Phenylethynyl-terminated imide oligomers derived from thioetherdiphthalic anhydride isomers with decreased melt viscosities

A series of oligomers based on the mixture of thioetherdiphthalic anhydride isomers and 4,4′-oxydianiline with 4-phenylethynylphthalic anhydride as reactive endcapping reagent were prepared. The calculated molecular weights were in the range of 1150–5070 g mol⁻¹ with different degrees of polymerization (n = 1, 3, 5, 7, and 9). The effect of molecular weight of the aromatic oligomers on their processability and solubility as well as the thermal and mechanical properties of the thermal-cured polyimides (PIs) was systematically investigated. The typical oligomer (Oligo-1) could be melted at temperatures of 289–334°C to yield stable molten fluid with melt viscosity below 1.0 Pa s. The melt viscosity of the oligomers increased with the increasing molecular weight. After thermally curing at 370°C, the thermoset PIs exhibited good thermal properties. The glass transition temperatures of oligomers measured by differential scanning calorimetry were in the range of 286–326°C, and the temperature of 5% weight loss was higher than 524°C. The cured films also showed good mechanical properties with tensile strength above 72 MPa and modulus more than 2.5 GPa.