Isotactic polypropylene temperature gradients

Among the three known crystalline structures (a, j3, y) of isotactic polypropylene (PP), the -modification is certainly the most fascinating one. While the stable a-structure develops under standard process conditions, the occurrence of the /J-form has to be forced (i) by directional crystallization in a temperature gradient field [1-3] (ii) by shear-induced crystallization [4-12] or (iii) by the addition of specific nucleating agents [13-31]. This latter technique is preferred at the industrial scale. 

Polypropylene (PP) has two crystalline forms. A monoclinic crystal (a-type) is obtained by slow crystallization from the molten state and a hexagonal crystal ( -type) is formed by annealing while a temperature gradient is maintained across the sample. Figure 5.25 A presents DSC heating curves of an isotactic PP film prepared by temperature gradient annealing [23]. The film was pressed at 473 K and cooled to 443 K, where the pressure was released. The... 

A) DSC heating curves of isotactic polypropylene, prepared by temperature gradient annealing, as a function of scanning rate (1) 1.25 (11) 2.5 (111) 5 ... 

Yi C, Petermarm J, Wittich H (1997) Tianscrystallization in fiber-reinforced isotactic polypropylene composites in temperature gradient. J App Polym Sci 65 67-75... 

Fig. 22 Chromatogram of a blend of isotactic polypropylene (305 kg/mol) and linear polyethylene (126 kg/mol). Stationary phase Nucleosil 500,250 mm x 4.6 mm I.D. Mobile phase gradient of EGMBE and TCB dotted line), temperature 140°C detector PL-ELS KKX) sample solvent n-decanol injection volume 50 pL concentration 1 mg/mL. (Reprinted from [152] with permission of Elsevier Limited)...

Lovinger AJ, Chua JO, Gryte CC. Studies on the a and p forms of isotactic polypropylene by crystallization in a temperature gradient J Polym Sci B 1977 15 641-656. 

Figure 7.4 SpheruUtic crystallization of iPP in the temperature gradient of 35°C/mm earlier (b) and later (a) stages. Reprinted from Pawlak, A., Piorkowska, E. Crystallization of isotactic polypropylene in a temperature gradient. Coll. Polym. Sci. 2001,279,939-946, fig. 5. Copyright 2001, with kind permission from Springer Science + Business Media.

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