Ken-ichi Niihara

Self-Assembled ABC Triblock Copolymer Double and Triple Helices†

In a block-selective solvent, the insoluble block or blocks of a block copolymer agglomerate to form nanometer-sized micelle-like aggregates, which are stabilized against further agglomeration by the soluble block(s). Depending on the composition of the copolymer, the interfacial tension between the solvent and the insoluble block(s), and other factors, the shape of the aggregates formed can be spherical, vesicular, tubular, cylindrical, etc. The shape diversity of such aggregates facilitates their applications in nanofabrication, lithography, cell culturing, and drug delivery. Most of the previous solution self-assembly studies of block copolymers were performed for AB diblock copolymers. The natural choices for solvents have been blockselective solvents, which solubilize one block of an AB diblock copolymer, but not the other. With ABC triblock copolymers, the solvent choice becomes much more interesting. Traditionally, solvents selective for one terminal block (A or C) or for two consecutive blocks (A and B or B and C) were used. 17] With rare exceptions, 14, 18–22] the use of such solvents led to core–shell–corona spherical or cylindrical aggregates. More recently, solvents selective for the A and C terminal blocks have been used, and the use of such solvents has led to aggregates with interesting coronal-chain segregation patterns. 24] We report in this paper the self-assembly of an ABC triblock copolymer in solvents that are good for C, poor for B, and marginal for A. We report also our surprising discovery that, after a long period of sample ageing, the selfassembled aggregates were double and sometimes triple helices. Even more surprising, such structures were formed in three different solvent systems that we have tested so far. The triblock copolymer used was poly(n-butyl methacrylate)-block-poly(2-cinnamoyloxyethyl methacrylate)-blockpoly(tert-butyl acrylate) or PBMA350-b-PCEMA160-bPtBA160 consisting of 350 BMA units, 160 CEMA units, and 160 tBA units. The precursor to this polymer was prepared by anionic polymerization and had a low polydispersity index of 1.06 (see the Supporting Information).

Elucidation of the Reinforcing Mechanism in Carbon Nanotube/Rubber Nanocomposites

High-performance sealants using rubber composites containing multiwalled carbon nanotubes (MWNTs) were developed in order to probe and excavate oil in deeper wells. However, the stress-strain behavior and the reinforcing mechanism of highly concentrated MWNT/rubber composites subjected to large deformation remain largely unexplored. Here we report on the complete stress-strain relationships of MWNT/rubber composites under uniaxial tension before rupture, with a suggestion of a novel reinforcement effect of high concentration of MWNTs. A theoretical model is developed to understand the reinforcing mechanism and estimate the mechanical properties of MWNT/rubber composites under large deformation. We have demonstrated that persistence length and reorientation of MWNTs during stretch have a significant impact on mechanical properties, such as the modulus of the rubber composite. These results provide guidelines for developing MWNT-reinforced composites to achieve desired nonlinear and extreme mechanical performance for a wide range of applications.

Three-dimensional structure of a polymer/clay nanocomposite characterized by transmission electron microtomography

Three-dimensional (3D) morphology of a polymer/clay nanocomposite, an organophilic montmorillonite (MMT) dispersed in poly(ethylene-co-vinylacetate) (EVA), was examined by transmission electron microtomography (TEMT). Using this technique, individual clay layers dispersed in the EVA matrix were clearly visualized. A volume fraction of the clay layers evaluated from the 3D reconstructed image agreed well with that calculated from the weight of the MMT component in the MMT/EVA system. The individual clay layers were digitally extracted by a newly developed 3D particle algorithm. A size distribution of the clay layers was directly obtained from the 3D reconstruction. Anisotropy of each clay layer was characterized by the determination of three semi-axes of an approximating ellipsoid with the same volume. One of the representative semi-axis of the ellipsoid was used to estimate average orientation of the MMT layers in the ultra-thin section used in the TEMT experiment. Thus, the combination of quantitative TEMT and 3D structural analysis is shown to be a powerful tool to investigate a relationship between the MMT distribution and a variety of physical properties of the nanocomposites.

Direct Three-Dimensional Observations of Order-Order Transition from Gyroid to Cylindrical Structures in a Block Copolymer

An order-order transition (OOT) from the bicontinuous double-gyroid (G) structure to the hexagonally-packed cylindrical (C) structure in a poly(styrene-block-isoprene) (SI) block copolymer was investigated by transmission electron microtomography (TEMT). The coexistence of the G and C microdomains during the OOT was observed. The boundary structures between the G and C microdomains were directly three-dimensionally visualized. It was found that the {220} plane of the G structure changed to the {110} plane of the newlygenerated C structure, consistent with Matsen’s prediction from his computer simulations based on self-consistent field (SCF) theory.

Thermostable Natural Rubber with Cellular Structure Using Thin Multiwalled Carbon Nanotubes

Rubber is an essential material in areas such as the transportation, civil engineering and construction, electronics, and medical equipment industry because of its unique properties, particularly its elasticity. Specifically, rubber with an extremely low elastic modulus (in the 1–100 MPa range) has an excellent ability to revert to its original shape after being stretched or compressed. Such elasticity enables rubber to be used in tires, hoses, belts, sealants, and packing materials. At present, the total amount of rubber consumed in the world is about 2000 tonnes per year. However, the market share of natural rubber (NR) is limited to 40 %. With progress in green technology, the consumption of NR is expected to increase rapidly, because NR is one of the most green yet versatile engineering materials. For example, its manufacturing energy is one-tenth that of synthetic rubber because NR is a naturally produced, highmolecular-weight hydrocarbon. In addition, the rubber tree provides a wide variety of medical supplies, fertilizer powder, and timber; a rubber tree also has the ability to absorb 9000 tonnes of CO2 per year. [2]