含水量與溫度對高分子/纖維素水凝膠電解質特性的分子動力學研究

Abstract

近年來，鋰離子電池已廣泛應用於現代科技產品中。然而，其使用的液態電解質存在潛在的安全風險。在此背景下，高分子凝膠電解質 (GPE)因具非液態特性，因而成為極具潛力的替代電解質之一。目前相關的研究多著重於提升GPE的離子導電率，而鮮少系統性地探討含水量對GPE機械性質等的關鍵影響，同時也缺乏明確的最佳化標準。本論文以分子動力學模擬探討含水量與溫度對複合凝膠電解質性能的影響，並引入纖維素作為補強材料，旨在建立最適含水量條件，並釐清微觀機制對宏觀性質的調控，作為GPE設計的理論依據。儘管本研究以GPE應用作為出發點，所探討之高分子/水/纖維素三元系統亦涵蓋水凝膠常見設計元素，研究成果具延伸至生醫、感測與智慧軟體等應用的潛力，對力學與結構調控理解同樣具啟發性。本論文的第二章首先討論模擬理論與模型建構的過程，涵蓋PVA、PEO、PAN與PVDF等高分子及纖維素之分子模型建立，並構築具不同含水量的系統。模擬採用COMPASS力場與NVT平衡條件，評估系統在多種含水量與溫度下的熱力學與機械性質。在第三章中，我們針對含水量與溫度兩項變因進行論。首先，我們分析含水量對系統的影響。結果顯示，在40%含水量時 (除PVDF/CEL外)，各系統之內聚能密度達局部極大化，結合能與機械性質亦表現最佳。而當含水量超過50%時，水分子開始主導鍵結行為，使系統性質由高分子支配轉變為水相控制。此外，透過徑向分布函數與氫鍵幾何參數 (數量、角度與長度)之分析，證實了水分子在不同含水量下對微觀結構的調控機制。其次，本研究探討機械性質最優化時所對應的含水量條件，溫度 (298-378 K)對內聚能密度的影響、結合能與機械性質之關係。結果顯示，內聚能密度與結合能整體隨溫度上升而下降，惟結合能並非單調遞減，尤其在338-358 K之間依高分子性質出現非線性變化。氫鍵之間的動態平衡與相變行為亦與溫度密切相關，進一步揭示微觀運動對結合能非線性行為的影響。最後在第四章中，我們將模擬結果與現有文獻數據進行比對。PVA-纖維素於不同含水量下所模擬之楊氏模量變化趨勢，與實驗文獻中所觀察到的實驗結果一致，均呈現先下降後上升的行為。另模擬所得之蒲菘比與玻璃轉移溫度等參數亦與實測數據相符，證實所採用之多初始態統計平均法可有效降低小晶胞模擬誤差，提升結果重現性與可信度。

In recent years, lithium-ion batteries have been widely adopted in modern technological products. However, the liquid electrolytes used in their batteries pose potential safety risks. Against this backdrop, gel polymer electrolytes (GPEs) have emerged as a promising alternative due to their non-liquid characteristics. Current research has primarily focused on enhancing the ionic conductivity of GPE, with limited systematic exploration of the critical influence of water content on mechanical properties and other key aspects of GPEs. Moreover, there is a lack of clear optimization standards. This thesis employs molecular dynamics simulations to investigate the effects of water content and temperature on the performance of composite GPEs, and introduces cellulose as a reinforcing material. The aim is to establish the optimal hydration condition and clarify the microscopic mechanisms regulating their macroscopic properties, thereby providing a theoretical foundation for future GPE design. Although the work is rooted in GPEs, the investigated polymer/water/cellulose ternary system also reflects key structural features commonly found in hydrogel materials. Consequently, the insights gained from this work may apply to broader hydrogel-related applications, such as biomedical engineering, sensing, and soft robotics, where mechanical stability and structural regulation are equally crucial. In Chapter 2, the simulation theory and model construction process are first discussed, covering the establishment of molecular models for polymers such as PVA, PEO, PAN, PVDF, as well as cellulose, and constructing systems with different water contents. The simulation uses the COMPASS force field and NVT ensemble conditions to evaluate the thermodynamic and mechanical properties of the system under various hydration levels and temperatures. In Chapter 3, we discuss the effects of two key variables: water content and temperature. First, we analyzed the effects of water content on the system. The results show that at 40% water content (except for the PVDF/CEL system), the cohesive energy density of all systems reaches a local maximum, and the bonding energy and mechanical properties also exhibited optimal performance. When the water content exceeds 50%, water molecules begin to dominate the intermolecular interactions, causing the system properties to transition from polymer-dominated to water-phase dominated. Furthermore, through analyses of the radial distribution function and hydrogen bond geometric parameters (bond number, angle, and length), we confirmed the regulatory mechanism of water molecules on the microstructure at different hydration levels. Second, this study investigated the hydration condition corresponding to the optimal mechanical performance, the effects of temperature (298–378 K) on cohesive energy density, and the relationship between binding energy and mechanical properties. The results showed that the cohesive energy density and binding energy decrease overall with increasing temperature, but the binding energy does not decrease monotonically, particularly exhibiting nonlinear changes between 338-358 K depending on polymer properties. The dynamic equilibrium between hydrogen bonds and phase transition behavior are closely related to temperature, further revealing the influence of molecular motion on the nonlinear behavior of binding energy. Finally, in Chapter 4, we compare the simulation results with existing literature data. The simulated Young’s modulus of PVA/cellulose systems at different hydration levels show a trend consistent with the experimental results observed in the literature, exhibiting a decrease followed by an increase. Additionally, the simulated parameters such as the Poisson’s ratio and glass transition temperature (Tg) are in agreement with the measured data, validating that multi-initial-state statistical averaging method effectively reduces small-cell-size simulation errors, thereby enhancing the reliability and reproducibility of the results.