Transitions of Aggregation States for Concentrated Carbon Nanotube Dispersion

Because of the lack of appropriate techniques for the measurement of concentrated dispersions, dispersion states of carbon nanotube (CNT) dispersions have been evaluated for dilute dispersions by assuming the dispersion state being unchanged by dilution. In this paper, it is clarified that this assumption does not hold true at a high concentration region by a direct measurement of size distribution and anisotropy for CNT dispersions in a wide concentration region. CNT dispersions showed a dispersion-state transition as a form of rotation restriction at a certain concentration. In addition to this, CNT dispersions whose solutes have a large specific surface area showed another dispersion-state transition at a certain concentration as a form of aggregation growth. To prove these dispersion-state transitions from another point of view, the difference in sheet resistance of conducting layers made from different CNT dispersions coated on a glass substrate was investigated. It was confirmed that their sheet resistance also showed a clear difference. This difference can be explained from the viewpoint of dispersion-state transitions induced by the drying process

Fabrication and Structural Characterization of Module-Assembled Amphiphilic Conetwork Gels

Structural analysis of inhomogeneity-free poly­(ethylene glycol)–poly­(dimethyl­siloxane) (PEG–PDMS) amphiphilic conetwork gels has been performed by the complementary use of small-angle X-ray and neutron scattering. Because of the hydrophobicity of PDMS units, the PEG–PDMS gels exhibit a microphase-separated structure in water. Depending on the volume fraction of PDMS, the microphase-separated structure varies from core–shell to lamellar. The obtained X-ray and neutron scattering profiles are reproduced well using a core–shell model together with a Percus–Yevick structure factor when the volume fraction of PDMS is small. The domain size is much larger than the size of individual PEG and PDMS unit, and this is explained using the theory of block copolymers. Reflecting the homogeneous dispersion conditions in the as-prepared state, scattering peaks are observed even at a very low PDMS volume fraction (0.2%). When the volume fraction of PDMS is large, the microphase-separated structure is lamellar and is demonstrated to be kinetically controlled by nonequilibrium and topological effects

SANS and DLS Study of Tacticity Effects on Hydrophobicity and Phase Separation of Poly(<i>N</i>‑isopropylacrylamide)

The tacticity effect on phase separation process of poly­(<i>N</i>-isopropylacrylamide) (PNiPAM) aqueous solutions was investigated by dynamic light scattering (DLS) and small angle neutron scattering (SANS) measurements. SANS measurement revealed that hydrophobicity of PNiPAM consisting of meso- and racemo-isomers increased with increasing the meso-content. This result is in accordance with the result of the previous experimental and simulation study on NiPAM dimers (DNiPAM) and trimers (TNiPAM) [Katsumoto, Y.; J. Phys. Chem. B 2010, 114, 13312−13318, and Autieri, E.; J. Phys. Chem. B 2011, 115, 5827–5839]; i.e., meso-diad is more hydrophobic than racemo-diad. In addition, a series of scattering experiments revealed that the ratio of meso-diad does not affect the static structure or the shrinking behavior of a single chain, but strongly affects the aggregation behavior. The PNiPAMs with low meso-content suddenly associate around the phase separation temperature, while that of the high meso-content gradually aggregate with increasing temperature. We propose that phase transition behavior of PNiPAM aqueous solutions can be controlled by changing the stereoregularity of the polymer chain

Structural Analysis of Lipophilic Polyelectrolyte Solutions and Gels in Low-Polar Solvents

Lipophilic polyelectrolyte gels capable of large swelling in low-polar solvents (3 ≤ ε ≤ 10) were developed by Ono et al. (Nature Mater. 2007), where ε is the dielectric constant. These gels were prepared by introducing tetraphenylborate as a lipophilic anion (tetrakis­(3,5-bis­(trifluoromethyl)­phenyl)­borate; TFPB^–) and tetraalkylammonium with long alkyl chains as a lipophilic cation (tetra­(<i>n</i>-butyl)­ammonium; TBA⁺) into a poly­(octadecyl acrylate) (pODA) backbone chain. Here, we investigated the structure of the lipophilic polyelectrolyte gels and corresponding polymer solutions in CH₂Cl₂ with small-angle neutron scattering (SANS) and dynamic light scattering (DLS). From SANS, it was revealed that individual pODA chain is regarded as a rod with the cross-section radius of 15 Å and the length of ca. 160 Å and is little changed by introduction of charges or cross-linking. In addition to this, it was revealed from SANS measurements that the second virial coefficient of pODA in CH₂Cl₂ was positive. In combination with DLS measurements, we observed several characteristic features similar to polyelectrolyte aqueous systems such as (i) the clear appearance of slow diffusional motion in polymer solutions, (ii) an increase of diffusion coefficient in gels, and (iii) an increase of osmotic modulus in solutions and gels when ionic groups are incorporated in pODA. These experimental findings clearly show that [TBA⁺]­[TFPB^–] dissociates enough and pODA, accompanying these ionic groups, acts as a polyelectrolyte even in a low-polar solvent such as CH₂Cl₂ (ε = 8.9). It is concluded that the good compatibility of pODA with CH₂Cl₂ and the introduction of dissociable ionic groups into pODA result in high-swelling capability of the lipophilic polyelectrolyte gels