Birefringence of amorphous polyoxides: Stress-optical behavior of poly(3-methyltetrahydrofuran)

The stress-optical behavior of an unswollen elastomeric network of poly(3-methyltetrahydrofuran) was measured for elongation ratios a in the range 1.182–1.549, at several temperatures between 20 and 60°C. No evidence of strain-induced crystallization was found; moreover, the dependence of birefringence Δn on true stress f/A was linear in the interval of α investigated. Values of Δa ranged from 2.4 to 2.8 in units of 10^?24 cm³, in the temperature range studied, with a temperature coefficient 3.1 × 10^?3 K^?1. Theoretical calculations carried out with the rotational-isomeric-state model gave values of Δa noticeably smaller than the experimental results; however, a small increase in the backbone valence angles θ improved the theoretical result of Δa without worsening that of the dipole ratio. Analysis of the Δa results seems to corroborate the conclusion obtained through the study of dipole moments concerning the preference for nucleophilic attack on the less hindered α carbon in the monomer. Theoretical and experimental values of the temperature coefficient of Δa were in clear disagreement; a qualitative explanation for this discrepancy is discussed.