An equilibrium study on the distribution of structural defects between the lamellar and amorphous portions of poly(vinylidene fluoride) and (vinylidene fluoride-tetra fluoro ethylene) copolymer crystals

Abstract

Samples of poly(vinylidene fluoride) (PVF₂) and (vinylidene fluoride-tetra fluoroethylene) (VF₂-VF₄) copolymer were etched with a chromium-based etching reagent. The etching rate was lower for the VF₂-VF₄ copolymer samples than for the PVF₂ samples. The melting point and enthalpy of fusion increased with increased etching time of the etched specimen. This was also true for the melt-quenched (etched) samples, whose values were always lower than those obtained from the direct run of the etched samples. The scanning electron micrographs of specimens etched for 24 h indicated that only the amorphous portion was etched without affecting the crystalline lamella. The sequence distribution of the PVF₂ and VF₂-VF₄ copolymer crystals were determined by ¹⁹F NMR measurements of the samples and their etched species. The observed probabilities (P_obs), calculated from the integrated area of the NMR peaks, indicated that the crystalline lamella had a more oriented chain structure than that of the amorphous overlayer portion. The head-to-head defects calculated from the aforementioned sequence analysis indicated a greater propensity in the amorphous portion than in the crystalline lamella. The equilibrium constant (K) for the distribution of defects between the lamella and amorphous portion of the crystal varied from 0.7 to 0.9. It was higher at a higher quenching rate of the crystallization, and in the isothermal crystallization, it also had a substantially high value, indicating the equilibrium inclusion of defects in the crystal. The distribution constant increased with an increase in the defect content in the chain and decreased with an increase in the defect size. The sequence distribution data, analyzed through a suitable melting-point depression equation, indicated a defect energy of 2.25 kcal/mol for the -phase PVF₂ crystals and 0.68 kcal/mol for the β-phase VF₂-VF₄ copolymer.