Viscoelasticity and birefringence of polyisoprene

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Abstract
The complex Young's modulus, E*(ω), and the complex strain‐optical coefficient, O*(ω), which is the ratio of the birefringence to the strain, were measured for polyisoprene (PIP) over a frequency range of 1 ∼ 130 Hz and a temperature range of 22 ∼ −100°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* = ER* + EG* and O* = CRER* + CGEG*, where ER* and EG* were complex quantities and CR and CG were constants. The CR value, equal to the ordinary stress‐optical coefficient measured in the rubbery plateau zone, was 2.0 × 10−9 Pa−1. The CG value, defined as the ratio O″/E″ in the glassy zone, was −1.1 × 10−11 Pa−1. The EG*, which was the major component of E* in the glassy zone, showed almost the same frequency dependence as that of polystyrene and polycarbonate. The ER*, which was dominant in the rubbery zone, was described well by the bead‐spring theory. The temperature dependence of the EG* was stronger than that of the ER*. This difference caused the breakdown of the thermorheological simplicity for E* and O* around the glass‐to‐rubber transition zone. © 1995 John Wiley & Sons, Inc.