A Lower Transition Point for Polymethyl Methacrylate at 30°–32° C

Abstract

IN view of the use of polymethyl methacrylate (PMMA) for the production of surgical and dental prostheses, its properties at body temperature and those encountered in the mouth are of particular importance. Furthermore, the necessity for the incorporation of metal inserts and porcelain teeth makes its thermal properties of unusual interest. The thermal expansion of both commercial PMMA (‘Perspex’) and samples prepared by dental techniques was examined by means of a specially constructed dilatometer. Increase in length was recorded by means of a dial gauge reading to 0.0005 mm, and temperatures were measured at three points along 70 mm specimens by means of a Leeds and Northrup potentiometer. Specimens were arranged in a silica vessel which in later experiments was filled with water to determine the effect of water absorption on the thermal properties of PMMA. Increase in length was plotted against rise in temperature between 20° and 40° C. The resulting graphs showed a change in upward slope between 30°–32° C. The thermal properties were studied daily for periods up to 35 days with the specimens immersed in water and for similar periods with the same specimen drying out. Graphs prepared as before showed no alteration in the position at which an upward change in slope occurred, although the thermal expansion was greater for saturated specimens than for dry ones. Several second-order phase transition points have been recorded¹ for PMMA, but so far as we can ascertain, no reference to one in the temperature range of 30°–32° C has previously been reported.