Intrinsic birefringence of poly(trimethylene terephthalate)

Highly oriented poly(trimethylene terephthalate) (PTT) fiber has a low birefringence that is unexpected for an aromatic polyester with a high refractive index. To explain this observation, the intrinsic birefringence Δ n of PTT crystal was calculated from its bond polarizabilities to be 0.029. This Δ n is almost an order of magnitude smaller than poly(ethylene terephthalate)'s value at 0.22, although both polymers have nearly identical crystal refractive indices. The small Δ n is due to the arrangement of PTT's methylene groups in gauche conformations, causing the chain‐repeating unit to be tilted ∼53° away from the c axis toward the basal plane. Because of the small Δ n , the crystalline‐phase orientation made only a small contribution to the overall birefringence despite the fiber's high crystallinity and orientation. To understand the effect of the number of methylene groups on polyester optical anisotropy, the Δ n 's of a series of poly( m ‐alkylene terephthalates) with m = 2–5 were compared and correlated with ψ: an angle made by the normal of the benzene ring with the crystal's axis. As ψ′ decreases, Δ n of the polyesters diminishes.