Free Radical Initiated Low Temperature Crosslinking of Phenylethynyl (PE) End-capped Sulfonated Oligomides

A relatively low-temperature crosslinking method for phenylethynyl (PE) end-capped oligomides was developed. PE end-capped oligomides are typically cured into crosslinked polyimides at 370 C for about 1 h. The addition of a low viscosity mixed-solvent of N-methylpyrrolidinone (NMP)/dimethyl ether of polyethylene glycol (M ¼ 250 g/mol), NMP/DM-PEG- 250, or NMP/polyethylene glycol (M ¼ 400 g/mol), NMP/PEG- 400, as film forming medium for PE-end-capped oligomides was investigated. Fourier transform infrared spectroscopy and 13C NMR showed that the mixed solvent addition was effective for achieving low-temperature crosslinking of the ethynyl endcaps over the temperature range 200–250 C. The low temperature crosslinking process was explained by thermolysis of the PEG molecules over this temperature range forming free radical species such as CH2CH2O or CH2CH2 which initiate cure of the ethynyl groups resulting in a cross linked polyimide membrane. The PEG solvents also provide a radical source for the degradation polymerization of the solvents to a water and NMP insoluble polymer, which formed a miscible blend with the crosslinked membrane. Glass transition temperature (differential scanning calorimetry) data and thermo gravimetric analysis data provide evidence for the miscible blend