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

摘要

The crystallization of well-defined poly(L-lactide)-b-poly(epsilon-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 L_(44)~(11)C_(56)~(14) and L_(60)~(12)C_(40)~9,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 deg C (for 30 min) and then at 30 deg C (for 15 min) was measured.At 100 deg 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 deg 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 L_(60)~(12)C_(40)~9 revealed an initial melt mixed morphology at 165 deg C that upon cooling transformed into a transient microphase-separated lamellar structure prior to crystallization at 100 deg C.