Morphological Transformation of PS-b-PEO Diblock Copolymer by Selectively Dispersed Colloidal CdS Quantum Dots
Morphological Transformation of PS-b-PEO Diblock Copolymer by Selectively Dispersed Colloidal CdS Quantum Dots
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DOI：
10.1021/ma034800g
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发表时间：
2003-09
期刊：
影响因子：
5.5
通讯作者：
Siao-Wei Yeh;K. Wei;Ya-Sen Sun;U. Jeng;K. Liang
中科院分区：
文献类型：
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作者：
Siao-Wei Yeh;K. Wei;Ya-Sen Sun;U. Jeng;K. Liang
嵌段共聚物是通用的平台材料，因为它们可以在适当的组成和条件下自组装成具有10至100 nm之间的周期厚度的各种纳米结构，这归因于不相容嵌段之间的微相分离。1 -3近年来，许多研究涉及纳米结构嵌段共聚物作为纳米模板、用于光刻的4-6纳米掩模、7和光子晶体8。具体而言，Reiter 4 b，c对通过非对称PB-bPEO结晶产生的具有长程有序的纳米尺度表面图案的工作是一个有趣的工作。在另一种情况下，Russell等人利用毛细管力将不同尺寸的CdSe纳米颗粒分选到PSb-PMMA多孔模板中。5a几个小组还报道了使用嵌段共聚物作为纳米模板来生长Co、Ag和Au纳米线5 b-d或控制纳米颗粒6 b、d如TiO 2和Pd的空间位置。此外，Register等人的具有大面积纳米级图案化的嵌段共聚物光刻也已被报道。这些研究促使我们研究嵌段共聚物和纳米粒子之间的相互作用，特别是对嵌段共聚物的形态的纳米粒子的影响。对于尺寸接近其玻尔激子半径（通常在1和10 nm之间）的半导体纳米颗粒，尺寸相关的带隙导致可调的光学性质。9，10这些半导体纳米颗粒被称为量子点（QD），因为它们的可调光学性质可以通过量子力学预测。然而，未用表面活性剂处理或未与聚合物链键合的纳米颗粒将形成大的聚集体。使用纳米结构的嵌段共聚物作为模板来选择性地控制半导体纳米颗粒在其中一个嵌段中的空间位置可能会导致几种潜在的应用。例如，相分离的嵌段共聚物中的周期性高折射率对比度域可用于光子晶体应用。具有高效发光的纳米颗粒可以与发光器件中的有机层结合。11然而，将纳米颗粒掺入嵌段共聚物中将导致比Balazs小组预测的原始状态更复杂的嵌段共聚物形态，12该小组分别使用自洽场理论和密度泛函理论来描述聚合物和纳米颗粒，来预测形貌和相图。在这篇文章中，我们报道了CdS量子点在二嵌段共聚物PS-b-PEO的PEO嵌段中的选择性分布，以及由此产生的形态变化。具体地，CdS QD诱导PEO域从六方填充的圆柱体变为体心立方或简单立方球体，如方案1所示。据我们所知，这是第一个研究有关的形态转变的嵌段共聚物的纳米粒子。对于本研究，分子量比为125 K/16.1K的不对称聚苯乙烯-b聚（环氧乙烷）二嵌段共聚物（PS-b-PEO）购自Polymersource Inc.该PS-b-PEO中PEO的体积分数为0.11，多分散性为1.04。CdS纳米颗粒是用巯基乙醇作为表面活性剂通过在甲醇中使乙酸镉（Cd（Ac）2·2 H2O）、硫化钠（Na 2S）和巯基乙醇（HSC 2 H4 OH）反应来合成的，随后修改动力学捕获方法。10过滤后，收集CdS QD，然后分散在N，N-二甲基甲酰胺（DMF）中。在我们的情况下，如方案1所示，含有化学活性羟基表面的表面活性剂改性的CdS变得亲水，并且它们的基本性质在表1中给出。在表1中，CdS QD的平均尺寸为约2.5nm，如从图1中CdS/PS-b-PEO的SAXS曲线确定的，因为在散射由形状因子主导的较高Q区域（Q > 0.07 Ω-1）中，SAXS曲线可以使用半径为2.5nm的球形形状因子来建模。根据紫外-可见光谱的起始吸收计算的CdS尺寸通常代表CdS在DMF中的尺寸分布曲线中的较大尺寸。6此外，从X射线衍射获得的CdS尺寸是晶体尺寸，其是最小的，并且对应于我们具有多晶CdS的事实。随后，在搅拌下将CdS/DMF加入到先前制备的PS-b-PEO/DMF溶液中。将该混合物在323 K下真空缓慢干燥，然后在383 K下保持24 h，之后获得CdS/PS-b-PEO纳米复合膜。纯PS-b-PEO膜的制备与CdS/PS-b-PEO膜相似，除了缺少CdS之外。已使用热重分析检查是否存在残留DMF溶剂，* 应将通信地址发送至：电话886-35731871;传真886-35-724727;电子邮件khwei@cc.nctu.edu.tw。方案1. PS-b-PEO二嵌段共聚物通过选择性分散的胶体CdS QD 7903 Macromolecules的形态转化2003，36，7903-7907
Block copolymers are versatile platform materials because they can self-assemble into various nanostructures with period thicknesses between 10 and 100 nm under the appropriate compositions and conditions, owing to microphase separation between incompatible blocks.1-3 In recent years, a number of studies involving applications of nanostructured block copolymer as nanotemplate,4-6 nanomasks for lithography,7 and photonic crystal8 have been reported. Specifically, the work on nanometer-scale surface pattern with long-range order created by crystallization of asymmetric PB-bPEO by Reiter4b,c is an interesting one. In another case, sorting out different sizes of CdSe nanoparticles into PSb-PMMA porous template with capillary force was carried out by Russell et al.5a Several groups also reported the use of block copolymer as nanotemplates to grow Co, Ag, and Au nanowires5b-d or to control the spatial locations of nanoparticles6b,d such as TiO2 and Pd. Moreover, block copolymer lithography with large area nanoscale patterning by Register et al.7a,b has also been reported. These studies motivated us to investigate the interaction between the block copolymer and nanoparticles, particularly on the effect of nanoparticles in the morphology of block copolymers. For semiconductor nanoparticles with sizes close to their Bohr exciton radius (typically between 1 and 10 nm), the size-dependent band gap results in tunable optical properties.9,10 These semiconductor nanoparticles are termed quantum dots (QDs) because their tunable optical properties can be predicted by quantum mechanics. Nanoparticles that are not treated with a surfactant or bonded to polymer chains will, however, form large aggregates. The use of nanostructured block copolymers as templates to selectively control the spatial location of semiconductor nanoparticles in one of the blocks may lead to several potential applications. For instance, periodic high refractive index contrast domains in phase-separated block copolymers can be used in photonic crystal applications. Nanoparticles with highly efficient luminescence can be combined with organic layers in light-emitting devices.11 The incorporation of nanoparticles into block copolymers, however, would lead to more complicated block copolymer morphologies than their pristine state as predicted by Balazs’group,12 which used a self-consistent-field theory and a density functional theory for describing the polymer and the nanoparticles, respectively, to predict the morphology and phase diagram. In this Communication, we report the selective distribution of CdS QDs in the PEO block of a diblock copolymer, PS-b-PEO, and the resultant morphological changes. Specifically, CdS QDs induce the PEO domains to change from hexagonally packed cylinders to body-centered-cubic or simple-cubic spheres, as shown in Scheme 1. To our knowledge, this is the first study concerning the morphological transformation of block copolymer by nanoparticles. For the present study, an asymmetric polystyrene-bpoly(ethylene oxide) diblock copolymer (PS-b-PEO) with a molecular weight ratio of 125K/16.1K was purchased from Polymersource Inc. The volume fraction of PEO in this PS-b-PEO is 0.11, with a polydispersity of 1.04. CdS nanoparticles were synthesized with mercaptoethanol as the surfactant by reacting cadmium acetate dehydrate (Cd(Ac)2‚2H2O), sodium sulfide (Na2S), and mercaptoethanol (HSC2H4OH) in methanol, following a modification of the kinetic trapping method.10 After filtration, CdS QDs were collected and then dispersed in N,N-dimethylformamide (DMF). In our case, surfactant-modified CdS containing a chemically active hydroxyl surface, as shown in Scheme 1, became hydrophilic, and their basic properties are given in Table 1. In Table 1, the averaged size of CdS QDs is about 2.5 nm as determined from SAXS curves of CdS/PS-b-PEO in Figure 1 because in the higher Q region (Q > 0.07 Å-1) where the scattering is dominated by form factor, the SAXS curves can be modeled using a sphere form factor of radius 2.5 nm. The sizes of CdS calculated from the onset absorption of the UV-vis spectrum typically represent the larger size in the size distribution curve of CdS in DMF.6 Moreover, the CdS size obtained from the X-ray diffraction is the crystal size, which is the smallest and corresponds to the fact that we have polycrystal CdS. Subsequently, CdS/DMF was added to a previously prepared PS-b-PEO/DMF solution under stirring. This mixture was dried slowly under vacuum at 323 K and then maintained at 383 K for 24 h, after which the CdS/PS-b-PEO nanocomposite film was obtained. Preparation of pure PS-b-PEO films is similar to that of CdS/PS-b-PEO, except the lack of CdS. Thermal gravity analysis had been used to check the presence of the residual DMF solvent, and there was * To whom correspondence should be addressed: Tel 886-35731871; Fax 886-35-724727; e-mail khwei@cc.nctu.edu.tw. Scheme 1. Morphological Transformation of PS-b-PEO Diblock Copolymer by Selectively Dispersed Colloidal CdS QDs 7903 Macromolecules 2003, 36, 7903-7907