【作者】 赵俊鹏；

【导师】 张广照；

【作者基本信息】 中国科学技术大学 ， 高分子化学与物理， 2010， 博士

【摘要】 两亲性或者温敏性共聚物在水溶液中的聚集和自组装行为,因其在纳米催化,生物医药,先进材料等领域的潜在应用价值而成为近年来高分子学科中的研究热点。由于共聚物的分子结构和成分比例是控制和调节其聚集行为,特别是聚集体形态和性质的重要因素,因而合成各种具有明确的分子结构,窄分散且成分可调节的共聚物可以为建立在水溶液聚集行为方面结构和性质间的关系奠定实验基础。在本文中,我们利用阴离子聚合和活性自由基聚合技术制备了一系列具有明确分子结构的两亲／温敏性嵌段／接枝共聚物。其中,利用无金属阴离子聚合技术合成两亲／温敏性接枝共聚物是本文的重点。同时,我们利用激光光散射和荧光光谱等方法对这些聚合物的一些溶液性质进行了研究。主要工作及成果归纳如下：1.首先我们合成了聚苯乙烯-b-(聚对羟基苯乙烯-g-聚环氧乙烷),PS-b-(PHOS-g-PEO)。通过常规的烷基锂引发体系我们制备了聚苯乙烯-b-聚对叔丁氧基苯乙烯(PS-b-PtBOS),通过该前体聚合物中的PtBOS在酸性条件下的水解得到了最终用作主链的两嵌段共聚物PS-b-PHOS。随后,利用一种磷腈类化合物(t-BuP4)和主链上的酚羟基(PhOH)反应所形成的无金属多基团大分子引发体系,引发环氧乙烷(EO)的阴离子开环聚合将PEO侧链接枝到主链上。我们合成了一系列大分子量窄分散的星状嵌段-接枝共聚物,并且发现即使t-BuP4和PhOH的投料比(摩尔比)低到0.2的时候,仍然可以得到可控的聚合反应。利用动／静态光散射以及荧光光谱我们对这种两亲性的嵌段-接枝共聚物的溶液性质进行了研究,包括水溶液中的临界胶束化浓度(CMC)和所形成聚集体的结构特征参数。由于较高的PEO含量和星状分子结构,这些PS-b-(PHOS-g-PEO)嵌段-接枝共聚物在水溶液中所形成的聚集体具有较低的密度和聚集度,并且其结构类似于超支化的纳米团簇。2.利用无金属多基团大分子引发体系,我们通过连续加料法合成了侧链为聚环氧丙烷-b-聚环氧乙烷(PPO-b-PEO)两嵌段共聚物的刷状接枝聚合物。为了使研究体系简单化,我们使用聚对羟基苯乙烯(PHOS)均聚物作为主链,使用t-BuP4来一同形成环氧单体阴离子开环聚合的引发体系。通过改变主链聚合物和侧链单体的投料比,我们可以控制侧链的长度和成分。通过改变两种侧链单体的投料顺序,我们得到了两种具有相反分子结构的产物,即连在主链上的侧链嵌段为PPO或者PEO。我们合成了一系列大分子量、窄分散且具有“核-壳”结构的刷状共聚物。利用动／静态光散射和荧光光谱研究了这些刷状聚合物在水溶液中的温度响应行为。在逐步升温的过程中我们观察到了两个阶段的变化,即温度诱导的分子内塌缩／缔合以及分子间的聚集。对于PPO在分子外层的刷状共聚物来说,分子间的聚集相对于另一种分子结构要更显著一些；但是它在高温下形成的聚集体中并不存在紧密的疏水微区；另外,所有这些刷状聚合物即使在低温下也处于未完全溶剂化的状态。这些现象都可归因于这种特殊的“核-壳”结构刷状分子构型。3.得用t-BuP4和PHOS所生成的多基团大分子引发体系,通过同时加料法合成了侧链为聚(环氧丙烷-ran-环氧乙烷)无规共聚物,P(PO-r-EO),的刷状接枝聚合物。通过改变两种环氧单体的投料比,我们制备了两个侧链长度相同但成分不同的样品。光散射和荧光光谱结果显示这种刷状共聚物在低温下的水溶液中处于近乎完全溶剂化的状态,并且在逐步升温过程中经历轻微的分子内收缩／缔合和非常显著的分子间聚集。在50℃以上,两个样品的溶液都变得非常浑浊并且紧接着出现宏观相分离。这表明这种刷状聚合物不能在高温下形成稳定的纳米尺度的聚集体。这些现象可以归结于两种环氧单体在侧链上的分布以及刷状的分子结构。4.通过可逆加成-裂解链转移(RAFT)自由基聚合,利用PEO大分子链转移剂(PEO-CTA),制备了双亲水性的两嵌段聚合物聚环氧乙烷-b-聚(N-异丙基丙烯酰胺),PEO-b-PNIPAM。聚合物被设计成具有高度不对称的分子结构,即一个很短的PEO嵌段加上一个很长的PNIPAM嵌段。利用动／静态光散射我们系统地研究了这种聚合物在稀水溶液中的温度诱导聚集行为,即聚合物的成分、浓度(Cp)、溶液的升温速率对于聚合物所形成聚集体的大小、聚集度和形貌的影响。在慢升温过程中,含有较长PNIPAM嵌段的共聚物在任何被测浓度下所形成的聚集体都表现出相同的形貌,即“平头”胶束。然而,对于含有相对较短PNIPAM嵌段的共聚物来说,聚集体的形貌表现出很高的浓度依赖性。我们观察到聚集体会从球形胶束拉长变为椭球形胶束甚至柱状胶束,而且在最高的浓度下会形成囊泡。快升温中形成的聚集体具有较为的结构特征,包括较小的尺寸、较低的聚集度、较高的密度和不同的形貌。对于这些现象我们在热力学和动力学上给出了一定的解释。5.利用阴离子聚合技术制备了一系列以PEO为亲水段的两亲性嵌段共聚物,并且利用动／静态光散射系统地研究了超声对于这些聚合物在水溶液中所形成的聚集体的影响。用一系列聚环氧乙烷-b-聚异戊二烯(PEO-b-PI)和聚环氧乙烷-b-聚苯乙烯(PEO-b-PS)两嵌段共聚物,以及聚环氧乙烷-b-聚异戊二烯-b-聚环氧乙烷(PEO-b-PI-b-PEO)三嵌段共聚物作为代表性两亲高分了。我们发现超声能很有效地破坏胶束间的缔合,将水溶液中最初形成的大聚集体分解成单分散的具有明确“核-壳”结构的胶束。除了超声作用的时间以外,我们也将疏水嵌段的种类和成分比例、共聚物溶液制备方法和共聚物的浓度作为辅助因素进行了研究和探讨。更多还原

【Abstract】 The aggregation and self-assembly behavior of amphiphilic/thermoresponsive polymers have attracted great academic interests there years, due to the potential applications related to nanocatalyst, biomedicine, advanced materials, etc. The molecular structure and composition have appeared to be the fundamental factors that regulate the aggregation behavior, especially the structural characteristics and physical properties of the polymeric aggregates. Therefore, it is necessary to synthesize copolymers with various well-defined molecular structures, narrow molecular weight distributions and tunable molecular compositions, so as to establish structure/properties relationships for the aggregation behavior in aqueous solutions. In our work, we have prepared series of amphiphilic/thermoresponsive block/graft copolymers with well-defined molecular structures, via anionic polymerization and living radical polymerization. Additionally, we have also investigated the physical properties, especially the aggregation behavior in aqueous solutions, of these copolymers. The main work and results are summarized as follows:1. We first present the synthesis of polystyrene-block-poly(p-hydroxystyrene-graft-ethylene oxide), PS-b-(PHOS-g-PEO), amphiphilic block-graft copolymers. The backbone diblock copolymers (PS-b-PHOS) were prepared by lithium-based anionic polymerization, followed by post polymerization acid hydrolysis of the poly(p-tert-butoxystyrene), PtBOS, precursor block. The PEO side chains were synthesized by metal-free anionic ring-opening polymerization of ethylene oxide (EO), using the phosphazene base (t-BUP4) and the phenolic hydroxyl groups (PhOH) in the backbones as the complex multifunctional initiating system. In all cases, star-like block-graft copolymers with high molecular weights and low polydispersities were synthesized. Well-controlled polymerization was achieved even with the molar ratio of t-BuP4 to PhOH being equal to 0.2. Dynamic and static light scattering and fluorescence spectroscopy studies were carried out to investigate the solution behavior of the amphiphilic block-graft copolymers, including the critical micelle concentration and structural characteristics of the aggregates formed in aqueous solutions. Because of the high PEO content and the star-like macromolecular architecture, the PS-b-(PHOS-g-PEO) block-graft copolymers form highly swelled aggregates with low aggregation numbers, having nanostructures resembling hyperbranched clusters.2. Thermoresponsive brush copolymers with poly(propylene oxide)-block-poly(ethylene oxide) side chains were synthesized via a "grafting from" technique. Near monodisperse poly(p-hydroxystyrene) was used as the backbone, and the brush copolymers were prepared by sequential metal-free anionic ring-opening polymerization of the oxyalkylene monomers, using the phosphazene base (t-BuP4) and the phenolic hydroxyl groups in the backbone to generate the complex multifunctional initiating system. The length and composition of the side chains were varied by changing the feed ratios of the backbone and the side chain monomers. By inverting the sequence of the monomer addition, two different molecular structures were achieved, with either poly(propylene oxide) or poly(ethylene oxide) linked to the backbone. In all cases, brush copolymers with high molecular weights and low molecular weight distributions were synthesized. The thermoresponsive behavior of the brush copolymers in dilute aqueous solutions was investigated by dynamic/static light scattering and fluorescence measurements. Temperature-induced intramolecular chain contraction/association and intermolecular aggregation could both be observed at different stages of the heating process. Intermolecular aggregation was more pronounced for the sample with the poly(propylene oxide) blocks located at the periphery. The results from fluorescence spectroscopy indicate the incompletely solvated state of the brush copolymer in aqueous solution at low temperature and the absence of compact hydrophobic domains in some of the aggregates, due to the core-shell brush-like molecular structure of the copolymers.3. Thermoresponsive brush copolymers with poly(propylene oxide-ran-ethylene oxide) side chains were synthesized via the "grafting from" technique discussed above. Poly(p-hydroxystyrene) was used as the backbone, and the brush copolymers were prepared by random copolymerization of mixtures of oxyalkylene monomers, utilizing metal-free anionic ring-opening polymerization, with the phosphazene base (t-BuP4) being the polymerization promoter. By controlling the monomer feed ratios in the graft copolymerization, two samples with the same side chain length and different compositions were prepared, both of which possessed high molecular weights and low molecular weight distributions. The results from light scattering and fluorescence spectroscopy indicated that the brush copolymers in their dilute aqueous solutions were near completely solvated at low temperature, and underwent slight intramolecular chain contraction/association and much more profound intermolecular aggregation at different stages of the step-by-step heating process. Above 50℃, very turbid solutions, followed by macrophase separation, were observed for both of the samples, which implied that it was difficult for the brush copolymers to form stable nanoscopic aggregates at high temperature. All these observations were attributed, at least partly, to the distribution of the oxyalkylene monomers along the side chain and the overall brush-like molecular architecture.4. Double hydrophilic poly(ethylene oxide)-b-poly(N-isopropylacrylamide) (PEO-b-PNIPAM) block copolymers were synthesized via reversible addition-fragmentation chain transfer (RAFT) polymerization, using a PEO-based chain transfer agent (PEO-CTA). The molecular structures of the copolymers were designed to be asymmetric with a short PEO block and long PNIPAM blocks. Temperature-induced aggregation behavior of the block copolymers in dilute aqueous solutions was systematically investigated by a combination of static and dynamic light scattering. The effects of copolymer composition, concentration (Cp), and heating rate on the size, aggregation number, and morphology of the aggregates formed at temperatures above the LCST were studied. In slow heating processes, the aggregates formed by the copolymer having the longest PNIPAM block, were found to have the same morphology (spherical "crew-cut" micelles) within the full range of Cp. Nevertheless, for the copolymer having the shortest PNIPAM block, the morphology of the aggregates showed a great dependence on Cp. Elongation of the aggregates from spherical to ellipsoidal or even cylindrical was observed. Moreover, vesicles were observed at the highest Cp investigated. Fast heating leads to different characteristics of the aggregates, including lower sizes and aggregation numbers, higher densities and different morphologies. Thermodynamic and kinetic mechanisms were proposed to interpret these observations, including the competition between PNIPAM intrachain collapse and interchain aggregation.5. The effect of sonication on the size and structure of polymeric aggregates formed by amphiphilic block copolymers was studied by the combination of dynamic and static light scattering. Poly(ethylene oxide)-b-polyisoprene, poly(ethylene oxide)-polystyrene diblock copolymers and poly(ethylene oxide)-b-polyisoprene-b-poly(ethylene oxide) triblock copolymer were used as typical polymeric amphiphiles. Sonication was found to be an effective method to break up intermicellar associations and split large polymeric aggregates, present initially in the aqueous solutions, into monodisperse micelles. The content and type of hydrophobic block, copolymer solution-preparation protocol, and copolymer concentration were also investigated as co-factors in conjunction to the effect of sonication time.更多还原