Martínez-Veracoechea Francisco J, Escobedo Fernando A
School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA.
J Chem Phys. 2006 Sep 14;125(10):104907. doi: 10.1063/1.2345652.
Particle-based molecular simulations of pure diblock copolymer (DBC) systems were performed in continuum space via dissipative particle dynamics and Monte Carlo methods for a bead-spring chain model. This model consisted of chains of soft repulsive particles often used with dissipative particle dynamics. The gyroid phase was successfully simulated in DBC melts at selected conditions provided that the simulation box size was commensurate with the gyroid lattice spacing. Simulations were concentrated at conditions where the gyroid phase is expected to be stable which allowed us to outline approximate phase boundaries. When more than one phase was observed by varying simulation box size, thermodynamic stability was discerned by comparing the Helmholtz free energy of the competing phases. For this purpose, chemical potentials were efficiently simulated via an expanded ensemble that gradually inserts/deletes a target chain to/from the system. These simulations employed a novel combination of Bennett's [J. Comput. Phys. 22, 245 (1976)] acceptance-ratio method to estimate free-energy differences and a recently proposed method to get biasing weights that maximize the number of times that the target chain is regrown. The analysis of the gyroid nodes revealed clear evidence of packing frustration in the form of an (entropically) unfavorably overstretching of chains, a phenomenon that has been suggested to provide the structural basis for the limited region of stability of the gyroid phase in the DBC phase diagram. Finally, the G phase and nodal chain stretching were also found in simulations with a completely different DBC particle-based model.
通过耗散粒子动力学和蒙特卡罗方法，在连续空间中对纯双嵌段共聚物（DBC）系统进行了基于粒子的分子模拟，用于珠子 - 弹簧链模型。该模型由通常与耗散粒子动力学一起使用的软排斥粒子链组成。在选定条件下，成功地在DBC熔体中模拟了螺旋状相，前提是模拟盒尺寸与螺旋状晶格间距相称。模拟集中在预期螺旋状相稳定的条件下，这使我们能够勾勒出近似的相界。当通过改变模拟盒尺寸观察到多个相时，通过比较竞争相的亥姆霍兹自由能来辨别热力学稳定性。为此，通过一种扩展系综有效地模拟化学势，该系综逐渐将目标链插入系统/从系统中删除。这些模拟采用了贝内特[《计算物理杂志》22，245（1976）]接受率方法的新颖组合来估计自由能差，并采用了最近提出的一种方法来获得使目标链重新生长次数最大化的偏置权重。对螺旋状节点的分析揭示了明显的堆积受挫证据，表现为链的（熵上）不利过度拉伸，这一现象被认为为DBC相图中螺旋状相稳定性的有限区域提供了结构基础。最后，在使用完全不同的基于DBC粒子的模型的模拟中也发现了G相和节点链拉伸。
