以分子動態模擬研究PTT(聚對苯二甲酸丙二脂)結晶初期行為

Abstract

以分子動態模擬研究聚對苯二甲酸丙二脂(PTT)結晶初期行為。藉由模擬恆溫結晶及拉伸程序，我們證實結晶核前驅物(precursor of nuclei)的存在，並發現其成長主要受到高分子主鏈的旋轉能量(torsional energy)及分子間凡得瓦爾作用力(van der Waals interaction)的影響。我們成功預測聚對苯二甲酸丙二脂的玻璃轉移溫度(Tg)及熔點(Tm)，並發現在恆溫結晶程序中，結晶核前驅物的總量在這兩溫度區間快速增加，接著維持週期性波動。此外；高分子主鏈藉由旋轉的分式重新排列，趨向結晶的結構。在拉伸程序中，有向性的晶核前驅物(oriented precursor)的總量大大的提升。系統中，高分子主鏈的旋轉角度分佈快速變成直鏈狀(trans-trans-trans-trans)構形。並在個別的有向性的晶核前驅物中，發現兩組旋轉角趨向結晶結構的轉變速率不同。其內部結構成長的三個因素:高分子鏈段的個數、旋轉角度(torsional angle)及排列緊密程度相互競爭或妥協，使結晶前趨物結構趨向結晶結構。由結果支持結晶前趨物在結晶初期扮演重要的角色。

Atomistic molecular dynamics simulations are performed to study the initial crystallization process of poly(trimethylene terephthalate) (PTT). The structure development of ordering structures (nuclei precursors) in the isothermal and stress-induced crystallization process has been observed in our simulations. The formation of nucleus precursors is found to be driven mainly by the torsional and van der Waals forces. The thermal properties, such as the glass transition temperature (Tg) and the melting temperature (Tm), determined from our simulation are in good agreement with experimental values. In isothermal processes, it is found that, between these two temperatures, the amount of precursors quickly arises during thermal relaxation period soon after the system is quenched and starts to fluctuate afterwards. The variation of precursor fraction with temperature exhibits a maximum between Tg and Tm, resembling temperature dependence of crystallization rate for most polymers. In addition, the backbone torsion distribution for segments within the precursor preferentially reorganizes to the trans-gauche-gauche-trans (t-g-g-t) conformation, the same as that in the crystalline state. On the other hand, during stress-induced crystallization, the amount of stress-induced precursor increases in all regions of temperature. The torsional distribution of the polymer backbone for segments rapidly rearrange to the t-t-t-t conformation in bulk phase. Within oriented precursors, the response of the torsional angle induced by stress is faster than that only induced by thermal stimulation, especially trans in