嵌段共聚物胶束溶液的结晶自由能
Crystalline free energies of micelles of diblock copolymer solutions
摘要：
我们报告的相对稳定性和各种胶束晶体的AB-二嵌段共聚物在溶液中的非热模型的结构行为的表征。我们采用了以前开发的粗粒化表示的链映射每个共聚物的软哑铃。由于这种强烈的自由度的减少，我们能够调查大的聚集系统，并为特定的长度比的块f = MA/（MA + MB）= 0.6，以定位有序-无序的胶束系统的转变。在过渡之上，发现机械和热性能取决于模拟箱中每个晶格位置的颗粒数量，以及最近用于多占据晶体的方法的应用（B.M. Mladek等人，物理修订版信函99，235702（2007））是正确定义平衡态所必需的。在这个方案中，我们计算了两个约化密度ρ/ρ \ast = 4，5和几个立方结构如FCC，BCC，A_（15）的自由能。在这两种密度下，BCC对称性被发现对应于无约束自由能的最小值，即在所考虑的少数对称性中的稳定对称性，而A15结构几乎是简并的，这表明本系统更倾向于在较少堆积的结构中结晶。在接近熔化的ρ/ρ \ast = 4时，林德曼比相当高（~ 0.29），空位浓度约为6%。当ρ/ρ \ast = 5时，稳定BCC结构的机械稳定性增加，空位浓度相应降低。发现电晕层厚度与芯半径的比率与聚（苯乙烯-b-异戊二烯）（22-12）在异戊二烯选择性溶剂中的实验数据非常一致，该聚（苯乙烯-b-异戊二烯）（22-12）也被报道以BCC结构结晶。
英文摘要：
We report a characterization of the relative stability and structural behavior of various micellar crystals of an athermal model of AB-diblock copolymers in solution. We adopt a previously devel- oped coarse-graining representation of the chains which maps each copolymer on a soft dumbbell. Thanks to this strong reduction of degrees of freedom, we are able to investigate large aggregated systems, and for a specific length ratio of the blocks f = MA/(MA + MB) = 0.6, to locate the order-disorder transition of the system of micelles. Above the transition, mechanical and thermal properties are found to depend on the number of particles per lattice site in the simulation box, and the application of a recent methodology for multiple occupancy crystals (B.M. Mladek et al., Phys. Rev. Lett. 99, 235702 (2007)) is necessary to correctly define the equilibrium state. Within this scheme we have performed free energy calculations at two reduced density ρ/ρ\ast = 4,5 and for several cubic structures as FCC,BCC,A15. At both densities, the BCC symmetry is found to correspond to the minimum of the unconstrained free energy, that is to the stable symmetry among the few considered, while the A15 structure is almost degenerate, indicating that the present sys- tem prefers to crystallize in less packed structures. At ρ/ρ\ast = 4 close to melting, the Lindemann ratio is fairly high (~ 0.29) and the concentration of vacancies is roughly 6%. At ρ/ρ\ast = 5 the mechanical stability of the stable BCC structure increases and the concentration of vacancies ac- cordingly decreases. The ratio of the corona layer thickness to the core radius is found to be in good agreement with experimental data for poly(styrene-b-isoprene)(22-12) in isoprene selective solvent which is also reported to crystallize in the BCC structure.
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