Hyeong Yong Song

Nonlinearity from FT-rheology for liquid crystal 8CB under large amplitude oscillatory shear (LAOS) flow

This study systematically investigated the nonlinear stress behavior of liquid crystal (8CB, 4-4′-n-octyl-cyanobiphenyl) in lamellar smectic A phase under large amplitude oscillatory shear (LAOS) flow. To investigate the nonlinear stress response under LAOS flow, the nonlinearity (I3/1) from Fourier transform-rheology as a function of applied shear time (3600 s) was calculated according to changes in both strain amplitude γ0 and frequency ω. The storage modulus G′(t) and loss modulus G″(t) from the conventional rheometer program under various LAOS flow conditions decreased and reached equilibrium as a function of time. This could be attributed to shear alignment of the lamellar smectic A structure. On the contrary, with G′(t) and G″(t), the nonlinearity I3/1(t) showed three different behaviors depending on the magnitude of strain amplitude: (1) Region I: Increased (increased and reached equilibrium), (2) region II: Increased and decreased (showed maximum value; decreased and reached equilibrium), and (3) ...

Effect of urea on heat-induced gelation of bovine serum albumin (BSA) studied by rheology and small angle neutron scattering (SANS)

This paper reports the effects of urea on the heat-induced gelation of bovine serum albumin (BSA), which was studied by the tube inversion method, rheological measurements, and small-angle neutron scattering (SANS). An increase in the urea concentration accelerated the rate of gelation because the protein molecules have already been unfolded to some extent during sample preparation in the urea solution. In addition, the BSA solution in the presence of urea underwent a sol-gel-sol transition during the time sweep test at a constant temperature of 80oC. On the other hand, the BSA solution without urea turned into a hard and brittle gel that did not return to the solution state during isothermal heating at a constant temperature of 80oC. Aggregation and re-bonding of the denatured and unfolded protein chains led to gel formation. Urea added to the protein denatures its tertiary and secondary structures by simultaneously disrupting the hydrogen bonds, hydrophobic interactions, and altering the solvent properties. Furthermore, urea induces thermoreversible chemical interactions in BSA solutions leading to the formation of a gel with dynamic properties under these experimental conditions.

Effects of silica nanoparticles on copper nanowire dispersions in aqueous PVA solutions

In this study, the effects of adding silica nanoparticles to PVA/CuNW suspensions were investigated rheologically, in particular, by small and large amplitude oscillatory shear (SAOS and LAOS) test. Interesting, the SAOS test showed the complex viscosities of CuNW/silica based PVA matrix were smaller than those of PVA/CuNW without silica. These phenomena show that nano-sized silica affects the dispersion of CuNW in aqueous PVA, which suggests small particles can prevent CuNW aggregation. Nonlinearity (third relative intensity ≡ I3/1) was calculated from LAOS test results using Fourier Transform rheology (FT-rheology) and nonlinear linear viscoelastic ratio (NLR) value was calculated using the nonlinear parameter Q and complex modulus G*. Nonlinearity (I3/1) results showed more CuNW aggregation in PVA/CuNW without silica than in PVA/CuNW with silica. NLR (= [Q0(ϕ)/Q0(0)]/[G*(ϕ)/G*(0)]) results revealed an optimum concentration ratio of silica to CuNW to achieve a well-dispersed state. Degree of dispersion was assessed through the simple optical method. SAOS and LAOS test, and dried film morphologies showed nano-sized silica can improve CuNW dispersion in aqueous PVA solutions.

The Study on the Rheological Properties of Polymer Matrix for MIF (Molded-In Foaming) Process

In order to select polymer matrix for MIF (Molded-In Foaming) process, in this study, we investigated rheological properties of commercial polymers, SBC (Styrene-Butadiene Copolymers, K-resin KK38) and SBS (StyreneButadiene-Styrene, KTR 101 and KTR 301). In time sweep test, the rheological properties (Gᐟ, Gᐥ, η*) of SBS at 155 and 170 C display almost constant value as a function of time from 0 s to 1800 s. On contrast, the rheological properties of SBS at 185 and 200 C exponentially increase as a function of time. It could be due to gelation of SBS at high temperature conditions. These increment of rheological properties are not observed in SBC. From LAOS (large amplitude oscillatory shear) test, the nonlinear rheological properties of SBS at 155 and 200 C after 1800 s are compared. The nonlinear rheological properties at 155 C show simple strain thinning behavior such as linear homopolymer, however, the nonlinear rheological properties at 200 C show 2 times strain thinning behavior (Payne effect). It well match with the gelation of SBS at 200 C. From rheological studies, it is confirmed that the proper polymer matrix for MIF process (low rheological properties at initial time and high rheological properties after process) is SBS KTR 301.

Decomposition of Q0 from FT-rheology into elastic and viscous parts: Intrinsic-nonlinear master curves for polymer solutions

In the medium amplitude oscillatory shear (MAOS) test, intrinsic nonlinearity Q0(ω) derived from Fourier-transform (FT)-rheology is an important material function for investigating the nonlinear responses of complex fluids. However, Q0 consists of elastic and viscous components without phase information, such as complex modulus |G*| obtained from the small amplitude oscillatory shear (SAOS) test. In this study, we decomposed Q0 into Q 0 ′(ω) and Q 0 ″(ω), as done for storage modulus G′ and loss modulus G″ in the SAOS test. Furthermore, intrinsic-nonlinear tan δ (≡tan δ3,0), which provides third-harmonic phase information, was calculated by dividing Q 0 ″ by Q 0 ′. First, we defined mathematically elastic nonlinearity Q 0 ′ and viscous nonlinearity Q 0 ″, and then investigated the physical meanings of Q 0 ′ and Q 0 ″. Second, we applied the intrinsic parameters Q 0 ′ and Q 0 ″ to monodisperse polystyrene solutions. All intrinsic-nonlinear master curves of Q 0 ′(ω) and Q 0 ″(ω) for model solutions showed similar behavior in the terminal regime (at low frequencies). Unentangled polymer solutions had the same intrinsic-nonlinear master curves. However, although the intrinsic-nonlinear master curves of entangled polymer solutions superimposed well at low De (near the terminal regime), they deviated at high De due to different entanglement densities. Therefore, two characteristic times, MAOS relaxation time and inflection time (τN and τinf), were determined from intrinsic-nonlinear master curves by comparing with terminal relaxation time and Rouse time (τL and τR) obtained from linear master curves. The results showed that intrinsic nonlinearities from the MAOS test are sensitive to relaxation processes (terminal and Rouse) of polymer chains. Finally, master curves were compared with predictions by the molecular stress function (MSF) model and the Pom-Pom model. The single-mode predictions of these two models described behavior changes qualitatively. However, both failed to achieve quantitative predictions of Q 0 ′(ω) and Q 0 ″(ω). On the other hand, the multimode MSF model agreed well with experimental data from the terminal regime to the inflection time scale under the terminal relaxation mode assumption.In the medium amplitude oscillatory shear (MAOS) test, intrinsic nonlinearity Q0(ω) derived from Fourier-transform (FT)-rheology is an important material function for investigating the nonlinear responses of complex fluids. However, Q0 consists of elastic and viscous components without phase information, such as complex modulus |G*| obtained from the small amplitude oscillatory shear (SAOS) test. In this study, we decomposed Q0 into Q 0 ′(ω) and Q 0 ″(ω), as done for storage modulus G′ and loss modulus G″ in the SAOS test. Furthermore, intrinsic-nonlinear tan δ (≡tan δ3,0), which provides third-harmonic phase information, was calculated by dividing Q 0 ″ by Q 0 ′. First, we defined mathematically elastic nonlinearity Q 0 ′ and viscous nonlinearity Q 0 ″, and then investigated the physical meanings of Q 0 ′ and Q 0 ″. Second, we applied the intrinsic parameters Q 0 ′ and Q 0 ″ to monodisperse polystyrene solutions. All intrinsic-nonlinear master c...

Investigation of foaming cell development for epoxy resin with blowing and curing agent by rheological properties

The effects of the curing and blowing agent concentration in an epoxy resin were investigated using rheological, mechanical, and optical methods. The curing time from the time sweep test decreased with increasing amount of curing agent at the fixed blowing agent concentration. On the other hand, with increasing blowing agent at a fixed curing agent concentration, the curing time showed a local minimum value. Minimization of the curing time is very useful for reducing the processing time. Axial normal forces as a function of time showed a relationship between the contraction force by the curing process and the expansion force by foaming process. From the axial normal force measurements, it could be categorized quantitatively into three parts: the curing dominant region (negative axial normal force development), transition region, and the foaming dominant region (positive axial normal force development). At the transition region, the axial normal force development was delayed because the foaming process was disturbed by the contraction. Mechanical and structural analysis were conducted for the fully cured and foamed epoxy resin. The completely developed epoxy foams with the high curing agent concentration become brittle. On the other hand, they contained well-distributed unit cell foams inside. This is because the fast curing process interrupts the coalescence of the closed foams. Overall, the optimal curing and blowing agent concentration for the epoxy resin could be determined from rheological analysis during the process and mechanical and structural analysis for fully cured and foamed epoxy resin.

A comparative study of the effects of cone-plate and parallel-plate geometries on rheological properties under oscillatory shear flow

In this study, the effects of cone-plate (C/P) and parallel-plate (P/P) geometries were investigated on the rheological properties of various complex fluids, e.g. single-phase (polymer melts and solutions) and multiphase systems (polymer blend and nanocomposite, and suspension). Small amplitude oscillatory shear (SAOS) tests were carried out to compare linear rheological responses while nonlinear responses were compared using large amplitude oscillatory shear (LAOS) tests at different frequencies. Moreover, Fourier-transform (FT)-rheology method was used to analyze the nonlinear responses under LAOS flow. Experimental results were compared with predictions obtained by single-point correction and shear rate correction. For all systems, SAOS data measured by C/P and P/P coincide with each other, but results showed discordance between C/P and P/P measurements in the nonlinear regime. For all systems except xanthan gum solutions, first-harmonic moduli were corrected using a single horizontal shift factor, whereas FT rheology-based nonlinear parameters (I3/1, I5/1, Q3, and Q5) were corrected using vertical shift factors that are well predicted by single-point correction. Xanthan gum solutions exhibited anomalous corrections. Their first-harmonic Fourier moduli were superposed using a horizontal shift factor predicted by shear rate correction applicable to highly shear thinning fluids. The distinguished corrections were observed for FT rheology-based nonlinear parameters. I3/1 and I5/1 were superposed by horizontal shifts, while the other systems displayed vertical shifts of I3/1 and I5/1. Q3 and Q5 of xanthan gum solutions were corrected using both horizontal and vertical shift factors. In particular, the obtained vertical shift factors for Q3 and Q5 were twice as large as predictions made by single-point correction. Such larger values are rationalized by the definitions of Q3 and Q5. These results highlight the significance of horizontal shift corrections in nonlinear oscillatory shear data.