EFFECT OF FREE SURFACES ON THE GLASS TRANSITION TEMPERATURE OF THIN POLYMER FILMS

James Forrest, Kari Dalnoki-Veress, Jim Stevens, and John Dutcher,
Phys. Rev. Lett. 77, 2002-2005 (1996); ibid, 77, 4108 (1996).

ABSTRACT

We have performed the first measurements of the glass transition temperature T_g for thin freely-standing polystyrene (PS) films. We have used Brillouin light scattering (BLS) to measure T_g for freely-standing films of different thicknesses. We find that the T_g values are reduced from the bulk value for films with thicknesses h < 700 angstroms. The reduction in T_g is a linear function of the film thickness, with a reduction of 70 K for a film with h = 290 angstroms. These measurements are the first to characterize unambiguously the effects of the free surface on the glass transition temperature of thin polymer films. The 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 presence of the substrate.

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