FTIR Dichroism Study of Orientation Relaxation Using Isotopically Labelled Polystyrene Stars.

Stress in a polymeric material is attributable to orientational anisotropy of the chain, as induced by a strain. This anisotropy, and its relaxation, can be measured directly by infrared dichroism, with the added advantage that the orientation of specifically labelled parts of the molecule can be followed. In this paper IR dichroism has been used to determine the orientational relaxation of six-arm polystyrene stars. Block copolymers of perdeuterated and protonated styrene were used to separate the relaxation of the region surrounding the branch point, the arm centers, and the chain ends. The results are compared with analogous measurements on linear chains, and the effects of branching are discussed. The orientation and its relaxation in the neighborhood of the branch point appears to be higher than the ends in the early stages, but to relax more rapidly. Residual orientation of the chain ends provides evidence of incomplete relaxation to a value correlated with the orientation of the surrounding medium.