A natural length scale for the glass transition of conjugated polymer film

A study is reported of the anomalous glass transition temperature, T-g, in thin free-standing conjugated polymer films revealed by confocal Raman scattering. Full details of the dependence of T-g on film thicknesses d ranging from less than to greater than the polymer average end-to-end distance R-EE are presented. In contrast to ultrathin films (d < R-EE), which always exhibit a reduction of their T-g values due to the chain confinement effect, thick films within a certain range of thicknesses distinguish themselves by presenting anomalous enhancement of two T-g values; one is interpreted as at the interface, and the other in the core. This study is strongly suggestive of a new length scale L, which lies in the range of most polymer electronic devices studied over which the physical properties of the film are greatly perturbed by the presence of free surfaces.