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 Δ n on true stress f / A was linear in the interval of α investigated. Values of Δ a ranged from 2.4 to 2.8 in units of 10 −24 cm 3 , 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 Δ a noticeably smaller than the experimental results; however, a small increase in the backbone valence angles θ improved the theoretical result of Δ a without worsening that of the dipole ratio. Analysis of the Δ a 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 Δ a were in clear disagreement; a qualitative explanation for this discrepancy is discussed.