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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, 1 H T 1ρ , 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 /3-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 1 H T 1ρ 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 /3-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).