Cold crystallization and multiple melting behavior of poly(l-lactide) in homogeneous and in multiphasic epoxy blends

Cold crystallization and melting of poly(l-lactide) (PLLA) blended with an uncured or with an amino-cured epoxy resin (diglycidyl ether of bisphenol-A [DGEBA]) were investigated. It was found that the uncured PLLA/DGEBA blends were miscible, as they exhibited a single composition-dependent glass transition temperature (Tg). Melting point depression measurements indicated the existence of some type of interaction between the blend components, which was confirmed by Fourier transform infrared spectroscopy. Depending on the crystallization conditions and on the blend composition, a mixture of α and α′ crystals have been detected in PLLA and in uncured DGEBA/PLLA blends when crystallized from the glassy state. At high DGEBA contents, preferably imperfect α crystals are formed. On the contrary, at low DGEBA contents, the α′ form predominates and an exotherm associated to the α′–α transformation appears on the differential scanning calorimetry (DSC) scan before the main melting peak. Upon curing, the system transforms from a homogeneous mixture with a single refractive index into an opaque multiphasic one, as revealed by the existence of two Tgs in the DSC scans. These cross-linked immiscible blends displayed a single crystallization exotherm which scarcely changed with composition, and PLLA cold crystallized mainly into the α′ form from an almost pure PLLA phase; subsequently, the α′ crystals transform into the α form just before melting during the DSC scan.