Polymorphism in Syndiotactic Polypropylene:  Thermodynamic Stable Regions for Form I and Form II in Pressure−Temperature Phase Diagram

• Published in: Macromolecules Vol. 34; no. 22; pp. 7730 - 7736
• Main Authors: Rastogi, S, La Camera, D, van der Burgt, F, Terry, A. E, Cheng, S. Z. D
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 23.10.2001
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Structure, morphology and analysis; Pressure effect; Phase diagram; Temperature effect; Styrene polymer; Experimental study; Syndiotactic polymer; Polymorphism
• ISSN: 0024-9297; 1520-5835
• DOI: 10.1021/ma0109119

Polymorphism is a general phenomenon observed in polymers. Syndiotactic polypropylene also exhibits polymorphism, and four limited-ordered modifications have been proposed so far. These modifications are commonly known as form I, II, III, and IV. Form I is normally obtained on cooling an isotropic melt at atmospheric pressure. By in-situ X-ray studies performed at elevated pressure−temperature, we show that, for the first time, form II can be also obtained on cooling the isotropic melt at high pressures. When investigated by NMR, the form II thus obtained is found to be free from conformational defects. We observed that above 1.5 kbar the melting temperature of form II is higher than that of form I. This is in contradiction to the melting behavior of the two different forms below 1.5 kbar; that is, the melting temperature of form II (crystallized at pressures greater than 1.5 kbar) is found to be always lower than for form I below 1.5 kbar. On cooling from the isotropic melt, below 1.5 kbar, the sample crystallizes in the ordered form I. These findings suggest presence of a thermodynamically stable region for form II in the pressure−temperature phase diagram. The difference in crystallization kinetics of the two phases has been also followed at different pressures.