On melt-crystallization of polytetrafluoroethylene and of random fluorinated copolymers of tetrafluoroethylene

Through differential scanning calorimetry, isothermal crystallization from the melt of polytetrafluoroethylene (PTFE) has been investigated. PTFE was regarded as one of the polymers for which crystallization is so rapid that the samples crystallize during the cooling from the melt to the selected cr ystallization temperature. By contrast, we now report that a stochastic beh avior is observed for isothermal melt-crystallization of PTFE. In fact, on cooling very quickly the samples from the molten state to the selected crys tallization temperature, crystallization during the cooling is randomly obs erved. Therefore, repeating the experiments until crystallization on coolin g was absent, it was possible to investigate isothermal melt-crystallizatio n of PTFE. However, crystallization is very fast; in fact, crystallization kinetics can be followed just for very low undercoolings, while as the unde rcooling becomes as large as about 15 degrees C, only secondary crystalliza tion is observed. In both cases, the data have been examined through the we ll-known Avrami analysis, taking into account the different physical meanin g of the obtained parameters. For the first cases (actual crystallization k inetics) very low, noninteger Avrami exponents have been obtained. They hav e been related to the fractal dimension of the crystallites and their value s to the morphological observations on PTFE. For the second cases, the typi cal low values of Avrami exponents of secondary crystallization are obtaine d. Moreover, isothermal melt-crystallization of random fluorinated copolyme rs of tetrafluoroethylene with either hexafluoropropylene or perfluoromethy lvinylether as comonomers has been studied and compared with that of PTFE. (C) 1999 John Wiley & Sons, Inc.