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

分子构建块故意连接成非常规则的三维网络的材料是人们越来越感兴趣的对象。这种兴趣部分源于通过自组装方法合成这种材料的能力。根据所使用的构建块，可以合成各种各样的材料。在一个极端的情况下，一个单独的碳原子构成了钻石，这是自然界中发现的最坚硬的材料。在另一个极端，一个很长的碳氢化合物链作为构建块导致非常软的塑料。在这两者之间，中等大小和硬度的构建块将会产生令人兴奋的新材料，这种材料结合了金刚石的一些强度和橡胶的弹性。我们的初步研究表明，由两种硬度或化学亲和力不同的构建块组成的网络可以形成橡胶，这种橡胶可以模仿超坚韧天然材料（如鲍鱼壳和蜘蛛丝中的粘合剂）的弹性反应。这项研究是对其他小组的实验工作的补充。它将填补需要计算模型的空白，帮助确定感兴趣的特定材料结构。因此，这项研究将为设计超级坚韧的“橡胶钻石”提供指导，这可能成为一种新材料的范例。从长远来看，这可能会影响到广泛的材料相关行业。这项工作还将为代表性不足的群体参与研究和编写教材提供机会。由于由有序和非晶畴组成的弹性体的变形阻力与重新排列这些畴的自由能量成本有关，因此假设可以通过使用允许控制底层有序-无序相变（由变形驱动）的自由能垒的构建块来优化规则网络的弹性特性。因此，计划是协同利用链的自组装特性，能够形成熵驱动的液晶有序（如半柔性链）和焓驱动的微分离有序相（如嵌段共聚物），以调整端链交联网络的非线性弹性行为，没有或最小的缺陷。目的是调节与锯齿弹性响应相关的自由能垒的高度和数量。分子动力学模拟将用于研究不同嵌段共聚物（其嵌段在焓亲和或主链柔韧性上不同）和非理想结构和缺陷对弹性行为的影响。

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.