Vashisth Aniruddh, Ashraf Chowdhury, Zhang Weiwei, Bakis Charles E, van Duin Adri C T
Department of Engineering Science and Mechanics , The Pennsylvania State University , 212 Earth and Engineering Sciences Building , University Park , Pennsylvania 16802 , United States.
Department of Mechanical Engineering , The Pennsylvania State University , 136 Research East Building, Bigler Road , University Park , Pennsylvania 16802 , United States.
J Phys Chem A. 2018 Aug 16;122(32):6633-6642. doi: 10.1021/acs.jpca.8b03826. Epub 2018 Aug 1.
Various methods have been developed to perform atomistic-scale simulations for the cross-linking of polymers. Most of these methods involve connecting the reactive sites of the monomers, but these typically do not capture the entire reaction process from the reactants to final products through transition states. Experimental time scales for cross-linking reactions in polymers range from minutes to hours, which are time scales that are inaccessible to atomistic-scale simulations. Because simulating reactions on realistic time scales is computationally expensive, in this investigation, an accelerated simulation method was developed within the ReaxFF reactive force field framework. In this method, the reactants are tracked until they reach a nonreactive configuration that provides a good starting point for a reactive event. Subsequently, the reactants are provided with a sufficient amount of energy-equivalent or slightly larger than their lowest-energy reaction barrier-to overcome the barrier for the cross-linking process and form desired products. This allows simulation of cross-linking at realistic, low temperatures, which helps to mimic chemical reactions and avoids unwanted high-temperature side reactions and still allows us to reject high-barrier events. It should be noted that not all accelerated events are successful as high local strain can lead to reaction rejections. The validity of the ReaxFF force field was tested for three different types of transition state, possibly for polymerization of epoxides, and good agreement with quantum mechanical methods was observed. The accelerated method was further implemented to study the cross-linking of diglycidyl ether of bisphenol F (bis F) and diethyltoluenediamine (DETDA), and a reasonably high percentage (82%) of cross-linking was obtained. The simulated cross-linked polymer was then tested for density, glass transition temperature, and modulus and found to be in good agreement with experiments. Results indicate that this newly developed accelerated simulation method in ReaxFF can be a useful tool to perform atomistic-scale simulations on polymerization processes that have a relatively high reaction barrier at a realistic, low temperature.
已经开发出各种方法来对聚合物交联进行原子尺度模拟。这些方法大多涉及连接单体的反应位点，但这些方法通常无法捕捉从反应物通过过渡态到最终产物的整个反应过程。聚合物交联反应的实验时间尺度从几分钟到几小时不等，这是原子尺度模拟无法达到的时间尺度。由于在实际时间尺度上模拟反应计算成本很高，在本研究中，在ReaxFF反应力场框架内开发了一种加速模拟方法。在这种方法中，跟踪反应物直到它们达到非反应性构型，该构型为反应事件提供了一个良好的起点。随后，为反应物提供足够量的能量——等同于或略大于其最低能量反应势垒——以克服交联过程的势垒并形成所需产物。这使得能够在实际的低温下模拟交联，这有助于模拟化学反应并避免不需要的高温副反应，同时仍然使我们能够排除高势垒事件。应该注意的是，并非所有加速事件都是成功的，因为高局部应变可能导致反应被拒绝。针对三种不同类型的过渡态（可能用于环氧化物的聚合）测试了ReaxFF力场的有效性，并观察到与量子力学方法有良好的一致性。进一步实施加速方法来研究双酚F二缩水甘油醚（双酚F）和二乙基甲苯二胺（DETDA）的交联，获得了相当高的交联百分比（82%）。然后对模拟的交联聚合物进行密度、玻璃化转变温度和模量测试，发现与实验结果吻合良好。结果表明，ReaxFF中这种新开发的加速模拟方法可以成为一种有用的工具，用于在实际低温下对具有相对高反应势垒的聚合过程进行原子尺度模拟。
