Influence of poly(acrylic acid) molar mass on the fracture properties of glass polyalkenoate cements

The failure behaviour of glass polyalkenoate cements was investigated using a linear elastic fracture mechanics (LEFM) approach. Cements were based on four model glasses with varying reactivity and four poly(acrylic acid)s (PAA)s with number average molar masses (Mn) ranging from 3.25 × 104 to 1.08 × 105. Cement properties were studied at time intervals of one, seven and twenty eight days. Compressive strengths (σc) of the cements increased with increasing fluorine content of the glass, with increased molar mass of the PAA and with ageing time. The Young's moduli increased with time, but were lower for cements based on the fluorine free glass. Moduli values were independant of PAA molar mass. The un-notched fracture strength (σf) of the cement increased with the molar mass of the PAA and with ageing time. Glass composition did not appreciably influence the un-notched fracture strength. The fracture toughness (KIC) increased with the molar mass of the PAA and with ageing time, but reduced with increasing fluorine content of the glass. The toughness (GIC) was dependant on molar mass. The influence of molar mass was not as great as predicted by the reptation chain pull-out model for fracture. The molar mass dependence of toughness was greatest with the lower fluorine content glasses. The plastic zone size at the crack tip increased with the molar mass of the PAA. However the plastic zone size decreased with ageing time for all the cements studied and was smaller for the more reactive higher fluorine content glasses.