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

Samples of poly(vinylidene fluoride) (PVF2) and (vinylidene fluoride-tetra fluoroethylene) (VF2-VF4) copolymer were etched with a chromium-based etchi ng reagent. The etching rate was lower for the VF2-VF4 copolymer samples th an 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 th ose obtained from the direct run of the etched samples. The scanning electr on micrographs of specimens etched for 24 h indicated that only the amorpho us portion was etched without affecting the crystalline lamella. The sequen ce distribution of the PVF2 and VF2-VF4 copolymer crystals were determined by F-19 NMR measurements of the samples and their etched species. The obser ved probabilities (P-obs), calculated from the integrated area of the NMR p eaks, indicated that the crystalline lamella had a more oriented chain stru cture than that of the amorphous overlayer portion. The head-to-head defect s calculated from the aforementioned sequence analysis indicated a greater propensity in the amorphous portion than in the crystalline lamella. The eq uilibrium 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 cr ystallization, it also had a substantially high value, indicating the equil ibrium inclusion of defects in the crystal. The distribution constant incre ased with an increase in the defect content in the chain and decreased with an increase in the defect size. The sequence distribution data, analyzed t hrough a suitable melting-point depression equation, indicated a defect ene rgy of 2.25 kcal/mol for the alpha-phase PVF2 crystals and 0.68 kcal/mol fo r the beta-phase VF2-VF4 copolymer. (C) 2000 John Wiley & Sons, Inc.