Transition Temperature of Polymethyl Methacrylate at Ultrasonic Frequencies

Abstract

PULSE method measurements by Krishnamurthi and Sastry¹ and Melchor and Petrauskas², of the variation of ultrasonic velocities with temperature for polymethyl methacrylate (‘Perspex’), showed that the transition temperature was independent of frequency (0.5–10 Mc./s.). Protzman³, using an optical diffraction method, found that the transition point for ‘Plexiglas II’ (unplasticized ‘Perspex’) varied from 63° C. at 3 Mc./s. to 49° C. at 11 Mc./s. The authors claim an accuracy of ± 1 per cent for their results, which are shown in Fig. 1, together with some of my own results. The following comments are made on these results.