非平衡態分子動力學模擬短鏈線性高分子之奈米流變行為與性質

Abstract

流體達到分子尺度(molecular level)，在基礎與技術的層面上，深度地瞭解流體的熱力學狀態與流變學性質之變化，是很重要的。本研究透過非平衡態分子動力學模擬模擬(non-equlibrium molecular dynamics simulations)，完成了三個基本的流變學流場，穩態剪切流動(steady state shear flow)、簡諧震盪剪切流動(oscillatory shear flow)以及奈米擠出流場(nano-extrusion flow)。穩態剪切流動在不同溫度、壓力與密度下，呈現液態n-hexadecane的流變學之材料參數(material functions)，包含黏度(viscosity)、第一與第二正應力係數(first and second normal stress coefficients)對剪切率(shear rate)之關係，同時觀察到剪切漸稀(shear thinning)現象，以及瞭解非平衡態熱力學狀態(non-equilibrium thermodynamic states)之變化，特別地發現在高剪切率下有剪切膨脹(shear dilatancy)行為。關於簡諧震盪剪切流動，可以得到動力性質(dynamic properties)，包含儲存模數與損失模數(storage and loss moduli)以及相位差(pahse angle)，並且也證實n-hexadecane流體具有線性黏彈性(linear viscoelasticity)與熱流變簡單性(thermorheological simplicity)。利用儲存模數與損失模數對頻率之動力頻譜(dynamics spectra)，可以判別出流體在不同溫度下之相狀態，即近似固態(Solid-lke)、液態(liquid-like)與膠態(gel-like)。進一步，擴展到複雜的流場—短鏈的聚乙烯流體在奈米擠出流動—探究分子鏈是如何通過一個突然縮收噴嘴，並且呈現流體在噴嘴內的速度與溫度之分佈。對於恆溫的流體在噴嘴內，其表觀黏度與剪切率之關係，將會出現牛頓高原(Newtonain plateau)與剪切漸稀之特徵。

Knowledge of how molecular fluids’ thermodynamic and rheological properties change is of considerable importance from both theoretical and industrial perspective. Using non-equilibrium molecular dynamics simulations, we performed three computer experiments of basic flow fields, namely, steady state shear flow, oscillatory shear flow, and nano-extrusion flow. In steady state shear flow of liquid n-hexadecane under different temperatures, pressures, and densities, material functions—the shear rate dependence of viscosity, first and second normal stress coefficients—present shear thinning phenomena. At extreme shear rates, specific behavior of shear dilatancy is observed in the variations of non-equilibrium thermodynamic states. More importantly, viscoelastic material functions of oscillatory shear flows—storage and loss moduli, and phase angle—can be obtained while two significant corroborated manifestations are that the n-hexadecane fluid undoubtedly possesses linear viscoelasticity and thermorheological simplicity. Viscoelastic spectra, the frequency dependence of storage and loss moduli, determine the phase of the fluid: a solid-like state, liquid-like state, and gel-like state. Furthermore, nano-extrusion flows of linear short polyethylene chains that were uniformly extruded by a constant-speed piston into surrounding vacuum from a reservoir through an abrupt contraction nozzle were built. Since the length of a molecular chain is larger than the nozzle diameter, we are interested in understanding how a molecular chain flows through the nozzle region. Molecular motions are explored during contraction flow while the velocity and temperature profiles in the nozzle region are presented. Similar to capillary rheometer, the relationship between the apparent shear viscosity and shear rate for the isothermal nozzle region emerges obviously both important characteristics—first-Newtonian plateau and shear thinning slope.