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This study investigated the non-linear rheological behaviors of silver nanowire (AgNW) suspensions containing silica nanoparticles (SiNPs) dispersed in aqueous polyvinyl alcohol (PVA) solutions under large amplitude oscillatory shear (LAOS) flows with various methods including LAOS moduli, Fourier-transform (FT) rheology, and the sequence of physical processes (SPP). The microstructures of the suspensions depended on the ratio of SiNP and AgNW concentrations (φSi/φAg). Lower φSi/φAg ratios yielded entangled AgNWs, whereas high φSi/φAg ratios induced AgNW–SiNP bundle formation due to strong attraction between SiNPs and AgNWs. Non-linear rheological behaviors were classified into three new LAOS types (A, B, and C) based on distinct microstructures including entangled networks of individual AgNWs, stiff AgNW–SiNP bundles, and a combination of both. Type A, with dominant entangled AgNWs, displayed two-step strain thinning attributed to flocculated network formation. Network disruption aggravated the non-linearities due to strain-induced structure formations, followed by the appearance of minima in FT intensities due to disentangled and aligned AgNWs. Type B, comprising AgNW entangled networks and AgNW–SiNP bundles owing to higher SiNP contents, exhibited broad one-step strain thinning. Type C, with dominant stiff bundles, presented the simplest network structures, resulting in one-step strain thinning. Furthermore, intracycle structural changes during LAOS flows were examined using two SPP parameters: instantaneously recoverable elasticity G t , max ′ and internal area of SPP moduli. The SPP analysis also exhibited different behaviors depending on the LAOS types. Thus, employing FT rheology and SPP methods, complex microstructures containing PVA, AgNWs, and SiNPs and their structural changes during LAOS flows were investigated. [ABSTRACT FROM AUTHOR]

This study focused on the nonlinear rheological characterization of three types of cellulose nanofibril (CNF) suspensions under large amplitude oscillatory shear (LAOS) flow. Three different CNFs were produced, two by mechanical fibrillation alone under different conditions [here named microfibrillated cellulose (MFC) and U-CNF] and the other by mechanical fibrillation after carboxymethylation (CM-CNF). MFC and U-CNF had broad width distributions, whereas CM-CNF had narrower fibril width and width distribution due to the presence of charged carboxymethyl groups. Nonlinear stress responses of the prepared suspensions were analyzed using the sequence of physical processes method. All CNF suspensions exhibited intracycle rheological transitions composed of three physical processes: (1) structure recovery, (2) elastic deformation to early stage yielding, and (3) late-stage yielding. MFC and U-CNF suspensions exhibited similar rheological transitions overall. However, CM-CNF suspension had a higher network recovery rate within a shorter time and showed an additional yielding step due to the complex interplay between recovery and yielding dynamics. This result originated from complete nanofibrillation and charged functional groups on fibril surfaces. Rapid reformation of effective fibril–fibril contacts in CM-CNF suspension was attributed to electrostatic repulsions and complete nanosized lateral dimensions. In addition, excitation frequency was found to influence intracycle rheological transitions. A range of intracycle rheological transitions became narrower on increasing frequency because the time period for each transition was not enough under faster flow conditions. In particular, the characteristic yielding step of CM-CNF suspension disappeared on increasing frequency, which suggested that high-frequency excitation might be unfavorable for the nonlinear viscoelastic characterization of soft materials under LAOS flow. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Monodisperse linear and 3-arm star polyisoprene (PI) melts were investigated under dynamic oscillatory shear flow. Master curves of linear rheological properties, G ′ ( ω ) and G ″ ( ω ), obtained by small amplitude oscillatory shear (SAOS) testing and nonlinear rheological properties, Q 0 ( ω ), obtained by medium amplitude oscillatory shear (MAOS) testing, were created using the same shift factors by applying the time–temperature superposition (TTS) principle. Linear master curves of 3-arm star PIs were broader than those of linear PIs because of the contribution made by an additional relaxation mode due to arm retraction. This difference was clearer in nonlinear master curves. In nonlinear master curves ( Q 0 ( ω )), linear PIs showed only one peak corresponding to relaxation of the backbone chain, whereas two weak local peaks were observed for 3-arm star PIs. One peak at lower frequency reflected backbone relaxation and the other at higher frequency reflected branch relaxation. These findings confirm that Q 0 is a highly sensitive parameter for characterizing the effect of one branch by detecting backbone and branch relaxation modes. Based on the hierarchical relaxation concept of branched polymers, three characteristic relaxation times of 3-arm star PIs were determined from a nonlinear master curve: backbone relaxation time ( τ b ) from local maximum Q 0 at lower frequency, backbone Rouse time ( τ R,b ) from local minimum Q 0 , and arm relaxation time ( τ a ) from local maximum Q 0 at higher frequency. Backbone relaxation times ( τ b ) from nonlinear master curves almost coincided with linear viscoelastic terminal relaxation times ( τ w ) from linear master curves within the limits imposed by experimental error. These relaxation times, especially τ R,b , were used for molecular stress function (MSF) model predictions of 3-arm star PIs as a criterion parameter to determine the terminal relaxation mode of the backbone chain, and τ R,b was found to be a reasonable fitting parameter for the MSF model. Fitting results showed that the weak peak arising from the one branch could be quantified using the MSF model. However, both linear and 3-arm star PIs had the same β value of 1, meaning a linear topology without a branch. It was concluded that the effect of only one branch could not be detected using β values. Our results suggest the possibility of using the nonlinear parameter Q 0 for determining model parameters for constitutive model equations. [ABSTRACT FROM AUTHOR]

Effects of silica nanoparticles withdifferent natures (hydrophilicityand hydrophobicity) on (80/20) PP/PS blends were investigated vialinear and nonlinear rheological properties. The hydrophilic silicananoparticle was fumed silica OX50 while the two hydrophobic oneswere precipitated silica D17 and fumed silica R202. SEM images revealedthat hydrophilic OX50 could not improve morphological properties ofthe blends. On the other hand, the two hydrophobic silica nanoparticles(R202 and D17) improved morphological properties. TEM examinationshowed that OX50 silica nanoparticles aggregated inside PS droplets,thereby making breakup of PS (dispersed) phase into smaller sizesmore difficult. D17 and R202 improved morphological properties regardlessof the different droplet size reduction mechanisms, and rheologicalproperties improved as a result. Both linear rheological propertiesfrom SAOS (small-amplitude oscillatory shear) tests and nonlinearrheological properties from LAOS (large-amplitude oscillatory shear)tests were obtained. The nonlinear–linear viscoelastic ratio(NLR ≡ normalized nonlinear rheological properties/normalizedlinear rheological properties) was used to quantify the degree ofdroplet dispersion and distinguish the effects of silica particleson the morphology of PP/PS blends. Previous research has observedan inverse correlation between NLR and droplet size. PP/PS/OX50 blendswith no alteration of droplet size showed constant NLR values (≅1)with increasing concentration of OX50 (hydrophilic silica). However,NLR values of PP/PS blends with hydrophobic silica nanoparticles (D17and R202) were much larger than 1 (NLR > 1) and increased withsilicaconcentration, which is consistent with morphological evolution, i.e.,reducing droplet size. However, NLR values of PP/PS/R202 blends wererelatively larger than those of PP/PS/D17 blends despite smaller dropletsizes. This can be attributed to a different morphology microstructure,i.e., R202 located in PP matrix phase and D17 at interface betweenPP and PS. Therefore, the NLR value of PP/PS/silica blend could bedue to the combined effects of the interface between droplets (PP/PSblend) and particle–polymer interactions (PP/silica nanocomposites).Especially, R202 showed larger NLR values due to PP/R202 nanocomposites.Based on these findings, relative NLR (= NLRPP/PS/silica/NLRPP/silica) is proposed as an effective measurementof droplet size information in PP/PS blends by eliminating the effectsof PP/silica nanocomposites. Relative NLR matched well with dropletsize evolution from the SEM results. [ABSTRACT FROM AUTHOR]

Linear and nonlinear rheological properties of cellulose nanofiber (CNF) suspensions were measured under small and large amplitude oscillatory shear (SAOS and LAOS) flow. Four different CNFs were produced, two by only mechanical disintegration and two with chemical pretreatments. Linear viscoelastic properties distinguished chemically treated CNFs from two untreated fibers via a different scaling exponent of the elastic modulus. However, different mechanical fibrillation degree was not characterized via linear viscoelastic properties. In contrast, nonlinear viscoelastic properties reflected both effects of chemical pretreatments and mechanical fibrillation. More fibrillated CNFs exhibited nonlinear rheological phenomena at larger deformations. In addition, chemically treated CNFs exhibited greater network stiffness and higher network recovery rates due to the presence of charged functional groups on the fiber surfaces. A material-property co-plot showed that network stiffness and recovery rate were in a trade-off relationship. [Display omitted] [ABSTRACT FROM AUTHOR]

Linear and nonlinear rheological properties of model comb polystyrenes (PS) with loosely to densely grafted architectures were measured under small and medium amplitude oscillatory shear (SAOS and MAOS) flow. This comb PS set had the same length of backbone and branches but varied in the number of branches from 3 to 120 branches. Linear viscoelastic properties of the comb PS were compared with the hierarchical model predictions. The model underpredicted zero-shear viscosity and backbone plateau modulus of densely branched comb with 60 or 120 branches because the model does not include the effect of side chain crowding. First- and third-harmonic nonlinearities reflected the hierarchy in the relaxation motion of comb structures. Notably, the low-frequency plateau values of first-harmonic MAOS moduli scaled with M w − 2 (total molecular weight), reflecting dynamic tube dilution (DTD) by relaxed branches. Relative intrinsic nonlinearity Q0 exhibited the difference between comb and bottlebrush via no low-frequency Q0 peak of bottlebrush corresponding to backbone relaxation, which is probably related to the stretched backbone conformation in bottlebrush. [ABSTRACT FROM AUTHOR]

In this study, azidoformate (AF) is investigated as a reactive functional group to mediate noncatalytic C–H insertion reactions to interconnect polypropylene (PP) chains to generate long-chain branches (LCBs). 4,4′-Isopropylidenediphenyl azidoformate (DAF) with two terminal AF groups is synthesized, which is first mixed with propylene/α-olefin copolymer at 100 °C. Then, the prepared mixture with a predetermined amount of DAF is reactively melt mixed with PP at 165 °C. This unique two-step preparation strategy affords PP with prominent rheological properties of LCBs, e.g., shear thinning and strain hardening. The rheological characteristics are easily controlled by adjusting the amount of DAF to afford an effective tool to prepare high-melt-strength PPs. The strength of this strategy is distinct, including a simple/typical melt processing procedure, no degradation of PP chains, and no purification/solvent recycling steps. The C–H insertion capability of azidoformate groups suggests an expandability of this methodology as a platform technology to prepare modified polymers. • Long-chain branched polypropylene is prepared via reactive melt mixing procedures. • Noncatalytic C–H insertion capability of azidoformate is utilized. • Propylene/α-olefin copolymer is employed as a dispersant of di (azidoformate). • The amount of branches is controlled by adjusting the amount of di (azidoformate). • Elastic and strain hardening melt behaviors of the resulting polymer are observed. [ABSTRACT FROM AUTHOR]