(217bl) Toughened Epoxy Thermosets Through Rearrangements of Network Molecular Architecture

Thermosetting polymers are turning out to be prominent as they are widely using as high performance composite materials and durable adhesives. The highly crosslinked molecular architecture make this group of materials strong but brittle. One key goal is to enhance the material’s toughness without sacrificing strength, glass transition temperature (Tg) and modulus. This aim could be possible through rearrangement of crosslinkings in a polymer network. Few approaches were proposed to alter thermosetting network topology. In the first approach, a novel technique referred to as “Reactive Encapsulation of Solvent” (RES) is introduced to adjust the molecular arrangement within a polymer network. SEM micrographs from fracture surfaces of tensile coupons indicated the success of RES technique in developing molecular-scale uncrosslinked regions in fully cured thermosets. The desired locally connected nano-scale features further referred to as “protovoids”. Density measurement and freeze fracture surface analysis revealed that the resulting thermosets are not mesoporous in their relaxed states and thus the molecular-scale cavities appear in response to mechanical load and deformation. Experimental results showed that the modified samples have volumetric energy capacity about 2.5 times larger than the unmodified samples. It was also observed that the modified specimens could bear higher uniaxial compressive stress (about 2.5 times greater) than the original samples. Additionally, dynamical mechanical analysis supported the claim that the RES technique does not alter glassy modulus and T_g. 

Overall, toughened epoxy materials were synthesized. The enhancement was merely attributed to the arrangement of crosslinkings in highly crosslinked polymer networks and thus major properties such as stiffness and strength as well as Tg have not been affected. The impact of this study will enable us to design more sustainable and durable multifunctional composite materials and adhesives.