CP/MAS C NMR analysis of the structure and hydrogen bonding of melt-crystallized poly(vinyl alcohol) films

The structure and hydrogen bonding of the melt-crystallized atactic poly(vinyl alcohol) (A-PVA) films, which were carefully prepared without significant thermal degradation, have been characterized by CP/MAS ¹³C NMR spectroscopy. The ¹³C spin-lattice relaxation analysis has revealed that there exist three components with different T_1C values, the crystalline, less mobile noncrystalline and mobile noncrystalline components, in good accord with the results for different PVA samples previously reported. It should be noted that the T_1C values of the crystalline and noncrystalline components are appreciably smaller for the melt-crystallized films than those for the un-annealed and annealed samples prepared by casting from the aqueous solution. The ¹³C NMR spectra of the crystalline and noncrystalline components are separately recorded by using the difference in T_1C and their CH lines are successfully resolved into three and seven constituent lines by the least-squares curve fitting, respectively. Moreover, the statistical analysis of the integrated intensities of the constituent lines thus obtained enables to determine the probability f_a for the formation of intramolecular hydrogen bonding in the successive two OH groups along each chain and another probability f_t of the trans conformation for the crystalline and noncrystalline components. It is found that the f_a value is relatively larger for the melt-crystallized films than those for the un-annealed and annealed samples. On the basis of these results, the features of the melt-crystallization and the resulting crystalline-noncrystalline structure are discussed by particularly considering effects of intra- and inter-molecular hydrogen bonding on the crystallization.