Chain dynamics, conformations, and phase transformations for form III polymorph of isotactic poly(1-butene) investigated by high-resolution solid-state C-13 NMR spectroscopy and molecular mechanics calculations

摘要

High-resolution solid-state C-13 NMR spectroscopy and MM3 molecular mechanics calculations have been applied to investigate side-chain conformations, side- and main-chain dynamics, and phase transformations of the form III polymorph of isotactic-poly(l-butene) (i-PB). MM3 calculations indicate that the t-g' side-chain conformation is much more stable than the g-g and g-t side-chain conformations in the crystal lattices of form III. The 13C cross-polarization magic-angle spinning (CPMAS) NMR spectrum at 199 K shows that the methyl group in the side chain adopts only the t-g' conformation. Upfield shift for the 13C methyl signal above 338 K indicates dynamic disorder in the side-chain conformations. The 13C two-dimensional exchange NMR reveals that the polymer chains in form III consist of two components. The 74% polymer chains execute the 90degrees helical jump motion with an activation energy of 79.3 +/- 5.6 kJ/mol, while the residual chains are immobile. It is found that the 90degrees helical jump motion begins to occur at the temperature very close to the glass-transition temperature (T-g). Upfield shifts for the 13C main-chain signals at 338 K indicate the motional mode change from the 90degrees helical jump motion to "more or less" rotational or jump motion with different angle displacements. Above 338 K, two phase transformations from form III to forms Il and I' are confirmed in the 13C CPMAS spectra. The H-1 spinlattice relaxation times in the rotating frame show that the transformation from form III to form I' proceeds within the form III phase via the crystal-to-crystal process, whereas the line-shape analyses of the 13C direct-polarization magic-angle spinning (DPMAS) NMR spectra indicate that the transformation from form III to form II proceeds via the quasi-melting(recrystallization process. The frozen chain conformations in the amorphous phase below T-g are also investigated on the basis of the 13C chemical shifts. It is suggested that most of the main chains adopt the helical (tg')(n) conformation, and the residual part adopts the (tt) conformation. [References: 44]