No abstract

ABSTRACT:Moderate increases (Ç 50-75%) in the toughness of bismaleimides (BMIs) were achieved with very low-molecular-weight ( Ç 1000 g/mol) imide thermoplastics at low levels of thermoplastic loading ( Ç 10-20%). The thermoplastic was introduced into the BMI using a simple, one-pot, reactive solvent approach. In this approach, the reactive diluent of a two-part BMI was used as the reaction solvent for the thermoplastic synthesis. The BMI monomer was then dissolved in the thermoplastic reaction solution to yield a low-viscosity homogenous prepolymer. The viscosity of the thermoplastic solution was Ç 6 Pa S at 55ЊC. The effect of thermoplastic loading and molecular weight on viscosity was determined by rheology, and the fracture toughness of neat resin plaques was determined by compact tension. Increasing the thermoplastic loading increased prepolymer viscosity without improving toughness, while increasing the thermoplastic molecular weight increased the toughness by only 25% more than the lowestmolecular-weight thermoplastic, yet increased viscosity fivefold. Fracture surfaces showed no obvious phase separation by scanning electron microscopy.

The fracture toughness of epoxy thermosets was increased by up to 220% using very low-molecular-weight (ϳ 1000 g/mol) imide thermoplastic. The objective was to produce a low-viscosity prepolymer that could be easily autoclave-processed to give a tough thermoset. Here, an homogenous epoxy prepolymer was prepared by first synthesizing very low-molecular-weight linear aromatic imide (ϳ 1000 g/mol) directly in a liquid allyl phenol reactive solvent, followed by dissolution of the epoxy (Epon® 825) and the cure agent (DDS) directly in the thermoplastic solution. The allyl phenol both cures into the epoxy network, through phenol functional groups, and accelerates the cure. The viscosity of the pure epoxy was 1.4 Pa ⅐ S at 30°C. The prepolymer formulations ranged from ϳ 5-33 Pa ⅐ S at 30°C, but all reduced to less than 1 Pa ⅐ S at 90°C. The onset of cure is well above 90°C so the prepolymer viscosity is within the range for autoclave processing. The cured resin plaques were not transparent, but phase-separated domains were not found by scanning electron microscopy, indicating that the domain size is below the detection limit of the instrument. The reactive solvent causes a decrease in both the T g and the high temperature modulus of the thermoset. Introduction of the thermoplastic results in partial recovery of the T g and modulus.