Homopolymers, crystallization kinetic melting temperatures

The development of the -modification is controlled by the relative crystallization thermodynamics and kinetics of the stable a-modification and of the smectic phase towards the metastable / -phase. For PP homopolymers, it is generally accepted that under isothermal conditions, the a-phase grows more rapidly at temperatures below 105 and above 140 °C than its counterpart, which in turn is more prone to develop in between these two temperatures in the presence of selective -promoters [52,70,122]. An elegant way to get fully nucleated /3-PP specimens would consist of pressing /3-PP pellets above their melting temperature (ideally more than 250 °C to erase any a-nuclei in the system), cool the melt quickly up to a crystallization temperature in between 100 and 130 °C, let the sample crystallize, and then quench it to room temperature [70]. However, such a processing method is too time-consuming to be of industrial relevance. 

Typically, polymer-grade l-LA with high chemical purity and optical purity (i.e., over 98-99% l-LA and less than 1-2% d-LA) is used for commercial PLA production. When l-LA is dehydrated at high temperature into L-lactide, some l-LA may be converted into d-LA. d-LA mixed with l-LA contributes to meso-lactide (the cyclic dimer of one d-LA and one l-LA) and heteropolymer PLA (with both d-LA and l-LA units). Heteropolymer PLA exhibits slower crystallization kinetics and lower melting points than homopolymer PLA (of pure l-LA units or pure d-LA units). 

In a similar fashion, DSC isothermal scans were recorded in order to study the crystallization kinetics of the PPDX homopolymer after melting the samples for 3 min at 150 °C and quenching them (at 80 °C/min) to the desired crystallization temperature (7(.). After the crystallization was complete, the inverse of the half -crystallization time, (i.e., the time needed for 50% relative conversion to the crystalline state [31,60]), was taken as a measure of the overall crystallization (nucleation and crystal growth) rate and its dependence on the crystallization temperature was analyzed. 

The overall crystallization kinetics of copolymers and their blends have been studied by DSC in the temperature range 1(X)-132°C [69]. For all examined blend compositions a single crystallization exotherm was observed at each T, whereas crystallization of mechanical mixtures of the copolymers showed separated exotherms of each component, thus supporting that the crystallization of melt mixed blends occurred from a homogeneous melt. The overall crystallization rate of copolymers was found to be affected by the copolymer structure and lower than that of PP homopolymer (BP < EP < PP), while the crystallization rate of the blends was intermediate between those of pure components (Fig. 10.12). The kinetics were analyzed by means of the Avrami equation (Eq. 10.14) the calculated values of the Avrami exponent for the blends, with average values of n from... 

Approaches used for crystallization in homopolymers may be used to calculate the change in melting temperature due to finite crystal thickness (Thompson-Gibbs equation), lamellar crystal surface energies (Flory-Vrij theory), and growth rates (kinetic nucleation theory). Details can be obtained from [1]. 

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