Local and global dynamics of atactic polypropylene melts by multiple fieldC-13 nuclear magnetic resonance

C-13 NMR T-1, NOE, and T-1 rho were measured for an atactic polypropylene ( aPP) melt at 5, 25, 75, and 125 MHz C-13 Larmor frequencies from 43 degrees C to 260 degrees C. The data were fit very well with a model for C-H vecto r reorientation composed of very fast librational relaxation, intermediate segmental relaxation, and a normal mode contribution described by Rouse dyn amics modified for entanglement effects. The longest relaxation time calcul ated from this fit agrees reasonably well with the value obtained from visc oelastic measurements on similar systems. The amplitude of the normal mode contribution for aPP is much smaller than that found for polyethylene in a previous study. The temperature dependence of the segmental dynamics is ver y similar to that of the viscosity indicating that conformational transitio ns are the fundamental motions for flow for aPP. The width of the distribut ion of relaxation times of segmental dynamics remains unchanged at about 1. 3 decades from T-g + 45 to T-g + 250 K. (C) 2000 American Institute of Phys ics. [S0021-9606(00)51131-3].