Melting of Conformationally Disordered Crystals (α′-Phase) of Poly(l-lactic acid)

• Published in: Macromolecular chemistry and physics Vol. 215; no. 11; pp. 1134 - 1139
• Main Authors: Androsch, René, Schick, Christoph, Di Lorenzo, Maria Laura
• Format: Journal Article
• Language: English
• Published: Weinheim Blackwell Publishing Ltd 01.06.2014; Wiley; Wiley Subscription Services, Inc
• Subjects: Applied sciences; Constants; Crystallization; Crystals; Exact sciences and technology; Formations; Heating; Melting; melting and crystallization; Organic polymers; Phase transformations; Physicochemistry of polymers; poly(l-lactic acid); Polylactic acid; polymorphism; Properties and characterization; Recrystallization; reorganization; melting and crystallization; Lactic acid polymer; Melting; Optically active polymer; Heating rate; Recrystallization; Semicrystalline polymer; Experimental study; reorganization; poly(L-lactic acid); Alpha form; Polymorphism
• ISSN: 1022-1352; 1521-3935
• DOI: 10.1002/macp.201400126

Melting and reorganization of conformationally disordered crystals (α′‐phase) of poly(l‐lactic acid) (PLLA) are analyzed as a function of the rate of heating in a wide range between about 10−1 and 103 K s−1. It is found for the first time that the reorganization of conformationally disordered α′‐crystals into stable α‐crystals can be suppressed by fast heating. Heating of α′‐crystals of PLLA at a constant rate, faster than 30 K s−1, leads to its complete melting between 150 and 160 °C and suppression of formation of α‐crystals on continuation of heating. Non‐isothermal reorganization of α′‐crystals into α‐crystals only occurs when heating at a rate slower than 30 K s−1. It is evidenced that isothermal reorganization of α′‐crystals into α‐crystals at 150–160 °C proceeds via melting followed by recrystallization rather than a solid–solid phase transition. It is shown for the first time that the reorganization of conformationally disordered α′‐crystals of poly(l‐lactic acid) (PLLA) into stable α‐crystals can be suppressed by heating at a rate faster than 30 K s−1. Isothermal reorganization of α′‐crystals into α‐crystals at 150–160 °C proceeds via melting followed by recrystallization rather than a solid–solid phase transition.