Crystallization Competition in a Double Crystallizable Poly(butylene Terephthalate)/Poly(ethylene Glycol) Block Copolymer at High Supercooling

Crystallization competition in block copolymers with two different crystallizable blocks gives rise to multiple potential crystallization pathways, of which many remain to be discovered. The isothermal crystallization kinetics of a commercial double crystallizable poly(butylene terephthalate)/poly(ethylene glycol) (PBT/PEG, 30/70 w%) multiblock copolymer was studied via fast scanning calorimetry (FSC) as well as FSC combined with synchrotron wide-angle X-ray scattering (WAXS). The crystallization rate as a function of the isothermal crystallization temperature (Tiso) exhibited two overlapping bell-shaped curves, corresponding to PBT and PEG crystallization. At the highest Tiso, only PBT crystallized. In the overlap region, where both blocks crystallized in competition, earlier PBT crystallization stimulated ensuing PEG crystallization. In contrast, earlier PEG crystallization suppressed subsequent PBT crystallization by inducing mobility restrictions. At the lowest Tiso, only PEG crystallized, leaving immobilized PBT in the amorphous state and implying that double crystalline PBT/PEG morphologies cannot be created isothermally at very low Tiso. These block copolymer findings were rationalized and complemented by comparison with the crystallization kinetics of the parent homopolymers.