Viscoelastic magnetic fluid (VMF) is a kind of a smart and functional fluid in which magnetic nano particles are uniformly suspend. The rheological behavior of VMF is investigated, in the present study, in view of effects from magnetic property in the fluid. The dynamic rheological properties of VMF in externally applied uniform magnetic fields are measured by a laboratory-made cone-plate rheometer. In particular, the effects of the magnetic field on the viscoelastic properties (the complex dynamic modulus; storage modulus G′ and loss modulus G″ ) of VMF are studied. In order to verify the gains from experimental investigation, a theoretical analysis is also presented in the present study. Experimental results indicates that the viscosity and elasticity of VMF are significantly influenced by both applied magnetic field and concentrations of the magnetic particles. Theoretical analysis qualitatively explains the findings from the experiments, indicating that VMF has dominant characteristic related to the concentration of magnetic particles associated with their network formation in the applied magnetic field.
A Host-Guest gel is a supramolecular network formed by the dynamic crosslinking between β-cyclodextrin and adamantane molecules, and has a novel tough and self-healing properties. In our previous study, we showed that this gel has a topological permanent crosslinking in addition to the dynamic crosslinking, and these two types of crosslinking brings a unique swelling property such as attaining an equilibrium swollen state. In the present study, we examined the swelling behavior and equilibrium swollen states of the Host-Guest gel. The analysis of the swelling process revealed the dynamic crosslinking hardly affect the diffusion on polymer chain under the swelling process. On the other hand, analysis of the equilibrium swollen state with and without a competitor of Guest molecule showed that the dynamic crosslinkings can trap the entanglements of polymer chains even in the equilibrium state because the dynamic crosslinkings are anchored by the permanent crosslinkings and consequently stored in the equilibrium state. These results indicate that the swelling properties of the Host-Guest gels are dominated by both dynamic and permanent crosslinkings in the network.
We investigated changes to the linear viscoelastic properties of a mixture comprising polycarbonate (PC) containing 3 wt.% of a multiwalled carbon nanotube (MWCNT) and high-density polyethylene (HDPE), also containing 3 wt.% MWCNT, during post-processing annealing. The oscillatory shear moduli — i.e., the storage modulus G′ and the loss modulus G′′ — gradually increased with residence time in a rheometer (i.e., annealing). The samples were prepared by compression-molding at low temperature, during which the MWCNTs became oriented by the applied squeeze flow. The marked increase in the oscillatory moduli may be attributed to the formation of a conductive MWCNT network owing to Brownian motion. Furthermore, the moduli increased more rapidly during high-temperature annealing. These increases can be expressed by a simple equation using only one characteristic time, i.e., the time required for MWCNT redistribution by Brownian motion. This characteristic time is considered one of the factors that control the structure of composites containing MWCNTs.
We have investigated the relation between the polymer and low-mass molecule (LM) dynamics in antiplasticized polycarbonate (PC). Antiplasticization results in enhancement of the Young’s modulus of polymeric materials by adding LMs. Comparison of the dynamic mechanical properties and dielectric relaxation behavior suggests that the LMs are immobile below room temperature (~ 25 ºC) and mobile above room temperature. The LM dynamics shows a similar trend to the local chain motion of PC, indicating the dynamical relation between the two components. Additionally, we investigated the effect of the LM size on antiplasticization and the local chain motion of PC by using alkyl cyano-biphenyl with various carbon numbers and 4-pentyl-4′-cyanoterphenyl. The LM size does not affect the degree of modulus enhancement of PC at the room temperature. However, mechanical relaxation of the local chain motion of PC shows wider temperature dependence for shorter LM size. The activation energy of molecular motion decreases with decreasing LM size, indicating that restriction of the local chain motion of PC and the LMs becomes weaker for smaller LM size.
We investigated the structure and viscoelastic properties of a series of poly(ether-block-amide) thermoplastic elastomers consisting of hard polyamide segments and soft polyether segments with no ester linkages. All thermoplastic elastomers used in this study exhibited heterogenous morphologies with crystalline lamella in the solid state. The heterogeneities and solubilities in the amorphous regions of the elastomers depended on the sample composition. We did not observe a clear, slow relaxation mechanism characteristic of phase-separated liquids for the dynamic moduli of samples in the molten state. This result shows that the melted samples had a homogeneous structure due to the good miscibility between the polyamide and polyether segments.