Isothermal Crystallization Rates

Other crystallization parameters have been determined for some of the polymers. The dependence of the melting temperature on the crystallization temperature for the orthorhombic form of POX (T = 323K) and both monoclinic (T = 348K) and orthorhombic (T = 329K) modifications of PDMOX has been determined (284). The enthalpy of fusion, Aff, for the same polymers has been determined by the polymer diluent method and by calorimetry at different levels of crystallinity (284). for POX was found to be 150.9 J/g (36.1 cal/g) for the dimethyl derivative, it ranged from 85.6 to 107.0 J/g (20.5—25.6 cal/g). Numerous crystal stmcture studies have been made (285—292). Isothermal crystallization rates of POX from the melt have been determined from 19 to —50 C (293,294). Similar studies have been made for PDMOX from 22 to 44°C (295,296). 

The stereosequence length also has a marked effect on the isothermal crystallization kinetics of the propylene oxide polymers. These studies and analysis of results on crystallization kinetics will be described in detail in another communication. Here we summarize briefly the main conclusions of the effect of stereosequence length on the isothermal crystallization rates. 

Gibbs-Thomson equation Hoffman-Weeks approach Isothermal crystallization Non-isothermal crystallization Rates of crystallization... 

FIGURE 7 Dependence of overall isothermal crystallization rate on average weight of molecular weight (g mol ) of PHB at corresponding crystallization temperature (Chan et al., 2004 Gunaratne and Shanks, 2006). 

The slope of the plots of the cooling crystallization function versus T has been suggested as a criterion for the overall non-isothermal crystallization rate. The non-isothermal crystallization of PPS is hastened by the presence of the VB phase, whereas, neither the type of nucleation nor the geometry of crystal growth changes, and no reduction of the PPS degree of crystallinity could be noticed. 

The presence of small additions of UHMWPE resrrlts in an increased initial crystalh-zation temperature of HDPE and an acceleration of the isothermal crystallization rate." The fold surface free energy of polymer chairts in HDPE/UHMWPE composites was lower than that in neat HDPE." UHMWPE promotes the nucleation rate of HDPE and it is an effective nucleating agent in HDPE matrix." ... 

The isothermal crystallization rate for polymers can be determined by DSC. The experimental procedure is given below, using polypropylene samples as an example. 

By measuring the fraction of crystallized material, with time t, a value of the constant n equal to 4 was evaluated by Miyata and Masuko [37] who observed, in nonisothermal crystallization experiments from the melt, a remarkable increase in the polymer crystallization at decreasing cooling rates and a maximum value of the isothermal crystallization rate at 105°C. A similar trend was observed by lannace and Nicolais [36]. However, these results could be affected by the polymer thermal degradation induced at a low cooling rates, as previously described by Migliaresi et al. [27]. 

Choi WJ, Kim SC (2(X)4) Effects of talc orientation and non-isothermal crystallization rate on crystal orientation of polypropylene in injection-molded polypropyloie/ethylene-propylene rubber/talc blends. Polymer 45 2393-2401... 

X(.t) = relative crystallinity at time t Z = isothermal crystallization rate constant n = Avrami index... 

In order to bypass such limitations, the overall crystallization kinetics may be determined by DSC. However, in this case, both primary nucleation and crystal growth will make a contribution to the overall isothermal crystallization rate [6,7,12]. Ideally, it would be better to determine both spherulitic growth rate and overall isothermal crystallization kinetics in separate experiments, if possible. In the literature, the most commonly reported [30] type of isothermal crystallization kinetic data is that measured by DSC, because it is the easier to obtain. The DSC experimental approach can be very useful and in some cases, the DSC data thus obtained can provide not only the overall crystallization rate but also the separation of the individual contributions of the primary nucleation and growth rate (more details to follow). 

For this set of PE-h-PS diblock copolymers, several polymer nucleation and growth theories were applied to overall DSC isothermal crystallization rate data [61, 130,269,270]. These results indicate that the behavior of the PE-6-PS system is complex. Although the Lauritzen-Hoffman theory was developed for describing crystal growth only, it has been employed to describe overall crystallization data since it is capable of fitting the data remarkably well [269]. In these cases, such as when isothermal crystallization kinetics data obtained by DSC is employed, the energy barrier for crystallization reflected in Kg (denoted by us in this case KJ) contains contribution from both primary nucleation and crystal growth. 

Here, the non-isothermal crystallization rate, K T), is related to the isothermal crystallization rate, k T), by ... 

Equation 9.28 can be used to describe the kinetics of non-isothermal crystallization process imder quiescent conditiom However, the crystallization process in the spinning filament is non-quiescent, and the molecular orientation developed imder the tensile stress affects the crystallization rate. Therefore, the traditional non-isothermal crystallization rate, K T), must be replaced with the non-isothermal, stress-induced crystalhzation rate, K T,J), where / is the orientation factor. K T,J) also is called the total crystallization rate. With the total crystallization rate, Equation 9.28 can be rewritten to give ... 

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