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

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 10 3 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.