Measurement of supercritical CO2 plasticization of poly(tetrafluoroethylene) using a linear variable differential transformer


From measurements utilizing a linear variable differential transformer (LVDT), poly(tetrafluoroethylene) is found to undergo unusually high linear dilation in CO2 at high temperatures and pressures. The extent of dilation and the dilational profiles are shown to depend strongly on molecular weight and crystallinity. It is also shown that the conditions at which melting and sintering occur can be identified from LVDT measurements. After the first heating cycle in CO2, unsintered samples (PTFE-230, -600, -1000, and U-PTFE) show a Tm identical to PTFE (327°C) instead of the higher pre-CO2 Tm. This observation indicates that these samples were sintered during the LVDT measurements. ΔHf was constant for pre-CO2 PTFE-230, -600, and -1000, but after sintering, crystallinity decreased with increasing molecular weight. In contrast to the precipitous drop for U-PTFE, ΔHf for PTFE-230, -600, and -1000 remained above 70 J/g. Chain entanglement effects on crystallization are modest for these low molecular weight materials. For lower molecular weight samples (PTFE-230, -600, and -1000), Tm depression and melt flow were observed. For the high molecular weight samples (PTFE and U-PTFE), Tm depression in CO2 also occurs, but exceptionally high molecular weight and chain entanglement preclude melt flow. Both PTFE and U-PTFE are unusual in that melting is signaled by an increase in dilation rather than the usual loss of shape or "slumping" due to melt flow. Despite the hydrostatic pressure effect, PTFE melting point depression in supercritical CO2 was found to be approximately 30°C. © 2008 American Chemical Society.

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