Supercritical CO2-assisted Toughening Of Polypropylene And Polystyrene: Process, Microstructure And Mechanical Properties

In a conventional supercritical carbon dioxide (scCO2) batch foaming process, CO2is dissolved in a polymer matrix, which increases the free volume between molecular chains and chain mobility, and consequently affects its crystallization behavior. After the saturation process, CO2is depressurized quickly to induce foaming in the polymer matrix. This type of process normally produces microcellular foams with an average cell size of about10?m and a cell density of more than109cells/cm3. Properties and applications of polymer foams depend very much on cell morphologies. As a widely used general-purpose polymer due to its excellent performance-to-price ratio, polypropylene (PP) and polystyrene (PS) have been widely studied and their foaming has also been subjected to intense studies. In this work, scCO2induced foaming of PP or PS is systematically studied with emphasis on the relationship between process, microstructure and mechanical properties.The first part of the thesis deals with the toughening of iPP by scCO2induced crystallization for the fine separation of rigid crystalline domains and soft amorphous ones in the polymer matrix. The results indicate that the toughness of injection molded iPP specimens can be significantly improved without loss of strength by controlled shearing, CO2induced re-crystallization and adequate cooling. Under shear, a high degree of orientation can be obtained with "shish-kebab" crystals formed in the shear zone. During the subsequent CO2treatment, a crystal network morphology may be formed as a result of an increase in the number of the primary lamellae and that of crosshatched subsidiary lamellae, which leads to an increase in the toughness. In addition, quenching in ice-water of scCO2treated iPP promotes the formation of nano-sized mesomorphic phase domains in the shear zone, which further toughens the iPP. The impact strength of the best toughened iPP is more than12times that of the original one while retaining the tensile strength and modulus.Moreover, the highly oriented iPP with "shish-kebab" and "spherulite" are used for CO2foaming to investigate the effect of crystalline structure on the formation of cell nucleation and growth. The impact property is also studied. A nanocellular foam is achieved by CO2foaming of shish-kebab crystalline structure in the solid state. The underlying principle is localized cell nucleation and growth in amorphous domains confined by the shish-kebab crystalline domains. Therefore, nano-cells are generated in amorphous domains confined by shish-kebab crystalline domains which cannot foam. At a chosen CO2pressure, the cell morphology depends very much on the foaming temperature, as the crystal morphology depends on the temperature when the CO2pressure is constant. Moreover, the impact tests indicate that the nanocellular bubbles among the network crystals structure can further toughen the scCO2treated iPP.In addition, the effect of the foaming conditions and the cell structural parameters of PS foams on the mechanical properties are studied systematically. PS foams with isotropic cell morphology and oriented cell one are prepared. For the isotropic cell morphologies, the mechanical properties of the PS foams increase with increasing relative density. When the relative density is constant, the cell size does not affect the tensile strength and modulus but has a slight effect on the impact strength. The relative impact strength increases with decreasing cell size. The results indicate that the solid area fraction on the fracture surface (fs) and the cell walls are the key parameters for toughening PS foams. For oriented foams, the cell morphologies oriented perpendicular to the impact direction could significantly enhance the impact properties of PS foams. Moreover, the oriented cells along the tensile direction and the oriented molecular chains sheared by the bubbles may both lead to the improvement of strength in the oriented direction. On the other hand, the cells oriented in the other two directions result in the poor impact properties, because their larger fs and worse dispersion of cell walls compared with those of isotropic foams.Finally, a two-step depressurization batch process is developed to produce bi-modal cell structure PS foams by using scCO2as the blowing agent. This unique cell structure with both small and large cells homogenous distribution throughout the entire volume of the foam sample might have particular properties which include both advantages of small cells and large cells. The results indicate that the bi-modal cell structure foams can be achieved by depressurization in two distinct steps and can be significantly affected by the process parameters. For both bi-modal foams and uniform foams, the relative impact strength increases with increasing relative density. When the relative density is constant, the impact strength of bi-modal foams decreases with a decrease in fs. Moreover, when fs is80%, the relative impact strengths of bi-modal foams are always higher than those of the uniform one, with relative density ranging from0.2to0.5. It indicates that the bi-modal cell structure could have stronger absorption of impact energy than the uniform one.