The effects of molecular orientation on the physical aging and mobility of polycarbonate?solid state NMR and dynamic mechanical analysis

Solid-state NMR and dynamic mechanical (DMA) measurements were performed on a series of uniaxially hot-drawn bisphenol-A polycarbonate samples in order to determine the effects of stretching on the structure, mobility, and local orientation environment. Proton spin-lattice relaxation times, 1H T1ρ, for the phenylene carbon protons were fitted to a biexponential decay function, and both the long and short relaxation times initially increased with stretching. Intensity data indicated an increase in the number of short relaxation time protons and a decrease in the number of long relaxation protons with orientation. Similarly, DMA spectra showed that the β-relaxation strength also increased with drawing, which implied an increase in the number of localized segmental relaxations. It is theorized that the long and short 1H T1ρ relate to protons within tightly packed "cooperative domains," and to those with greater localized free-volume, respectively. Stretching is known to distort the free-volume distribution, causing a decrease in the mean free-volume but an increase in the number of larger, more elliptical holes. This is expected to cause a decrease in the α-transition mobility (due to larger cooperative domains) and an increase in the β-mobility (due to the increase in the number of β-relaxing segments associated with the larger free-volume holes). These predictions are consistent with results recently reported by Shelby and Wilkes on the physical aging and creep behavior of these samples (M. D. Shelby & G. L. Wilkes, Polymer 1998, 39, 6767; M. D. Shelby & G. L. Wilkes, J Polym Sci Part B: Polym Phys 1998, 36, 2111). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 32–46, 2001