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Abstract:
Crystallization from melt of poly(vinylidene fluoride) was studied by thin film chip calorimetry at cooling rates from 500 to 100,000 Ks−1 and isothermally down to 76°C. At ca. 70°C, for cooling rates higher than 2000 Ks−1, there appears a change in crystallization from high temperature α phase to low temperature β phase. The amorphous state is preserved at cooling rate 100,000 Ks−1. Analysis of the crystallization kinetics with Ziabicki model reveals maximum of the steady-state crystallization rate of β phase as 2200 s−1 at 22°C, and the highest crystallization rate of α phase as 200 s−1 at 70°C. Approximation of the temperature dependent steady-state crystallization rate with the Turnbull and Fisher nucleation model results in the equilibrium melting temperatures 227 and 173°C for the α and β phase, respectively, and in the energy barrier for short-distance transport, ED, as 70–80 kJ mol−1 at high supercooling.

Keywords:
Poly(vinylidene fluoride), Ultra-fast calorimetry, Crystallization kinetics

Abstract:
Melt-crystallization of polyvinylidene fluoride (PVDF) was investigated in non-isothermal mode at ultra-high cooling rates ranging between 30–3000 K/s as well as at constant temperatures after quenching at 6000 K/s. An increase of the cooling rate above 150 K/s leads to the formation of betaphase manifested by a low temperature shoulder of crystallization exotherm in addition to the alphamodification. At the cooling rates above 2000 K/s there is only low temperature exothermic peak that is attributed to the crystallization of pure betamodification. Isothermal crystallization was possible to realize at 110 oC as the lowest, resulting in form. Much higher crystallization rate in submicrogram samples, as compared to standard DSC experiments, is also reported.

Keywords:
Polyvinylidene fluoride, Crystallization, Ultra-fast calorimetry, Polymorphism

Abstract:
Crystallization of polypropylene (PP) can result in formation of different crystal modifications depending on external conditions. The mechanisms of formation of various crystal modifications in polypropylene are still under discussion.We have investigated non-isothermal melt crystallization of isotactic polypropylene at cooling rates ranging from 1 up to 180,000 K/min using two types of differential scanning calorimeters-standard device Perkin–Elmer DSC Pyris-1 and ultra-fast calorimeter. Additional results were obtained by means of wide angle X-ray scattering and optical microscopy. At cooling rates below 6000 K/min there is only one exothermic peak corresponding to simultaneous crystallization ofalpha andbeta modifications. At cooling rates higher than 6000 K/min there is additional low temperature DSC peak corresponding to formation of mesomorphic phase. At the rates higher than 36,000 K/min there is no trace of formation of any ordered phase. In our opinion this complex behavior observed during crystallization of polypropylene can be explained using the concept of metastable phases. An increase ofbeta content in samples with quinacridone pigment has been observed only at very low cooling rates, corresponding to high temperatures of crystallization and low homogeneous nucleation rate.

Keywords:
Polypropylene, Crystallization, Utra-fast calorimetry, Crystallographic modifications, Metastable phases