This thesis describes the self-assembly of silica nanospheres (SNSs) of ten-nanometer scale in the liquid phase into one-dimensional (1D) chain-like, ring-like, and three-dimensional (3D) hollow nanostructures using different block copolymers (BCPs) as the mediators. This research is based on the findings by Fukao et al. (J. Am. Chem. Soc. 2009) that SNSs undergo 1D assembly in the liquid phase in the presence of F127, a PEO-PPO-PEO (PEO: poly(ethylene oxide); PPO: poly(propylene oxide)) BCP. Although the effects of F127 were not clarified at that stage, their findings suggest that BCPs provide a powerful self-assembly strategy towards novel assembled nanostructures. The overview of self-assembly, the properties of BCPs and silica nanoparticles, and the background of this thesis are described in Chapter 1. Chapter 2 presents the investigations conducted for further clarifying the polymers' effects on 1D assembly of SNSs. The effects of several PEO-PPO-PEO BCPs with different PEO and PPO segment lengths are examined. In Chapter 3, the effects of lysine, an organic small molecule contained in the F127-SNS 1D assembly system, are examined. In Chapter 4, the EOVE-MOVE (EOVE: ethoxyethyl vinyl ether; MOVE: methoxyethyl vinyl ether) BCPs are employed as the mediators for the self-assembly of SNSs. Ring and chain-like nanostructures are obtained with SNSs of different sizes. In Chapter 5, the PPO-PEO-PPO BCPs are used and vesicles composed of a monolayer of SNSs are obtained under hydrothermal conditions. Different surface topographies of the vesicles can be obtained in the presence of polymer. Finally, conclusions and perspective of the thesis are given in Chapter 6.

Self-assembly of nanoparticles (NPs) is essential for the development of advanced technologies. Polymers, as a particular class of soft materials, provide a powerful self-assembly strategy for fabricating nanostructures. Polymers in colloidal systems induce a wealth of interactions, which can be manipulated to assemble the colloidal NPs. The surface chemistry of silica NPs has been well studied. Facile methods have been developed by Yokoi et al. (J. Am. Chem. Soc. 2009) and Snyder et al. (Langmuir 2007) to synthesize stable colloidal suspensions of silica nanospheres (SNSs) with controllable nano-scaled sizes. Such SNSs have great potentials as ideal building blocks for self-assembly to diverse nanostructures. Fukao et al. (J. Am. Chem. Soc. 2009) found that the SNSs assemble in the liquid phase to form chain-like nanostructures in the presence of F127. The SNSs are typical isotropic particles lacking obvious anisotropic interactions. It was supposed that synergistic effects between F127 and SNSs might lead to the 1D assembly of SNSs; however, the details were unknown. The F127-SNS 1D assembly system provides an ideal model for the study of polymer-mediated self-assembly. One of the outstanding advantages of this system is that F127 and SNSs associate with each other through surface adsorption, which means that synthetically-challenging procedures such as surface modifications are not necessary. Therefore, the self-assembly principles obtained from the current system can be transferred to other polymer-NP systems. In addition, the PEO-PPO-PEO BCPs with different PEO and PPO segment lengths are commercially available, which enables us to preform detailed investigations into the polymers' effect practically. This thesis has two objectives: to elucidate the effects of BCPs on the self-assembly of NPs and to explore novel assembled nanostructures through the BCP-mediated NP assembly approach. SNSs developed by Yokoi et al. (J. Am. Chem. Soc. 2006) are employed as the model NPs, and different classes of BCPs are used to mediate the self-assembly of SNSs.

In Chapter 2, the effects of PEO-PPO-PEO BCPs on 1D assembly were investigated to further clarify the mechanisms of 1D assembly. The F127-mediated 1D assembly of SNSs was found to be pH-sensitive and the optimal pH of 1D assembly (pH1D) in the presence of F127 is around 7.5. pH should have significant effects on SNSs which are charged, whereas it should have little effect on F127 which is nonionic. 1D assembly of NPs is generally a result of delicate balance of attractive and repulsive interactions. Here, pH1D may serve as an index of such balance of interactions. To clarify the effects of F127 on the 1D assembly of SNSs, different PEO-PPO-PEO BCPs, P123, L64, P65, and F68, were employed to mediate the assembly of SNSs. All the investigated polymers lead to 1D assembly of SNSs; however, different pH1D is observed for different polymers. It is found that pH1D shifts from ~9.4 to ~7.5 with increasing hydrophilic-lipophilic balances (HLBs) of the polymers. The HLB value is a measure of the polymers' relative hydrophilicity. Therefore, the balance of interaction is affected by the relative hydrophilicity of the polymer. Relationships between pH1D and the polymers' number of NEO and NPO units further suggest that the polymers may contribute steric and hydrophobic interactions to the 1D assembly of SNSs. Finally, a model is proposed to explain the 1D assembly of SNSs mediated by the PEO-PPO-PEO BCPs.

Lysine, a basic amino acid as the catalyst for the synthesis of the SNSs, is contained in the F127-SNS assembly system. Lysine has -NH2 and -OH groups; therefore, it can interact with both F127 (EO units) and SNSs (≡SiOH groups) via hydrogen bonding interactions. Lysine and SNSs also electrostatically interact through the NH3+ and SiO- groups. Small molecules interacting with both the BCP and NPs have been reported to affect the self-assembly in BCP-NP nanocomposites. To further clarify the mechanism of 1D assembly, the F127-mediated assembly of SNSs in the absence and presence of lysine was examined in Chapter 3. The absence of lysine is realized by synthesizing SNSs using primary amines (n-butylamine, n-propylamine) or ammonia as the base catalysts. The F127-mediated 1D assembly of SNSs proceeds even in the absence of lysine, suggesting that lysine is not essential to 1D assembly. The presence of basic amino acids in the suspension tends to shift pH1D to higher values. The basic amino acids may increase interparticle attractions through electrostatic shielding effects.

In Chapter 4, a novel class of EOVE-MOVE BCPs developed by Aoshima and co-workers (J. Polym. Sci., Part A: Polym. Chem. 2008) were employed to mediate the self-assembly of SNSs. The EOVE and MOVE segments are thermoresponsive, having the lower critical solution temperature at 20 ℃ and 60 ℃, respectively. The EOVE and MOVE segments carry ether oxygens; therefore, they should interact with the SNSs via hydrogen bonding interactions. It is found that the SNSs with a diameter of 15 nm assemble into ring nanostructures composed of 3-6 particles in the presence of the EOVE100-b-MOVE310 BCP at pH 7.8. Several BCPs with different EOVE and MOVE segment lengths also lead to the ring formation; however, an EOVE-MOVE random copolymer does not. Therefore molecular structure of the BCP is important for the ring assembly. SNSs with a diameter of 30 nm are found to assemble into 1D chain-like nanostructures in the presence of the EOVE100-b-MOVE310 BCP at pH 7.8. This gives important insights into the mechanisms of ring assembly: (1) the size of SNSs critically affects the mode of assembly, and (2) the ring assembly may be a particular case of 1D assembly.

In Chapter 5, the SNSs were found to assemble into vesicles composed of a monolayer of SNSs in the presence of 25R4, a PPO-PEO-PPO-type BCP. The assembly favorably proceeds in the presence of 1.8-2.2 wt% 25R4 under hydrothermal conditions (110-190 ℃) at pH 8.8-9.4. The mode of assembly is strongly affected by the molecular structure of the BCP. The SNSs assemble into vesicles in the presence of 17R4, another PPO-PEO-PPO BCP, whereas they assemble into ill-defined spheres or aggregates in the presence of PEO-PPO-PEO-type BCPs (L121, P123, L64, P65, F127, and TR-704). 25R4 further mediates the anisotropic organization of SNSs confined in the surface of the vesicles. This anisotropic assembly along with the Gibbs-Thomson effect reconstructs the surface topography of the vesicles.

In this thesis, several BCPs were employed to mediate the self-assembly of SNSs. Chapters 2 and 3 provide insights into the mechanism of 1D assembly of SNSs mediated by PEO-PPO-PEO BCPs. In Chapter 4, the ring assembly of isotropic SNSs in the liquid phase is achieved for the first time. In Chapter 5, vesicles composed of a monolayer of NPs, which had been achieved only through complicated or synthetically challenging processing, are obtained with a relatively simple method. In addition, an usual assembly phenomenon, i.e. the anisotropic assembly of SNSs confined in the curved surface of the vesicles, is identified. This thesis revealed that the BCP-mediated self-assembly of SNSs is an effective route for fabricating novel assembled nanostructures. Because the SNSs and the BCPs associate with each other through surface adsorption (hydrogen bonding), the self-assembly principles may be transferred to other NP-polymer systems. Nowadays, synthetic polymers with a wide range of compositions, topologies, and functionalities are available. These polymers, including commercial or tailored-made, shall lead to many interesting nanostructures assembled from NPs of metal oxides, noble metals, semiconductors, and so on.

ナノメートル領域で構造が制御された材料は従来の原理を超えた機能を発現する可能性があり注目を集めている。コロイドナノ粒子の自己集合はナノ材料を築く上で有効な手段のひとつである。電場・磁場等の外場、あるいは鋳型を利用したナノ粒子の配列制御はさかんに行われている。一方、複雑な自己集合性を示すソフトマターであるブロックコポリマーがナノ粒子配列の媒体として利用されている。ブロックコポリマーをナノ粒子表面に化学修飾して粒子を配列させる例、あるいはブロックコポリマーが液晶を形成する濃厚系においてナノ粒子と混合し、配列させる例が報告されている。しかし、ブロックコポリマーとナノ粒子の希薄混合系についてはほとんど検討されておらず、このような系が粒子の配列制御に有効であるかどうかの理解は不十分であった。

本博士論文ではブロックコポリマーによるシリカナノ粒子の自己集合現象に着目し、ポリマーと粒子を比較的希薄な条件で混合した系における自己集合現象の解明と、新規な自己集合構造の創製を行ったものである。本博士論文は「Self-Assembly of Silica Nanospheres Mediated by Block Copolymers(ブロックコポリマーによるシリカナノ粒子の自己集合)」と題し、Chapters 1―6より構成されている。先行研究によって開発されたシリカナノ粒子は、コロイド安定性を持ち、ナノメートル領域でのサイズ制御が容易であることから、モデル粒子として利用している。はじめにナノ粒子がポリマーの存在下、液相中で一次元状に自己集合する現象についてその機構を詳細に検討している。続いて、異なる構造を有するポリマーを用いた場合にリング状、あるいは中空状の粒子配列が得られることを述べている。

Chapter 1では自己集合現象、ブロックコポリマーやシリカの特性、コロイド粒子間相互作用など、本研究の背景を述べている。特に、先行研究で見出されたポリエチレンオキシド (PEO) とポリプロピレンオキシド (PPO) からなる両親媒性トリブロックコポリマーF127 (PEO―PPO―PEO) が誘起するシリカナノ粒子の一次元配列に関して詳しく述べ、ブロックコポリマーの役割を解明することの必要性、およびブロックコポリマーを利用して新しいナノ自己集合構造を得るための課題を示し、本論文の位置づけを明らかにしている。

Chapter 2では、F127によるシリカナノ粒子の一次元配列現象におけるポリマ-の役割について述べている。親水性のPEOと疎水性のPPOの鎖長が異なる種々のPEO-PPO-PEOブロックコポリマ-を使うことによって、一次元配列を与える最適なpH条件がブロックコポリマ-の親水性-疎水性比あるいは各ブロックの鎖長と関連することを明らかにしている。ゼ-タ電位測定により、ブロックコポリマ-はシリカナノ粒子に静電遮蔽効果を与えることが示されている。以上の実験結果に基づき、一次元配列現象の機構が提案されている。

Chapter 3ではシリカナノ粒子の一次元配列現象における塩基性アミノ酸の役割について述べている。先行研究で開発されたシリカナノ粒子のコロイド溶液には、合成の際に触媒として用いた塩基性アミノ酸が含まれている。本章では、有機アミンあるいはアンモニアを触媒にして合成したシリカナノ粒子を用いた系でも、F127による一次元配列が達成可能であることを示している。塩基性アミノ酸の存在は一次元配列に必須ではなく、一次元配列体を得るための最適pHに影響すると結論づけている。

Chapter 4ではエトキシエチルビニルエ-テル (EOVE) とメトキシエチルビニルエ-テル (MOVE)からなる両親媒性ジブロックコポリマ- (EOVE-MOVE) によるシリカナノ粒子の特異な配列構造について述べている。ポリマ-の濃度、pH、あるいは温度等の条件を最適化することによって、シリカナノ粒子が液相中でリング状に配列することを示している。ブロックコポリマ-とランダムコポリマ-の効果を比較し、ブロックコポリマ-の分子構造がリング状配列に重要であることが示されている。さらに、シリカナノ粒子の粒径を増大させると、配列様式がリング状から一次元状へと変化することが示されている。リング状配列の経時変化を追うことにより、本現象のスキ-ムが提案されている。

Chapter 5ではPPO-PEO-PPO型のトリブロックコポリマ-を用いたシリカナノ粒子の配列について述べている。ポリマ-濃度、pH、あるいは温度等の条件を最適化することによって、シリカナノ粒子が単層のベシクル構造へと自己集合する。ベシクルの表面における粒子の配列パタ-ンがポリマ-に媒介されることにより経時的に変化することが示されている。ポリマ-の分子構造や、各種パラメ-タの効果が詳細に検討され、ベシクル形成のスキ-ムが提案されている。シリカ粒子ベシクルの構造解析や安定性の評価などが行われている。

Chapter 6では本研究で得られた結果を総括している。

以上、本論文はブロックコポリマ-によるシリカナノ粒子の自己集合現象の解明と新規な自己集合構造の創出に関する成果をまとめたものである。本成果は基礎、応用両面で有用なものであり、工学的に高い価値を有し、化学システム工学の発展に寄与するところが大きい。

よって本論文は博士(工学)の学位請求論文として合格と認められる。