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  • 學位論文

以耗散粒子動力學法研究高分子與高分子及高分子與奈米粒子混合物之表面偏析現象

Studies of surface segregation of polymer-polymer and polymer-nanoparticle blends by dissipative particle dynamics

指導教授 : 諶玉真

摘要


高分子-高分子或高分子-奈米粒子(nanoparticle)混合物的薄膜在過去幾十年已被廣泛的應用在各種領域。然而,對於此兩類材料的均勻度的控制仍然相當難達成。在本篇論文裡,我們採用耗散粒子動力學(dissipative particle dynamics)來研究影響此類材料相行為的因素。透過了解表面偏析(surface segregation)與本體聚集(bulk aggregation)的機制,控制標的物種(高分子或奈米粒子)在薄膜裡的分佈是可以達成的,本篇論文內容可分為以下三部分: (1) 針對雙成分無熱(athermal)高分子-高分子混合物的奈米級薄膜的表面偏析現象做一探討研究。此混和物包含了下面4種類型:線性(linear)/線性、星狀(star)/線性、瓶刷型(bottlebrush)/線性、類硬桿型(rod-like)/線性高分子混合物。此系統中的表面偏析現象乃是由純熵(entropic)效應所驅動並可歸納為兩種機制。對線性/線性、星狀/線性高分子混和物而言,較小的高分子傾向偏析至邊界因為其排外體積(excluded volume)小於母體(matrix)高分子。對瓶刷型/線性、類硬桿型/線性高分子混合物而言,擁有較長持續長度(persistent length)的高分子會偏析至邊界是因為該類高分子傾向留在空乏區並與邊界牆對齊。應用上述結果對於控制高分子混合物的薄膜均勻性與表面特性有相當大的助益。 (2) 對無熱高分子-奈米粒子混合物薄膜的表面偏析與本體聚集現象做一探討研究,該薄膜是由兩面無熱牆所構成且奈米粒子的形狀為球體或正方體。上述兩種現象均由純熵效應所驅動,同時也可以解釋為空乏吸引力(depletion attraction)驅動,其強度受奈米粒子的大小改變而有深遠的影響。在一特定的奈米粒子體積分率下,小奈米粒子傾向表面偏析,然而,大奈米粒子則是本體聚集。此兩種相態的交錯(crossover)起因於粒子-牆與粒子-粒子之間的空乏吸引力的競爭。若是粒子-牆主導則會形成表面偏析,相對的,粒子-粒子主導則會形成本體聚集。正方體相對於球體有較多的接觸面積,其空乏吸引力相對比球體強,因此,表面偏析與本體聚集的交錯現象發生在較小的正方體。 (3) 在奈米球(nanosphere)或奈米正方體(nanocube)組成的無熱高分子-奈米粒子混合物薄膜中,對該系統由空乏吸引力驅動的的相行為做一探討研究。在低濃度的奈米粒子條件下可以觀察到表面偏析現象,相對的,在高濃度時下則是看到本體聚集。奈米粒子的表面偏析量(surface excess)與聚集數(aggregation number)可以藉由調整其濃度來控制。當使用表面粗糙化(surface-roughened)或高分子接枝(polymer-grafted)的奈米粒子時,因為該系統缺乏空乏吸引力故可獲得均勻分布的高分子-奈米粒子混合物薄膜。因此,添加表面粗糙化的奈米粒子於高分子-平滑(smooth)奈米粒子混合物薄膜中可以用來調整其薄膜的特性。在高分子-奈米球與高分子-奈米正方體混合物的系統中,本體聚集皆被壓制。然而,表面偏析在高分子-奈米球混合物的系統中是被壓制的,但是,在高分子-奈米正方體混合物的系統中表面偏析則是被提升的。其原因在於完全不同的奈米粒子形狀造成了不同的空乏吸引力,進而導致截然不同的相態。

並列摘要


Thin films of polymer-polymer blend or polymer-nanoparticle blend (PNB) have been utilized in many applications for last few decades. However, the uniformity control of both materials is still difficult to achieve. In this study, we adopt dissipative particle dynamics to investigate the factors affecting the phase behaviors of such materials. By understanding the mechanisms of surface segregation and bulk aggregation, it is achievable to control the distribution of target species (polymers or nanoparticles). This thesis is organized into three major parts. First, surface segregation of binary athermal polymer blends confined in a nanoscale thin film has been investigated. The polymer blend includes linear/linear, star/linear, bottlebrush/linear, and rod-like/linear polymer systems. The segregation is driven by purely entropic effects and two mechanisms are found. For linear/linear and star/linear polymer blends, the polymers of smaller size are preferentially segregated to the boundary because their excluded volumes are smaller than those of matrix polymers. For bottlebrush/linear and rod-like/linear polymer blends, the polymers with larger persistent length are preferentially segregated to the boundary because they favor to stay in the depletion zone by the alignment with the wall. These consequences are of great importance to the control of the homogeneity as well as the surface properties of polymer blend thin film. Secondly, surface segregation and bulk aggregation in a thin film of athermal PNB have been explored. The thin film is confined between two athermal walls and the shape of the nanoparticles is spherical or cubic. Both phases are driven purely by the entropic effect, i.e. depletion attraction, which depends significantly on the nanoparticle size. At a specified particle volume fraction, surface segregation dominates for small nanoparticles but bulk aggregation emerges for large ones. The crossover between the two phases is a result of the competition between particle-wall and particle-particle depletion attractions. The dominance of the former leads to surface segregation while the control of the latter results in bulk aggregation. Since nanocubes possess more contact areas and thus exhibit stronger depletion attractions than nanospheres do, the crossover from surface segregation to bulk aggregation occurs at smaller particle size for nanocubes. Last, the phase behavior of an athermal film of PNB driven by depletion attraction is studied for nanospheres and nanocubes. Surface segregation is observed at low nanoparticle concentrations while bulk aggregation is seen at high concentrations. Surface excess and the aggregation number can be controlled by tuning the nanoparticle concentration. As surface-roughened or polymer-grafted nanoparticles are used, uniform PNBs are acquired due to the lack of depletion. Thus, the addition of surface-roughened nanoparticles into polymer-smooth nanoparticle blend can be employed to tune the phase characteristics. It is found that bulk aggregation is suppressed for both polymer-nanosphere and polymer-nanocube blends. However, surface segregation is impeded for polymer-nanosphere blend but enhanced for polymer-nanocube blend owing to the distinct influence of the nanoparticle shape on depletion.

參考文獻


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