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Melt crystallization of ferroelectric copolymers of poly(vinylidene-fluoride-trifluoroethylene)
This dissertation is a study of the phase transition and melt crystallization of ferroelectric copolymers of poly(vinylidene fluoride-trifluoroethylene) (P(VDF-TrFE)) containing 83 mol% poly(vinylidene fluoride). The microstructure of crystallized P(VDF-TrFE) are characterized by vibrational spectroscopy to determine their chain conformation distribution, wide-angle X-ray diffraction (WAXD) to measure the crystal structure and determine the content of $\alpha$ and $\beta$ crystal phases. Differential scanning calorimetry (DSC) is used to measure the transition temperatures and their enthalpies. P(VDF-TrFE) is a polymorphic system exhibiting a Curie transition below its melting temperature, at which the copolymer undergoes a long range conformational change from a predominantly trans structure ($\beta$ phase) which can be ferroelectric, to a structure which has a mixture of trans and gauche conformations and is called the paraelectric $\alpha$ phase. Variable temperature FTIR studies have been made to follow the phase transition and to understand the multiple phase behavior that has been suggested for these copolymers. Factor analysis has been applied to the FTIR data to determine the number of crystal phases present for this copolymer composition. Melt crystallization under nonisothermal and isothermal conditions has been used to modify the structure of the copolymers and change both the Curie and melting temperatures. It is found that with slow cooling rates from the melt through the crystallization temperature, there is a stabilization of the paraelectric $\alpha$ phase which is then retained to different degrees on passing through the Curie transition on cooling. Under all crystallization conditions the copolymers exhibit two Curie transitions on cooling whose temperatures and intensities vary with cooling rate and crystallization time. The Curie temperature on cooling and subsequent heating is decreased upon slower cooling, reflecting an increase in the gauche chain conformers as evidenced by FT-Raman spectroscopy. WAXD data show that there are $\alpha$ and $\beta$ phase crystals present in samples that have been cooled slowly, accompanied by an increase in interplanar spacing which indicates that the $\beta$ phase also contains gauche defects. Longer isothermal crystallization at 135$\sp\circ$C shows also that there is an increase in $\alpha$ phase content with longer crystallization time, though when crystallization is carried out at 135$\sp\circ$C there is always a coexistence of $\alpha$ and $\beta$ phases in the final room temperature structure. Highly oriented P(VDF-TrFE) copolymers were prepared by solid-state coextrusion to aid in understanding the vibrational assignments in the copolymer. Infrared dichroism and WAXD were used to assess the sample orientation and measure the transition moment angles for some of the vibrations in the spectrum.
Bakeev, Katherine Antolin, "Melt crystallization of ferroelectric copolymers of poly(vinylidene-fluoride-trifluoroethylene)" (1993). Doctoral Dissertations Available from Proquest. AAI9408254.