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

Abstract

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 crystallization temperature. By contrast, we now report that a stochastic behavior is observed for isothermal melt-crystallization of PTFE. In fact, on cooling very quickly the samples from the molten state to the selected crystallization temperature, crystallization during the cooling is randomly observed. Therefore, repeating the experiments until crystallization on cooling was absent, it was possible to investigate isothermal melt-crystallization of PTFE. However, crystallization is very fast; in fact, crystallization kinetics can be followed just for very low undercoolings, while as the undercooling becomes as large as about 15°C, only secondary crystallization is observed. In both cases, the data have been examined through the well-known Avrami analysis, taking into account the different physical meaning of the obtained parameters. For the first cases (actual crystallization kinetics) very low, noninteger Avrami exponents have been obtained. They have been related to the fractal dimension of the crystallites and their values to the morphological observations on PTFE. For the second cases, the typical low values of Avrami exponents of secondary crystallization are obtained. Moreover, isothermal melt-crystallization of random fluorinated copolymers of tetrafluoroethylene with either hexafluoropropylene or perfluoromethylvinylether as comonomers has been studied and compared with that of PTFE.

Supercritical CO2 processing and annealing of polytetrafluoroethylene (PTFE) and modified PTFE for enhancement of crystallinity and creep resistance

“…After annealing at ambient pressure in N 2 either at 290 or 330 8C, DH f was not distinguishable from as received samples within experimental error. Evidently, thermal annealing is insufficient to increase the degree of crystallinity perhaps due to extremely rapid crystallization [28].…”

Extreme thermal behaviors of polytetrafluoroethylene and random tetrafluoroethylene fluorinated copolymers

“…In fact, it was reported that for PTFE, crystallization is so rapid that the sample crystallizes during the cooling from the melt to the selected crystallization temperature 25. Nevertheless, in an article in press, 26 we have shown that it is possible, by carefully controlling the experimental conditions, to perform isothermal melt‐crystallization experiments even for this polymer. Moreover, in the past, the equilibrium melting temperature of PTFE was determined from data of melting temperature as a function of the lamellar thickness, 27 a method strictly related to that used by Hoffmann and Weeks 22.…”

Studies on Wear Properties of Polystyrene in the Presence of Fibrillated Network of Polytetrafluoroethylene

“…Several authors have reported that superheating is higher (Tm ≈ Tm0 + 10 ∘ C, where the equilibrium melting temperature Tm0 has been reported in the range of 332-336 ∘ C) [15][16][17][18]. Pucciariello et al applied MDSC in the quasi-isothermal mode to investigate melting and crystallization of PTFE [19,20]. They confirmed the strong differences in the melting temperature of meltcrystallized and as-polymerized PTFE.…”