Fractography of highly crosslinked polymers: the influence of filler particles

Abstract

The fracture of phenol formaldehyde polymers may result in a surface morphology in which tracks are aligned in the direction of crack propagation. Fibrous features may be formed which in some cases lie on the tracks, but which in other cases may be displaced [1]. A similar fracture surface morphology was observed in several other highly crosslinked glass polymers [2]. It has been suggested that this morphology is due to the intersection of microcracks generated by a fortuitous change in the direction of the principal tensile stress just ahead of the propagating crack tip [3]. This suggests the possibility of forming isolated tracks and features by inclusion of filler particles which may serve to change the direction of the local principal tensile stress by acting as stress concentrators. In a first at tempt to find such an effect it would seem advantageous to work with low volume fractions of anisometric particles covering a wide range of sizes visible by scanning electron microscopy (SEM). A mixture of dimethacrylate monomers, convenient for present purposes, is used currently as a base for tooth restorative composites. A highly viscous monomer, the adduct of bisphenol A and glycidyl methacrylate ( B I S G M A , 7 5 w t % ) was thinned with triethylene glycol dimethacrylate (25%) and photosensitizers were added [4]. A silanated lithium aluminium silicate [5] was admixed with negligible sedimentation. The mixture was polymerized by exposure (60sec) to an intense ( ~ 1200Win 2) source of visible light (400 to 600 nm). Cylindrical glassy