Segmental Dynamics in Entangled Linear Polymer Melts
Segmental Dynamics in Entangled Linear Polymer Melts
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DOI：
10.1021/ma202759v
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发表时间：
2012-04-24
期刊：
影响因子：
5.5
通讯作者：
Larson, Ronald G.
中科院分区：
文献类型：
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作者：
Wang, Zuowei;Likhtman, Alexei E.;Larson, Ronald G.
我们提出了缠结线性聚合物熔体中链段动力学的分子动力学（MD）和滑弹簧模型模拟。分析了柔性链和半柔性链的节段方向自相关函数和均方节段位移的时间相关行为，特别关注这些动态量之间的比例关系。两种模拟方法在不同粗粒度级别的有效组合使我们能够探索从 Z 近似到 2 到 90 个纠缠的链长度的链动力学。对于给定的 Z 链长度接近 15，访问的时间尺度跨越 10 多年，涵盖了所有有趣的弛豫机制。获得的单体均方位移 kW 的时间依赖性与管理论预测非常一致。一阶和二阶分段方向自相关函数 C-1(t) 和 C-2(t) 的结果表明，C-2(t) 与 C-1(t)(m) 具有清晰的幂律关系，其中 m = 3、2 和 1，分别在初始、自由 Rouse 和纠缠（约束 Rouse）状态下。返回原点假设导致纠缠态中分段方向自相关函数与 g(1)(t) 之间成反比，该假设通过约束 Rouse 态中 C-1(t) 与 g(1)(t)(-1) 成比例与 t(-1/4) 成比例的模拟结果令人信服地得到了验证，其中对于良好纠缠链，C-1(t) 和 g(1)(t) 都相当不敏感达到约束释放的效果。然而，二阶相关函数 C-2(t) 对约束释放效应表现出更强的敏感性，并且经历了从自由劳斯状态到纠缠状态的长期交叉。该交叉区域的时间比 C-1(t) 的交叉区域长至少十年。仅在链长度为 90 个纠缠的滑弹簧模拟中观察到 C-2(t) 与 t(-1/4) 成比例的预测时间缩放行为，而较短的链显示出更高的缩放指数。报告的模拟工作可用于理解核磁共振实验的观察结果。
We present molecular dynamics (MD) and slip-springs model simulations of the chain segmental dynamics in entangled linear polymer melts. The time-dependent behavior of the segmental orientation autocorrelation functions and mean-square segmental displacements are analyzed for both flexible and semiflexible chains, with particular attention paid to the scaling relations among these dynamic quantities. Effective combination of the two simulation methods at different coarse-graining levels allows us to explore the chain dynamics for chain lengths ranging from Z approximate to 2 to 90 entanglements. For a given chain length of Z approximate to 15, the time scales accessed span for more than 10 decades, covering all of the interesting relaxation regimes. The obtained time dependence of the monomer mean square displacements, kW, is in good agreement with the tube theory predictions. Results on the first- and second-order segmental orientation autocorrelation functions, C-1(t) and C-2(t), demonstrate a clear power law relationship of C-2(t) similar to C-1(t)(m) with m = 3, 2, and 1 in the initial, free Rouse, and entangled (constrained Rouse) regimes, respectively. The return-to-origin hypothesis, which leads to inverse proportionality between the segmental orientation autocorrelation functions and g(1)(t) in the entangled regime, is convincingly verified by the simulation result of C-1(t) proportional to g(1)(t)(-1) proportional to t(-1/4) in the constrained Rouse regime, where for well-entangled chains both C-1(t) and g(1)(t) are rather insensitive to the constraint release effects. However, the second-order correlation function, C-2(t), shows much stronger sensitivity to the constraint release effects and experiences a protracted crossover from the free Rouse to entangled regime. This crossover region extends for at least one decade in time longer than that of C-1(t). The predicted time scaling behavior of C-2(t) proportional to t(-1/4) is observed in slip-springs simulations only at chain length of 90 entanglements, whereas shorter chains show higher scaling exponents. The reported simulation work can be applied to understand the observations of the NMR experiments.