Toward Soft Diamond: Molecular Modeling for the Engineering of Novel Super-tough Materials

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Materials where molecular building blocks are purposely connected in very regular three-dimensional networks are the object of increasing interest. This interest stems in part from the ability to synthesize such materials via self-assembly methods. Depending on the building block used, widely varied materials could be synthesized. At one extreme, a single carbon atom as building block leads to diamond, the hardest materials found in nature. At the other extreme, a very long hydrocarbon chain as building block leads to very soft plastics. In between, building blocks of intermediate size and stiffness would lead to exciting new materials that combine some of the strength of diamond with the elasticity of rubbers. Our preliminary studies have revealed that networks made of two types of building blocks that differ in stiffness or chemical affinity would form rubbers that mimic the elastic response of super-tough natural materials such as the adhesive in abalone shells and spiders' silk. This study is complementary to experimental efforts by other groups. It will fill a gap in the need for computational models that help identify specific material configurations of interest. The research will hence provide guidelines for the design of super-tough "rubbery diamonds" that could become a new materials' paradigm. In the long term, this could impact a broad range of materials related industries. The work will also provide opportunities for the involvement of underrepresented groups in research and the development of educational materials. Since the resistance to deformation in an elastomer composed of ordered and amorphous domains is associated with the free energy cost of rearranging those domains, it is postulated that the elastic properties of regular networks can be optimized by using building blocks that allow control of the free-energy barriers of the underlying order-disorder phase transitions (driven by deformation). Accordingly, the plan is to synergistically leverage the self-assembling properties of chains that are capable of forming entropy-driven liquid crystalline order (like semiflexible chains) and enthalpy-driven micro-segregated ordered phases (like block copolymers) to tune the non-linear elastic behavior of end-chain crosslinked networks with no or minimal defects. The aim is to modulate the height and number of free-energy barriers associated with a sawtooth elastic response. Molecular dynamics simulations will be used to investigate the effects on elastic behavior of different block copolymers (whose blocks differ in either enthalpic affinity or backbone flexibility) and of non-ideal architectures and defects.