Synthesis and properties of acetylene-terminated isoimide and imide oligomers exhibiting excellent solubility behaviors in low boiling point solvents

A series of acetylene-terminated isoimide and imide oligomers were prepared based on 2,3,3,4-biphenyltetracarboxylic dianhydride, 4,4-(hexafluoroisopropylidene)diphthalic anhydride, 3,4-oxydianiline, and 1,3-bis(3-aminophenoxy)benzene as monomers. 3-Ethynylaniline was used as an end-capping reagent and trifluoroacetic anhydride served as a dehydrating agent. The main oligomer structures were confirmed by Fourier transform infrared spectroscopy, and the thermal imidization was characterized by differential scanning calorimetry. All uncured isoimide and imide oligomers exhibit excellent solubility characteristics in organic solvents such as N,N-dimethylacetamide and N-methyl-2-pyrrolidone, and most of the oligomers can be dissolved in low boiling point solvents such as acetone and ethyl acetate (more than 45 wt%). In comparison with imide analogs, the isoimide oligomers exhibited an increased solubility in low boiling point solvents which can be attributed to their unique asymmetric structures. The properties of the cured films and adhesives were evaluated by dynamic mechanical thermal analysis, thermogravimetric analysis, as well as tensile shear strength tests of the polymer adhesives. The cured films exhibited extremely high glass transition temperatures (T-g) of up to 341.6 degrees C and 5% weight loss temperatures (T-d5) of up to 518.2 degrees C in a nitrogen atmosphere. Taken in concert, the results obtained indicate that all cured films featured an excellent thermal stability and a particularly high thermal-oxidative stability. One of these oligomers was selected and formulated to form a thermosetting adhesive for further investigation of the adhesion properties. This was done in an effort to improve the high-temperature performance characteristics of composite materials.