Homopolymers, crystallization kinetic temperatures

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). 

A related problem of interest is when a polymer is not crystalline as prepared, but is potentially crystallizable. This situation is commonly encountered in crys-tallizable copolymers, and is also found in homopolymers. Some typical examples of this phenomenon are found in poly(styrene) synthesized by means of alfin type catalysts,(50) poly(methyl methacrylate), prepared by either free-radical or ionic methods,(39,51,52) and poly(carhonate).(53) Treatment with particular solvents or diluent at elevated temperatures can induce crystallinity in these polymers. The reason for the problem is kinetic restraints to the crystallization process. Treatment with appropriate diluents alleviates the problem. The principles involved, and the diluent requirements will he enunciated in the discussion of crystallization kinetics. For present purposes it should be recognized that the crystallizability of a polymer, particularly a copolymer, cannot be categorically denied unless the optimum conditions for crystallization have been investigated. Thus, in light of the previous discussion regarding the minimum concentration of chain units required for crystallization, and the need to have favorable kinetic conditions, the lack of crystallization in any given situation needs to be carefully assessed. 

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... 

Taking into account all of the results presented above, we can conclude that in order to be sure that homogenous nucleation is indeed present (even when first-order crystallization kinetics is encountered), the crystallization rate must exhibit a dependence on the volume of the droplets or on the cube of the particle diameter. Additionally, even in extremely small droplets comparable to only a few chains in size, the nucleation still occurs within the interior of the droplets. Furthermore, the homogeneous nucleation event is independent of the molecular weight and of the molecular architecture (at least when comparing homopolymers and diblock copolymers). The homogeneous nucleation temperature is a function of the particle size. In certain cases, when the droplets size is nanometric, modifications of the crystal structure of the polymer as compared with that usually observed in the bulk have been reported. The effects of superficial nucleation are important and should be taken into consideration. 

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. 

---