The molecular basis of fracture in crosslinked glassy polymers

Crosslinked polystyrene (XPS) and poly(methyl methacrylate) (XPMMA) were prepared by photopolymerization of the respective monomers in the presence of reversible crosslinkers, acrylic acid anhydride (AAA), and methacrylic acid anhydride (MAA). Fracture studies on the crosslinked samples were carried out using a Dental Burr Grinding Instrument( DBGI). The fracture energy in all cases showed a maximum around 1.5-5.0 mol % crosslinker. The samples were decrosslinked by hydrolysis using dilute aqueous ammonium hydroxide solutions to determine the number of chain scissions as a result of grinding. The number of chain scissions increased asymptotically with crosslink density in the range of a 0.0-10.0 mol % crosslinker. The number of bonds activated per scission, obtained from the calculated total chain scission energy (after subtracting the chain pullout energy) and the experimental number of chain scissions, remained fairly constant for AAA-PS and AAA-PMMA at 312 +/- 150 bonds and 202 +/- 50 bonds, respectively, in the region below the fracture energy maximum. In an attempt to explain the fracture energy increases, increasing physical entanglements with crosslinking is considered. (C) 1997 John Wiley & Sons, Inc.