Recent progress of molecular rheology for various polymeric systems by a rheo-optical method using simultaneous measurements of stress and birefringence is briefly reviewed. The advantage of the method is that the stress can be disassembled into its components following the birefringence response. The scope of this review is to elucidate the significance of the viscoelastic segment and also to extend the stress-optical rule to inhomogeneous systems. Firstly, effects of highly branched structures on the viscoelastic segment size is examined with bottle-brush type poly(macromonomer) and highly crosslinked thermoset resins. These studies indicate the importance of internal motions of the viscoelastic segments such as bending and tension modes in addition to the reorientation mode of the segments. These modes can be separately estimated with the aid of the rheo-optical measurements. The estimation of the bending mode is particularly important for the relaxation dynamics of tightly entangled systems of semiflexible polymers. Secondly, the extension of the stress-optical rule for inhomogeneous systems is discussed. In these systems, the concept of phase stresses and strains is quite important to understand their molecular rheology, and the stresses can be estimated from the rheo-optical measurements.

Original experimental and computational models for PSAs for flexible displays have been studied to clarify the property target for the PSA development. Possible defects in the devices, crack formation, delamination, and residual strain were addressed by a new theory, “Multiple-Neutral-Plane Phenomena”, with an originally developed software using Vector Finite Element Method. Peel adhesion values were found to be correlated with tan δ from DMA multiplied by tensile modulus from stress-strain curves, which offered a prediction equation for peel strength. The study has enabled a PSA material producer to develop PSAs for the application.

For the application of polymer gels, it is necessary to control their various physical properties independently and precisely. However, the heterogeneity of polymer gels hinders the precise control over the network structures, as well as verification of molecular models. To understand the structure-property relationship of polymer gels, I have developed a novel polymer gel with regular network structure (Tetra-PEG gel). In this review, I introduce our achievements for the fundamental properties of polymer gels, and show one of the medical applications for the artificial vitreous body using the TetraPEG gel, the Oligo-Tetra-PEG gel system.

In the research field of molecular rheology, one of the most challenging tasks is to understand multi-chain interactions, including topological interactions, frictional interactions, and chain associations. Our research in the past ten years have been placed on polymer dynamics under the multi-chain interactions, in particular the topological and frictional interactions between miscible components having large dynamic asymmetry, and physical associations including ionic interactions and hydrogen-bondings.

The paper reports a novel idea of an experimental approach to estimate the apparent planar elongational viscosity and the relaxation time of low viscosity fluids. A two-dimensional swirling contraction channel is designed to maintain a constant elongation rate to the flow direction. The hydrodynamic focusing creates a stable planar elongation flow field. The flow is injected into the center of the channel and a sheath flow is injected through both sides. The sample fluid is stretched in the flow field, and then forms the ribbon-shaped filament. In the swirling contraction part, a static force balance between the hoop force caused by the planar elongational stress and the centrifugal force determines the radial position of the filament. The distribution of the planar elongational stress can be estimated by observing this position, and then the apparent planar elongational viscosity was calculated using developed simplified expressions. On the other hand, when a channel with a constant width swirling part is used, the elongational stress generated in the straight contraction part gradually decreases passing through the constant width part. The sample filament position is then shifted based on the stress relaxation. The relaxation time can be evaluated by observing this variation.