Poly bulk crystallization rates

Since most applications of PLA-based materials are in the sohd state, flie detailed knowledge of the composition, bulk structure, and conformation of these materials is crucial. For example, it is weU known that poly(L-lactide) (PLLA) obtained from LL-lactide is usually molded at 100-120 C in industrial melt processing because of the higher crystallization rate. It has been reported that when it is crystallized within this temperature region a mixture of crystals, a- and P-forms, is formed [2]. Although infrared and Raman spectroscopy is a very promising analytical technique, as will be... 

Vanroy et al. [13] showed that the crystallization rate of films of poly (ethylene terephthalate) capped between aluminum layers increases upon confinement and that samples totally composed by adsorbed chains do not crystallize within extremely long experimental times (>10 folds the bulk crystallization time). In these extreme conditions, the entropic penalty required for the formation of stems is excessively high, indicating that the rate of transition from the amorphous to the crystalline state drops to extremely low values. 

Napolitano and coworkers studied the cold crystallization of poly(ethylene terephthalate) (PET) and poly(L-lactide) (PLEA) films confined between two Al layers (i.e., sandwiched films) [50, 53]. They reported that, after thermal annealing at a temperature where the maximum crystallization rate in the bulk was achieved, crystallization was inhibited when the thickness of the films was less than 20 nm for PET or less than 10 nm for PLEA. Such a threshold corresponds to the thickness of the adsorbed layers (i.e., a great reduction in the molecular mobility compared to the bulk) formed between the two solid substrates. Judging from the ratio between the thicknesses of the adsorbed layers and corresponding Rg of the polymers, their adsorbed layers are considered as the interfacial sublayer in our classifications. Hence, the discrepancy is noticed between the uncapped PE and PEO interfacial sublayers, where we can clearly see the seaweed structures (Fig. 6.5a), and the sandwiched PET and PLEA interfacial sublayer. This contrast... 

The rates of crystallization for three polymers follow Equation (13) as Figure 6 shows for PHBV, poly(ethylene terephthalate) (PET), and PEO (Chan et al., 2011). The relevant data are listed in Table 2. The comparison of Ec uation (11) and Equation (13) shows that kJi/AH is proportional to the ratio of surface and bulk energy of the critical nucleus. 

The dissolution of polymers in a solvent is irsually a slow process. It is preceded by swelling, which is controlled by the diffusion rate of solvent molecules into the bulk polymer. To accelerate dissolution, it is useful to stir the system, to work with airy precipitates of polymer sample instead of bulk particles and to increase temperature. Crystallized polymers usually dissolve after melting of their crystallites at elevated temperature. After temperature has been decreased, some of them (for example polyolefins) precipitate, while other ones (for example poly(ethylene oxide)s) may remain molecularly dissolved. 

---