GROWTH AND HEALING OF A SURFACE CRACK IN POLY(METHYL METHACRYLATE) UNDER CASE-II DIFFUSION OF METHANOL

The behaviour of a pre-crack inserted in a surface swollen layer forme d by case II diffusion of methanol in poly(methyl methacrylate) (PMMA) was investigated with reference to the distribution of internal compr essive stress induced under constraint by the inner glassy core. The e lastic analyses of internal stress were conducted by a finite element method using the data for the mechanical properties and the swelling s trains preliminarily obtained for the sheet specimen overall swollen i n methanol. The growth of the inserted crack in the surface swollen la yer under static tension in methanol at 20 degrees C was almost arrest ed for a long period at a position where the internal compressive stre ss in the tensile direction took the maximum magnitude near the bounda ry between the swollen layer and the glassy core. After being released from the internal compressive stress, the crack rapidly progressed to lead to a general fracture going across the boundary. A relatively lo ng crack inserted in the thinner surface swollen layer completely disa ppeared in methanol at 40 degrees C during 3 min under no external loa d. Crack healing, however, was not observed either in methanol at the lower temperature of 20 degrees C or in the entirely swollen specimen, even at 40 degrees C. These results suggest that both high temperatur e above the glass transition temperature of swollen PMMA and the signi ficant internal compressive stress are required for crack healing. (C) 1997 Elsevier Science Ltd.