Crystalline structure and molecular mobility of PVDF chains in PVDF/PMMA blend films analyzed by solid-state 19F MAS NMR spectroscopy

The crystalline structure and molecular mobility of poly(vinylidene fluoride) (PVDF) blended with poly(methyl methacrylate) (PMMA) have been investigated using solid-state 19F magic angle spinning (MAS) NMR spectroscopy. The origin of the characteristic crystalline peaks observed for three typical polymorphs of PVDF (α-, β- and γ-forms) are explained based on the density functional theory (DFT) calculations of a model compound. Variations in the crystalline conformation of PVDF chains in the blends, the degree of crystallinity and the molecular mobility in the amorphous phase are quantitatively analyzed using the NMR spectral shapes and the 19F spin-lattice relaxation time in the rotating frame (T1ρF). The dominant crystalline form in the blends of [PVDF/PMMA]=[80/20], [70/20] and [50/50] is the α-form containing a small amount of the β-form, whereas films made with a [65/35], [60/40] and [55/45] ratio consist of the β- and γ-form. This finding indicates that PVDF chains possess more elongated conformations along their polymer chains in the latter blends. In contrast, no crystalline structure is found in [45/55], [40/60] and [20/80], which indicates that PVDF and PMMA chains are miscible at the molecular level. A minimum value of T1ρF is observed for [60/40], and the T1ρF value increases as the PVDF fraction decreases. The restricted molecular motion in the amorphous phase at lower PVDF fractions with elevated Tgs is reflected in the increase in spectral widths and the T1ρF values.