Langmuir 2003, 19, 10399-10402
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Thickness Dependence of the Glass Transition Temperature in Thin Films of Partially Miscible Polymer Blends L. Hamon,† Y. Grohens,*,‡ and Y. Holl§ Groupe Physique Chimie du Vivant, Laboratoire Milieux Nanome´ triques, Universite´ d’Evry, Rue du Pe` re Jarlan, 91025 Evry Cedex, France, Laboratoire Polyme` res et Proce´ de´ s, Universite´ de Bretagne Sud, Rue St-Maude´ , BP 92116, 56325 Lorient Cedex, France, and Institut Charles Sadron, 6, rue Boussingault, 67083 Strasbourg Cedex, France Received June 27, 2003. In Final Form: September 25, 2003 The thickness dependence of the glass transition temperature, Tg(h), has been investigated using ellipsometry at a variable temperature for thin films of partially miscible stereoregular poly(methyl methacrylate) (PMMA)-low-molecular-weight poly(ethylene oxide) (PEO) blends. These values were compared to Tg(h) values of pure stereoregular PMMA thin films, and the miscibility of PMMA/PEO systems is assessed by the so-called plasticizing effect of PEO, that is, ∆Tg*. In thin-film geometry, the miscibility of i-PMMA is enhanced compared with that in the bulk and becomes higher than that for s-PMMA in contrary to the bulk behavior of the blends. The conformation energy ∆E of the PMMA chains in the thin film increase much more for s-PMMA than for i-PMMA, indicating larger gauche to trans conformational rearrangement for the former isomer in the confined geometry. These large local modifications of the s-PMMA conformation in the thin film could result in modification of the mixing entropy and, therefore, be the main reason for the lower miscibility of PEO in s-PMMA as compared to that in i-PMMA.
Introduction In many experimental studies over the past decade, it has been observed that the glass transition temperature of thin polymer films, Tg(h), differs from that of the bulk polymers, Tg(bulk).1-17 Tg in thin polymer films is dependent on the film thickness, and the difference between Tg(bulk) and Tg(h) increases with decreasing film thickness. In the case where the interactions between the polymer and the substrate are weak, Tg(h) usually * Author to whom correspondence should be addressed. † Universite ´ d’Evry. ‡ Universite ´ de Bretagne Sud. § Institut Charles Sadron. (1) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhys. Lett. 1994, 27, 59. (2) Orts, W. J.; van Zanten, J. H.; Wu, W.; Satija, S. K. Phys. Rev. Lett. 1993, 71, 867. (3) Wu, W.; van Zanten, J. H.; Orts, W. J. Macromolecules 1995, 28, 771. (4) Wallace, W. E.; van Zanten, J. H.; Wu, W. Phys. Rev. E 1995, 52, R3329. (5) van Zanten, J. H.; Wallace, W. E.; Wu, W. Phys. Rev. E 1996, 53, R2053. (6) Forrest, J. A.; Dalnoki-Veress, K.; Stevens, J. R.; Dutcher, J. R. Phys. Rev. Lett. 1996, 77, 2002. (7) Forrest, J. A.; Dalnoki-Veress, K.; Dutcher, J. R. Phys. Rev. E 1997, 56, 5705. (8) Mattsson, J.; Forrest, J. A.; Bo¨rjesson, L. Phys. Rev. E 2000, 62, 5187. (9) Kukao, K.; Miyamoto, Y. Europhys. Lett. 1999, 46, 649. (10) Xie, L.; DeMaggio, G. B.; Frieze, W. E.; DeVries, J.; Gidley, D. W.; Hristov, H. A.; Yee, A. F. Phys. Rev. Lett. 1995, 74, 4947. (11) DeMaggio, G. B.; Frieze, W. E.; Gidley, D. W.; Zhu, M.; Hristov, H. A.; Yee, A. F. Phys. Rev. Lett. 1997, 78, 1524. (12) Kim, J. H.; Jang, J.; Zin, W. C. Langmuir 2000, 16, 4064. (13) Grohens, Y.; Brogly, M.; Labbe, C.; David, M.-O.; Schultz, J. Langmuir 1998, 14, 2929. (14) Prucker, O.; Christian, S.; Bock, H.; Ruhe, J.; Franck, C. W.; Knoll, W. Macromol. Chem. Phys. 1998, 199, 1435. (15) Fryer, D. S.; Nealey, P. F.; De Pablo, J. J. Macromolecules 2000, 33, 6439. (16) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Faraday Discuss. 1994, 98, 219. (17) Grohens, Y.; Sacristan, J.; Hamon, L.; Reinecke, H.; Mijangos, C.; Guenet, J. M. Polymer 2001, 42, 6419.
decreases with decreasing thickness.1,4,6-9,12,14,15 On the other hand, Tg of films coated on strongly interacting substrates increases with decreasing film thickness because of the hydrogen bonds between the polymer and the substrate.3,5,16,17 Nevertheless, this generally reported trend is not consistent with all published results.13 In the case of stereoregular poly(methyl methacrylate)s (PMMAs), which are known to develop rather strong attractive interactions with the SiOx substrate, the isotactic form exhibits a strong increase of Tg(h), whereas the syndiotactic form shows a Tg(h) depression. Consequently, other parameters such as chain conformations, thermal history, or entanglements should be taken into account in the studies of the Tg thickness dependence.18 The concept of physically mixing two or more polymers to obtain new materials has been of widespread scientific interest and commercial utilization over the past two decades. Several studies have been published on blends of poly(ethylene oxide) (PEO) and PMMA.19-31 Numerous works reported that the miscibility domain ranges between (18) Grohens, Y.; Hamon, L.; Reiter, G.; Soldera, A.; Holl, Y. Eur. Phys. J. E 2002, 8, 217. (19) Silvestre, C.; Cimmino, S.; Martuscelli, E.; Karasz, F. E.; MacKnight, W. J. Polymer 1987, 28, 1190. (20) Zawada, J. A.; Ylitalo, C. M.; Fuller, C. G.; Colby, R. H.; Long, T. E. Macromolecules 1992, 25, 2896. (21) Russel, T. P.; Ito, H.; Wignall, G. D. Macromolecules 1988, 21, 1703. (22) Eunsook, J.; Taikyue, R. J. Polym. Sci., Part A: Polym. Chem. 1990, 28, 385. (23) Schantz, S. Macromolecules 1997, 30, 1419. (24) Talibuddin, S.; Wu, L.; Runt, J.; Lin, J. S. Macromolecules 1996, 29, 7527. (25) Hopkinson, I.; Kiff, F. T.; Richards, R. W.; King, S. M.; Farren, T. Polymer 1995, 36, 2183. (26) Chen, X.; Yin, J.; Alfonso, G. C.; Pedemonte, E.; Turturro, A.; Gattiglia, E. Polymer 1998, 39, 4929. (27) Straka, J.; Schmidt, P.; Dybal, J.; Schneider, B.; Spevacek, J. Polymer 1995, 36, 1147. (28) Parizel, N.; Laupreˆtre, F.; Monnerie, L. Polymer 1997, 30, 3719. (29) Marco, C.; Fatou, J. G.; Gomez, M. A.; Tanaka, H.; Tonelli, A. E. Macromolecules 1990, 23, 2183. (30) Lu, X.; Weiss, R. A. Macromolecules 1992, 25, 3242.
10.1021/la0351461 CCC: $25.00 © 2003 American Chemical Society Published on Web 11/05/2003
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10 and 30 wt % PEO.19-24 PMMA stereoregularity is one of the major factors affecting the miscibility of the blend. s-PMMA/PEO systems are miscible at the molecular level in the amorphous state until 30% PEO, whereas highly isotactic PMMA gives rise to a phase separation for lower PEO content.19,29 The blends obey the classical Fox equation in their miscible range of composition. In a recent publication, we have studied the behavior of stereoregular PMMA/PEO oligomer blends and found out that, for 10 wt % PEO, the blends are miscible whatever the polymer tacticity.32 Surface segregation in polymer blends has been investigated and is well described theoretically for bulk or thick films,33 but only few results exist for thin films in the nanometer range.34 Very recent results35 show that the miscibility of PMMA/PEO polymer blends in thin films was enhanced compared with that in the bulk. In this study, to further investigate the thickness dependence of Tg in thin films, Tg(h) was measured in compatible blends of (91:9) PMMA/PEO by variable temperature ellipsometry as a function of the thickness (in the range 25-120 nm) and compared to the Tg(h) values of pure PMMA thin films. The Tg depression resulting from the plastisizing effect of PEO oligomers was compared in thin films and in the bulk and discussed in terms of PMMA/PEO miscibility. Experimental Section Stereoregular PMMAs were purchased from Polymer Source, Inc., Canada. The tacticity of the PMMA samples was (i) 97% isotactic for i-PMMA and (ii) 80% syndiotactic for s-PMMA. Their molecular weight was 35 kg/mol with a polydispersity of 1.02. The PEO oligomer (400 g/mol) so-called E9 was purchased from Aldrich. The PMMA/E9 blends of one compositional ratio, 91:9 (w/w), were prepared by dissolving the two polymers in chloroform. The solution was stirred for 24 h. The thin films were prepared by spin coating on (111) silicon wafers. The substrates were treated by an argon-water plasma during 6 min at 80 W prior to solution deposition. This treatment provides homogeneously hydroxylated high energy surfaces, which were studied by water contact angle measurements (advancing and receding angles) with variability in the range of (1°, and no hysteresis is observed. The desired thickness of the polymer films was achieved by varying the concentration of the solution (3-15 g/L). The spin-coated samples were studied only after annealing at Tg(bulk) + 70 °C, for at least 24 h, and cooled to room temperature at a constant rate. Before and after Tg measurements, optical microscopy and atomic force microscopy (AFM) were used to observe the topography of the samples. There were no remarkable changes in sample topography after Tg measurement; therefore, polymer dewetting did not occur under these experimental conditions. The Tg(bulk) of the bulk polymer samples was investigated by differential scanning calorimetry at a heating rate of 2 °C/min. Spectroscopic ellipsometry experiments were performed using a Sopra ES4M apparatus working in a wavelength range from 0.4 to 0.8 µm, equipped with a hot stage. During the kinetic ellipsometric scans, performed at 2 °C/min, the ellipsometric angles (ψ, ∆) were continuously monitored. The experimental data points of cos ∆ allowed us to identify two temperature ranges represented by two intersecting lines of different slopes. The slopes were fitted using a linear least-squares routine. The experimental glass transition temperature, Tg(h), is defined as the temperature where the two lines intersect. (31) Rao, G. R.; Castiglioni, C.; Gussoni, M.; Zerbi, G.; Martuscelli, E. Polymer 1985, 26, 811. (32) Hamon, L.; Grohens, Y.; Soldera, A.; Holl, Y. Polymer 2001, 42, 9697. (33) Jones, R. A. L.; Kramer, E. J. Polymer 1993, 34, 115. (34) Kim, J. H.; Jang, J.; Lee, D. Y.; Zin, W. C. Macromolecules 2002, 35, 311. (35) Jeong, U.; Ryu, D. Y.; Kho, D. H.; Lee, D. H.; Kim, J. K.; Russel, T. P. Macromolecules 2003, 36, 3626.
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Figure 1. Raw ellipsometric data: cos ∆ as a function of the temperature for the i-PMMA and (91:9) i-PMMA/E9 blend. The kink in the curve represents the Tg value of the thin film.
Results and Discussion The topography and the surface chemical composition of thin films of PMMA/PEO systems were studied by Affrossman et al.36 In their blends, where PMMA was largely predominant (90%), the surface was rather smooth and the chemical composition of the surface is identical to that of the bulk. Their PEOs had molecular weights ranging between 10 and 120 kg/mol, which allowed PEO crystallization at room temperature. In our study, as a result of the oligomeric character of E9, its melting point is close to 10 °C and is, therefore, in the molten state at room temperature. After annealing of the samples at Tg,PMMA + 70 °C, it can be assumed that the film of polymer blend is in an amorphous state (Tamb > Tm,E9). Indeed, no organized structures were detected by AFM at the surface of the films. AFM can also be used to characterize the polymer dewetting process that can occur in thin films.37 Thermally growing holes corresponding to dewetting have been observed for our systems in the range of compositions close to the limit of miscibility of the PMMA/PEO blends. Roughness measurements through the root mean square (rms) on the AFM (1 µm × 1 µm, 512 dots/line) pictures can help in the assessment of the film stability.
∑(Zi - Zaverage)2/N
rms ) x
(1)
Films of pure PMMA present a rms ) 0.31 ( 0.02 nm whatever the PMMA tacticity. The presence of 9% of E9 in the film does not yield a large modification in the rms (rms ) 0.21 ( 0.01 nm) whatever the thickness range (25-150 nm). Particular investigations have been made on larger AFM pictures (10 µm × 10 µm, 512 dots/line) without any remarkable change of roughness: rms ) 1.08 ( 0.14 nm. Raw ellipsometric data such as cos ∆ can be plotted as a function of the temperature as shown in Figure 1. The kink in the curves is considered as the thickness-dependent glass transition temperature of the thin films, Tg(h).1,6,13 No modification in the width of the transition is observed between pure PMMA and the blends, and only one transition is observed. The comparison between the evolution of Tg(h) with the film thickness for stereoregular thin films of pure PMMA and the Tg(h) of blends of (91:9) i-PMMA/E9 (Figure 2) and (91:9) s-PMMA/E9 (Figure 3) (36) Affrossman, S.; Kiff, T.; O’Neill, S. A.; Pethrick, R. A.; Richards, R. W. Macromolecules 1999, 32, 2721. (37) Reiter, G. Europhys. Lett. 1993, 23, 579.
Glass Transition Temperature
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Figure 4. ∆Tg* calculated using eq 2 (see text) for i-PMMA/E9 and s-PMMA/E9 thin films versus the film thickness.
Figure 2. Glass transition temperature for thin films of pure i-PMMA and (91:9) i-PMMA/E9 versus the film thickness. Experimental points can be fitted using an exponential function: Tg ) y0 + Ae-h/t.
Figure 3. Tg for thin films of pure s-PMMA and (91:9) s-PMMA/ E9 versus the film thickness. Experimental points can be fitted using an exponential function: Tg ) y0 + Ae-h/t.
highlights similar trends. The addition of PEO does not modify the evolution of Tg(h) with the thickness, namely, the increase of Tg(h) for i-PMMA toward lower thickness and the decrease of Tg(h) for s-PMMA. The unexpected very different behaviors of the two PMMA stereoisomers Tg(h) values in a confined geometry were already demonstrated and tentatively explained by a difference in the densities of the monomers in direct interaction with the substrate.13 According to some of the concepts put forward to explain the deviation to the bulk behavior of thin films, the previously mentioned result for the PMMA/PEO blends is somehow unexpected. Indeed, even though the surface tensions of the PMMA and PEO are both close to 40 mJ/ m2, the strong difference in the molecular weight between the two blend components accounts for the segregation of the lowest-molecular-weight compound, that is, PEO, to the surface or interface for entropic reasons.38 In the case of polymer blends, the widespread layer model in which the layer of strongly restricted mobility in direct contact (38) Jones, R. A. L.; Richards, R. W. Polymers at Surfaces and Interfaces; Cambridge University Press: Cambridge, U.K., 1999.
with the surface is the only significant contribution to the Tg(h) modifications does not hold to explain the present data. Thus, PEO and PMMA are known to both adsorb strongly on silica, and, therefore, coadsorption may lead to a modification of the configuration of the adsorbed layer. The only dependence of Tg(h) on the composition and configuration of the adsorbed layer is rather inconsistent with our results. Polymer chains in the overall thickness of the thin film, even those having no contact with the silicon surface, may participate in the Tg modification. It is worth mentioning that, for a given thickness, a decrease of Tg between pure PMMA and PMMA/PEO can be observed. The Tg depression, so-called plastisizing effect, of E9 on PMMA is due to the low Tg of PEO (Tg,E9 ) -71 °C). Such a phenomenon has been reported by Kim et al.34 for a blend of polystyrene and poly(2,6-dimethyl-1,4phenylene oxide) (PS/PPO) in thin films on the overall range of compositions. Whatever the blend composition, a decrease of Tg(h) with the film thickness is observed identical to that observed for pure PS. PPO induces an increase of Tg for a given thickness because of its high Tg. The thickness at which the Tg(h) modification occurs is independent of the blend composition. This is consistent with our results because this thickness is around 50 nm with or without E9 and corresponds to 8REE according to the molecular weight of our PMMA. To point out the plastisizing effect of E9 on the stereoregular PMMA, eq 2 allows the calculation of ∆Tg*:
∆Tg* )
∆Tgtf ∆Tg
) bulk
Tg,PMMAtf - Tg,PMMA/PEOtf Tg,PMMAbulk - Tg,PMMA/PEObulk
(2)
with ∆Tgtf ) variation of Tg between thin films of pure PMMA and thin films of PMMA/E9 blend and ∆Tgbulk ) plastisizing effect of 9% of E9 in the bulk blends. ∆Tgbulk ) 18 and 34 °C for i-PMMA/E9 and s-PMMA/E9, respectively. The higher value of ∆Tg*, the so-called plasticizing effect, for s-PMMA is probably due to the more intimate mixture of PMMA and PEO chains at the molecular level. The evolution of ∆Tg* as a function of the thickness allows the comparison of the plasticizing effect of E9 in the bulk or in the thin film. ∆Tg* ) 1 indicates a distribution of PEO in the PMMA that is identical in the bulk and the thin film. Values of ∆Tg* < 1 represent a possible segregation of PEO at interfaces or nanophase separation in the bulk of the film. One can observe in Figure 4 that the ∆Tg* values strongly depend not only on the PMMA tacticity but also on the film thickness.
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∆Tg* is higher for the i-PMMA/E9 system than for s-PMMA/E9, but for both blends, ∆Tg* increases when the thickness decreases. These evolutions of ∆Tg* with PMMA tacticity are unexpected because blends containing s-PMMA exhibit a larger miscible range, that is, 30%, than i-PMMA, for which the miscibility window is restricted to only 10%.32 Thus, i-PMMA/E9, which is at the limit of miscibility, exhibits higher values of ∆Tg* than s-PMMA/E9 and, therefore, a higher plasticizing effect in thin-film geometry. In other words, surface segregation of PEO at interfaces is assumed to be larger in thin films of s-PMMA than in those of i-PMMA. The other fascinating feature in these thin polymer blend films is that the plastisizing effect increases for both stereoisomers with decreasing film thickness. Because the surface energy and molecular weight are very similar for the two PMMA stereoisomers, these factors are not significant in the previously mentioned effects. Entropic contribution to the miscibility in thin films is probably relevant and has to be further discussed in light of our previous studies of these systems. In a recent study,18 we have shown that there was a strong increase in the conformation energy, ∆E, of PMMA chains in thin films, whatever the stereoregularity. However, this ∆E increases much more for s-PMMA than for i-PMMA, indicating larger gauche to trans conformational rearrangements for the former isomer in the confined geometry. The discrepancy in the chain organization at interfaces originates in the adsorption of PMMA chains at the surface, which is directly influenced by the local stiffness of some chain segments and their mobility. Fast solvent evaporation and the spin-coating
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process can also influence the PMMA chain conformation in the thin film. Interactions between PMMA and PEO are of the van der Waals type and involve the carbonyl carbon of PMMA and the oxygen of the ether function of PEO.30 To maximize its interactions with PMMA in the bulk, PEO has to adopt a planar zigzag conformation, whereas the helix conformation is known to be the most stable form of this polymer.31 It can be assumed that large local modifications of s-PMMA conformations in the thin film could result in a decrease of the number of PMMA/PEO contacts and, therefore, a decrease of the negative contributions (decrease of ∆Hm and ∆Sm) to the free enthalpy of mixing, ∆Gm. A specific entropic contribution to the Flory interaction parameter, χ, cannot be ruled out either. The large chain conformational reorganization in the thin films might be the main reason for the lower miscibility of PEO in s-PMMA as compared to that in i-PMMA. Conclusion It has been shown that the miscibility in blends can be tuned by using the thickness and the polymer stereoregularity in thin-film geometry. It was found that the PEO miscibility in i-PMMA thin-film geometry is enhanced compared with that in the bulk, whereas the inverse is observed for s-PMMA. The application of such properties can be to achieve a better control of the surface friction or wettability by specific segregation of one component of the blend. Moreover, polymer blends can fruitfully contribute to understanding the fundamentals of the thickness dependence of Tg. LA0351461
