SOLVENT CRAZING AS A STRESS-INDUCED SURFACE ADSORPTION AND BULK PLASTICIZATION EFFECT

摘要

In solids with low bulk modulus, like polymer glasses, concentration of stress due to surface defects and other kinds of inhomogeneities can induce substantial surface tension reduction and bulk plasticization in extremely localized regions. This effect becomes important in the presence of organic liquids. As a liquid comes into contact with the polymer surface and as stress is applied, a point is reached when the work to draw new surface area becomes minimal. In addition, at least up to a diffusion limited extent, bulk salvation can also take place locally. At this point, solvent crazing initiates in the presence of a triaxial stress field, and craze fibrils are easily drawn with additional deformation. For van der Waals (WL) interactions between polymer and solvent, two different expressions for solvent craze initiation have been derived. The first is derived assuming interfacial tension reduction (ITR) is the dominant mechanism for solvent craze initiation. The second is derived assuming that flaw tip bulk plasticization (BP) is the main mechanism for solvent craze initiation. Existing experimental data on six different glassy polymers was also examined with respect to the above two expressions. A relatively good functional relation (straight line passing near the origin) was found for a wide spectrum of glassy polymers and apolar (WL) liquids, for both the (ITR) and (BP) cases. Additional assumptions made in this analysis, especially about the stress concentration factor and the bulk modulus of the materials, indicate a better correlation with interfacial tension reduction data, in the case of (WL) liquids. However, better controlled experiments would be necessary to properly identify the mechanisms of environmental crazing, even in the case of (WL) liquids. (C) 1995 John Wiley & Sons, Inc. [References: 21]