Structure Modification of Fluorocarbon Ether Bibenzoxazole Polymers to Improve Hydrolytic Stability

摘要

Hydrolytically and thermooxidatively stable fluorocarbon ether bibenzoxazole (FEB) polymers were synthesized by the acetic acid-promoted polycyclocondensation of fluorocarbon ether bis (o-aminophenol) monomers with novel fluorocarbon ether dithioimidate ester monomers. The dithioimidate esters were derived from hexafluoropropylene oxide (HFPO) or, preferably, from a combination of tetrafluoroethylene oxide (TFEO) and HFPO. The latter class of TFEO:HFPO-derived dithioimidate esters was tailored to impart improved hydrolytic stability to the resultant FEB polymers while minimizing undesirable, concomitant increases in polymer glass transition temperatures (Tg). The polymers were obtained in moderate-to-high molecular weight and exhibited inherent viscosities in the range of 0.20 to 0.41 dl/g. Polymer structures were verified by elemental and infrared spectral analysis. Lower polymer Tg's were obtained with increased fluorocarbon ether content, a minimum value of -48 C (-55 F) being achieved. High thermooxidative stability of the HFPO- and TFEO:HFPO-based FEB polymers was substantiated by thermogravimetric analysis and isothermal aging studies in air. After isothermal aging in air for 200 hours at 288 C (550 F) and 260 C (500 F), respresentative polymers exhibited weight losses of 17 and 7 percent, respectively. Hydrolytic stability of the polymers was evaluated by exposure to a 95 percent relative humidity, 93 C (200 F) environment for prolonged periods of time. Based on comparative polymer Tg, inherent viscosity and infrared spectral data, the HFPO- and TFEO:HFPO-based FEB polymers exhibited vastly improved hydrolytic stability over analogous TFEO-based FEB polymers.