Mechanical Engineering - Theses
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ItemMiscibility, morphology and physical properties of blends of polycarbonate with a range of thermoplastic polyesters(University of Melbourne, 1991)This study aims to gain an understanding of the relationship between miscibility, morphology and physical properties of polycarbonate (PC)/thermoplastic polyester blends. Blends of PC with 6 polyesters were prepared. The blends were compounded using a single screw extruder and then injection moulded to form test pieces. A transesterification inhibitor was added to prevent transesterification occurring during the melt processing. The miscibility of the blends was determined using Differential Scanning Calorimetry and Dynamic Mechanical Thermal Analysis. The blends exhibited a range of miscibilities from completely miscible to almost immiscible. The miscibility of some of the blends was found to be dependent on the method of blend preparation, and whether or not transesterification had occurred. Transmission Electron Microscopy of Ruthenium Tetroxide stained sections of the blends was used to study their morphology. As would be expected, the partially miscible blends showed phase separation, whereas the miscible ones did not. The morphology of the partially miscible blends was found to be dependent on the viscosities of the homopolymers and the shear rate applied to the blend during melt processing. Small Angle X-ray Scattering was used to study how the spherulitic morphology of the semicrystalline polyesters was effected by blending them with PC. As the miscibility of the blend increased, an increasing amount of PC became incorporated between the lamellae of crystalline polyester. The resistance to fracture of the blends was determined under both constant strain rate and impact conditions. Tensile tests and Scanning Electron Microscopy of fracture surfaces was used to aid the interpretation of the fracture results. Under constant strain rate conditions, the fracture toughness of the partially miscible blends was dependent on the adhesion between its phases. If the adhesion was poor, the blend had a low resistance to fracture. If the adhesion was good, the fracture toughness was dependent on the morphology of the blend and the relative ductilities of the homopolymers. If the blend consisted of domains of ductile material dispersed through a brittle matrix, its fracture toughness was synergistic. However, if the blend consisted of rigid domains dispersed through a ductile matrix, its resistance to fracture was poor. If the two phases had similar ductilities, the fracture toughness of the blend was additive. Miscible blends were found to have poor resistance to fracture under constant strain rate conditions. These blends had greater than additive densities, indicating that they contain less free volume than their corresponding homopolymers. This results in reduced molecular mobility, which embrittles the material, because it is unable to undergo energy dissipative plastic deformation. The fracture toughness of one particular miscible blend was further reduced by the poor adhesion between the crystalline phase of one of the polymers and the mixed amorphous phase. The impact strength of the blends was found to increase as the miscibility of the blend increased.