BIREFRINGENCE OF AMORPHOUS POLYMERS .8. VISCOELASTICITY AND BIREFRINGENCE OF POLYISOPRENE

The complex Young's modulus, E*(omega), and the complex strain-optical coefficient, O*(omega), which is the ratio of the birefringence to the strain, were measured for polyisoprene (PIP) over a frequency range of 1 similar to 130 Hz and a temperature range of 22 similar to -100 degrees C. The imaginary part of O*, O'', was positive at low frequencies and negative at high frequencies. The real part, O', was always positive and showed a maximum. The complicated behavior of O* could be understood by the assumption that E* = E*(R) + E*(G) and O* = C(R)E*(R) + C(G)E*(G), where E*(R) and E*(G) were complex quantities and C-R and C-G were constants. The C-R value, equal to the ordinary stress-optical coefficient measured in the rubbery plateau zone, was 2.0 X 10(-9) Pa-1. The C-G value, defined as the ratio O''/E'' in the glassy zone, was -1.1 X 10(-11) Pa-1. The E*(G), which was the major component of E* in the glassy zone, showed almost the same frequency dependence as that of polystyrene and polycarbonate. The E*(R), which was dominant in the rubbery zone, was described well by the bead-spring theory. The temperature dependence of the E*(G) was stronger than that of the E*(R). This difference caused the breakdown of the thermorheological simplicity for E* and O* around the glass-to-rubber transition zone. (C) 1995 John Wiley & Sons, Inc.