The Differential Fast Scanning Calorimetry Research On The Secondary Crystallization Of Polyvinylidene Fluoride

Polyvinylidene Fluoride (PVDF) is a kind of semi-crystallized polymers which has high strength, rigidity and tenacity, and it is stable under acidic and alkaline circumstances and y-radiation resistant. It has been accepted as excellent thermoplastics and membrane materials. Its β-form crystal has a good piezoelectric and thermoelectric property, which has been widely used in vary research areas, such as:medical measurement, esthesiometer, mechanical measurement, energy harvesting and structural modal measurement, and the crystallization of PVDF has a great effect on these applications. Because the crystallization speed of PVDF is very fast, the standard differential scanning calorimetry can only detect the crystallization of PVDF in high temperature region; while the fast differential scanning calorimetry(FDSC) that established by our laboratory can detected the crystallization of PVDF in low temperature region for its high cooling rate. So that we can supplement and improve the theory of the crystallization of PVDF.In this thesis, we use DFSC to determine the process of secondary crystallization of PVDF. First, we made a comparison between DFSC and standard differential scanning calorimetry (DSC), and we found that the position of melting peak of PVDF is different from each other. That mainly because our DFSC machine can detect the transition process of amorphous phase to crystalline phase, which could not be detected by standard DSC machine, and for these reason, the formations of crystal under these two methods are different. And then, we only observed one melting peak of PVDF on DSC machine while the melting temperature is442.8K, but we can find two melting peak on our FDSC machine, the position of melting peak in high temperature region (420.5K) was close to the melting peak that detected by DSC machine among these two peaks, the second melting peak was found in low temperature region, this peak was the melting peak of the secondary crystallization of PVDF, and the position of melting peak varied with crystallization time and temperature changed.We have set multiple groups of program to determine the secondary crystallization of PVDF. Firstly, PVDF is annealed for same times at different temperature under20000K/s cooling rate, and it has been found that the melting peak shift to high temperature as the annealing temperature increased, that means the secondary crystallization is temperature dependent. Then, PVDF is annealed for different time at280K under20000K/s cooling rate, and it has been found that the secondary crystal became more and more stable as the annealed time grew, and the melting peak did not change when the annealed time reached a specific value, which means the secondary crystallization of PVDF was basically completed. That was the time dependent of secondary crystallization of PVDF. We also found that the progress of secondary crystallization of PVDF is mainly comprised two periods. The crystallization speed in first period is faster than that in the second period, the crystallization speed of this period has temperature dependence, for the crystallizing rate became slower as the annealed temperature got higher. While the crystallization speed in second period is slower, and it is not temperature dependent, and we are still working on the mechanism of this period.