Department of Chemical and Biomolecular Engineering, University of Melbourne, VIC 3010, Australia.
J Phys Chem B. 2010 Aug 19;114(32):10520-3. doi: 10.1021/jp101793c.
Polymers present a fascinating range of mechanical properties in the extension, compression, and flow of soft condensed matter. Entropy determines the physics of the elastic material behavior of polymeric systems in deformation. Classical models of polymer dynamics have assumed chain entanglement and resulting reptation in concentrated polymer solutions. Here, we present a thermodynamic treatment of interacting chains in solution with increasing concentration from the dilute to concentrated regimes. As the polymer chain concentration increases above the critical overlap, the chains must either compress or overlap and entangle, resulting in a decrease in chain configurational entropy. The free energy of chain entanglement is shown to be less favored than compression at concentrations above the critical overlap. Elastic forces act on the chains to reduce the dimensions to the ideal random walk size with increasing concentration. At significantly higher concentrations, the free energies reach an asymptote where chain compression and entanglement are simultaneously possible. Entanglement and reptation are shown to be statistically improbable in the semidilute regime, and it is concluded that the compression of polymer chains is favored at semidilute concentrations.
聚合物在软凝聚态的拉伸、压缩和流动中呈现出一系列迷人的力学性能。熵决定了聚合物系统在变形过程中弹性材料行为的物理学。聚合物动力学的经典模型假设了链缠结和在浓聚合物溶液中的蠕动。在这里，我们提出了一种热力学处理方法，用于研究从稀溶液到浓溶液中相互作用链的浓度增加。当聚合物链浓度超过临界缠结时，链必须压缩或重叠缠结，导致链构象熵的减少。链缠结的自由能显示出在高于临界缠结的浓度下，比压缩更不利。弹性力作用在链上，使链的尺寸随着浓度的增加减小到理想的无规行走尺寸。在浓度显著增加的情况下，自由能达到一个渐近值，在这个值上，链的压缩和缠结同时成为可能。缠结和蠕动在半浓溶液中被证明是统计上不可能的，因此得出结论，在半浓溶液中，聚合物链的压缩是有利的。
