Abstract
The morphology of block copolymer can be tailored by blending with the corresponding homopolymer if the homopolymer is uniformly solubilized in the microdomain. Depending on the molecular weight of the homopolymer, the blend system may exhibit “wet-brush” phase behavior or “dry-brush” phase behavior in the melt. In this report, we study the morphology and crystallization kinetics of a series of wet-brush blends of a symmetric polybutadiene-block-poly( ε -caprolactone) （ PB-b-PCL ） and a low molecular weight homopolymer PB. In the study of melt state morphology, we found that as the weight fraction of PB increases, the microdomain structure of the blend system changes from lamellae(wPB=0.5~0.6) to hexagonally-packed PCL cylinder(wPB=0.7~0.75) and liquid-like packed PCL sphere (wPB=0.8~0.85). The crystallization behavior of PCL is strongly affected by this morphological transformation; their crystallization rate was highly sensitive to the microdomain morphology. The fixed cooling rate experiment in the DSC revealed that freezing temperature exhibits one to one correlation with the meet to the melt state morphology. The isothermal crystallization kinetics in wPB=0.7 was properly described by the classical Avrami equation, indicating that the crystallization started from heterogeneous nucleation followed by long-range crystal growth. There has a maximum in crystallization rate Due to the competition between crystallization driving force and mobility of PCL block. We also found an unusual increase in crystallization rate at –41 ℃,because at very low crystallization temperature, homogeneous nucleation dominated crystallization rate and increased overall crystallization rate. After PCL crystallized, melt state morphology were all destroyed and become the morphology consisting of alternating amorphous PB and crystalline PCL lamellae, show the “breakout” crystallization mode.