多尺度模擬探討高分子奈米複合材料熱傳導與機械性質研究

Investigation of Thermal Conductivity and Mechanical Properties of Polymer Nanocomposites by Multiscale Simulation

黃天榮(Huang, Tien Jung)

指導教授 ： 陳信龍

國立清華大學/工學院/化學工程學系/博士(2016年)

摘要

傳統上，研究人員使用試誤法發展新高分子及奈米複合材料，因此需要不斷地試誤，這是需要耗費大量的時間、金錢與成本物力。本論文目的在發展出一種多尺度模擬方法，用以預測機械性質及高分子奈米複合材料的熱傳導性能，藉以促進實務應用之材料的設計。本論文所討論的系統包含：（1）聚（對苯硫醚）（PPS）/石墨薄片（GF）的複合材料；（2）聚丙烯（PP）/碳纖維（CF）的複合材料；（3）具方向性的PBT及PPS；和（4）液晶高分子和尼龍6的複合材料。
利用實驗測量和非平衡分子動力學模擬（NEMD），來研究不同的石墨薄片（GF）重量分率的石墨薄片（GF）/ 聚（對苯硫醚）（PPS）複合材料的熱導率。由NEMD模擬獲得之不同GF重量分率的熱傳導變化，相匹配於從熱壓法得到之樣品的實驗結果。一旦GF 的TC值從MD模擬結果而得，就可以利用經驗方程式，麥克斯韋 - 歐肯模型（Maxwell-Eucken model），來預測不同GF重量級分的TC變化。由麥克斯韋 - 歐肯模型預測的TC值，與用實驗和MD模擬方法預測的TC值一致。
我們已經研究了聚丙烯（PP）/ 碳纖維（CF）複合材料的界面強度，通過實驗觀察以及使用分子動力學（MD）模擬，以決定四個PP / CF複合材料的最佳的化學官能基團。第一，利用模擬退火程序建構PP/CF、PP-MAH/CF、PP-MAH/CF-NH2 (2%) 及PP-MAH/CF-NH2 (5%)的結構，以獲得穩定界面的結構，並進一步採用這些結構來評價界面結合強度。我們發現，相對於原始結構的結晶程度，在界面之PP和PP-MAH的結晶程度被顯著地改善。結果顯示，利用每單位面積的相互作用能量和拉伸模擬的機械強度，改質PP/官能化石墨烯的強度較高。最後，改質PP和利用雙極基官能化的石墨烯複合材料的MD模擬結果，提供經濟且快捷的方法，以在進行實驗之前，評估的高分子複合材料系統的機械性能。
針對具有方向性的高分子系統進行強化傳熱的研究。針對具有方向排列之分子鏈的PPS 及 PBT，進行分子動力學模擬和熱導率的計算。模擬預測發現PPS和PBT的熱傳導率比起原本無序排列的結構系統，其熱傳導率提升了十倍，結果顯示有序排列的結構系統，就能夠減少加入成本較高的碳奈米管或石墨等材料，一樣能夠提升它們的熱傳導率。
使用多尺度模擬方法，包括分子動力（MD）、粗粒化分子動力（CGMD）及耗散粒子動力（DPD），來研究液晶高分子（p-HBA、對羥基苯甲酸、LCP）和尼龍6（nylon 6）的混合特性。LCP的聚集被認為會形成被分散在尼龍6主體內的短纖維。利用溶解度參數計算來進行相溶性分析，發現該LCP分散相可以提高材料的機械強度。這預測結果與實驗結果一致。同時計算秩序參數，來表徵不同混合成分和溫度的結構順序。有秩序排列促進LCP和尼龍6在界面彼此相互作用，而導致能獲得改善的機械強度。

關鍵字

多尺度模擬 ； 高分子 ； 奈米複合材料 ； 熱傳導 ； 機械性質 ； 分子動力 ； 介觀模擬

並列摘要

Traditionally, researchers develop a new polymer and nanocomposite by use of trial and error approach, which should continuously try again and again and need to spend a lot of time, money and material costs. This thesis is directed to developing the multiscale simulation method for predicting the mechanical properties and thermal conductivity of polymer nanocomposites to facilitate the design of materials for practical applications. The systems studied include (1) poly (p-phenylene sulfide)(PPS) / graphite flake (GF) composites, (2) polypropylene (PP)/carbon fiber (CF) composites, (3) oriented PBT and PPS and (4) liquid crystal polymer and nylon 6 material composites.
The thermal conductivities (TC) of graphite flake (GF)/ poly (p-phenylene sulfide)(PPS) composites at different GF weight fractions were investigated by experimental measurement and by non-equilibrium molecular dynamics simulation (NEMD).The TC variation at different GF weight fractions obtained by the NEMD simulation matches well with the experimental findings for the samples from the hot press method. An empirical equation, the Maxwell-Eucken model, can be used to predict TC variations at different GF weight fractions once the GF TC value is derived from the MD simulation results. The TC values predicted by the Maxwell-Eucken model agree well with those by experimental and MD simulation approaches.
We have investigated the interfacial strengths in polypropylene (PP)/carbon fiber (CF) composites through experimental observation as well as using molecular dynamics (MD) simulation to determine the optimal chemical functionalization groups for four PP/CF composites. First, the structures of PP/CF, PP-MAH/CF, PP-MAH/CF-NH2 (2%) and PP-MAH/CF-NH2 (5%) were constructed to obtain stable interface structures by the simulated-annealing procedure, and these structures were further used to evaluate the interface bonding strength. We found that the degrees of crystallinity of PP and PP-MAH at the interfaces are significantly improved when compared to those of the pristine structure. The results show, through the interaction energy per unit area and the tensile simulation mechanical strength, that the strength of the modified PP/ functionalized-graphene is higher. Finally, the MD simulation results of the modified PP and functionalized-graphene composites by polar groups provide an economical and quick way to assess the mechanical properties of a polymer composite system before conducting an experiment.
The enhancement of heat transfer was studied for oriented polymer systems. Molecular dynamics simulations and calculations of thermal conductivity was conducted for PPS and PBT having an ordered alignment in molecular chain. The simulation results predicted was that the thermal conductivity of PPS and PBT could be increased by 10-fold compared to that associated with the isotropic state. The result showed that the thermal conductivity can also be enhanced in ordered arrangement of the polymer chains, so that the amount of the nano-additions for enhancing the thermal conductivity such as graphite or carbon nanotube can be reduced.
A multi-scale simulation method, including molecular dynamics, CGMD and DPD, was employed to study the blend of liquid crystal polymer (p-HBA, para-hydroxyl benzoic acid, LCP) and nylon 6. The aggregation of LCP was considered to form short fiber dispersed in the nylon 6 matrix. The utilization of the solubility parameter calculation for compatibility analysis revealed that the LCP dispersed phase could enhance the mechanical strength of materials. This prediction was consistent with the experiment results. The order parameter was also calculated to characterize the structural order under different blend compositions and temperatures. Ordered arrangement facilitated LCP and Nylon 6 to interact each other at the interface leading to obtain improved mechanical strength.

並列關鍵字

Multiscale Simulation ； Polymer ； Nanocomposites ； Thermal Conductivity ； Mechanical Properties ； Molecular dynamics ； Mesoscale simulation

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