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標題: | 高分子鏈長及鍵強度對熱誘導式相分離動力學之影響-耗散粒子動力學模擬 Effects of Chain Length and Bond Strength on Phase Separation Dynamics-a Dissipative Particle Dynamics Simulation Analysis |
作者: | Yung-Hsu Wu 吳永旭 |
指導教授: | 王大銘(Da-Ming Wang) |
關鍵字: | 耗散粒子動力學,高分子薄膜成膜機制,相分離動態機制,合併行為,高分子鏈糾結, Dissipative particle dynamics,polymer membrane formation,phase separation dynamics,coarsening,polymer entanglements, |
出版年 : | 2008 |
學位: | 博士 |
摘要: | 本研究利用適用於微米尺度的模擬方法-耗散粒子動力學,探討增加高分子溶液中的高分子鏈長以及高分子鏈段間的彈性係數,如何影響高分子溶液經過熱誘導式相分離後的高分子濃度振盪、高分子結構變化、以及高分子富相尺寸成長等動態性質,以提供有利於控制高分子薄膜成膜結構的資訊。
模擬結果顯示,在稀薄高分子溶液的spinodal decomposition相分離中,高分子富相結構由初期的雙連續結構,因進行界面能量釋放而轉變為球狀結構的過程相對快速。而在亞濃高分子溶液的spinodal decomposition中,雙連續結構則能維持較長時間,當高分子鏈段長度增加時,形成的雙連續結構較為緻密;而透過提高分子鏈段間的彈簧係數以增加高分子鏈鍵強度後,使高分子運動進行局部的合併,進而產生局部高分子碎片的結構。在相分離的造成的濃度振盪方面,提高高分子鏈鍵強度會抑制相分離進行,而提高高分子鏈長及高分子鏈鍵強度,皆會減緩在相分離階段高分子富相及高分子貧相兩相間的界面張力釋放。在高分子的合併過程中,高分子富相的尺寸與相分離時間的關係遵守冪次法則,本研究透過分析其成長指數探討鏈長及鍵強度對合併機制的影響,當高分子鏈較短且鍵強度較低時,高分子富相的合併初期以成長指數1/3進行成長,表示合併是透過擴散進行,而後期成長指數轉換為2/3,說明後期的合併行為由界面張力引起的流動。高分子鏈長的增加使前期擴散主導階段的高分子富相尺寸降低,成長指數降為1/4,說明鏈長的提高將限制高分子以爬行方式進行運動進而減慢高分子的合併,而高分子鏈間鍵強度的增加則使擴散主導階段的成長指數降低至1/5,代表鏈段的鍵強度增加將牽制相分離時的高分子運動;此外,鏈長及鍵強度的改變不影響合併行為後期的流動階段成長機制,代表在流動主導的合併過程中,是由界面張力的釋放主導,而無法顯現鏈長或鍵強度對合併行為的影響。當鏈長及鍵強度同時提升時,相分離受到高分子在亞濃溶液中形成的網狀結構所抑制,利用Frenkel-Flory-Rehner理論,說明高分子網狀結構在溶液中形成的鏈糾結使需要發生相分離的高分子與溶劑間的作用力參數提高。在合併過程中,當鏈長及鍵強度同時提高時,會減緩由擴散機制主導轉為流動主導的轉換時間。 本研究並探討提高高分子與溶液間的作用力參數對於鏈糾結效應的影響,隨著作用力參數的提高,高分子鏈糾結對相分離動態的影響相對減輕,相分離受到熱力學驅動力主導,使得鏈糾結效應對相分離動態的影響較低。 Dissipative particle dynamics (DPD) was employed to perform three- dimensional simulations to investigate the phase separation dynamics of polymer solutions with different polymer chain length and bond strength. How the chain length and bond strength influenced the concentration fluctuation, morphology evolution and coarsening mechanism of polymer-rich regions during the phase separation were presented. The results show that phase separation dynamics was suppressed by the chain length effects and the hindrance effect resulted from enhanced spring between polymer segments. On morphology evolution, the chain length effect sustained the bicontinuous structure, and the bond strength effect due to the increase of spring constant resulted in local aggregation of polymer-rich regions.. The growth of polymer-rich domains was analyzed to study the effects of chain length and bond strength on coarsening mechanism. For the polymer solution composed of shorter and more flexible chains, a two–stage coarsening was observed with the crossover of the domain growth exponent from 1/3 to 2/3 during the course of phase separation. The crossover reflected that the growth mechanism altered from diffusion to interfacial-tension driven flow. When the chain flexibility was kept the same but the chain length increased, the growth exponent was reduced to 1/4 in the diffusion-dominating coarsening regime, while effects on the growth exponent in the flow–dominating regime were absent. Besides, the concentration fluctuation as well as the dissipation of interfacial energy during phase separation was suppressed. When the chain length was kept short but the bond strength was enhanced by increasing the spring constant between the polymer segments, the growth exponent approached 1/5 in the diffusion-dominating regime. Nevertheless, the entanglement effects in the flow-dominating coarsening regime were not remarkable. The chain length effect slowed down the domain growth, which could be explained by that polymer chains could only perform reptation when chain entanglements occurred. Moreover, when both the effects of chain length and bond strength were enhanced, polymer networks composed of longer chains with stronger bond strength imposed an energy barrier for phase separation to occur, which corresponded to the Frenkel-Flory-Rehner hypothesis. As a result, the polymer solution with longer chains with stronger bond strength can only undergo phase separation when a larger quench depth was employed to initiate the phase separation, reflecting that the entanglement effects derived from the increase of chain length and bond strength were alleviated with the deeper quench. |
URI: | http://tdr.lib.ntu.edu.tw/jspui/handle/123456789/40486 |
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