Elder Robert M, Knorr Daniel B, Andzelm Jan W, Lenhart Joseph L, Sirk Timothy W
U.S. Army Research Laboratory, Aberdeen Proving Ground, Maryland 21005, USA.
Soft Matter. 2016 May 11;12(19):4418-34. doi: 10.1039/c6sm00691d.
Protective equipment in civilian and military applications requires the use of polymer materials that are both stiff and tough over a wide range of strain rates. However, typical structural materials, like tightly cross-linked epoxies, are very brittle. Recent experiments demonstrated that cross-linked poly(dicyclopentadiene) (pDCPD) networks can circumvent this trade-off by providing structural properties such as a high glass transition temperature and glassy modulus, while simultaneously exhibiting excellent toughness and high-rate impact resistance. The greater performance of pDCPD was attributed to more facile plastic deformation and nano-scale void formation, but the chemical and structural mechanisms underlying this response were not clear. Here, we use atomistic molecular dynamics to compare the molecular- and chain-level properties of pDCPD and epoxy networks undergoing high strain rate deformation. We quantify the tensile modulus and yield strength of the networks as well as the prevalence and characteristics of nanovoids that form during deformation. Networks of similar molecular weight between cross-links are compared. Two key molecular-level properties are identified - monomer flexibility and polar chemistry - that influence the behavior of the networks. Increasing monomer flexibility reduces the modulus and yield strength, while strong non-covalent interactions (e.g., hydrogen bonds) that accompany polar moieties provide higher modulus and yield strength. The lack of strong non-covalent interactions in pDCPD was found to account for its lower modulus and yield strength compared to the epoxies. We examine the molecular-level properties of nanovoids, such as shape, alignment, and local stress distribution, as well as the local chemical environment, finding that nanovoid formation and growth are increased by the monomer rigidity but decreased by polar chemistry. As a result, the pDCPD network, which has a stiff chain backbone with nonpolar alkane chemistry, exhibits more and larger nanovoids that grow more readily during deformation, which could account for the higher toughness and more ductile behavior observed in pDCPD.
民用和军事应用中的防护装备需要使用在很宽的应变率范围内既坚硬又坚韧的聚合物材料。然而，典型的结构材料，如高度交联的环氧树脂，非常脆。最近的实验表明，交联聚（二环戊二烯）（pDCPD）网络可以通过提供诸如高玻璃化转变温度和玻璃态模量等结构性能来规避这种权衡，同时展现出优异的韧性和高应变率抗冲击性。pDCPD的更佳性能归因于更易发生的塑性变形和纳米级空洞的形成，但这种响应背后的化学和结构机制尚不清楚。在这里，我们使用原子分子动力学来比较pDCPD和环氧树脂网络在高应变率变形过程中的分子和链级特性。我们量化了网络的拉伸模量和屈服强度以及变形过程中形成的纳米空洞的普遍性和特征。比较了交联点之间分子量相似的网络。确定了两个关键的分子级特性——单体柔韧性和极性化学——它们影响网络的行为。增加单体柔韧性会降低模量和屈服强度，而极性基团伴随的强非共价相互作用（如氢键）会提供更高的模量和屈服强度。发现与环氧树脂相比，pDCPD中缺乏强非共价相互作用导致其模量和屈服强度较低。我们研究了纳米空洞的分子级特性，如形状、排列和局部应力分布，以及局部化学环境，发现纳米空洞的形成和生长因单体刚性而增加，但因极性化学而减少。因此，具有刚性链主链和非极性烷烃化学的pDCPD网络表现出更多、更大的纳米空洞，这些纳米空洞在变形过程中更容易生长，这可以解释在pDCPD中观察到的更高韧性和更具延展性的行为。
