Preparation And Characterization Of PLLA/PCL Composite Spherulites By Thermally Induced Phase Separation

With the broad applications of porous PLLA materials as scaffolds in tissue engineering, biomaterials or drug carriers, there are high demands to improve the physical properties of PLLA materials. To overcome the disadvantages of fragility and poor stability of PLLA, blending is a good solution. PCL is a sort of toughening materials because of its good chain flexibility. So PLLA/PCL composite material can improve the toughness of PLLA material. This research fabricated a series of PLLA/PCL composite spherulite materials with varied morphologies by quenching PLLA/PCL-THF solutions to a metastable state to induce liquid-liquid phase separation. The results are summarized below:(1) Through changing the quenching conditions such as quenching time and quenching temperature, the concentration of polymer solution and the blending ratio, analyze the properties and crystallization process of the PLLA/PCL materials. The results indicated that no sheaves were observed when the quenching time was not long enough. With increasing quenching time, the PLLA sheaves appeared and the radian increased. With decreasing quenching temperature, the morphology of sheaves transformed from bouquets to cauliflowers then changed back, bouquets. When the concentration of polymer solution increased, the radius and radian of the sheaves were both increased. With increasing the blending ratio of PLLA/PCL, the morphology of composite spherulites from sheaves changed to microspheres, which were composed of a large amount of sheaves. When the blending ratio was 5:1, the degree of crystallization of PLLA reached to 64.44%, which increased by 23.40% compared to the pure PLLA system (40.41%). "Namely, PCL acted as nucleating agent and promoted the crystallization of PLLA in this system.(2) The mechanism of the formation of PLLA sheaves or spherulites was examined by the isothermal and non-isothermal crystallization of PLLA/PCL solutions using DSC. The good linear double-logarithmic plots were obtained by the Avrami equation, and the Avrami index n and K value were calculated. The results showed that the crystallization mechanism was heterogeneous nucleation and 3D growth. Jeziorny and Mo methods were utilized to the analysis of non-isothermal crystallization process. The apparent Avrami exponent n is 4.48. The non-isothermal crystallization activation energy △E was negative value (-89.55 kJ/mol), which means that the crystallization process of PLLA in THF solutions was a barrierless and spontaneous process and the higher concentration, the easier crystallization, the stronger activity ability of molecular chain, the more heat is released.(3) The porosity of the PLLA/PCL porous films was 96.8% and its contact angle was 142.84°, which increased by 63.36° compared to the PLLA solvent cast film, indicating that the porous has developed to be a type of super hydrophobic material. The specific surface area of the composite porous composite spherulite film was 10.60 m2/g, while the pure PLLA porous film was 3.65 m2/g. The adsorption capacity of the composite porous composite spherulite films was over 11.5 g/g towards silicone oil, etc, which larger than the pure PLLA porous films and ten times larger than the cast PLLA films. Due to the super hydrophobic property and capillary effect, the composite films can absorb the oil layer on water surface quickly and effectively, thus they can be a great oil-water separation material.