Slow Relaxation of Spin-Cast Poly(methyl methacrylate) Confined in Thin Films
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Slow Relaxation of Spin-Cast Poly(methyl methacrylate) Confined in Thin Films Helen Richardson, Michele Sferrazza and Joseph L. Keddie Department of Physics, University of Surrey, Guildford, Surrey GU2 7XH, UK ABSTRACT Although the volume relaxation of bulk homopolymer glasses is thoroughly understood, many questions remain about the volume relaxation of polymers cast from solvent. Furthermore, in polymer thin films, the possible effects of confinement, surfaces and interfaces on relaxation are largely unexplored. Measurements of the film thickness of spin-cast poly(methyl methacrylate) thin films over extended periods of time, using spectroscopic ellipsometry, reveal that the characteristic time for relaxation increases with film thickness. This result is consistent with the idea of enhanced molecular mobility in thin films, and it might reflect the conformation and a reduced extent of entanglements of the polymers when spin-cast from dilute solution. INTRODUCTION Understanding structural relaxation in polymers is important for predicting long-term changes in their physical properties, such as bulk and shear moduli, density, refractive index and dielectric strength. Indeed, there exists a vast body of literature describing relaxation behavior in bulk systems [1-4], but to-date relatively few studies have been carried out on polymers confined in thin films [5]. Of these studies, most have concerned films cooled from the polymer melt and very few have concerned the relaxation of unannealed spin-cast polymer films, although it has been recognized that these are metastable forms of soft matter of scientific interest [6]. A question of fundamental importance is whether the dynamics of relaxation in confined polymers is the same as in bulk polymers [7]. The modes of relaxation in a polymer near an interface with a substrate or the atmosphere are likely to differ from those modes in the bulk [5]. Hence, the distribution of relaxation times is likewise expected to differ. Moreover, studying the relaxation behaviour in unannealed, spin-cast films is potentially very useful because it may help to explain a number of phenomena such as de-wetting [8], drying [9] and stress relaxation [10]. Structural relaxation in thin films is difficult to study experimentally. Volume changes during the relaxation of a polymer glass are typically of the order of 1 part in 104 [3,4], and so a non-invasive experimental technique for studying thin films must be sensitive to dimensional changes of less than 1 nm. The technique must be fast enough to detect thickness changes occurring over a few minutes, while it must also be stable enough to observe changes occurring over several days. Neutron [11] and X-ray [12] reflection have been shown to have sufficient resolution to probe small dimensional changes in thin films. They are thus appropriate techniques for this type of experiment. Spectroscopic ellipsometry, which likewise offers subnm resolution of film thickness [13], can obtain measurements in less than a minute but can also monitor
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