For linear polymethyl Methacrylate, it may be observed that with high molecular weight the glass transition temperrature is almost independant of molecular weight but with decreasing molecular weight induced by radiation decreases quite sharply. On the other hand, the flow temperature is dependant on molecular weight and is reduced markedly with increasing radiation dose. A relation between molecular weight and flow temperature is proposed,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648698&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648704&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648700&type=, where M is molecular weight, T,f, is flow temperature in K.,There are obvious flow temperature and viscous liquid state for cross-linking polymethyl methacrylate except high cross-linked polymer with M,c, (average molecular weight between cross-links) 2.5 × 10,4,. An effect of cross-linking on glass transition is observed. Additional cross-link will cause an additional restriction to the flexiblity of the molecules. Hence it may be processed that:,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648705&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648708&type=,http://html.publish.founderss.cn/rc-pub/api/common/picture?pictureId=42648707&type=,where T,g, and 121 are the glass transitions of cross-linked and linear polymer respectively.,Because glass transition temperature determines the upper limit of temperature of glass-like thermoplastic and the T,g, value raises 20-30℃, then the outstanding improvement of cross-linked polymethyl methacrylate, as compared with the linear materials, is its increased heat resistance. Moreover this polymer exhibits a considerable possibility for heat processing because T,f, raises only 3-10℃ higher than that of commercial linear polymer.