Yingling Yaroslava G, Garrison Barbara J
Department of Chemistry, 104 Chemistry Building, The Pennsylvania State University, University Park, Pennsylvania 16802, USA.
J Phys Chem B. 2005 Sep 1;109(34):16482-9. doi: 10.1021/jp0527711.
A coarse-grained model has been developed for molecular dynamics simulations of the interaction of light with polymeric materials. The photon energy can result in a vibrational excitation (photothermal process) or disruption of a chemical bond (photochemical process) in a polymer. In the latter case, the formation of active radial sites and the occurrence of chemical reactions have to be taken into consideration. The novel feature of this model is the incorporation of chemical reactions into the united atom approximate representation of the polymer structure, which permits the study of laser ablation, degradation, or the effect of various chemical reactions on large time and length scales. The chemical reactions are included in the model in a probabilistic manner as in the kinetic Monte Carlo method. This model adopts physically and experimentally known quantities such as enthalpies and probabilities of reactions. Properties such as laser irradiation time, laser fluence, and wavelength are explicitly included. Moreover, no chemically correct interaction potential is required to incorporate the effects of chemical reactions on the dynamics of the system after energy deposition. We find that the model provides a plausible description of the essential processes. The laser-induced pressure relaxation is the main mechanism responsible for the onset of polymer ablation. Since the pressure relaxation processes are slow, there is a delay in the onset of ablation after the end of the laser pulse as is observed experimentally. The vaporization processes are not efficient for material removal, and the effect is minimal for both photochemical and photothermal processes. A lower fluence is needed for the onset of ablation with photochemical processes than photothermal processes.
已经开发了一种粗粒度模型，用于对光与聚合物材料相互作用进行分子动力学模拟。光子能量可导致聚合物中的振动激发（光热过程）或化学键的断裂（光化学过程）。在后一种情况下，必须考虑活性自由基位点的形成和化学反应的发生。该模型的新颖之处在于将化学反应纳入聚合物结构的联合原子近似表示中，这使得能够在较大的时间和长度尺度上研究激光烧蚀、降解或各种化学反应的影响。化学反应以概率方式包含在模型中，类似于动力学蒙特卡罗方法。该模型采用了诸如反应焓和概率等物理和实验已知的量。明确包含了诸如激光照射时间、激光能量密度和波长等属性。此外，在能量沉积后，无需化学上正确的相互作用势来纳入化学反应对系统动力学的影响。我们发现该模型对基本过程提供了合理的描述。激光诱导的压力弛豫是聚合物烧蚀开始的主要机制。由于压力弛豫过程缓慢，如实验观察到的那样，在激光脉冲结束后烧蚀开始存在延迟。汽化过程对于材料去除效率不高，并且对于光化学和光热过程其影响都很小。与光热过程相比，光化学过程开始烧蚀所需的能量密度更低。
