Highly crosslinked/thermally stabilized

Third, the introduction of crosslinks between chains confers insolubility and increased solid state rigidity, often accompanied by improved thermal stability. High degrees of crosslinking confer ceramic-type properties on the solid, whether the backbone atoms are carbon atoms or inorganic species. 

Fusion of the llptlnlte macerals In bituminous coals commences at lower temperatures and reaches a much greater extent than that of the aromatic macerals (Figures 4 and 5). The greater thermal stability Indicated by the much higher fusion temperatures of the bituminous llptlnltes compared to brown coal llptlnltes can be explained In terms of these materials having a more highly crossllnked macromolecular structure than the llptlnltes In the brown coals. This Increase with coallflcatlon could be the consequence of In situ crosslinking of material or the selective loss of llptlnlte fractions that are less crossllnked and therefore less Inherently stable ... 

Thermal stability as measured by these ramped TGA experiments of the sort previously described are not the definitive test of a polymer s utility at elevated temperature. Rather, for a polymer to be useful at elevated temperatures, it must exhibit some significant retention of useful mechanical properties over a predetermined lifetime at the maximum temperature that will be encountered in its final end use application. While many of the bisbenzocyclobutene polymers have been reported in the literature, only a few have been studied in detail with regards to their thermal and mechanical performance at both room and elevated temperatures. Tables 7-10 show some of the preliminary mechanical data as well as some other physical properties of molded samples of polymers derived from amide monomer 32, ester monomer 40, diketone monomer 14 and polysiloxane monomer 13. The use of the term polyamide, ester etc. with these materials is not meant to imply that they are to be regarded as merely modified linear thermoplastics. Rather, these polymers are for the most part highly crosslinked thermosets. 

The DSC of primary acetylenic monomer 68 showed the presence of two distinct exotherm peaks. The first peak of 202 °C was attributed to the acetylene reacting separately with a second acetylene while the exothermic peak of 270 °C was proposed to be a benzocyclobutene reacting with a second benzocyclobutene. The resulting polymer was believed to be a highly crosslinked network and to some extent the Tg of 380 °C would support this contention. Interestingly, this homopolymer had poor thermal stability at 343 °C in air, with a retention of 41 % of its weight after 200 h. 

Throughout this chapter the chemical concepts employed to synthesize and cure addition poly(imides) have been discussed and their use as matrix resins for fiber composites has frequently been mentioned. The most important property of the imide backbone structure is the inherent thermal stability. The target of achieving the temperature performance of linear poly(imide) has not been reached, because of the aliphatic nature of the reactive endgroups, and because of the low molecular weight of the imide backbone required for processing. Future developments of addition polyimides will, as in the past, focus on the requirement of high thermal and thermal oxidative stability of the crosslinked... 

The crosslinked triazine (or cyanurate) polymers distinguish themselves by high glass temperatures and improved thermal stability. 

---