Isotactic polypropylene spherulitic crystallization

Samuels, R. J. Yee, R.Y. (1972). Characterization of structure and organization of beta-form crystals in type-iii and type-iv beta-isotactic polypropylene spherulites. Polym. Sci. PartA-2 Polymer Phys., Vol.lO, Issue 3, pp. 385-432. 

Neat isotactic polypropylene (iPP) crystallized from melt exhibits spherulitic morphology of the crystalline phase (72,73). In some cases and under very specific conditions, cylindrites, axialites, quadrites, hedrites, and dendrites may be formed of iPP (74). In general, crystallization from quiescent melts results in spherulitic morphology, whereas crystallization fi-om melts subjected to mechanical loads results in cylindrites (75). Crystalline supermolecular structure caused by oriented crystal growth from heterogeneous surfaces is commonly termed transcrystallinity (76). 

Fig. 2.4 DSC melting endotherms of high crystallinity polypropylene (7) crystallized frtnn the melt for 3 h at 145 °C, (2) partially melting scan (3) fully remelted after the partial melting From figure 3, Weng J, Olley RH, Bassett DC and Jaaskelainen P, Changes in the Melting Behavior with the Radial Distance in Isotactic Polypropylene Spherulites. J. Polym. Sci. Polym. Phys. 41 2342-2354. Copyright 2003 by John Wiley Sons, Inc. Reprinted by permission of John Wiley Sons, Inc...

In the crystallization of isotactic polypropylene from the melt, the number and size of the spherulites (and hence the rate of crystallization) can be influenced by the addition of certain nucleating agents.The smaller the spherulites, the greater is the transparency of the polypropylene film.The mechanical properties can also be affected in some cases. 

The effect of heterogeneous nucleation on the crystallization of isotactic polypropylene from the melt can be easily established as follows. A small amount of powdered polypropylene is well mixed with about 0.1 wt% of sodium benzoate in a mortar or by means of an analytical mill. Some of the mixture is transferred with a spatula to a microscope slide and melted at about 250 °C on a hot block. A cover slip is pressed on to the melt with a cork to obtain as thin a film as possible.The sample is held at 200-250 °C for some minutes and then allowed to crystallize at about 130 °C on the hot stage of the microscope an unadulterated polypropylene sample is crystallized in the same way. Both samples are observed under a polarizing microscope during crystallization,the difference in spherulite size between nucleated and untreated polypropylene can be seen very clearly. An ordinary microscope can also be used by placing polarizers on the condenser and eyepiece, and adjusting these to give maximum darkness. 

Khoury, F. The spherulitic crystallization of isotactic polypropylene from solution on the evolution of monoclinic spherulites from dendritic chain-folded crystal precursors. J. Res. Nat. Bur. Stand. 1966, A-70, 29. 

Norton, D.R. Keller, A. The spherulitic and lamellar morphology of melt crystallized isotactic polypropylene. Polymer 1985, 26, 704. 

Fig. 1.9 AFM tapping mode images of a spherulitic texture in isotact polypropylene. The sample was crystallized to completion at 145°C and subjected to permanganic etching prior to examination. Image (a) shows topography while (b) contains phase information. Scale bars 5 iim.

Presence of TCL changes the properties of crystalline matrix. Transcrystallization of isotactic polypropylene in the presence of different fibers has been thoroughly analyzed. Gray as the first one provided detailed description of isotactic polypropylene behavior in the presence of wood fibers using polarized light microscopy. He observed that when melted polymer is cooled down, it crystallizes in spherulite forms in nonisothermal and isothermal conditions, creating additionally a TCL. 

The crystallization behavior is quite different if the two polymers are immiscible in the melt. Figure 6.20 (64) shows droplets of polyisobutylene dispersed in isotactic polypropylene. On spherulite formation, the droplets, which are noncrystallizing, become ordered within the growing arms of the crystallizing component. 

Thus, if our consideration is valid and dependence of Gtr and d are determined only by the proposed mechanism, at constant values of Gr and AE, then the dependence of InGR on 1/d should be a straight line from which values of IuGr, corresponding to d. %4D (intercept on ordinate axis) and N =C4(2Ym) (o)/(Ag) kT may be found. Indeed, the dependence of the rate of spherulitic crystallization for gutta-percha and isotactic polypropylene on the interlayer thickness is linear. ... 

Isotactic polypropylene displays a highly unusual ability to induce epitaxial crystallization of a number of different polymers with their chain axes tilted at large angles, 40—80°, relative to the helix axis direction of the polypropylene substrate. The same polymer shows an epitaxy, i.e. homoepitaxy, at an angle of 80° (Fig. 7.30). The homoepitaxy is responsible for crosshatching, a structure typical of the monoclinic a structure. This feature causes the lamellar branching and the optical complexity, with both positive and negative spherulites, typical of isotactic polypropylene (section 7.4). 

Pawlak A and Piorkowska E (1999) Effect of negative pressure on melting behavior of spherulites in thin films of several crystalline polymers, J Appl Polym Sd 74 1380-1385. Nowacki R, Kolasiuska J and Piorkowska E (2001) Cavitation during isothermal crystallization of isotactic polypropylene, J Appl Polym Sci 79 2439-2448. 

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