Static and Dynamic\nThermomechanical Properties of\nPhase-Separated Epoxy Networks with Tuned Microstructures

Polymerization-induced phase separation\nis a useful method for\nthe construction of heterogeneous epoxy networks with properties exceeding\ntheir homogeneous counterparts. In this work, we examine the static\nand dynamic thermomechanical properties of phase-separated epoxy networks\nsalient to their application as encapsulants. Three heterogeneous\nepoxy-amine networks with nano-, meso-, and macro-phase-separated\nmorphologies comprised of hard and soft domains are compared to a\nrigid, unstructured network. The glass transition profiles of the\nheterogeneous networks are complex, spanning many decades in the frequency\ndomain. The nanophase-separated morphology leads to higher coefficient\nof thermal expansion, yet surprisingly is characterized by reduced\nresidual stress. Under both quasi-static and dynamic compression (strain\nrates of order 10<sup>–3</sup> and 10<sup>3</sup> s<sup>–1</sup>, respectively), the nanophase-separated network also exhibits higher\nmodulus and strength. In split-Hopkinson bar experiments, the energy\ndissipation characteristics of the epoxy networks were nearly identical.\nCuriously, however, the Hugoniot response of the macro-phase-separated\nnetwork determined by ballistic shockwave analysis indicates a remarkable\nability of this material to mitigate shockwave propagation in comparison\nto many homogeneous and heterogeneous polymer materials. Collectively,\nthis work reveals several previously unreported phenomena with respect\nto structure–property relationships in phase-separated epoxy\nnetworks, illustrating the potential value of systematically tuned\nmicrostructures for optimization of application-specific physical\nproperties.