Effect of Salt Concentration and Valence on Behavior of Polyelectrolyte Brush Studied by Molecular Dynamics Simulations

以分子動力學模擬研究鹽濃度及鹽價數對高分子電解質刷行為的影響

謝沃質(Vijeth Sathyanarayana Guptha)

指導教授 ： 曾繁根 ； 蕭百沂

國立清華大學/工學院/奈米工程與微系統研究所/碩士(2013年)

摘要

In this study, a fully charged polyelectrolyte brush system is examined by means of molecular simulations. The simulation is conducted using Langevin dynamics to investigate the conformation and swelling of the polyelectrolyte (PE) chains inside a brush system under different conditions such as changing the valance of added salt and changing grafting density of chains.
We observe that addition of the monovalent salt induces the structural change of the PE brush. This is due to the screening effect of salt ions surrounding the PE brushes. In this case, the stiffness of the chain gradually decreases from rod-like regime to coil/globule like regime, upon increasing charge ratio of the solution. Consequently, the thickness of the brush decreases monotonically. In multivalent (divalent and trivalent) salt solution, the structural transition is quite different compared to monovalent case. In the region where charge ratio <1, the height of the brush decreases upon increasing the salt concentration. At =1, the chain reach a minimum height, indicating a critical point. Then in >1, the chains swell due to the electrostatic interaction. A charge inversion occurs because of the condensation of multivalent cations which are tightly bound on the PE chains. The effective chain charge reverses its sign and it is the electrostatic repulsion inside the chains which causes the chain swelling.
We also change the grafting density of the PE brush system to study the brush behavior. The height of the brush decreases with increase in grafting density at a given charge ration in the region <1. At charge ratio =1, the chain attains a minimum height in all the grafting density and the value almost the same in all the grafting densities under multivalent (divalent and trivalent) salt solution. But in monovalent case, the brush continues to collapse. In the charge ratio >1, the brush swells and the swelling becomes stronger with the valance in multivalent salt solution. The results show that the brush height can be controlled by changing the valence of the added salt in the solutions or by changing the chain grafting density.

關鍵字

高分子電解質刷

並列摘要

無資料

並列關鍵字

Polyelectrolyte Brush

參考文獻

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