Gary E. Spilman

Crosslinking chemistry for high-performance polymer networks☆

Abstract A new thermally reactive monomer has been designed and synthesized that brings novel crosslinking chemistry to high-performance polymers. This monomer (XTA) is a derivative of terephthalic acid and was based on the thermal chemistry of benzocyclobutene. Various model compounds have been synthesized to investigate substituent effects on benzocyclobutene reactivity. Irreversible reaction exotherms around 350°C were observed in these model compounds using differential scanning calorimetry. Based on these studies, polyaramid and poly(aryl ether ketone) XTA copolymers were synthesized. The formation of an insoluble network resulted after heat treatment of these polymers.

Thermally crosslinkable thermoplastic PET-co-XTA copolyesters

Abstract A series of thermally crosslinkable polyester copolymers were synthesized by incorporation of a benzocyclobutene-containing terephthalic acid derivative (XTA) into polyethylene terephthalate (PET). The cyclobutene moiety on the XTA monomer allows for reactive crosslinking at temperatures ∼350°C requiring no catalyst and causing no change in mass. Copolymers were synthesized containing 1, 5, 10, 20, 50, and 100 mol%o XTA. Crosslinking occurred above the melting temperature (∼250°C) yet below the degradation temperature (∼400°C), providing a window for melt processing of the copolymer. To demonstrate this point fibres were melt spun. The PET-co-XTA copolymers show systematic variations in the glass transition, recrystallization, melting and degradation temperatures as a function of benzocyclobutene content. The degradation and melting temperature both decrease slightly with increased XTA, while the recrystallization and glass transition temperature were relatively insensitive to XTA content. Thermal gravimetric analysis (TGA) indicated a decrease in the degradation temperature as higher amounts of XTA were incorporated, although an increase in the %char at 800°C was seen. This decrease in degradation temperature may be due to the generation of free radicals. Limiting Oxygen Index (LOI) measurements showed an increase in the oxygen content required to maintain a stable flame in copolymers with increasing amounts of XTA. LOI values ranged from 18 for neat PET to 35 for the copolymer containing 20 mol % XTA. Wide-angle X-ray scattering data showed little change in the crystalline structure, but decreasing crystallinity for PET for blends containing up to 20 mol% XTA. The 50 mol% XTA copolymer was amorphous, while the 100% XTA homopolymer (PEXTA) showed evidence of a new crystalline structure. Crystalline diffraction peaks showed reduced intensities in data recorded for heat treated samples, and there was evidence for new peaks in the copolymer containing 20 mol% XTA when heated near 300°C. Transmission electron microscopy of cross-sections through burned samples showed a highly crystalline char at the surface of XTA copolyesters. This crystalline char appeared to protect the underlying copolymer from further flame-induced degradation. Evidence for significantly increased adhesion of the copolymers to polyimide films was also obtained.

Thermally crosslinkable thermotropic copolyesters : synthesis, characterization, and processing

The synthesis, characterization, and processing of thermotropic copolymers composed of hydroxybenzoic acid (HBA), hydroxynaphthoic acid (HNA), and systematically varying amounts of hydroquinone (HQ) and crosslinkable terephthalic acid (XTA) are described. The XTA monomer contains a benzocyclobutene (BCB) group that lies dormant during synthesis and initial steps of processing, but that can be thermally activated to introduce covalent crosslinking between laterally adjacent macromolecules. The XTA-containing HBA/HNA copolymers all remain thermotropically liquid crystalline, and can be processed into oriented fibres by melt spinning. Rheological characterization reveals an increase in the viscosity and transition from liquid-like to solid-like behaviour as the crosslinking proceeds. X-ray diffraction reveals the changes in local organization with increasing XTA content. The microstructures of the XTA-containing copolymers (up to 20% XTA) in the condensed state are similar to those seen in HBA/HNA copolymers.