Nonisothermal Crystallization of Polytetrafluoroethylene in a Wide Range of Cooling Rates

Abstract

Compared to other semicrystalline polymers, PTFE demonstrates a very fast crystallization process on cooling. This study explores for the first time the nonisothermal PTFE ultrafast crystallization under tremendously fast cooling rates (up to 800,000 K·s(-1)) achieved by using fast scanning calorimetry (FSC) and ultra-fast scanning calorimetry (UFSC). Regular DSC was also used to get crystallization at slower rates. The data obtained on a wide range of cooling rates (over 8 orders of magnitudes) help to get new knowledge about crystallization kinetics of PTFE. Both FSC and UFSC data show that it is impossible to bypass the crystallization and thus to reach a metastable glassy state even for the fastest cooling rate employed (800,000 K·s(-1)). The crystals formed under such conditions are slightly less stable than those produced under slower cooling rates, as reflected by a shift of the melting peak to lower temperature. The difference in crystal morphologies was confirmed by SEM observations. The variation of the effective activation energy (Eα) with the relative extent of crystallization reveals that PTFE crystallization follows a transition from regime II to regime III around 315-312 °C. Corroborated temperature dependences of Eα obtained respectively for crystallizations under slow and fast cooling rates were combined and fitted to the theoretical dependence of the growth rate derived from the Hoffman-Lauritzen theory.

Physical Processes

“…The cold crystallization kinetics is limited primarily by diffusion so that cold crystallization data contain mostly information about this process and, thus, should afford its better description in the form of a more accurate value of U * . More importantly, this and other examples [108,119,120] clearly demonstrate that both melt and glass crystallization kinetics can be fitted successfully with a single set of the Hoffman-Lauritzen parameters.…”

Isoconversional Kinetics by Fast Scanning Calorimetry

“…The strong E α -dependence reveals the complexity of the crystallization mechanism. A progressive transition from nucleation to a crystal growth controlled process has been suggested [33]. These increasing negative values correspond to a nucleation control above 312 C. Very fast homogeneous and heterogeneous nucleation occurs which allows the formation of PTFE crystals even at the highest cooling rates used.…”

“…The approach has been explored further by combining DSC and FSC measurements in a study of the crystallization kinetics of polytetrafluoroethylene (PTFE) by Bosq et al [33,34]. The time scale of the experiments is about $1 h for DSC measurements and about $0.5 s for FSC measurements as shown in Fig.…”

“…By combining nonisothermal DSC and FSC data on crystallization of PTFE obtained in a wide range of cooling rates it is possible to cover the temperature range of more than 100 K, which is significantly wider than $3 K in regular isothermal DSC and $50 K in regular nonisothermal DSC measurements. The temperature range can be extended to more than 200 K by combining DSC, FSC and ultra-fast scanning calorimetry (UFSC) data [33]. UFSC was performed at the University of Rostock on a calorimeter described elsewhere [36,37].…”

“…UFSC was performed at the University of Rostock on a calorimeter described elsewhere [36,37]. It is noteworthy [34] that crystallization of PTFE has been so fast that it was impossible to avoid it even when cooling by UFSC at 800,000 K s À1 [33] and $10 6 K s À1 using UFSC. An advanced isoconversional method has been applied to the data on the PTFE crystallization during cooling from the melt.…”