外加直流電場下高分子電解質刷之電腦模擬研究

Polyelectrolyte Brushes in Direct-Current Electric Fields: a Computer Simulation Study

何雨璠(Yu-Fan Ho)

指導教授 ： 蕭百沂

國立清華大學/原子科學院/工程與系統科學系/碩士(2012年)

摘要

我們以朗日凡分子動力學模擬研究在不同嫁接密度和外加直流電場條件下的高分子電解質刷系統的構形和其反離子的分佈狀態，系統中電場的方向垂直於基板表面。
首先，我們發現當電場強度大於臨界電場值時，高分子電解質刷的構形開始產生明顯的改變。在低嫁接密度系統中，所有高分子鏈都會隨著電場增強而伸展其高度，此時電解質刷是由同質相所組成；然而，在中和高嫁接密度系統裡，電解質刷可以允許兩相同時並存。研究顯示在中嫁接密度系統下，當電場強度在|E**|<|E|<|E***|的區間，由於一部分高分子鏈沒有伸直其構形，所以電解質刷會分裂成疏密兩層結構。而在高嫁接密度系統中，只有很少數的高分子鏈能被電場拉伸，其他未受拉伸的高分子鏈會和反離子形成緊密層；即使在高電場條件下，高分子電解質刷也不會產生如中嫁接密度系統的兩相再合併情形。
再者，在低和中嫁接密度系統中，當電場強度超過臨界電場時，反離子的分佈狀態會出現明顯的變化。電場強度小於臨界電場時，由於反離子吸附於鏈上的效應，其分佈狀態形似高分子單體的分佈狀態。而電場強度大於臨界電場時，大部份的反離子會往基板表面移動，此時，反離子吸附於鏈上的比率也會降低。另外，我們也可經由計算側視密度圖和俯視密度圖，更加清楚了解高分子單體和反離子的分佈細節。
最後，我們發現臨界電場值隨嫁接密度增高而變大，而且遵循尺度關係律 。此外，我們也討論了在不同嫁接密度和電場強度條件下的系統相圖，讓我們對高分子電解質刷系統有更深入的認識。

關鍵字

高分子電解質刷 ； 嫁接密度 ； 臨界電場

並列摘要

We use Langevin dynamics simulations to investigate the conformation of polyelectrolyte (PE) brushes and the counterions distribution at different grafting densities in direct-current uniform electric fields |E|. The field direction is perpendicular to the substrate surface.
Firstly, we observe that |E| larger than a critical electric field |E*| can induce large conformational change of PE brushes. At low grafting densities, the height of chains stretch together with |E| and the brush is in a homogeneous phase. However, at intermediate and high grafting densities, two phases can coexist in the brush. For the intermediate grafting density case, the brush separates into dilute and dense, two-layer phases when |E**|<|E|<|E***| because a fraction of the chains are not stretched. For high grafting density, only few chains are stretched by the electric field. The majority of the chains are non-stretched and form a compact layer with the counterions. The bifurcated brush does not merge into a homogeneous phase in strong electric fields as at the intermediate grafting density.
Secondly, the profiles of the counterion distribution change significantly at low and intermediate grafting density when the field strength exceeds |E*|. Below |E*|, the counterion profiles match the monomer distributions due to ion condensation on the chains. Above |E*|, the majority of the ions move toward the substrate surface and the fraction of the condensed counterions decreases. The monomer and ion distributions are studied in detail through calculation of the side-view and top-view densities.
Finally, we find that |E*| increases with grafting density and follows the scaling law. A schematic phase diagram is presented, which gives a thorough picture of the PE brush sys-tem over the parameter space grafting densities and |E|.

並列關鍵字

Polyelectrolyte Brushes ； Grafting density ； Critical electric field

參考文獻

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