Probe Diffusion during Sol–Gel Transition of a Radical Polymerization System Using Isorefractive Dynamic Light Scattering

Probe diffusion in a gelation process provides unique information such as local viscosity and sol/gel fraction that general scattering and rheological measurements do not provide. In this study, we revisited a gelation process of radical copolymerization of monomers and cross-linkers by conducting a series of probe diffusion experiments with dynamic light scattering (DLS). By using an isorefractive solvent to the gel system, we exclusively monitored the dynamics of gold nanoparticles during its real-time gelation process at multiscattering angles. The obtained time-correlation functions (<i>g</i>²(τ) – 1) were analyzed by fitting with empirical stretched exponential functions. The ratio of mobile particles to the total particles, the relaxation time of mobile particles, and the heterogeneity of their dynamics were obtained as the fitting parameters. With those fitting parameters, the gel point, heterogeneity of local environment, and the local viscosity were evaluated. In addition, a unique up-and-down transition was found in the relaxation time, suggesting the local viscosity that the particles feel changes drastically around the gel point. This transition point in the relaxation time matches the gel point for homogeneous gels but showed a systematic deviation in heterogeneous gels by changing <i>q</i>^–1 and the size of probe particles

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

Probe Diffusion of Sol–Gel Transition in an Isorefractive Polymer Solution

The sol–gel transition of tetrafunctional polymers with mutual reactive end-groups was investigated by analyzing the dynamics of probe particles via dynamic light scattering. The dynamics of probe particles was exclusively observed by matching the refractive index of the solvent and the polymers. The sol–gel transition point, decreasing of sol fraction and increasing of gel fraction with the reaction, the onset of formation of closed structure inside branched polymer clusters, and a piece of evidence for the decrease of the local viscosity in postgel regime were observed via the dynamics of probe particles. In addition, a scaling relationship η_eff ∼ ε^{–1.13±0.06} was found in a wide range of cross-linking conversion (<i>p</i>) before the gel point, where η_eff is the effective viscosity estimated from probe particles’ dynamics and ε ≡ |<i>p</i> – <i>p</i>_c|/<i>p</i>_c is the relative distance from the sol–gel transition point (<i>p</i>_c is the cross-linking conversion at gel point)

Water-in-Ionic Liquid Microemulsion Formation in Solvent Mixture of Aprotic and Protic Imidazolium-Based Ionic Liquids

We report that water-in-ionic liquid microemulsions (MEs) are stably formed in an organic solvent-free system, i.e., a mixture of aprotic (aIL) and protic (pIL) imidazolium-based ionic liquids (ILs) containing the anionic surfactant dioctyl sulfosuccinate sodium salt (AOT). Structural investigations using dynamic light, small-angle X-ray, and small-angle neutron scatterings were performed for MEs formed in mixtures of aprotic 1-octyl-3-methylimidazolium ([C₈mIm⁺]) and protic 1-alkylimidazolium ([C_<i>n</i>ImH⁺], <i>n</i> = 4 or 8) IL with a common anion, bis­(tri­fluoro­methanesulfonyl)­amide ([TFSA^–]). It was found that the ME structure strongly depends on the mixing composition of the aIL/pIL in the medium. The ME size appreciably increases with increasing pIL content in both [C₈mIm⁺]­[TFSA^–]/[C₈ImH⁺]­[TFSA^–] and [C₈mIm⁺]­[TFSA^–]/[C₄ImH⁺]­[TFSA^–] mixtures. The size is larger for the <i>n</i> = 8 system than that for the <i>n</i> = 4 system. These results indicate that the shell part of MEs is composed of both AOT and pIL cation, and the ME size can be tuned by pIL content in the aIL/pIL mixtures

Solvated Structure of Cellulose in a Phosphonate-Based Ionic Liquid

We investigated the solvated structure of cellulose in a phosphonate-based ionic liquid (IL) solution utilizing scattering experiments and all-atom molecular dynamics (MD) simulations. Based on the high-energy X-ray total scattering experiment and MD simulations, a predominant interaction between cellulose and the IL was established, i.e., hydrogen bonding between the IL anion species and hydroxyl groups of cellulose. In addition, it was found that intramolecular hydrogen bonds existed within cellulose molecules, even when dissolved in the IL. Furthermore, the conformation of cellulose chains in the IL was investigated by a small-angle X-ray scattering experiment. As a result, it was found that cellulose molecules were dispersed at the molecular level and existed as rigid-rod-like polymers because of the intramolecular hydrogen bonds within the cellulose molecules. In dynamic light scattering experiments, a speckle pattern was observed for concentrated cellulose solutions. This indicated the existence of a physical-gel-like frozen inhomogeneity

Structure and Rheology of Wormlike Micelles Formed by Fluorocarbon–Hydrocarbon-Type Hybrid Gemini Surfactant in Aqueous Solution

The structure and rheological properties of wormlike micelles formed by a fluorocarbon–hydrocarbon-type hybrid gemini surfactant in an aqueous solution were investigated by means of small-angle X-ray scattering (SAXS) and viscoelastic measurements. The cross-sectional structure (the radius of the hydrophobic core and the thickness of the hydrophilic shell) and the aggregation number per unit axial length of wormlike micelles were evaluated by a model fitting analysis of SAXS profiles. Both parameters for the hybrid gemini surfactant were smaller than those of a corresponding hydrocarbon–hydrocarbon-type gemini surfactant. On the other hand, the viscosity of the hybrid gemini surfactant was higher than that of the hydrocarbon–hydrocarbon-type gemini surfactant. From the viscoelastic parameters, the steady state compliance, <i>J</i>_e, and the terminal relaxation time, τ_w, which were independently obtained by dynamic viscoelastic measurement, we revealed that a larger number of entanglements and a longer contour length of the hybrid gemini surfactant led to the higher viscosity. These results obtained by the rheological measurements were consistent with those obtained by SAXS analysis

Mesoscopic Structural Aspects of Ca²⁺-Triggered Polymer Chain Folding of a Tetraphenylethene-Appended Poly(acrylic acid) in Relation to Its Aggregation-Induced Emission Behavior

We recently reported that tetraphenylethene-appended poly­(acrylic acid) derivatives [PAA-TPE_<i>x</i> (<i>x</i> = 0.01–0.05)] provide a fluorescent Ca²⁺ sensor, where aggregation-induced emission (AIE) of the TPE pendants occurs in conjunction with Ca²⁺-triggered polymer-chain folding. On the basis of dynamic and static light-scattering data, here we discuss the hydrodynamic radius and molar mass of PAA-TPE_0.01 in the presence of Ca²⁺, Mg²⁺, or Na⁺ at various concentrations and elucidate the origin of the Ca²⁺ selectivity. In contrast to Na⁺, which exclusively triggered nonfluorescent interpolymer aggregation of PAA-TPE_0.01, Ca²⁺ and Mg²⁺ induced polymer-chain folding associated with AIE from the TPE pendants. Importantly, Ca²⁺ caused polymer-chain folding more effectively than Mg²⁺. Consequently, polymer aggregates formed in the presence of Ca²⁺ possessed a much higher inner density than those formed in the presence of Mg²⁺, leading to a more pronounced AIE behavior and, in turn, the Ca²⁺ ion selectivity over Mg²⁺

SANS Study on Critical Polymer Clusters of Tetra-Functional Polymers

A series of critical clusters was prepared by mixing two different kinds of tetra-functional poly­(ethylene glycol) (PEG) prepolymers carrying complementary end-groups. The structures of these critical clusters were investigated by small angle neutron scattering (SANS) in different dilution levels. Scaling laws for semidilute polymer solutions were observed in the solution of critical polymer clusters for <i>q</i> < ξ^–1, where <i>q</i> is the scattering vector and ξ is the correlation length. The fractal dimension of the critical clusters was estimated to be approximately 2.0, irrespective of the preparation condition of the critical clusters. For <i>q</i> > ξ^–1, the size distribution of the critical clusters influenced the scattering intensity. Assuming the validity of the scattering theory for the dilute solution of critical polymer clusters in the <i>q</i>-range <i>q</i> > ξ^–1, the Fisher exponent was estimated to be 1.90–2.25, which was found to depend on the preparation condition of the critical clusters

Microscopic Structure of the “Nonswellable” Thermoresponsive Amphiphilic Conetwork

We investigated the microscopic structure of the nonswellable hydrogel using small-angle neutron scattering (SANS). The hydrogel consisted of four-armed thermoresponsive prepolymer units embedded in a homogeneous network of four-armed poly­(ethylene glycol) (Tetra-PEG). The structure of the hydrogel was similar to that of the ordinary Tetra-PEG hydrogels at temperatures below 16.6 °C, whereas discrete spherical domains were formed at temperatures above 19.5 °C. The number of prepolymer units contained in one domain was much larger than unity, indicating that multiple thermoresponsive prepolymer units as well as Tetra-PEG units gathered to form a domain. Formation of domains much larger than a single prepolymer unit led to significant frustration of the matrix polymer network outside the domains. This frustration enhanced the elastic energy of the matrix network which would cancel the osmotic pressure and induce significant macroscopic shrinking. The selection mechanism of the domain size could qualitatively be explained by the balance between the interfacial and conformational free energies

Pressure Response of a Thermoresponsive Polymer in an Ionic Liquid

We investigated pressure effects on the lower critical solution temperature (LCST)-type phase behavior of a thermoresponsive polymer in an ionic liquid (IL) solution. The cloud point temperature of the IL solution increased monotonically with pressure, which was quite different from that of conventional aqueous polymer solutions reported in previous work, which exhibited a convex-upward-type pressure dependence. We compared the IL and aqueous systems and concluded that the difference results from their solvation mechanisms. Dynamic light scattering (DLS) measurements showed an appearance of a slow mode (corresponding to aggregation) in addition to the fast mode (corresponding to molecular dispersion) at the cloud point pressure, indicating an onset of pressure-induced phase separation. This work contributes to the fundamental understanding of the phase behavior of polymers in IL systems under high pressure