Film thickness dependence of the kinetic stability and glass transition temperature of ultrastable and rejuvenated PS films prepared by vapor deposition*

We have used ellipsometry to study the properties of ultrastable and rejuvenated films of near-oligomeric PS with thickness ranging from 10-100 nm.  The measured T_g of the rejuvenated films show a reduced T_g value with decreasing thickness as seen for many other PS samples.  While a T_g measured from cooling is not possible for the as-deposited stable glass system, the onset temperatures (T_ons) for rejuvenation show the same film thickness dependence such that the difference between T_g and T_ons is a constant temperature.  The thin films also show  evidence for a portion of the sample already being in the normal state before the first heating.  The results are discussed in terms of studies of thin films of stable molecular glass as well as thin films of high M_w PS produced by spin casting.