Crystallization kinetics and crystallization-induced morphological formation in the blends of poly(ε-caprolactone)-block-polybutadiene and polybutadiene homopolymer

The crystallization kinetics and the perturbation of the melt mesophase induced by crystallization in the blends of a symmetric poly(epsilon-caprolactone)-block-poly(1,4-butadiene) (PCL-b-PB) and a poly(1,4-butadiene) homopolymer (h-PB) have been investigated. The crystallization at low to moderate undercooling was found to break out the melt structure and transformed it into a crystalline lamellar structure. The mechanism of the morphological breakout in a cylinder-forming blend with the overall volume fraction of PB (f(PB)) of 0.78 was resolved by time-resolved simultaneous small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS). The results revealed two steps involved in the breakout process. At the initial stage of crystallization, the formation of crystallites within a portion of PCL cylinders deformed the microdomains and initiated local coalescence of the domains through conformational communication of the coronal PB blocks. After this stage the crystallinity started to develop rapidly, and the crystallization was proposed to occur via diffusion of PCL blocks to the crystal growth front, which then induced the formation of extended lamellar morphology. The isothermal crystallization kinetics of the cylinder-forming blend with f(PB) = 0.74 was also studied. Interestingly, the overall crystallization rate exhibited an abrupt increase at very large undercooling (Delta T similar or equal to 96 K). This anomalous acceleration of crystallization rate indicated that the crystallization kinetics was largely controlled by homogeneous nucleation when the morphological breakout was strongly restricted at very low temperatures.