以耗散粒子動力學法研究三段鏈共聚高分子在混合溶劑中之微胞型態

Abstract

本論文使用耗散粒子動力學(Dissipative Particle Dynamics)探討三段鏈共聚合高分子(ABC tri-block copolymer)在混合溶劑中的型態，其中A、B及C分別為親水、疏水及超疏水的鏈段，而混合溶劑中包含了共同溶劑(common solvent)及水(selective solvent)。本文研究的三段鏈共聚合高分子依照結構可以分為兩大類：線性(linear)和星狀(star)三段鏈共聚合高分子。我們分別改變高分子在系統中的體積比以及親水鏈段的長度，觀察在不同比例的混合溶劑中，共聚合高分子的型態變化。模擬的結果可以發現，混合溶劑中水含量增加至一個臨界值(critical water content, cwc)，三段鏈共聚合高分子會開始聚集形成微胞。而微胞的型態則隨高分子的結構不同而不同，如線性ABC三段鏈共聚合高分子會形成核-殼-暈微胞(core-shell-corona micelle)，而線性BAC三段鏈共聚合高分子及星狀高分子則會聚集成節蟲狀微胞(segmented-worm micelle)。無論是線性的或是星狀的高分子，提高高分子在溶液中的體積分率時，臨界水含量均會降低，此結果可由系統中之微胞中聚集數(aggregation number)隨水含量之變化曲線上清楚呈現。模擬結果亦顯示，改變線性和星狀三段鏈共聚高分子之親水鏈段長度並不會影響臨界水含量值。 在星狀和線性BAC三段鏈共聚合高分子系統中，都會在水含量夠高時出現結蟲(segmented worm)的型態，但節蟲微胞演化的過程中卻是不盡相同的。在線性BAC三段鏈共聚合高分子系統中，我們在改變混合溶劑的比例時，可以觀察到微胞型態由bi-core micelle、sandwich micelle、cylindrical micelle乃至於形成segmented-worm micelle，而在星狀共聚合高分子系統中，在超過臨界水含量值時，節蟲微胞便直接出現。 我們亦對系統進行中子散射的模擬，結果顯示中子散射強度隨水含量之變化曲線(intensity vs. water content)與微胞中聚集數隨水含量之變化曲線(aggregation number vs. water content)，兩者十分穩合。表示確實可以藉由中子散射強度的變化點，來代表微胞微結構改變的起始處。

In this work, we have investigated the morphologies of triblock copolymer in mixed solvents by using dissipative particle dynamics. The triblock copolymer consists of A, B and C blocks, which is hydrophilic, weakly hydrophobic and strongly hydrophobic. According to the work of Einsenberg, different micellar morphologies can be observed for diblock copolymers by adding different amount of selective solvent (water) in common solvents (dioxane and DMF). Here we focus our attention in studying how the morphologies of the aggregates evolve as water content increases. We have also estimated in our simulation the intensity of small angel neutron scattering of the system to identify change of states of micellar formation. Linear and star ABC triblock copolymers with two hydrophobic blocks and one hydrophilic block are examined in this work. It is found that critical water content (cwc), which is the water content for the first micelles to form, decreases as the volume fraction of polymer increases irrespective of the architecture of the triblock copolymer. Triblock copolymers with different architectures result in different micellar morphologies. For example, ABC triblock copolymers form typical core-shell-corona micelles and on the other hand, linear BAC as well as ABC star copolymers develop segmented-worm aggregates in purely selective solvents. For the same triblock copolymer, transformation between different morphologies can be observed as water content in the mixed solvent increases. Therefore we can tune the morphologies of copolymers by changing the solvent quality. It is also found that critical water content of the system is not affected by the length of the hydrophilic blocks. We have performed the calculation of the intensity of SANS in our simulation. Results show that sudden changes in the SANS consistently indicates transformations of the morphologies of micelles formed.