. After 1 h standing a second NMR spectra is recorded. A longer exposure does not change the calculated average tacticity. The polymer solution and the SiO2 substrate were brought into contact at two different temperatures, namely, 25 or 50 °C, before the NMR investigation. For the needs of one experiment, silica was modified by the chemisorption of C16 chlorosilanes. WinNMR software was used for the integration of the NMR peaks and for the spectral subtraction between the spectra recorded for PMMA added to silica and the silica spectra alone. Indeed, silica contains surface impurities which desorb and provide a signal as shown in Figures 1-3, which has to be subtracted to allow a quantitative determination of the average tacticity. The subtraction of the silica spectra was adjusted by leveling the intensity of the peaks at 1.25 ppm in both the PMMA/ silica and silica alone spectra. Thermogravimetry was used between 40 and 500 °C to determine from weight loss the amount of PMMA adsorbed on silica.
Results and Discussion The NMR spectra are focused on the R-methyl peaks, which are generally used to determine the PMMA tacticity in solution. The window of chemical shift of the NMR spectra chosen here is from 0.6 to 1.4 ppm. Figure 1 clearly shows the high content of isotactic mm triads of the i-PMMA sample in CDCl3 solution. After addition of SiO2, two new peaks appear due to the impurities desorbed from (11) Legrand, A. P.; Hommel, H.; Tuel, A.; Vidal, A.; Balard, H.; Papirer, E.; et al. Adv. Colloid Interface Sci. 1990, 33, 91.
10.1021/la9908384 CCC: $19.00 © 2000 American Chemical Society Published on Web 04/22/2000
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Carriere et al.
Table 1. Percentages of Stereoregular Sequences Calculated from the Integrated Intensities of 1H NMR r-Methyl Peaks for the Different Samplesa
sample
PMMA/g of SiO2, conditions
isotactic heterotactic syndiotactic sequences, sequences, sequences, % % %
a b c d
i-PMMA, Reference CDCl3 95 4 0.06, room temp 42 0 0.12, room temp 51 23 0.06, 50 °C 58 14 0.06, modified SiO2 93 2
1 58 26 28 5
e f g
a-PMMA, Reference CDCl3 13 37 0.06, room temp 6 39 0.12, room temp 5 41 0.06, 50 °C 5 38
50 55 54 57
h
s-PMMA, Reference CDCl3 7 18 0.06, room temp 0 25
75 75
a Reference represents the PMMA in CDCl before adsorption, 3 the concentrations are given in grams of PMMA for 1 g of SiO2. The conditions represent the temperature at which the adsorption is conducted or the use of chemically modified SiO2.
Figure 3. R-Methyl peaks of 1H NMR spectra of atactic PMMA in CDCl3: (A) a-PMMA in solution; (B) a-PMMA after silica addition (0.12 g of PMMA for 1 g of SiO2); (C) silica alone; (D) resulting spectra after spectral subtraction (D ) B - C).
Figure 2. R-Methyl peaks of 1H NMR spectra of syndiotactic PMMA in CDCl3: (A) s-PMMA in solution; (B) s-PMMA after silica addition (0.12 g of PMMA for 1 g of SiO2); (C) silica alone; (D) resulting spectra after spectral subtraction (D ) B - C).
presence of silica particles. Moreover, we assume that the small shifts of resonance in spectra before and after silica addition can be connected to susceptibility effects. However, the most interesting feature is that the peak assigned to the isotactic sequences exhibits a strong reduction compared to that of the syndiotactic triads. The integrated intensity of mm triads relative to the other triads decreases from 95 to 42 or 51%, depending on the PMMA to silica ratio, after adsorption when the experiment is done at room temperature. Since no change in the chain tacticity is allowed, the only explanation for the observed effect is that the NMR lines of the polymer segments directly bonded to the substrate are so broad that they escape detection in high-resolution spectrum. This effect was already reported in earlier studies.12,13 Strong specific interactions discussed elsewhere1 in terms of acid-base interactions are likely to involve the PMMA carbonyl groups and the silanol surface groups. Previous studies14 pointed out that the density of interfacial interaction of i-PMMA determined from FT-IR appears to be higher than for s-PMMA on a reactive substrate such as alumina. These observations are consistent with the strong lowering of the relaxation time of adsorbed i-PMMA chains as compared to s-PMMA calculated by molecular simulation.6 Therefore, the large decrease of the relative amount of isotactic triads observed in this study by 1H NMR after adsorption can be attributed to a preferential adsorption of these sequences at the SiO2 surface. Thus, the only sequences which still sensitively contribute to the NMR peaks are the nonadsorbed chains and also the free segments belonging to the physisorbed chains, namely, loops and tails. Furthermore, the increase after adsorption in the amount of syndiotactic triads from 1 to 26 or 58%, according to the ratio of PMMA to silica, is somewhat unexpected for a highly isotactic sample. However, these sequences can be viewed as “defects” in the chain stereoregularity highlighted here since they may concentrate in loops and tails which are the only part of the adsorbed layer contributing significantly to the NMR signal. It should be noticed that the adsorbed amount obtained from thermogravimetry is 0.03 g/g for 0.06 g of PMMA brought into contact with 1 g of silica (i-PMMA sample a
the silica surface, namely, methylene and methyl groups at 1.25 and 0.88 ppm, respectively. Otherwise, the NMR peaks assigned to PMMA change in shape and intensity. Thus, the slight broadening of the peaks results from the
(12) Miyamoto, T.; Cantow, H. J. Makromol. Chem. 1972, 162, 43. (13) Abd El-Hakim, A. A.; Ramadam, A. M.; Badram, A. S. Polymer 1992, 22, 4880. (14) Grohens, Y.; Schultz, J.; Prud′homme, R. E. Int. J. Adhesion Adhesives 1997, 17, 163.
Figure 1. R-Methyl peaks of 1H NMR spectra of isotactic PMMA in CDCl3: (A) i-PMMA in solution; (B) i-PMMA after silica addition (0.12 g of PMMA for 1 g SiO2): (C) silica alone; (D) resulting spectra after spectral subtraction (D ) B - C).
Stereospecificity in Adsorption
in Table 1). This represents an adsorption of 50% of the PMMA molecules initially introduced in solution. This value can be converted in a covering value of 2 PMMA repeat units/nm2, which is below the value of the plateau of the adsorption isotherm, namely, 6 units/nm2. Considering the i-PMMA sample b in Table 1, a decrease in the selectivity of the adsorption process must be envisaged from the 51% rather than 42% remaining isotactic sequences. Indeed, the higher concentration of PMMA, namely, 0.12 g/g with respect to the silica, increases the competition between molecules for the adsorbing silanol sites. The higher adsorbed amount by increasing the loops and tails reduces the ability of long isotactic sequences to be in close contact with the surface. When the adsorption is conducted at 50 °C, Table 1 shows that for i-PMMA sample c the residual isotacticity raises to 58%, whereas it is only 42% at 25 °C. It turns out that evidence of the stereospecificity of the adsorption process remains but is markedly lowered. The driving force for selective adsorption should then be not only the polymer stereoregularity alone but also the tacticity induced conformation of the PMMA in solution as will be discussed later. The same type of experiments have also been conducted on a chemically silane treated silica surface. The silane layer provides an inert low surface energy material which is expected to hinder the development of specific interactions between PMMA and the surface silanols groups. It can be observed from Table 1 that i-PMMA sample d adsorbed on the chemically modified SiO2 exhibits no stereospecific adsorption. However, another conclusion that may also be considered is that in the absence of specific interactions no reduction in the mobility of the molecular species is expected at the interface. Indeed, even if stereoselective physisorption occurs at the silanized silica surface 1H NMR will not be a relevant technique in that particular case. Besides, as observed in Table 1, the isotacticity of the a-PMMA sample e decreases from 13 to 6% or 5% at 25 and 50 °C, respectively. Thus, stereospecificity in the adsorption is still noticeable for a-PMMA; however, its extent becomes lower than for i-PMMA. Selectivity in the adsorption of s-PMMA can still be deduced from the integrated intensity of the NMR peaks as shown in Figure 2. Indeed, the percentage of isotactic sequences decreases from 7 to 0% after silica addition. However, no definitive conclusion can be drawn from these last results since the intensity of the NMR signal is close to the range of experimental error. The driving forces for the stereoselective adsorption are discussed in the following section. As shown in a previous paper,15 the density of PMMA-chloroform interactions determined from FT-IR is higher in diluted solution for syndiotactic macromolecules than for the isotactic ones at room temperature. From these results, one can assume a specific conformation of the stereoregular sequences in solvent according to their tacticity. These local conformations result in a difference in the solvation of the two PMMA isomers;16 namely, less solvent molecules are in interaction with i-PMMA chains more than with s-PMMA chains. The hindrance of polymer/solvent contacts in i-PMMA is also likely to be attributed to the existence a secondary structure which differs from that of s-PMMA. s-PMMA chains are claimed to form a helix with a larger (15) Grohens, Y.; Carriere, P.; Spevacek, J.; Schultz, J. Polymer 1999, 40, 7033. (16) Klein, M.; Guenet, J. M. Macromolecules 1989, 22, 3716.
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number of monomers per turn than the isotactic chains.17 The internal part of the s-PMMA helix is then supposed to be more accessible to the solvent molecules. The s-PMMA and the embedded solvent molecules form at room temperature so-called compounds (or complexes)16,18 which are known to provide in some cases special properties such as physical gel formation. The driving force for the specific adsorption of isotactic stereoregular sequences at a surface could be the possibility for the system to lower the interfacial tension between the silica and the PMMA solution through a segregation of the less solvated isotactic sequences at the silica/chloroform interface. Actually, the isotactic stereoregular PMMA would, therefore, be regarded as an amphiphilic copolymer. The hydrophobic/hydrophilic character not being provided, here, by a chemical difference in the chain but by a difference in the conformation/ solvation induced by the tacticity. Moreover, it may not be neglected that the high persistence length of the i-PMMA chain segments, as calculated by molecular simulation,19 can also be a strong argument for their preferential adsorption at an interface. The existence of conformational changes in solutions of synthetic polymers was first observed by Reiss and Benoit.20 Submitting a PMMA dilute solution to an increase in temperature has been shown15 to induce a conformational transition at 40 °C which is suggested to result in a modification of the secondary structure of the chain for both iso- and syndiotactic PMMA. From Table 1 sample c it can be observed that when the adsorption is conducted at 50 °C the isotacticity of the system increases as compared to the experiment conducted at room temperature. The conformational modifications in i-PMMA chains which occur above 40 °C lower the stereospecificity of the adsorption process since the percentage of isotactic sequences increases from 42 to 58%. It can be concluded from that experiment that the tacticity induced conformation is significant in the adsorption process. The predominant role played by a solvated secondary structure can therefore be reasonably assumed in the interfacial behavior of stereoregular PMMA. Conclusion H NMR has provided evidence of stereoselective adsorption of isotactic segments at the SiO2 surface. Increasing temperature and concentration lowers the preferential adsorption of isotactic sequences. The stereospecificity is most significant for i-PMMA and is also observed for a-PMMA and s-PMMA. Finally, the presence of reactive hydroxyl groups at the surface of the substrate is essential to ensure the development of specific interactions which are necessary to reduce the mobility of bonded sequences which will not be detected by NMR. Further experiments will be carried out on different solvents, substrates, and polymers to find out if the stereoselectivity in the adsorption process observed here is a concept which can be generalized to other systems. 1
Acknowledgment. Grant 203/96/1386 of the Grant Agency of the Czech Republic as well as support within the Barrande program (Czech-French cooperation) are gratefully acknowledged. LA9908384 (17) Spevacek, J.; Schneider, B. Adv. Colloid Interface Sci. 1987, 27, 81. (18) Saiani, A.; Spevacek, J.; Guenet, J. M. Macromolecules 1998, 31, 703. (19) Vacatello, M.; Flory P. J. Macromolecules 1986, 19, 405. (20) Reiss, C.; Benoit, H. C. R. Hebd. Sceances Acad. Sci. 1961, 253, 268.
