Yin De-Wei, Yan Qiliang, de Pablo Juan J
Department of Chemical and Biological Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706-1691, USA.
J Chem Phys. 2005 Nov 1;123(17):174909. doi: 10.1063/1.2102827.
The volumetric properties of highly-charged defect-free polyelectrolyte networks with tetrafunctional crosslinks are studied through molecular dynamics simulations in the canonical ensemble. The network backbone monomers, which are monovalent, and the counterions, which are mono-, di-, or trivalent, are modeled explicitly in the simulations, but the solvent is treated implicitly as a dielectric medium of good solvation quality. The osmotic pressure of the network-solvent system is found to depend greatly on the strength of electrostatic interactions. Discontinuous volume phase transitions are observed when the electrostatic interactions are strong, and the onset of these transitions shifts to higher solvent dielectricity as the counterion valency increases. The roles of the various virial contributions to the osmotic pressure are examined. The network elasticity entropy is found to behave nearly classically. As the network contracts and collapses with increasing strength of electrostatic interactions, the loss of counterion entropy leads to increased counterion osmotic pressure contributions via two mechanisms. The reduction in available configurational space increases the counterion translational entropy contribution to the ideal part of the osmotic pressure, and the greater number of counterion-monomer contacts formed due to counterion condensation and confinement increases the counterion excluded-volume entropy contribution to the excess part of the osmotic pressure. These observations contrast the decrease in the single ideal-gas-like counterion translational entropy contribution to the osmotic pressure predicted by the counterion condensation-charge renormalization theory. An accompanying decrease in the total electrostatic energy balances the loss of counterion excluded-volume entropy as the polyelectrolyte networks collapse in low-dielectric solvents. This interplay between the electrostatic energy and the counterion excluded-volume entropy appears to be responsible for the discontinuous volume phase transitions that are observed in polyelectrolyte networks. The structure of the polyelectrolyte network is also found to be affine in the swollen state, with constituent chains nearly fully extended, and nonaffine in the collapsed state, with the chains adopting a Gaussian conformation.
通过正则系综中的分子动力学模拟，研究了具有四官能交联的高电荷无缺陷聚电解质网络的体积性质。在模拟中明确建模了单价的网络主链单体和单价、二价或三价的抗衡离子，但将溶剂隐式处理为具有良好溶剂化质量的介电介质。发现网络 - 溶剂系统的渗透压很大程度上取决于静电相互作用的强度。当静电相互作用较强时，观察到不连续的体积相变，并且随着抗衡离子价数的增加，这些相变的起始点向更高的溶剂介电常数移动。研究了各种维里项对渗透压的作用。发现网络弹性熵的行为几乎是经典的。随着网络随着静电相互作用强度的增加而收缩和塌陷，抗衡离子熵的损失通过两种机制导致抗衡离子渗透压贡献增加。可用构型空间的减少增加了抗衡离子平动熵对渗透压理想部分的贡献，并且由于抗衡离子凝聚和限制而形成的更多抗衡离子 - 单体接触增加了抗衡离子排除体积熵对渗透压过量部分的贡献。这些观察结果与抗衡离子凝聚 - 电荷重整化理论预测的单个类理想气体抗衡离子平动熵对渗透压贡献的减少形成对比。当聚电解质网络在低介电溶剂中塌陷时，总静电能的相应减少平衡了抗衡离子排除体积熵的损失。静电能与抗衡离子排除体积熵之间的这种相互作用似乎是聚电解质网络中观察到的不连续体积相变的原因。还发现聚电解质网络的结构在膨胀状态下是仿射的，组成链几乎完全伸展，而在塌陷状态下是非仿射的，链采用高斯构象。
