Hybridization of a Bimodal Distribution of Copolymer Micelles
Hybridization of a Bimodal Distribution of Copolymer Micelles
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共聚物胶束双峰分布的杂化
DOI：
10.1021/acs.macromol.0c01419
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发表时间：
2020
期刊：
影响因子：
5.5
通讯作者：
Lodge, Timothy P.
中科院分区：
文献类型：
--
作者：
Zhao, Dan;Wang, En;Lodge, Timothy P.
用时间分辨动态光散射（DLS）和小角X射线中子散射（SAXS/SANS）研究了两种嵌段共聚物胶束在离子液体混合物1-乙基和1-丁基-3-甲基咪唑双（三氟甲基磺酰基）酰亚胺（[EMIM]和[BMIM][TFSI]，简称EMIM和BMIM）中的杂化作用.采用两种聚（甲基丙烯酸甲酯）-嵌段-聚（甲基丙烯酸正丁酯）（PMMA-b-PnBMA）共聚物，其中PnBMA和PMMA分别是核形成嵌段和电晕形成嵌段。这两个二嵌段具有相同的电晕嵌段分子量（25 000 g/mol），但核心嵌段长度相差2.2倍（24 000和53 000 g/mol）。两种聚合物在含有0-30重量% BMIM的混合离子液体溶剂中组装成球形胶束，尽管具有不同的尺寸。溶剂选择性随着BMIM含量的增加而降低。时间分辨SANS定量了每种共聚物的单聚物交换时间作为溶剂组成的函数。在最具选择性的溶剂（100%EMIM）中，较长链的交换速度比较短链慢104倍;这种差异减小到约100倍。30% BMIM中的50。然后将给定溶剂中的两种胶束溶液以相等的比例混合，并在几个月内监测共混胶束的结构演变。从以前的工作，我们知道，在平衡的两种共聚物应该形成一个均匀的人口的混合（“杂化”）胶束，与大小之间的前体胶束（虽然更接近较大的）。在更有选择性的溶剂中，光散射和中子散射都表明胶束的表观重均分子量（Mw）最初随时间增加，而SAXS显示平均胶束核心尺寸增加。这些观察结果反映了较短链从较小胶束到较大胶束的净转移，因为较短链的交换更容易，但在某种意义上，使系统进一步远离平衡。动态光散射监测的长时间演化表明，胶束杂化过程对溶剂选择性有很大的依赖性。在100%EMIM中，MW甚至在几个月后继续增加，表明在实验时间尺度内没有达到平衡。对于选择性较低的溶剂，胶束尺寸最终开始减小并接近平衡尺寸，表明两种分子量的单聚物交换是有效的。在选择性最低的30%BMIM溶剂中，胶束的平均流体动力学尺寸和胶束的平均流体动力学尺寸立即开始减小，并最终接近平衡胶束的值。然而，该过程必须涉及其他弛豫机制，例如，胶束融合/碎裂或胶束产生/湮灭，因为胶束的总数也需要调节。总的来说，这项工作暴露了不同的路线的可能性，以平衡一个给定的系统，从而强调了在嵌段共聚物胶束平衡的复杂性。
The hybridization of two diblock copolymer micelles in mixtures of ionic liquids, 1-ethyl and 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMIM] and [BMIM][TFSI], or EMIM and BMIM in short), was studied by time-resolved dynamic light scattering (DLS) and small-angle X-ray and neutron scattering (SAXS/SANS). Two poly(methyl methacrylate)-block-poly(n-butyl methacrylate) (PMMA-b-PnBMA) copolymers, where PnBMA and PMMA are the core- and corona-forming blocks, respectively, were employed. The two diblocks have the same corona block molecular weight (25 000 g/mol) but core block lengths differing by a factor of 2.2 (24 000 and 53 000 g/mol). Both polymers assemble into spherical micelles in mixed ionic liquid solvents containing 0–30 wt % BMIM, albeit with different sizes. The solvent selectivity decreases with increasing BMIM content. Time-resolved SANS quantified the unimer exchange time for each copolymer as a function of solvent composition. In the most selective solvent (100% EMIM), the longer chains exchanged ∼104times more slowly than the shorter ones; this difference was reduced to a factor of ca. 50 in 30% BMIM. The two micelle solutions in a given solvent were then mixed in equal proportions, and the structural evolution of the blended micelles was monitored over several months. From previous work, we know that at equilibrium the two copolymers should form a uniform population of mixed (“hybridized”) micelles, with size intermediate between the precursor micelles (albeit closer to the larger ones). In the more selective solvents, both light and neutron scattering show that the apparent weight-average molecular weight (Mw) of the micelles initially increases with time, while SAXS shows an increase in the average micelle core size. These observations reflect a net transfer of shorter chains from smaller to larger micelles, as the shorter-chain exchange is much more facile but, in a certain sense, takes the system further away from equilibrium. The long-time evolution monitored by DLS shows that the process of micellar hybridization depends greatly on solvent selectivity. In 100% EMIM,Mwcontinues increasing even after several months, indicating that equilibrium is not reached within the experimental time scale. For less selective solvents, the micelle size eventually begins to decrease and approaches the equilibrium size, indicating that a unimer exchange of both molecular weights is operative. In the least selective solvent, 30% BMIM, bothMwand the average hydrodynamic size of the micelles start to decrease immediately and ultimately approach the values of the equilibrium micelles. However, this process must involve other relaxation mechanisms, e.g., micelle fusion/fragmentation or micelle creation/annihilation, as the total number of micelles also needs to be adjusted. Overall, this work exposes the possibility of different routes to equilibrium for a given system and thereby underscores the complexity of equilibration in block copolymer micelles.
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DOI：
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2004
期刊：
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