探究聚3-烷基噻吩高分子系統中各項分子內/間勢能函數、側鏈長度及接枝密度對於自組裝行為的影響

Abstract

利用粗粒化分子動力學模擬探討P3AT高分子在不同接枝密度以及側鏈長度下的相行為變化。利用全原子模型建構P3AT高分子並且取得結構的細部資訊，之後利用波茲曼反轉技術擬合得到有效粗粒化模型力場。除了兩硫環分子間非鍵結作用力參數利用反式構型改變不同距離所得到的位能曲線圖修正。當加入兩面角限制，會有兩種不同的側鏈位向，其中接枝密度為100%及33%時側鏈位於反側，接枝密度為50%及25%時側鏈位於同側。在接枝密度為100%及50%時，硫環間的π–π作用力在P3AT的自組裝行為扮演最重要的角色。然而，我們分別在接枝密度為100%及50%時，觀察到單層層狀以及雙層層狀，隨著側鏈長度的增加觀察到層狀轉換為扭曲層狀結構。這並不足以為奇，因為較長的側鏈會增加主鏈的扭曲程度。當側鏈接枝密度的下降，不僅僅是硫環間的π–π作用力可以增加主鏈間的堆疊，而且側鏈間的空間也有助於側鏈間的交互穿插以及側鏈的聚集。因此，poly(3’-nonylthiophene), poly(3’-dodecylthiophene)及poly(3’-pentadecylthiophene)高分子在接枝密度為25%時，我們預測型態為多角柱狀，其中柱狀為側鏈間聚集並且由主鏈間的聚集所包覆。在接枝密度為25%時，當側鏈長度繼續增加，堆疊型態將由側鏈間聚集的柱狀型態轉換成較為穩定的雙層層狀。

The self-assembling behavior of conjugated regioregular poly(3’-alkylthiophene) (P3AT) polymers with various grafting density and length of alkyl side-chains are systematically examined by the coarse-grained molecular dynamics (CGMD) simulations. The effective CG interaction potentials are determined by taking into account the atomistic structural details from all-atom (AA) MD through the mapping route between the AAMD and CGMD data via the Iterative Boltzmann Inversion (IBI) methods. Except the non-bonded interaction potential between two thiophenes is modified by simply calculating the potential energy curve for them in a tran-orientation. With the dihedral angle constraint, two types of the grafted side-chains, which are located on both sides (anti-) and the same side (syn-) of the backbones, are modeled at GD = 100%, 33% and 50%, 25%, respectively. For densely grafted side-chains, such as when GD = 100% and 50%, the strong π-π attraction between the thiophene rings plays the most dominant role on the self-assembling behavior of P3AT molecules. Thus, we observe the formation of single-walled and double-walled lamellae, which however become more twisted with increasing the length of side-chains at GD = 100% and 50 %, respectively. This is not surprising as longer flexible alkyl side-chains cause the backbone thiophene rings to suffer a larger distortion degree. When the grafting density is reduced significantly, not only the π-π attraction between the thiophene rings can induce the ordered packing of main-chains, but also the significant space between the side-chains can facilitate the interdigitation and aggregation of side-chains. Accordingly, for the poly(3’-nonylthiophene), poly(3’-dodecylthiophene), and poly(3’-pentadecylthiophene) molecules with 25 % grafting density, we predict the formation of polygonal columns, which are formed by the packing of main-chains parallel to the column axes and the aggregation of the side-chains within the columns. When the side-chain length is increased further, these aggregated long side-chains can no longer be constrained within the polygonal minor-domains. Instead, double-walled lamellae become the most stable phase for P3AT molecules with 25 % GD.