Photoelasticity and structure of PMMA networks

Abstract

The optical anisotropyΔα (α=polarizability) of uniaxial deformed swollen and unswollen PMMA networks depends on the temperature. This is not in agreement with the Kuhn-Grun-theory. The temperature dependence and the negative values even above the glass transition temperatureTg can be explained by the existence of associations which cause a hindrance of the ester side group mobility. The swelling of the network in the optical isotropic solvent carbon tetrachloride does not degrade the associations and therefore not change the network structure; onlyTg decreases. The situation is more complicated if optical anisotropic solvents are used. Polar aliphatic and aromatic solvents degrade the associations to some extend and the birefringence is shifted to more positive values. An additional change is caused by specific polymer solvent interactions. Aromatic solvents form complexes with the ester side group and the aliphatic solvent chloroform completely degrades the associations, which can be explained by the formation of hydrogen bonds. Sign and magnitude of the optical anisotropy allow statements on the orientation of the solvent molecules with respect to the polymer chain or the ester side group.