Study On The Characteristic Temperature Of Polymer Crystallization By Thermal Analysis

The crystallization process of crystalline polymers will have a direct impact on the physical and mechanical properties of it.Therefore,the theory and research methods of polymer crystallization kinetics have been paid much attention by scholars.The calculation of polymer crystallization kinetics can be divided into isothermal and non-isothermal crystallization method.In this paper,a method for evaluating the characteristic temperature of polymer crystallization is proposed through the kinetic calculation and behavior analysis of the melt crystallization of polyvinylidene fluoride(PVDF).This method is also applied to the crystallization kinetics study of melt crystallization of low molecular weight poly(L-lactic acid)(PLLA)and the rapid cold crystallization of polypropylene(PP).The main content has the following sections:Firstly,the non-isothermal and isothermal crystallization behavior of PVDF was studied by differential scanning calorimetry(DSC).The non-isothermal crystallization kinetic parameters of PVDF were calculated using the Ozawa method,Jezinory method and Mo method,separately.The calculation results are basically the same.The average values of the Avrami exponent and Ozawa exponent are 2.33 and 1.93,respectively.And the coefficient a of the Mo equation is 1.23.This indicates that the non-isothermal melt crystallization mechanism of PVDF is mainly homogeneous nucleation and globular diffusion growth.The isothermal crystallization kinetic parameters of PVDF were calculated by Avrami equation and double exponential equation.The Avrami exponents were 2.06 and 2.36,respectively.The isothermal crystallization mechanism of PVDF is similar to non-isothermal crystallization,corresponding to a lower growth dimension.Secondly,considering the effect of temperature on the crystallization nucleation and crystal growth,the dependence of crystallization temperature on the the cooling/heating rate was discussed.The non-isothermal crystallization initiation temperature and peak temperature both have empirical relationship,T_p?lg|?|and T_o?lg|?|,with the cooling/heating rate.By obtaining the intersection point of two relationship curves,the melt crystallization characteristic temperature T_op of PVDF can be obtained as 153.6?C,indicating that only when the constant temperature is lower than characteristic temperature,obvious isothermal crystallization behavior can be observed.Finally,the Avrami equation was used to study the melt crystallization kinetics of low molecular weight PLLA.The results showed that the Avrami exponents of the non-isothermal crystallization process increased from 2.00 to 3.18 with the decrease of cooling rate,indicating its crystal growth mode changed from low dimension to multi-dimensional.The melt crystallization characteristic temperature of PLLA obtained by the intersection method was 142.0?C.Fast scanning calorimetry(FSC)was used to study the cold crystallization behavior of disorder polypropylene(PP).The results show that the inhibitory effect rapid heating rate on nucleation of cold crystallization is related to the initial nucleation state.The characteristic temperature of PP cold crystallization obtained by intersection method,which is-25.1?C.It indicates that the nucleation produced at a temperature higher than the kinetic characteristic temperature will promote the cold crystallization.