Pre-melt crystallization

Figure 8.5 DSC trace on heating of neat PLLA displaying glass transition, cold crystallization, pre-melt crystallization, and melting. Adapted with permission from [44],...

Thus, melting of a crystalline substance without superheating is a superficial effect. Pre-melting phenomena are apparently also related to the formation of liquid films on the surfaces of crystals, if not to other incidental causes (for example, impurities), and are not pertinent to Frenkel s theory. Heterophase fluctuations are quite large where the difference between two phases and the surface tension between them tend to zero—near the critical point and near the Curie point. The first case is commonly known, the second was earlier investigated quantitatively in Landau s fine work [13, 14]. 

The importance of pre-structuring the PE before ultra-drawing is shown in Figure 13, which reveals the radically different stress-strain behavior of compression molded UHMW-PE (R) versus solution-crystallized UHMW-PE (S). The compression molded sample contains many chain entanglements from melt crystallization and hence can only be drawn to a maximum draw ratio ( ) of 6 or 7. The solution-crystallized sample was drawable to draw ratios in excess of 70, because of the reduced density of chain entanglements. This sample was a cast PE film, with a mass fiaction 2 % PE in decalin, where residual solvent was extracted prior to... 

As LB. Rabinovich and his research team noted [2-5, 73], in the samples of polymer mixtures with plasticizers containing an excess of a low-molecular component over its solubility in the polymer the amoimt of the surplus plasticizer can be so large that its formed crystals will reinforce the saturated solution of the plasticizer in polymer thus preventing it from devitrification. In this case devitrification of the saturated solution will take place in the temperature range of the plasticizer surplus pre-melting phase. The ehitin-water sample containing 89,6 mass% of H2O (Figure 18) [4, 5, 72, 73] can serve as an example. 

Chiu, F., Shen, K., Tsai, S. H. Y, Chen, C. Pre-melting temperature effect on the isothermal melt crystallization and multiple melting behavior of syndiotactic polystyrene. Polym. Eng. Sci., 41(5), 881-889 (2001). 

Chiu, F-C., Peng, C-G., Fu, Q. Non-isothermal crystallization and multiple melting behavior of syndiotactic polystyrene— Pre-melting temperature effects. Polym. Eng. ScL, 40(11), 2397-2406 (2000). 

Tian G, Wu Q, Sun SQ, Noda 1 and Chen GQ, Study of Pre-melting and Crystallization Process of Biosynthesized Poly(3-hydroxybutyrate) Using Two-dimensional Fourier-Transform Infrared Spectroscopy. Chem J Chinese Universities, 2002, 8 1627-1631. 

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