Methyl methacrylate monomer–polymer equilibrium in solid polymer

Abstract 

Objective

Poly(methyl methacrylate) (PMMA) is commonly processed in dentistry by thermally initiating the free-radical polymerization of methyl methacrylate (MMA). Residual MMA, a tissue irritant, is a concern. The concentration of MMA ([MMA]) versus time and temperature was studied to identify optimum processing conditions.

Materials and methods

One hundred milligram portions of plain and dental PMMA powders were incubated (10–170°C, 1–384h), with and without 6.0μL MMA added. After incubation, [MMA] was determined by GC.

Results

For plain PMMA alone, equilibrium was attained in about 100h. The equilibrium data for log[MMA] versus 1/T was better fitted by a quadratic than a straight line, and formed an upper bound to the values of [MMA] when PMMA was incubated with MMA at temperatures >∼120°C. The response surface for [MMA] versus log(time) and reciprocal temperature was fitted. An ‘overshoot’ in the equilibration process was identified, and postulated to be due to a rapidly formed intermediate of unknown chemistry.

Significance

Minimization of the residual MMA in acrylic denture bases prepared by processing a mixture of PMMA and MMA is important for reasons of mechanical properties and irritancy. The response surface mapped here allows direct identification of the optimum processing conditions.

Keywords: Polymer equilibrium, Free-radical, Solid-state