Polyvinylidene Fluoride - Chlorotrifluoroethylene - Trifluoro Ethylene Preparation And Crystallization And Thermal, Mechanical Properties

Several poly(vinylidene fluoride-chlorotrifluoroethylene-trifluoroethylene) [P(VDF-CTFE-TrFE)] terpolymers, including 68 mol % VDF, were prepared by partial reduction of chlorine in poly(vinylidene fluoride-chlorotrifluoroethylene) [P(VDF-CTFE)] copolymers. The effect of CTFE content and crystallization conditions (1. crystallized from the solution in DMF at 35℃; 2. annealed at temperatures 5 -10℃below their respective melting points.) on crystallization behavior of the terpolymers was investigated by XRD, FT-IR, and DSC. Thermal and mechanical properties of the solution-cast terpolymer films have also been investigated in an integrated way by a series of thermal analysis techniques, including TGA, DMA and TMA.Results show that with increasing CTFE content the terpolymers contain less (even disappeared) of theβ-phase which has all-trans chain conformation, and more of theγ-phase is found, which becomes prominent with trans-trans-trans-gauche conformation. Crystallinity, crystal size, fusion enthalpy and melting temperatures of the terpolymers decrease with an increase in CTFE content. Compared with annealed terpolymers, the terpolymers crystallized from the solution at 35℃include more polar components that contain more trans conformation, but have lower crystallinity, melting temperatures, fusion enthalpies and smaller crystal size. These results suggest that crystallization from the solution may be helpful in forming polar crystal, whereas an annealing process at high temperature is beneficial in perfecting the crystal structure.The terpolymers exhibited a relatively stable behavior with a single degradation process at temperature above 400℃. A lower CTFE content gives the polymer slightly higher thermal stability probably due to the instability of C-Cl bond during heating. DMA detected a relatively weakαa relaxation and a broadαc relaxation in the samples of low CTFE content. These two relaxation processes completely mixed together in the samples of high CTFE content, revealing the crystalline structures in the polymer become a more imperfect and diffuse state as CTFE units increasing. The polymer with less CTFE units possesses an enhanced stiffness due to its higher degree of crystallinity. A contraction process after a slight amount of thermal expansion upon heating is detected by TMA, due to the release of internal tensile strain/stress generated during solidification of the films. The higher crystallinity of the polymer film generated the greater strain/stress, leading to the larger degree of shrinkage. Also, the higher melting point of the polymer with less CTFE units allows the film soften at a higher temperature.