CHAIN DYNAMICS IN THE CRYSTALLINE ALPHA-PHASE OF POLY(VINYLIDENE FLUORIDE) BY 2-DIMENSIONAL EXCHANGE H-2 NMR

Two-dimensional (2D) exchange deuteron NMR has been applied to study the ultraslow chain motion in the crystalline alpha-phase of poly(vinylidene fluoride) and specifically its very unusual alpha-relaxation that occurs in the crystalline regions at ca. 370 K. Without the need for interfacing a model of motion, this technique demonstrates that the chain motion associated with this alpha-relaxation is reorienting the C-H-2 bond directions by uniquely defined angles of 67 or 113-degrees. Owing to the high angular resolution of 2D exchange H-2 NMR (+/- 0.5-degrees), it is shown that these reorientation angles are most consistent with the crystalline structure of Takahashi et al. (Macromolecules 1983, 16, 1588). Considering also the dielectric relaxation results of Miyamoto et al. (J. Polym. Sci.: Polym. Phys. Ed. 1980, 18, 597), we arrive at a unique conclusion about the dynamics: the chain motion is characterized by an electric dipole moment transition only along the molecular direction and by a conformational change TGTGBAR <--> GBARTGT, yielding a reorientation angle of 113-degrees for the C-H-2 bond directions.