Crystallization in Poly(L-lactide)-b-Poly(e-caprolactone) Double Crystalline Diblock Copolymers: A Study using X-ray Scattering, Differential Scanning Calorimetry and Polarized Optical Microscopy

The crystallization of well-defined poly(l-lactide)-b-poly(e-caprolactone) diblock copolymers, PLLA-b-PCL, was investigated by time-resolved X-ray techniques, polarized optical microscopy (POM), and differential scanning calorimetry (DSC). Two compositions were studied that contained 44 and 60 wt % poly(l-lactide), PLLA (they are referred to as L4411C5614 and L6012C409, respectively, with the molecular weight of each block in kg/mol as superscript). The copolymers were found to be initially miscible in the melt according to small-angle X-ray scattering measurements (SAXS). Their thermal behavior was also indicative of samples whose crystallization proceeds from a mixed melt. Sequential isothermal crystallization from the melt at 100 °C (for 30 min) and then at 30 °C (for 15 min) was measured. At 100 °C only the PLLA block is capable of crystallization, and its crystallization kinetics was followed by both WAXS and DSC; comparable results were obtained that indicated an instantaneous nucleation with three-dimensional superstructures (Avrami index of approximately 3). The spherulitic nature of the superstructure was confirmed by POM. When the temperature was decreased to 30 °C, the PCL block was able to crystallize within the PLLA negative spherulites (with an Avrami index of 2, as opposed to 3 in homo-PCL), and its crystallization rate was much slower than an equivalent homo-PCL. Time-resolved SAXS experiments in L6012C409 revealed an initial melt mixed morphology at 165 °C that upon cooling transformed into a transient microphase-separated lamellar structure prior to crystallization at 100 °C.