Effect of Free Surfaces on the Glass Transition Temperature of Thin Polymer Films

Abstract

We report the first measurements of the glass transition temperature T g for thin freely standing polystyrene (PS) films. We have used Brillouin light scattering to measure T g for freely standing films of different thicknesses. We find that T g decreases linearly with film thickness h for h # 700 Å, with a reduction of 70 K for a film with h 290 Å. These measurements characterize unambiguously the effects of the free surface on T g of thin polymer films. Results are compared to similar results for supported PS films [Keddie et al., Europhys. Lett. 27, 59 (1994)], and we find that their measured values are influenced strongly by the substrate. [S0031-9007(96)01093-9] PACS numbers: 64.70. Pf, 68.60.Bs, 78.35.+c From organic liquids to metals to polymers, almost any substance can be transformed into a glassy state. One of the fundamental parameters describing a glass is the glass transition temperature T g . For temperatures greater than T g , the material is a viscous liquid. As the liquid is cooled below this temperature, the material forms an amorphous solid. Despite the technological significance of glass forming materials, the glass transition itself is poorly understood. In particular, it is not clear whether the glass transition is a thermodynamic transition [1] or a purely kinetic phenomenon [2]. Adam and Gibbs [3] introduced the concept of cooperative rearrangement in an attempt to unify these two views of the glass transition by demonstrating that such cooperativity, coupled with a thermodynamic glass transition, resulted naturally in system dynamics such as those described by the WLF equation [4] for temperatures near T g . Donth [5] has estimated the size of such cooperatively rearranging regions (CRR) to be of the order of 10 Å for a number of glass forming materials. The introduction of such a length scale suggests that studying samples with dimensions comparable to the CRR may lead to the observation of finite size effects.An attractive choice for studies of finite size effects on the glass transition is the use of polymer molecules. For polymers the molecule can be characterized by the endto-end distance R EE ϳ 2R g , where R g is the radius of gyration of the molecule. R EE is typically much larger than the size of the CRR, and R EE can be adjusted by changing the molecular weight M w of the molecules. Polymer molecules can be confined by preparing samples in a thin film geometry. The relevant length scale is the film thickness h which can be adjusted to be comparable to, much larger than or smaller than R EE so that the effects of chain confinement can be investigated. Glassy polymer films can be cast onto any substrate, even those that the polymer melt itself does not wet.The first direct measurements of the glass transition temperature in thin polymer films were performed recently by Keddie, Jones, and Cory [6]. For polystyrene (PS) films on hydrogen-passivated Si(111), the measured T g values were lower than the bulk value T g ͑bulk͒ for films with thicknesses h # 400 Å. Because the changes ...

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