Anisotropy of extremely monodisperse polymer stable glass thin films

In the twelve years since their introduction, ultrastable molecular glasses have become a topic of great interest. Thin films prepared by vapour deposition at substrate temperatures slightly below T_g can exhibit increased densities and stabilities similar to those expected for isotropic glassy films aged hundreds or thousands of years. However, these ultrastable glasses may not truly present the same state as aged glasses. In many ultrastable glasses, the samples display anisotropy, unless steps are taken to make films only in a narrow range of production parameters such as substrate temperature and deposition rate. Simulations have suggested (Lin et. al. J. Chem. Phys. 140, 204504 (2014)) that evaporated oligomeric samples may provide a way to avoid this anisotropy and produce truly isotropic stable glass. In this case, the length of oligomeric samples can greatly influence anisotropy. Using our recently demonstrated technique for making polymer stable glasses, we present an experimental study of extremely monodisperse, ultrastable polymeric/oligomeric thin films. We use spectroscopic ellipsometry to measure birefringence caused by anisotropy in such samples as a function of the stability (fictive temperature) and the oligomeric size.