Crystallization of Polytetrafluoroethylene in a Wide Range of Cooling Rates: Nucleation and Diffusion in the Presence of Nanosilica Clusters.

Polytetrafluoroethylene (PTFE) is a polymer that displays exceptional properties. This synthetic fluoropolymer is also known to crystallize very fast upon cooling. The present work highlights for the first time the influence of nanosilica clusters on PTFE crystallization at fast cooling rates (up to 5000 K·s-1). The silica was synthesized from aqueous silicate solution and the surface modification was performed using TriEthoxyFluoroSilane (TEFS). In order to understand the crystallization behavior of PTFE/silica nanocomposite at a fast cooling rate, the measurements were carried out by Fast Scanning Calorimetry (FSC). The data were consequently combined with the measurements performed by conventional Differential Scanning Calorimetry (DSC). Interestingly, the results displayed variation of the crystallization behavior for the nanocomposite at fast cooling rates compared to slow cooling rates. The differences in crystal morphologies were then observed by Scanning Electron Microscopy (SEM) after slow and fast cooling rates. Finally, the effective activation energies (Eα) obtained from the crystallization under various cooling rates were combined in order to obtain one set of Hoffman-Lauritzen parameters. This procedure allowed us to show that the crystallization of PTFE in the presence of silica is promoted or hampered according to the cooling rates employed.

Melt and glass crystallization of PDMS and PDMS silica nanocomposites.

Silica nanoclusters were homogeneously dispersed into an end-linked PolyDiMethylSiloxane (PDMS) matrix. Dynamic relaxation, glass and melt crystallization of end-linked PDMS-silica nanocomposites (PDMSnanoSi) were compared to those of PDMS. A particular emphasis is made on the kinetic aspects of these transitions by corroborating investigations conducted by means of Dynamic Mechanical Analysis (DMA) and Differential Scanning Calorimetry (DSC). Addition of silica nanoclusters does not modify the relaxation behavior of the amorphous phase and the glass transition kinetics. However, melt and glass crystallizations are significantly promoted in the presence of silica nanoclusters. The secondary crystallization process is more pronounced for PDMSnanoSi and higher crystal perfection due to structuring effects of silica nanoclusters is also highlighted. For the two systems, one set of Hoffman-Lauritzen parameters have been evaluated by combining melt and glass crystallization kinetic data.

Nonisothermal crystallization of polytetrafluoroethylene in a wide range of cooling rates.

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.