以電腦分子模擬探討PTT(聚對苯二甲酸丙二酯)纖維之結構與性質之關係

Abstract

本研究利用電腦分子模擬技術，探討PTT高分子性質與其分子結構之間的關係。本研究主要內容有二部分。首先是尋找最能代表真實PTT纖維的模擬參數(力場、樣品數、聚合度)。於力場選擇上，我們發現以DREIDING-Exp6-Ewald力場描述PTT系統，於單位晶格參數與密度平均誤差僅1.34%，因此以DREIDING-Exp6-Ewald力場描述PTT系統為最佳選擇。對於結構複雜度高的PTT非晶相系統，我們發現以10個獨立系統樣品數目所得的性質分佈與500個獨立系統樣品數目近似。此外，從一系列非晶相系統中PTT分子型態與其聚合度的分析上，發現PTT分子呈橄欖球型，且高分子鏈聚合度大於54，分子整體外觀構形隨聚合度上升分別以定比例放大及增加。同時我們也發現，PTT單體位能貢獻度在鏈聚合度大於54後即不再變動。因此我們確認以聚合度54(系統原子數5400)及樣品數10是描述PTT非晶相系統之最佳選擇。 本研究第二部分在探討PTT纖維結晶度與紅外線吸收光譜之關係。我們應用Hessian 矩陣法及velocity autocorrelation法計算3D分子振動模式，發現計算所得之特性吸收峰頻率與文獻記載之實驗值有一定比關係，比值約為0.85~0.91之間。同時，無論是晶相或是非晶相的PTT，其紅外線吸收光譜中主要的特性吸收峰均相同均不隨系統的尺寸（鏈聚合度及分子數）而改變。此外，我們發現PTT非晶相系統於升溫(300K至473K)及拉伸 (0%, 50%及100%)過程中，其晶相特性峰之強度會因結晶度提高而增強，非晶相特性峰之強度會因結晶度提高而降低。透過完美PTT晶相與非晶相系統紅外線吸收特性峰的比較，有助於未來PTT半晶相系統定量分析吸收峰強度與結晶度關係及兩者關連性。

We performed a series of atomistic molecular dynamics simulations to study the relation between the properties and the conformation of poly (trimethylene terephthalate), PTT. In the first part of this work, we investigate the dependence of properties on the degree of polymerization (DP) and number of PTT molecules in the system. We find that there is an apparent change of such dependences in both structure (end-to end distance, aspect ratio) and energetic properties as the DP of PTT is greater than 54. This indicates that simulation models with DP equal to or larger than 54 are capable of representing the polymeric material. In the second part of this work, we discuss the relation between the infrared (IR) spectrum and the degree of crystallization of PTT fibers. The IR spectrum are calculated from the Hessian matrix (0 K) and the Fourier transform of the velocity autocorrelation function (elevated temperatures). We find that the calculated IR frequencies are constantly greater than the experimental values by 10 to 15%. We have also identified the characteristic peaks of PTT fibers in the pure crystalline and pure amorphous states. We observe an increase and decrease in the intensity of crystalline and amorphous characteristic peaks respectively as an amorphous PTT is heated (from 300K to 473K) or stretched (up to 100% extension). This indicates the enhancement of crystillinity of PTT in the heating or drawing process.