Study of spin–lattice relaxation and local motion in dissolved poly[2,2‐propanediylbis(3,5‐dimethyl‐4, hydroxyphenyl)carbonate

Abstract

AbstractProton and carbon‐13 spin–lattice relaxation times are reported for 10‐wt % solutions of tetramethyl bisphenol‐A polycarbonate. The relaxation times for both nuclei were measured at two Larmor frequencies and as a function of temperature. These relaxation times are interpreted in terms of three motions: segmental motion, restricted rotational diffusion, and backbone methyl‐group rotation. The Hall–Helfand correlation function is used to describe the segmental motion. Internal rotation is described by the usual Woessner approach and restricted anisotropic rotational diffusion by the Gronski approach. As demonstrated by its higher activation energy, correlated segmental motion appears to be slower than the unsubstituted polycarbonate of BPA. In addition, the single‐transition processes seem to be still less important than correlated backbone transitions. Phenylene‐group rotation is described in terms of restricted rotational diffusion instead of complete anisotropic rotation. The time scale for backbone methyl‐group rotation is comparable to that in BPA, a fact indicative of weaker cooperativity between this motion and the other motions. Rotation of the methyl group attached to the phenylene ring is too fast to significantly contribute to relaxation except by partially averaging the dipole–dipole interactions. The higher activation energies for segmental motion observed in solution for this methyl‐substituted polycarbonate relative to the unsubstituted polycarbonate parallel a significant increase in the glass transition temperature observed for the substituted material. The restricted pheylene‐group rotation in solution is also parallelled by a large upward shift of the low‐temperature loss peak in the glassy polymer.