Oriented foaming of polystyrene with supercritical carbon dioxide for toughening
Polystyrene (PS) foams with isotropy cell morphology and oriented cell morphology were prepared to investigate the relationship between the cell morphologies and the mechanical properties (tensile and impact properties). For the isotropy cell morphologies, the tensile strength, tensile modulus and impact strength of PS foams all increase with the increase of relative density. When the relative density is constant, the cell size does not affect the tensile strength and modulus but has a modest effect on the impact strength. The solid area (cell walls) fraction on the fracture-surface and the cell walls finely dispersed by the bubbles are the main reasons for the toughening of isotropy PS foams. For oriented foams, the cell morphologies oriented perpendicular to the impact direction could significantly enhance the toughness of PS foams. The oriented bubbles and the matrix perpendicular to the fracture propagation direction are expected to absorbed large impact energy during the fracture process and make sample more ductile. The impact strength of the highly oriented PS foam in this work is about 1.5 times that of the unfoamed one while its relative density is 0.3. Small angle X-ray scattering (SAXS) measurements indicate that the molecular chains can be oriented by the shearing of the bubbles by oriented foaming, which leads to the improvement of tensile strength along oriented direction. In addition, the cells oriented parallel to the impact direction result in the poor impact properties.