| Blending two or more polymers together to achieve property complement is one of the important routes to obtain new polymer materials. However, many blends present two-phase structure due to the thermodynamic incompatibility between the component polymers. Compared with that of the blends with'sea-island'morphology, the co-continuous blends usually show better performance and potential applications in more fields because of their special sub-microcosmic/macroscopical network structure. The fabrication of porous materials by selective etching one of the component in the co-continuous blends is one of the further applications of such blend material. To obtain different pore sizes to meet different requirements such as drug controlled release and tissue engineering scaffold. it is necessary to deeply explore how the co-continuous phase morphology form and evolve during melt mixing.Therefore, two biocompatible polymers, ethylene-vinyl acetate copolymer (EVA) and poly(ε-caprolactone) (PCL) were used in this work to prepare EVA/PCL blends with various component ratio and viscosity ratio by blending. Then the phase behavior of the blends was analyzed by morphological and rheological approaches, and the dominant factors determining formation and evolution of the co-continuous phase morphology were studied by relating the immiscible morphology to the viscoelastic behavior of the blends. Through establishing the relationship between co-continuous phase morphology and processing conditions, the porous materials with various pore sizes based on EVA were fabricated successfully by selective solvent etching.(1) The Scanning Electron Microscopy (SEM) results showed that the EVA/PCL blends were thermodynamically immiscible. The component ratio affected the phase morphology of the blends significantly. The blends showed dispersed-continuous and co-continuous morphology at different component ratios. The results of rheological tests showed that the blends had a wide co-continuous region (PCL concentrations of 40~60 wt%) due to high viscosity ratio between PCL and EVA, and to higher elasticity and longer relaxation time of EVA. The models merely based on viscosity or elasticity can hence not be well used to describe the phase inversion point of the blends.(2) The bulk properties of component polymers strongly influenced the formation of co-continuous structure. The co-continuous morphology became clearer and the composition range increased as viscosity ratio reduced. In addition, the low interfacial tension between two components favored the formation of co-continuous morphology. The lower the interfacial tension was, the lower co-continuous phase concentration was, and the wider co-continuous composition range.(3) The processing conditions also affected the evolution of co-continuous phase morphology significantly. The shear flow reduced the co-continuous phase domain because the contribution of EVA to the capillary numbers highly exceeded the breakup tendency of PCL in shear flow, and as a result, the two phases were highly deformed but not break up. Moreover, in the isothermal annealing process, the co-continuous phase domain increased and the coalescence rate growed as temperature increased; in the nonisothermal annealing process, the co-continuous phase domain also increased with increasing annealing temperature. The coalescence effect is dominated by the decrease of viscosity of the blend systems.(4) The porous materials based on EVA or PCL prepared by the traditional solution casting-particle filter method showed poor porous morphology: wide pore size distribution, co-existance of the open-cell and the closed-cell structure and low pore connectedness. However,for those porous materials based on co-continuous blends, the pore connectedness and porosity were improved evidently. The pore size (1.5-120μm) could be controlled by designing and controling the processing and annealing conditions of the co-continuous blends. |
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