Optimization of fracture toughness of high Tg thermosets was done through systematic investigation of different formulations of reactive functional modifiers using soft particle impact modification. Important parameters like particle size, interparticle distance (IPD) were varied by altering cure kinetics and modifying the molecular architecture of the additives. The best performing systems showed an increase in fracture toughness of 70-80% with an optimum Rp of 1.3 μm and IPD of 0.4 μm at 15 vol% impact modifier.
In addition, a new platform of using block copolymer blends was studied for its feasibility to achieve non-spherical morphology for effective impact modification. The micromechanics of this approach is discussed and techniques like optical and scanning electron microscopy were used to investigate the toughness mechanisms. Block copolymers with different molecular weights, backbone structures and architectures were blended in different ratios and the fracture toughness achieved using non-spherical morphologies was compared to the spherical modifiers. The effect of particle size and shape on the fracture was evaluated by defining a metric called shape factor. Fracture toughness was seen to correlate well with the morphology and three-fold improvement was observed over the conventional spherical modifiers.
Fundamental studies on the effect of functional additives on network structure of thermosets were also done by measuring their physical and mechanical properties using techniques such as DSC, DMA and non-linear compression testing. Model non-stoichiometric networks were made with DGEBA and DDM with excess mole ratio of amine and epoxy groups. The trend in the properties of the networks at low strain regions in compression testing and before the Tg in DMA, exhibit effects similar to physical ageing due to the network being heterogenous and fragmented. At high strain regions and above the Tg, the effects of crosslink density and network connectivity are evident. The fracture toughness could be correlated well to the properties like Tg, yield stress, different moduli of the networks and the breadth of the α transition measured in the DMA, therefore demonstrating that these tools can be very effective in probing the network defects and correlating them to a non-linear engineering property like fracture toughness
