Dynamic birefringence of oligostyrene: A symptom of "polymeric" mode

The dynamic birefringence and the dynamic viscoelasticity of an oligostyren e, A1000, whose molecular weight (M-w = 1050) was comparable to the Kuhn se gment size, M-K, were examined near and above the glass-transition temperat ure in order to characterize polymeric features of very short chains with M similar to M-K. The complex shear modulus, G*(omega), was similar to that for supercooled liquids: No polymeric modes such as the Rouse mode were det ected at low frequencies of viscoelastic spectrum. On the other hand, the s train-optical coefficient was found to be negative in the terminal flow zon e and positive in the glassy zone. Because the negative birefringence of po lystyrene is originated by polymeric modes associated with chain orientatio n, the present results indicate that polymeric modes exist and become domin ant for birefringence in the terminal flow. The data were analyzed using a modified stress-optical rule: The modulus and the strain-optical ratio were separated into polymeric (rubbery) and glassy components. The total modulu s, G*(omega), was mostly due to the glassy component, G(G)*(omega), resulti ng in the positive birefringence. G(G)*(omega) for A1000 agreed with that f or high M polystyrenes when compared at a comparable reduced frequency scal e. The polymeric component, G(R)*(omega), giving rise to the negative biref ringence was lower than G(G)*(omega) over the whole frequency range but its contribution to the birefringence exceeded that of the glassy component at low frequencies because of the larger optical anisotropy and longer charac teristic relaxation time of the former. The limiting modulus of G(R)* at hi gh frequencies was about 3 times lower than that for high M polystyrenes, i ndicating that the main-chain orientation of the oligostyrene on instantane ous deformation was reduced compared with that of high M polystyrenes. (C) 2000 John Wiley & Sons, Inc.