Development of a Reactive Force Field for Simulating Photoinitiated Acrylate Polymerization

Light-driven and photocurable polymer-based additive manufacturing (AM) has enormous potential due to its excellent resolution and precision. Acrylated resins that undergo radical chain-growth polymerization are widely used in photopolymer AM due to their fast kinetics and often serve as a departure point for developing other resin materials for photopolymer-based AM technologies. For successful control of the photopolymer resins, the molecular basis of the acrylate free-radical polymerization has to be understood in detail. We present an optimized reactive force field (ReaxFF) for molecular dynamics (MD) simulations of acrylate polymer resins that captures radical polymerization thermodynamics and kinetics. The force field is trained against an extensive training set including density functional theory (DFT) calculations of reaction pathways along the radical polymerization from methyl acrylate to methyl butyrate, bond dissociation energies, and structures and partial charges of several molecules and radicals. We also found that it was critical to train the force field against an incorrect, nonphysical reaction pathway observed in simulations that used parameters not optimized for acrylate polymerization. The parameterization process utilizes a parallelized search algorithm, and the resulting model can describe polymer resin formation, crosslinking density, conversion rate, and residual monomers of the complex acrylate mixtures.以光驱动、光固化聚合物为基底的增材制造（Additive Manufacturing, AM）凭借优异的分辨率与精准度，拥有巨大的应用潜力。能够发生自由基链式聚合的丙烯酸酯树脂，因反应动力学速率快而被广泛应用于光聚合增材制造领域，且常作为开发其他光聚合基增材制造技术用树脂材料的起点。为实现对光聚合树脂的精准调控，必须深入理解丙烯酸酯自由基聚合的分子机制。本研究开发了一款经优化的反应力场（ReaxFF），用于丙烯酸酯聚合物树脂的分子动力学（MD）模拟，可准确刻画自由基聚合的热力学与动力学特性。该力场基于大规模训练集进行参数拟合，训练集涵盖了从丙烯酸甲酯到丁酸甲酯的自由基聚合反应路径的密度泛函理论（DFT）计算结果、键解离能，以及多种分子与自由基的结构和部分电荷。研究同时发现，针对未优化的丙烯酸酯聚合参数在模拟中出现的非物理错误反应路径进行力场训练，是至关重要的一环。参数化过程采用了并行搜索算法，最终得到的模型可描述复杂丙烯酸酯混合体系的树脂形成过程、交联密度、单体转化率以及残留单体含量。