空乏力引發之奈米粒子自組裝

Depletion-Induced Self-Assembly of Nanoparticles

胡斯瑋(Ssu-Wei Hu)

指導教授 ： 諶玉真

國立臺灣大學/工學院/化學工程學研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.02629

摘要

高分子奈米複合材料是以高分子為基材、奈米粒子為添加物製備而成的新穎材料，其具有諸多獨特的高功能性質。無論是在高分子溶液或其熔融態中，高分子複合材料提供了獨特的機械，電氣，光學和熱學性質。當高分子與奈米粒子間無特殊作用力時，奈米粒子會因空乏作用力(depletion interaction)而聚集。在本篇論文中，我們利用耗散粒子動力學（DPD）模擬法，針對由空乏作用力所引起之非等向性奈米粒子自組裝行為進行研究。內容可分為以下三部分:
(1)在第1部分（第3章）中，我們探討於高分子溶液中，由空乏引力所引起的親溶劑片狀奈米粒子的自組裝行為。藉由計算兩奈米粒子間作用力，我們證實既使當高分子溶液濃度很高時，空乏作用力仍正比於片狀奈米粒子的總表面積以及系統滲透壓。當加入不同濃度的非吸附性高分子於奈米粒子懸浮液中時，我們觀察到三種不同的平衡態:分散態(dispersion)、過渡態(pre-transition)、相分離態(phase separation)。於過渡狀態中，奈米粒子自組裝成有限大小的一維線狀結構，此聚集體的平均大小正比於奈米粒子濃度的平方根及高分子濃度。此外，類似於界面活性劑形成微胞的過程，當奈米粒子濃度逐漸提高，我們亦可得到片狀奈米粒子的臨界聚集濃度。藉由改變片狀奈米粒子及高分子濃度，我們可得到片狀奈米粒子的型態相圖。
(2)在第二部分(第四章)中，我們探討於高分子熔融態中，親油性奈米粒子的自組裝以及疏油性奈米粒子的分散行為。我們發現空乏作用力對於親油性奈米粒子的自組裝行為有顯著的影響，為了增加高分子的熵，親油性奈米粒子會傾向於聚集。此外，由於空乏作用力正比於粒子表面積，大片的片狀奈米粒子趨向於聚集而小片奈米粒子傾向分散。另一方面，由於奈米粒子於高分子熔融態中擴散較慢，阻礙了疏溶劑奈米粒子的聚集。進而造成疏油性奈米粒子於短時間內仍分散於高分子基質中。
(3)在第三部分(第五章)中，我們探討利用空乏力分離不同長度親溶劑柱狀奈米粒子的機制。針對單一種柱狀奈米粒子懸浮於特定高分子濃度中，藉由改變柱狀奈米粒子濃度及其長度或者系統溫度，我們可得到兩張形態相圖，包含柱狀奈米粒子濃度-長度圖及柱狀奈米粒子濃度-溫度圖。對於兩種不同長度的柱狀奈米粒子混和物，當其處於過度態中，我們觀察到兩種不同的聚集體包含單一成分聚集體(homogeneous cluster)、混和聚集體(mixed cluster)。在分離過程中，由於混和聚集體的存在會降低分離效果。因此，根據柱狀奈米粒子濃度-長度圖，我們提出了兩種形成單一成分聚集體的條件，並依照此兩種條件，我們認為利用稀釋及升溫的方法可有效增進奈米粒子分離效果。此方法適用於任何利用空乏力分離奈米粒子的系統中。

關鍵字

空乏作用力 ； 耗散粒子動力學 ； 奈米粒子自組裝

並列摘要

Polymer nanocomposites made by dispersing nanoparticles in a polymer matrix are versatile materials. Whether in a solution or in bulk, these materials have unique mechanical, electrical, optical, and thermal properties. In many cases, the entropic depletion attractions generated by the addition of non-adsorbing polymers present themselves and may significantly influence the self-assembly behavior of nanoparticles, especially when the particles are anisotropic. This thesis adopts dissipative particle dynamic (DPD) simulations to explore three interesting issues associated with the depletion-induced self-assembly behavior of anisotropic nanoparticles. The first part (Chapter 3) explores depletion attraction between two solvophilic nanodiscs and the aggregation behavior of nanodisc suspension. The depletion force caused by the addition of the polymer is proportional to the product of the osmotic pressure and the area of the nanodisc, even for concentrated polymer solutions. For a suspension of nanodiscs, three possible equilibrium states are observed upon polymer addition: dispersion, pre-transition, and phase separation. In the pre-transition regime, the critical aggregation concentration appears and finite sized clusters are mainly formed with a columnar structure, similar to the micellization associated with typical surfactants. Furthermore, the mean size of the columnar clusters seems to grow with (φD)½ and φP. At the end of this procedure, the phase diagram is obtained by varying nanodisc concentration (φD) and polymer concentration (φP ). The second issue (Chapter 4) investigates the self-assembly of organophilic nanoparticles and the dispersion of organophobic nanoparticles. Depletion attraction is found to play a major role in the aggregation of organophilic nanoparticles. Since depletion attraction is proportional to the area of the nanodisc, large nanodiscs are disposed to clustering, while small nanodiscs tend to disperse. On the other hand, slow aggregation kinetics, hindered by the energy barrier associated with depletion interactions and low nanoparticle diffusivity, is responsible for the low degree of aggregation for organophobic nanoparticles. The third topic (Chapter 5), studies the depletion-induced separation of solvophilic nanorods in a polymer solution. By varying the concentration (φR) and length (L) of the nanorods or by changing system temperature (T), two morphological phase diagrams including φR-L and φR-T are obtained for a given polymer concentration. For a binary nanorod mixture in the pre-transition state, the cluster can be homogeneous or mixed, with the latter hindering the size fractionation. The φR-L phase diagram, gives two criteria associated with the formation of homogenous clusters. Based on the criteria, two methods, for the improvement of fractionation resolution, dilution and temperature increment, are proposed. This methodology can be generally applied to fractionate nanoparticle dispersions of any kind.

並列關鍵字

depletion force ； dissipative particle dynamic ； self-assembly of nanoparticle

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