The Polymer Physics of CR-39 - The State of Understanding

CR-39 polymer is a highly cross-linked thermoset prepared by polymerisation of diethylene glycol bis(allyl carbonate) monomer. The polymerisation is initiated by radicals and proceeds via the addition reaction of allyl groups. The structure of the resulting polymer is a dense three-dimensional network consisting of polyallyl chains joined by diethylene glycol dicarbonate links. The average length of polyallyl chains is relatively short (DPW"80) as the result of degradative chain transfer specific for polymerisation of allyl groups. The formation of a network during polymerisation is a special case of the percolation process and can be modelled by a Monte Carlo simulation. The simulation predicts the distribution of cluster sizes, the gel point and the length distribution of the polyallyl chains as a function of monomer conversion. It can predict inhomogeneities in the cross-linking density which could explain some anomalous etching behaviour of CR-39. The polymerisation does not reach full conversion because the system enters the glassy state first. However, the residual mobility in the glassy state and the presence of chemically reactive groups are responsible for physico-chemical ageing of the polymer and the changes in the performance of CR-39 with time.