Toshikazu Takigawa

Shape Selection of Twist-Nematic-Elastomer Ribbons

How microscopic chirality is reflected in macroscopic scale to form various chiral shapes, such as straight helicoids and spiral ribbons, and how the degree of macroscopic chirality can be controlled are a focus of studies on the shape formation of many biomaterials and supramolecular systems. This article investigates both experimentally and theoretically how the chiral arrangement of liquid crystal mesogens in twist-nematic-elastomer films induces the formation of helicoids and spiral ribbons because of the coupling between the liquid crystalline order and the elasticity. It is also shown that the pitch of the formed ribbons can be tuned by temperature variation. The results of this study will facilitate the understanding of physics for the shape formation of chiral materials and the designing of new structures on basis of microscopic chirality.

Poisson's ratio of polyacrylamide (PAAm) gels

Abstract Poissons ratio ( μ 0 ) of polyacrylamide (PAAm) gels was estimated. The value of μ 0 for PAAm gels was found to be 0.457, which is close to that for poly (vinyl alcohol) (PVA) gels swollen in the mixture of dimethylsulfoxide (DMSO) and water, but is higher than the value for PVA hydrogels.

Dynamic viscoelasticity and critical exponents in sol‐gel transition of an end‐linking polymer

Evolution of viscoelasticity in an end‐linking gel system has been investigated. Storage and loss dynamic shear moduli, G’(ω) and G‘(ω), are determined for angular frequencies ω over more than 2.5 decades near the sol‐gel transition. Gelation conditions are varied widely to obtain critical exponents which are independent of reaction rates and paths. A viscoelastic scaling exponent n at the gel point (G’∼G‘∼ωn) has been observed as n=0.67–0.68. Reaction time dependent shift factors for relaxation time and compliance before the gel point are determined in creating modulus‐frequency master curves of G’ and G‘. Scaling of these shift factors yields critical exponents k and z for zero shear viscosity and steady state compliance as k=1.3 and z=2.7–2.8, respectively. On the other hand, the critical exponent for equilibrium shear modulus after the gel point is found to be 2.0–2.1, which is much smaller than the exponent for steady state compliance. It is suggested that links in the backbone mainly determine the c...

Pressure‐Responsive Polymer Membranes of Slide‐Ring Gels with Movable Cross‐Links

Polymer membranes comprising slide-ring gels with movable cross-links exhibit a nonlinear pressure-dependence in the fluidic flow rate. The proportional constant between the flow rate and pressure significantly changes at a critical pressure. The slide-ring gels are promising polymer membrane materials, which would allow for the on-off control of fluid permeability using an imposed pressure.

Effects of mechanical stress on the volume phase transition of poly(N-isopropylacrylamide) based polymer gels

The effects of mechanical stress on the volume phase transition of a poly(N-isopropylacrylamide) (PNIPA) gel as well as a copolymer gel composed of N-isopropylacrylamide (NIPA) and sodium acrylate (SA) were investigated in the relatively low stress region. The PNIPA gel without elongational stress showed the behavior close to the second order phase transition. The character of the first order transition became clear under tension, and the transition temperature increased with increasing applied stress. Similar behavior was observed for the NIPA-SA copolymer gel, but the copolymer gel showed the first order transition in the whole stress range investigated. The thermodynamical linear region, where the transition temperature varies linearly with applied stress, was narrower than the mechanical linear region determined by the stress–strain relation of the gels. The change in the transition behavior by the application of the mechanical stress originated chiefly from the volume change in the gels by the applie...

Structure and mechanical properties of poly(vinyl alcohol) gels swollen by various solvents

Abstract The swelling and mechanical properties of poly(vinyl alcohol) (PVA) hydrogel, and PVA gels obtained by swelling precursors in various solvents were investigated. On the basis of the experimental results, the structure of the gels in various solvents was estimated. PVA gels have a uniform structure with flexible PVA chains in a mixed solvent of dimethyl sulphoxide (DMSO) and water. On the other hand, those swollen in methanol, ethanol and formamide have a two-phase structure, which is composed of PVA-rich and solvent-rich phases. The PVA chains in the PVA-rich phase are crosslinked by hydrogen bonding.

Swelling and deswelling kinetics of poly(N-isopropylacrylamide) gels

Swelling and deswelling kinetics was investigated for three types of cylindrical poly(N-isopropylacrylamide) (PNIPA) gels differing in crosslink density. The temperature dependence curves of the volume of the gel specimens were different from one another. One of the gel specimens was considered as a critical gel showing the continuous volume phase transition. The volume change process of the specimens after a temperature jump was examined. In the deswelling processes with temperature jumps to temperatures higher than 35 degrees C, a phase separation was observed in the gel specimens and the volume change slowed down due to the homogenization after the phase separation. The value of the diffusion constant obtained without the phase separation decreased rapidly as temperature approaches the transition temperature. The rapid decrease for the critical gel indicates the emergence of the critical slowing-down. The value of the critical exponent for the correlation length suggests that the universality class for the volume phase transition of the critical PNIPA gel belongs to the class for the classical theory.

Mechanical properties of artificial tracheas composed of a mesh cylinder and a spiral stent.

Much work has been done on the materials used for mesh-type artificial tracheas, but a precise mechanical evaluation of these structures has not yet been performed. In the present study, we determined the mechanical properties of typical mesh-type artificial tracheas and compared them with those of native trachea. Four types of artificial trachea were made and used for the mechanical tests. The basic frame of all the specimens was composed of a mesh cylinder and a spiral stent. The specimen whose mesh was sealed with collagen sponge showed almost the same behavior in the force-strain curve under compression, suggesting that collagen sealing has little effect on mechanical properties. Agreement between measured and estimated mechanical properties was good, especially in the low strain region, suggesting that artificial tracheas can be designed in terms of mechanical properties by mainly considering the basic frame structure.

Change in Young’s modulus of poly(N-isopropylacrylamide) gels by volume phase transition

Abstract Mechanical properties of poly( N -isopropylacrylamide) (PNIPA) gels were examined both in swollen and collapsed state. Stress–strain curve of the gel in the swollen state was linear and the collapsed gel also showed almost linear stress–strain behavior. The initial Young’s modulus ( E 0 ) in the collapsed state was much higher than that in the swollen state. The number of cross-links increased largely by the introduction of the physical cross-links due to collapse of the gels.

Shape and chirality transitions in off-axis twist nematic elastomer ribbons

Using both experiments and finite element simulations, we explore the shape evolution of off-axis twist nematic elastomer ribbons as a function of temperature. The elastomers are prepared by cross-linking the mesogens with planar anchoring of the director at top and bottom surfaces with a 90° left-handed twist. Shape evolution depends sensitively on the off-axis director orientation at the sample midplane. Both experiments and theoretical studies show that when the director at midplane is parallel to either the ribbons long or short axes, ribbons form either helicoids or spirals depending on aspect ratio and temperature. Simulation studies show that if the director at midplane is off-axis, ribbons never form helicoids, instead evolving to distorted spiral shapes. Experimental studies for two samples with off-axis geometry show agreement with this prediction. Samples in all these geometries show a remarkable transition from right- to left-handed chiral shapes on change of temperature. Simulations predict off-axis samples also change their macroscopic chirality at fixed temperature, depending on the angular offset. These results provide insight into the mechanisms driving shape evolution and macroscopic chirality, enabling engineering design of these materials for future applications.