The micromechanics and microstructure of CO 2 crazes in polystyrene

Abstract

Although CO 2 at 1 atmosphere pressure is not a crazing and/or cracking agent for polystyrene (PS), we have established that it becomes one at higher pressure. Crazes grown from cracks in PS thin films in high pressure CO 2 are investigated using transmission electron microscopy (TEM). The fact that broken craze fibrils retract strongly upon exposure to high pressure CO 2 gas suggests that the primary effect of the CO 2 is plasticization, not surface energy reduction. Quantitative analyses of TEM micrographs of crazes grown at CO 2 pressures in the range 5 to 100 MPa at 34°C and 45°C have been carried out to find the craze fibril volume fractions v f ( x) and the surface displacements w( x) along each craze. From the fibril volume fraction profile along the craze, the dominant craze thickening mechanism of CO 2 crazes is shown to be the same as that for air crazes, i.e. the surface drawing mechanism, and not the fibril creep mechanism. The craze surface stress profile is computed from the craze surface displacements using a distributed dislocation analysis. These profiles all show a stress concentration at the craze tip which falls to a roughly constant value σ b , over the rest of the craze. The fracture toughness G Ic (and critical stress intensity factor K Ic ) for propagation of a crack in PS at these CO 2 pressures can also be computed. All these quantities ( V f , σ b , G Ic and K Ic ) show pronounced minima as a function of CO 2 pressure at 20 MPa, the same CO 2 pressure at which T g of the polymer also reaches a minimum. These minima are more pronounced at 45°C than at 34°C. The G Ic 's and K Ic 's are depressed by orders of magnitude at the minimum, which corresponds to the qualitative observation that CO 2 becomes a severe cracking agent at these pressures. These observations provide additional confirmation that the major mechanism for the environmental crazing and cracking of PS by CO 2 is plasticization of the craze fibrils and surfaces.