Melting of Conformationally Disordered Crystals (a'-Phase) of Poly(L-lactic acid)

Melting and reorganization of conformationally disordered crystals (??-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 fi rst time that the reorganization of conformationally disordered ??-crystals into stable ?-crystals can be suppressed by fast heating. Heating of ??-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 ?-crystals on continuation of heating. Non-isothermal reorganization of ??-crystals into ?-crystals only occurs when heating at a rate slower than 30 K s -1 . It is evidenced that isothermal reorganization of ??-crystals into ?-crystals at 150-160 °C proceeds via melting followed by recrystallization rather than a solid-solid phase transition.