Multiple Glass-Transition Temperatures in Thin Supported Films of

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Multiple Glass-Transition Temperatures in Thin Supported Films of Isotactic PMMA as Revealed by Enhanced Raman Scattering G. Vignaud,† J.-F. Bardeau,‡ A. Gibaud,‡ and Y. Grohens*,† Laboratoire Polyme` res, Proprie´ te´ s aux Interfaces et Composites (L2PIC), Centre de Recherche BP 92116, Rue Saint Maude´ , 56321 Lorient Cedex, France, and Laboratoire de Physique de l’Etat Condense´ , UMR CNRS 6087, Universite´ du Maine, 72085 Le Mans Cedex 09, France Received April 5, 2005. In Final Form: July 15, 2005 The glass-transition temperature, Tg, of isotactic PMMA thin films has been measured for four thicknesses by enhanced Raman spectroscopy and ellipsometry. This was made possible by inserting a silica spacer layer between the film and the substrate. The use of such a spacer drastically improves the sensitivity of Raman scattering measurements. The improvement in the sensitivity allows us to study phenomena involving changes in molecular dynamics, such as the phase transition, and to probe the existence in very thin films of several thickness-dependent transition temperatures, Tg(h). This in turn is interpreted as the occurrence in the film of a layered structure. The influence of the polymer concentration on the conformation of the surface adsorbed polymer layer and therefore on Tg(h) is discussed.

Introduction Ultrathin polymer films have shown deviations of the glass-transition temperature (Tg) from the bulk since Reiter1 reported the dewetting of polystyrene (PS) ultrathin films that are less than 20 nm thick at temperatures lower than Tg. At the same time, Keddie et al.2 showed the dependence of Tg as a function of thickness for spin-coated PS and poly(methyl methacrylate) (PMMA) films on silicon and gold by ellipsometry. Since then, many groups have explored this phenomenon using a variety of experiments such as ellipsometry,3-5 X-ray reflectometry,6 differential scanning calorimetry (DSC),7 Brillouin scattering spectroscopy (BLS),8 and dielectric relaxation spectroscopy (DRS).9-10 For supported PMMA films, it was shown that Tg(h) decreased when the film thickness (h) decreased except for strongly attractive surfaces that induced Tg to increase.11 More particularly, it was found that for isotactic PMMA, Tg(h) increased from 334 to 383 K when it was spin coated on a silicon wafer as ultrathin 20-nm-thick films.12 Specific interactions between the substrate and the polymer chains yielding to modification of Tg(h) were clearly shown in the case of stereoregular PMMA spin cast on silicon surfaces by monitoring the carbonyl peak of the infrared reflection absorption.13 Fryer * Corresponding author. E-mail: [email protected]. † Centre de Recherche BP 92116. ‡ Universite ´ du Maine. (1) Reiter, G. Europhys. Lett. 1993, 23, 579. (2) Keddie, J. L.; Jones, R. A. L.; Cory, R. A. Europhys. Lett.1994, 27, 59. (3) Beaucage G.; Composto R.; Stein R. S. J. Polym. Sci., Part B: Polym. Phys. 1993, 31, 319. (4) Forrest, J. A.; Dalnoki-Veress, K.; Stevens, J. R.; Dutcher, J. R. Phys. Rev. Lett. 1996, 77, 2002. (5) Kawana, S.; Jones, R. A. L. Eur. Phys. J. E 2003, 10, 223. (6) van Zanten, J. H.; Wallace, W. E.; Wu, W. L. Phys. Rev. E 1996, 53, R2053. (7) Pham, J. Q.; Mitchell, C. A.; Bahr, J. L.; Tour, J. M.; Krishanamoorti, R.; Green, P. F. J. Polym. Sci. 2003, B 41, 3339. (8) Mattsson, J.; Forrest, J. A.; Bo¨rjesson, L. Phys. Rev. E 2000, 62, 5187. (9) Hartmann, L.; Gorbatschow, W.; Hauwede, J.; Kremer, F. Eur. Phys. J. E 2002, 8, 145. (10) Kojio, K.; Jeon, S.; Granick, S. Eur. Phys. J. E 2002, 8, 167. (11) Sharp, J. S.; Forrest, J. A. Phys. Rev. E 2003, 67, 031805. (12) Grohens, Y.; Hamon, L.; Reiter, G.; Sodera, A.; Holl, Y. Eur. Polym. J. E 2002, 8, 217.

et al. widely changed the surface energies, γ, of the substrate surfaces coated with PS and PMMA.14 For high γ values, Tg(h) values in PS and PMMA ultrathin films were higher than in the bulk and increased monotonically with increasing γ. The molecular dynamics simulations performed by Torres et al. demonstrated that Tg(h) of ultrathin films with thickness less than 60 nm increased with an increase in the intermolecular potential between the polymer chains and the substrate.15 It was suggested that the increase of Tg was mainly due to the attractive force between polymer chains and the substrate. It is well known that the film thickness is directly related to the solution concentration used for spin coating and that an increase in PMMA concentration in solution can yield a brushlike configuration on a substrate whereas a lower concentration supports the appearance of flat adsorption with an increasing proportion of trans conformations.16 However, the role of the configuration of the adsorbed layer (i.e., conformation of the chains pinned at the filmsubstrate interface) in Tg(h) is still unclear. In this letter, we used micro-Raman spectroscopy based on engineered SiO2/silicon substrates17 (allowing an enhancement of the Raman signal of nanometric films) to probe the conformation of the stretching modes of isotactic PMMA as a function of temperature. After demonstrating that the intensities of these vibrational modes are temperature-dependent, we have undertaken more precise investigations of the effect of thickness on the glass-transition temperatures of thin supported films. We report the observation in thin films of two glass-transition temperatures Tg(h) that are thickness-dependent. These results are discussed in terms of a layered structure of the films together with the conformation of the PMMA adsorbed layer. A comparison (13) Grohens, Y.; Brogly, M.; Labbe, C.; David, M.-O.; Schultz, J. Langmuir, 1998, 14, 2929. (14) Fryer, D. S.; Peters, R. D.; Kim, E. J.; Tomaszewski, J. E.; de Pablo, J. J.; Nealey, P. F.; White, C. C.; Wu, W. L. Macromolecules 2001, 34, 5627. (15) Torres, J. A.; Nealey, P. F.; de Pablo, J. J. Phys. Rev. Lett. 2000, 85, 3221. (16) Konstadinis, K.; Thakkar, B.; Chakraborty, A.; Potts, L. W.; Tannenbaum, R.; Tirrell, M.; Evans, J. F. Langmuir, 1992, 8, 1307. (17) Baptiste, A.; Bulou, A.; Bardeau, J. F.; Nouet, J.; Gibaud, A. Langmuir 2004, 20, 6237.

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of Tg(h) measured by ellipsometry and Raman scattering is presented. Experimental Section Sample Preparation. The PMMA used in this study is an isotactic stereoregular polymer purchased from Polymer Source Inc., Canada. Its molecular weight was 39.5 kg/mol with a polydispersity equal to 1.38. The tacticity of the PMMA was 97% isotactic. Thin films were deposited by spin coating solutions of the polymer in chloroform onto a silicon wafer containing a silica (SiO2) spacer layer of about 100 nm. The substrates were treated for 4 h in an UV-ozone chamber prior to depositing polymer in order to remove organic contaminants and improve adhesion on the surface. The concentrations of PMMA solutions used were 26, 16, 12, and 7 g/L to reach the targeted thickness. The spincoated films were studied after 12 h of annealing under vacuum at 100 °C to facilitate the relaxation of the chains within the thin films. The thicknesses of the thin films were measured by X-ray reflectivity with a Philips X’pert reflectometer using Cu KR radiation. The thicknesses of the four prepared PMMA films were estimated to be 20, 30.2, 40, and 70.8 nm, respectively. Raman and Ellipsometry Measurements. Micro-Raman experiments were performed on a Jobin-Yvon T64000 Raman spectrometer in single-monochromator mode using a Notch Rayleigh rejection filter, a 600 lines/mm diffraction grating, and a cooled CDD detector. The λ0 ) 514.5 nm radiation of an argonkrypton ion laser was used for excitation with power limited to 4 mW on the sample to minimize possible deterioration. The experiments were performed with a confocal microscope equipped with a ×50 objective (0.55 numerical aperture), which yields a spot diameter of less than 5 µm on the sample. The spectra were recorded in an air atmosphere every 5 °C starting from room temperature to 140 °C. Spectroscopic ellipsometry experiments were performed by using a Jobin-Yvon UVISEL AGMS M200 apparatus working in the wavelength range from 248 to 827 nm, equipped with a hot stage. The incident and reflected angles were fixed at 70°. For each film, ψ and ∆ ellipsometric angles were measured as a function of the temperature at a fixed wavelength. The details concerning the equations yielding the ellipsometric angles are given in ref 27.

Results and Discussion The intensity of Raman scattering recorded from thin organic films (in back-scattering geometry) on bulk silicon wafers is usually very weak.17 Indeed at normal incidence, because of the reflection coefficient for the silicon substrate at λ0, the interference between the incident and specularly reflected beams at the silicon/film interface gives rise to standing waves with a node near the silicon surface involving an excitation field experienced by the film that is much weaker than the incident one. As recently reported,15 one way to enhance the Raman signal is to cast the polymer film onto engineered SiO2/silicon substrates. In our case, the modified silicon wafer yields an enhancement of the Raman signal of the vibrational modes of the PMMA film by a factor of up to 5. The Stokes Raman spectra were measured in the 12903200 cm-1 frequency range. Figure 1 shows the spectrum (18) Schneider, B.; Stokr, J.; Schmidt, P. Polymer 1979, 20, 705. (19) Willis, H. A.; Zichy, V. J. I.; Hendra, P. J. Polymer 1969, 10, 737. (20) Liem, H.; Cabanillas-Gonzalez, J.; Etchegoin, P.; Bradley, D. D. C. J. Phys.: Condens. Matter 2004, 16, 721. (21) De Gennes, P. G. Eur. Phys. J. E 2000, 2, 201. (22) Ellison, C. J.; Torkelson, J. M. Nat. Mater. 2003, 2, 695. (23) Forrest J. A.; Dalnoki-Veress K.; Dutcher J. R. Phys. Rev. E 1997, 56, 5705. (24) Grohens, Y.; Brogly, M.; Labbe, C.; Schultz, J. Polymer 1997, 38, 5913. (25) Fontana, B. J.; Thomas, J. R. J. Phys. Chem. 1961, 65, 480. (26) Semenov, A. N. Phys. Rev. Lett. 1998, 80, 1908. (27) Tompkins, H. G.; McGahan W. A. Spectrocopic Ellipsometry and Reflectrometry; Wiley: New York, 1999.

Figure 1. Raman spectrum of a 20 nm thin film of isotactic PMMA at 25 °C deposited onto a Si/SiO2 substrate.

Figure 2. Raman spectra of a 20-nm-thick film of isotactic PMMA in the 2800-3000 cm-1 region recorded at 85, 107, and 130 °C. Table 1. Assignment of the Raman Peaks of PMMA in the 2800-3000 cm-1 Region wavenumber (cm-1) 2842 2925 2953 3000

assignment18-19 combination band involving O-CH3 (affected by Fermi resonance) combination band involving O-CH3 and symmetric νs(CH2) symmetric νs(CH) stretching vibration of O-CH3 and C-CH3 and asymmetric νa(CH2) asymmetric νa(CH) stretching vibration of O-CH3 and C-CH3

of a 20-nm-thick PMMA film at room temperature. To follow the Raman intensities’ evolution as a function of temperature, we restricted the analysis to the 2750-3100 cm-1 range because in this range the Raman signal of the polymer is intense and does not overlap with any bands coming from the substrate. In this region, the spectrum shows well-defined peaks assigned to CH2, CH3, OCH3, and CCH3 stretching vibrations18,19 (Table 1). The fact that the intensity of these bands is temperature-dependent is clearly shown in Figure 2. These observations, revealing that these bands are very sensitive to conformational and structural modifications versus temperature, also attempt to demonstrate that micro-Raman spectroscopy using engineered substrates is a new powerful technique for studying the glass transition Tg of very thin polymer films. As the transition point is approached, changes in the peak positions, peak widths, and relative intensities provide significant clues regarding the molecular motions associ-

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Figure 3. Integrated intensities of bands in the 2750-3100 cm-1 region for isotactic PMMA films of different thicknesses as a function of temperature. In the inset, we show a schematic representation of chains in contact with the attractive surface upon increasing the polymer concentration (i.e., the thickness of the film).

ated with the heating of the polymer. These changes can be correlated with temperature-induced dynamics in the PMMA chain ensembles including mobility, chain conformation, and inter- and intramolecular interactions. Recently, confocal Raman spectroscopy has been used to measure Tg in free-standing films of PS.20 The values determined for Tg were in good agreement with previous results obtained by other techniques such as Brillouin scattering and ellipsometry. We now examine the role of interfacial interactions at the air/film and film/substrate interfaces. At a given temperature, the Raman scattering intensity was integrated over the 2750-3100 cm-1 range after subtracting the baseline. The temperature dependence of the integrated intensities for the four films is displayed in Figure 3. For the 70-nm-thick film, the integrated intensity peaks around 60 °C. This value is in good agreement with the Tg expected in bulk isotactic PMMA. In addition, a faint shoulder located at T ) 65 °C is also observed. This temperature is in agreement with the change in slope observed in the same film by ellipsometry, as shown in Figure 4. This indicates that this shoulder is not an artifact. This is further confirmed by the small error bars and by the progressive shift of this shoulder toward higher temperatures when the thickness of the film is decreased. For the 40-nm-thick film, this shoulder becomes a distinct peak located at 80 °C while simultaneously the peak located at 60 °C remains. This temper-

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Figure 4. Ellipsometric determination of the Tg(h) of isotactic PMMA films of various thicknesses.

ature-dependent peak is found at T ) 105 °C for the 20nm-thick film. This clearly shows that Raman spectroscopy allows the determination of the presence of several Tg’s in a thin film and is sensitive enough to detect local Tg variations related to changes in polymer-polymer chains’ dipolar interactions. The existence of several temperatures can be inferred to be due to the presence in the film of layers exhibiting different glass-transition temperatures. A simple model where Tg would vary continuously as a function of the depth is not fully consistent with our observations. Indeed, the molecules close to the free surface of the film are supposed to have enhanced mobility;21,22 therefore, a reduction of Tg for these molecules should be expected. Progressing in the film, the glass-transition temperature Tg should reach that of the bulk23,28 to increase significantly near the film substrate interface because of strong interactions between the polymer and substrate.2,6,24 In that case, we should not observe two distinct peaks as revealed in Figure 3 but a single broad peak centered on Tg(h). Therefore, according to the results shown in Figure 3, we propose that the film is composed of two distinct layers: the first layer consisting of chains pinned at the silicon surface yields Tg1(h) ranging from 75 to 110 °C and the second one, of free bulklike entangled chains yielding “regular” bulk Tg2 (62 °C). A close look at the data reveals a third peak at 55 °C for the 20 nm film. From our measurements, it is clear that this peak overwhelms the (28) Sills, S.; Overney, R. M.; Chau, W.; Lee, V. Y.; Miller, R. D.; Frommer, J. J. Chem. Phys. 2004, 120, 5334.

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error bars. Therefore, this peak is ascribed to a physical phenomenon that is still not fully understood but is assumed to be the signature of the chains at the free surface. This will be further investigated in a forthcoming paper. It is worth noting that only one Tg(h) was recorded from ellipsometry ranging from 65 to 107 °C for the 70 and 20 nm films, respectively, thus in total agreement with previous studies.9,12,13 The kink in the thermal expansion of the film recorded by variable-temperature ellipsometry and generally claimed to be the Tg(h) of the film is assumed to be an average of the different Tg’s observed by Raman. Several groups have proposed a multilayer models4,8,23 for the Tg(h) in which a mobility gradient starting from the polymer/substrate interface to the free interface can give rise to a deviation from the bulk behavior. Let us notice that if one can measure the Tg1(h) of polymer chains pinned at the silicon surface then observing its displacement toward higher temperatures with decreasing film thickness is unexpected. This might be ascribed to the thin film samples’ preparation path used to achieve different thicknesses by spin coating. The polymer solution concentration is the factor that is varied for this purpose. From adsorption experiments25 and simulations,26 it has been shown that in a good solvent the increase in polymer concentration in solution increases the number of chains in contact with the attractive surface and yields a decrease in the number of monomer/surface contacts per chains. The configuration of the adsorbed layer is mainly due to the energetic competition for the interface of the chains close to or above the overlap C* concentration. In our system, C*, which is defined as the

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concentration that separates the dilute and semidilute regimes in polymer solutions,29 is estimated to be 10 g/L. This means that the 70, 40, 30, and 20 nm films were cast from the solutions above, close to (30 nm) and below (20 nm) the critical concentration, respectively. Thus, a modification in the thickness of the film through the polymer concentration gives rise to changes in the conformation of polymer chains at the surface involving a change in the Tg of the adsorbed layer. We therefore claim that Tg1(h) is the glass-transition temperature of the adsorbed layer, which is 110 °C for flat pinned isotactic PMMA chains and only 65 °C for brushlike pinned chains according to the initial solution concentration. In conclusion, we have shown that micro-Raman spectroscopy is a powerful in-situ technique for investigating the glass transition of polymer thin films if the polymer films are deposited on engineered SiO2/silicon substrates. Local changes on the molecular scale were clearly evidenced in the Raman spectra, and two transition temperatures were observed. The two observed Tg’s for isotactic PMMA thin films are fully consistent with a layered model with the highest Tg ascribed to the pinned chains. We have also shown that the variation in this Tg is strongly dependent on the spin-coated solution concentration. In forthcoming studies, we will attempt to separate the concentration effects and the thin film thickness effects for the different Tg(h)’s recorded by Raman enhanced spectroscopy. LA050898B (29) de Gennes, P.-G. Scaling Concepts in Polymer Physics; Cornell University Press: Ithaca, NY, 1985.