Influence of molecular weight on thermal and mechanical properties of bisphenol A-based phthalonitrile resins

This effort assesses the correlation between chemical structures and performance-essential thermal, mechanical, and long-term stability properties of cross-linked thermosets. Resins of different molecular weights were prepared from the Bisphenol A based PEEK (TM)-like oligomeric phthalonitrile (BisA). Differential scanning calorimetry, which was used to investigate curing thermodynamics, indicated that BisA resins demonstrated positive correlation between increasing oligomer molecular weight and both resulting melting points and cure initiation conditions. Characterization of thermal properties using thermogravimetric analysis (TGA) indicated a similar molecular weight trend, with char yields ranging between 57% and 73%. Rheological studies of BisA of different molecular weights indicated significant viscosity increases in phthalonitriles that cross-linked from oligomers with higher molecular weights. Moreover, the n = 1 chain length resin exhibited a gel point at 100 degrees C lower than the n = 25 oligomer. Analysis of hardness of these cured polymers indicated that the resin cross-linked using the n = 1 oligomer was most brittle, while the thermoset derived from the n = 4 BisA demonstrated highest hardness. Aging of cured phthalonitriles indicated that the n = 10 remained most stable in long-duration high-temperature environments. This study suggests the use of preparing BisA thermosets from oligomers with different molecular weights as an effective strategy for improving toughness, albeit at the tradeoff of lower thermal stabilities.