Recoverable Shear Research Articles

Correlating the Segmental Relaxation Time of Polystyrene.

A previous related paper dealing with the density relaxation of polystyrene (PS) has shown that the equilibrium relaxation time (τeq) has a purely exponential temperature dependence (ETD) below ≈100 °C. Such an ETD is now also confirmed based upon available dielectric spectra data for PS. By combining the ETD behavior of τeq (or aT) at low temperatures with a VFTH behavior at higher temperatures (based mainly on available recoverable shear compliance data), a composite correlation for τeq (or aT) is developed, which is continuous with continuous slope at a crossover temperature that is found to be 99.22 °C, where τeq = 92.15 s. This composite representation is shown to describe (without any adjustable parameters) available independent data for the segmental relaxation time over a finite range both above and below Tcrossover (i.e., the glass transition temperature).

Development and implementation of shear wall finite element in OpenSees

Shear walls are widely used as lateral load-resisting systems in high-rise building structures. The design and nonlinear analysis of shear walls under seismic loads for recoverability are complicated and challenging, which demands nonlinear computer modeling procedures. This paper presents a novel finite element named Double Vertical Beam-column with Shell Element (DVBSE) for the nonlinear analysis of complex reinforced concrete shear walls with columns. The DVBSE is implemented in the open-source finite element program OpenSees. The theoretical formulations of DVBSE are described in detail. The accuracy of the developed DVBSE is established by comparing the finite element analysis results with existing experimental data on recoverable steel-GFRP composite bar reinforced concrete shear walls. The comparative study demonstrates that the proposed DVBSE is superior to the commonly used finite element model in simulating the responses of recoverable complex shear walls under seismic loads. These research findings contribute to further advancements in the development of new shear wall elements and lay a solid foundation for the nonlinear numerical simulation of innovative shear walls.

Effects of Thermal Cycling and Porosity on Phase Transformation of Porous Nanocrystalline NiTi Shape Memory Alloy: An Atomistic Simulation

Herein, the effects of thermal cycling and porosity on the martensitic phase transformation behavior of porous nanocrystalline (NC) NiTi shape memory alloys (SMAs) at the atomic scale are investigated by molecular dynamics simulation. The simulation results show that thermal cycling causes the NC NiTi SMAs to repeatedly undergo martensitic phase transformation and inverse phase transformation processes. The martensitic phase transformation capability is suppressed with increasing porosity and number of cycles, but the suppression effect of thermal cycling is weakened with increasing porosity. Moreover, the start temperature of martensitic transformation, residual martensitic phase, and the fraction of interstitial atoms increase with increasing porosity. Thermal cycling causes the disordered structures and shear plastic deformation to accumulate as cycling increases. Recoverable shear deformation is generated within the grain, but shear plastic deformation is mainly concentrated at both grain boundaries and pore surfaces. Residual martensitic phase increases after thermal cycling. These results provide important explanations and references for a deeper understanding of phase transformation behavior and pore effects caused by thermal cycling.

Nonhysteretic superelasticity and strain hardening in a copper bicrystal with a ∑3{112} twin boundary

Superelasticity in nanoscale materials is of profound fundamental interest. Despite extensive research into low-dimensional systems such as nanowires and nanobeams, no compelling evidence exists for superelasticity in high-dimensional systems. Here, we demonstrate that a copper bicrystal with a ∑3{112} twin boundary (TB) during shear loading and unloading cycles exhibits nonhysteretic superelasticity. The upper bound of total recoverable shear strain associated with superelastic deformation is 24.8%. ∑3{112} TB under shear loading dissociates into two phase boundaries (PBs) with different characteristics, constituted by two phases with FCC and 9R structures. The transformation between these two phases, assisted by the reversible migration of tilt PB during loading and unloading cycles, accounts for the superelasticity. The driving force for the superelasticity is the potential energy difference per atom of the two phases. However, the irreversible migration of mixed tilt/twist PB with energy dissipation is responsible for strain hardening. This work, the first observation of nonhysteretic superelasticity in a copper bicrystal, may shed light on the design of bulk materials with superelasticity.

Open boundary molecular dynamics of sheared star-polymer melts.

Open boundary molecular dynamics (OBMD) simulations of a sheared star polymer melt under isothermal conditions are performed to study the rheology and molecular structure of the melt under a fixed normal load. Comparison is made with the standard molecular dynamics (MD) in periodic (closed) boxes at a fixed shear rate (using the SLLOD dynamics). The OBMD system exchanges mass and momentum with adjacent reservoirs (buffers) where the external pressure tensor is imposed. Insertion of molecules in the buffers is made feasible by implementing there a low resolution model (blob-molecules with soft effective interactions) and then using the adaptive resolution scheme (AdResS) to connect with the bulk MD. Straining with increasing shear stress induces melt expansion and a significantly different redistribution of pressure compared with the closed case. In the open sample, the shear viscosity is also a bit lowered but more stable against the viscous heating. At a given Weissenberg number, molecular deformations and material properties (recoverable shear strain and normal stress ratio) are found to be similar in both setups. We also study the modelling effect of normal and tangential friction between monomers implemented in a dissipative particle dynamics (DPD) thermostat. Interestingly, the tangential friction substantially enhances the elastic response of the melt due to a reduction of the kinetic stress viscous contribution.

Viscoelastic properties and melt rheology of novel polyamide 6/fluoroelastomer nanostructured thermoplastic vulcanizates

The present article reports viscoelastic properties and melt rheology of novel nanostructured thermoplastic vulcanizates (TPVs) consisting of dispersed nanodimensional fluoroelastomer (FKM) particles into the continuous polyamide (PA6) matrix. The influence of dynamic vulcanization on the above properties of the blends was reported. The storage modulus and loss modulus were plotted against reduced frequency to understand the behavior of nanostructured TPVs. PA6/FKM TPVs showed pseudoplasticity and obeyed cross viscosity model. Insight into how the crosslink density of TPVs affects the rheological response was discussed. Also, a new relationship of zero shear viscosity (η 0) with the crosslink density of TPVs was proposed to gain insight into dynamic vulcanization. It was found that up to a certain amount of curative concentration, η 0 depends upon the crosslink density to a power equal to 3.4 and is roughly proportional to the crosslink density on further increase of curative concentration. van Gurp–Palmen plot of phase angle versus reduced complex modulus indicated the droplet–matrix morphology of the PA6/FKM blends which was further confirmed from field emission scanning electron microscopy and atomic force microscopy studies, where FKM nanoparticles (60–80 nm) were the dispersed phase. Various melt elastic properties of TPVs such as die swell, first normal stress difference, recoverable shear strain, and apparent elasticity were measured and analyzed. TPV extrudates were examined using surface finish and roughness values. TPVs exhibited smoother extrudate surface than reported earlier. Extrudate surface roughness from atomic force microscopy was used to analyze the processing behavior of TPVs.

Melt rheological properties of PBT/SEBS and reactively compatibilized PBT/SEBS/SEBS‐g‐MA polymer blends

ABSTRACTMelt rheological properties of PBT/SEBS and PBT/SEBS/SEBS‐g‐MA blends at SEBS volume fraction (Φd) = 0.00–0.38 were studied at 240°C, 250°C and 260°C using a capillary rheometer. The compatibilizer SEBS‐g‐MA addition resulted in significant reduction in the dynamic interfacial tension which in turn led to increased phase adhesion. The power law exponent n decreased with increasing Φd and increasing temperature for both the compatiblized and uncompatiblized blends. The consistency index of PBT/SEBS increased with increasing Φd but were smaller than those of PBT/SEBS/SEBS‐g‐MA blends. Melt elasticity such as die swell and first normal stress difference increased with Φd. Variations of first normal stress coefficient function (ψ1), recoverable shear strain (γR), relaxation time (λ), and shear compliance (Jc) values versus shear rate were analyzed. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 41402.

Rheological behavior of coir‐fiber‐filled polypropylene composites at constant shear stress

The rheological behavior of coir‐fiber‐filled polypropylene (PP) composite has been studied at constant shear stress. The shear stress versus shear rate relationship for the composite follows power law model of viscous flow. Unlike similar studies in the literature, the viscosity is treated as a stress‐independent parameter, which increases with the increase of fiber loading; but decreases with the rise of temperature. The SEM reveals that the fibers are loosely bound to the polymer matrix and the outer surface of the composite is rough and irregular, making it susceptible to high friction with the wall of the flow channel. With analogy to nth order chemical reaction, new formula has been derived for the activation energy of viscous flow, which is found to increase with the increase in the fiber content. The die‐swell ratio decreases with the increase of fiber loading, but increases with the rise of temperature. The elastics parameters of the composite such as the recoverable shear strain, the first normal stress difference, and the elastic strain induced by the stored energy in the capillary reservoir have been estimated based on the die‐swell data. POLYM. COMPOS., 36:51–61, 2015. © 2014 Society of Plastics Engineers

Melt Elastic Properties during Capillary Extrusion of PP Impact Copolymer/Styrene-Butadiene-Styrene Block Copolymer Blends

Our previous study reported on low temperature impact enhancement of PP impact copolymer (PPcp) with the addition of styrene-butadiene-styrene (SBS). In present study rheological properties of PPcp/SBS blend melts in shear and elonagtional flows have been correlated with morphology using image analysis and theoretical models. The influence of blend morphology on steady shear and melt elastic properties (die swell, first normal stress difference, recoverable shear strain and elastic shear modulus) evaluated on capillary rheometer showed strong correlation under shear and elongational flows. The melt elastic properties were also found to critically rely on the inter-particle distance of the SBS phase.

Control the discontinuity of the flow curve of the polyethylene by nanoclay and compatabilizer

A study on the melt elasticity behavior and extrudate characteristic of low density polyethylene (LDPE)/nanoclay composite and maleated polyethylene (MAPE) as compatabilizer was done. Extrusion studies were carried out in capillary rheometer. A microscope has been used to examine the surface characteristics of the extrudate by taking photographs. With a view to characterize melt elasticity of these nanocomposites, parameters such as die swell, principle normal stress, recoverable shear strain, and shear modulus were calculated. Compatibilizers were premade as block copolymers (BCPs), are of crucial importance during the formation and stabilization of polymer blends. These polymeric surfactants reduce the interfacial tension, R, between the blend components, and due to their ability to accumulate preferentially in the interface, they stabilize the obtained morphology against coalescence. Small amounts of these species in the range of parts of percent are already active.

Elastic properties of gluten representing different wheat classes

Traditional instruments used to evaluate dough and/or gluten rheological properties do not provide unambiguous separation of elastic and viscous behaviors. Recovery after shear creep and cyclic large deformation cyclic tensile testing were used here to decouple elastic and viscous effects. A large variation in the recoverable shear strain (∼7.2% to ∼28%) was seen for glutens from 15 U.S. popular common wheat cultivars with varying HMW subunits. Sedimentation values ranged from 29 to 57 ml for 12 hard wheat cultivars and 15 to 22 ml for three soft wheat cultivars. The tensile force at 500% extension ranged from 0.12 to 0.67 N for hard wheat glutens and from 0.10 to 0.20 for soft wheat glutens. However, the recoverable work after large extension was less than 40% of the total work of extension. In addition, recoverable work in tensile testing was highly correlated with the total work of extension ( r 2 = 0.97) and mixograph mix times ( r 2 = 0.81). Good to excellent bread volume was obtained for several cultivars from this sample set. This suggests that optimizing water absorption for mixing doughs to achieve maximal bread volume compensates for the wide range of viscoelastic behaviors of gluten.

The Rheological Properties of Modified Microcrystalline Cellulose Containing High Levels of Model Drugs

The rheological properties of different types of microcrystalline cellulose (MCC) mixed with model drugs and water have been evaluated to identify the influence of sodium carboxymethylcellulose (SCMC) added to the cellulose during preparation. A ram extruder was used as a capillary rheometer. The mixtures consisted of 20% spheronizing agent (standard grade MCC or modified types with 6% or 8% of low viscosity grade SCMC) and 80% of ascorbic acid, ibuprofen or lactose monohydrate. The introduction of SCMC changed all rheological parameters assessed. It produced more rigid systems, requiring more stress to induce and maintain flow. Degree of non-Newtonian flow, angle of convergence, extensional viscosity, yield and die land shear stress at zero velocity, and static wall friction were increased, but recoverable shear and compliance were decreased. The presence of SCMC did not remove the influence of the type of drug. The mixture of ibuprofen and standard MCC had the lowest values for shear stress as a function of the rate of shear, extensional viscosity, and angle of convergence, but the highest values for recoverable shear and compliance. The findings indicate that the system has insufficient rigidity to form pellets.

The influence of non-ionic surfactants on the rheological properties of drug/microcrystalline cellulose/water mixtures and their use in the preparation and drug release performance of pellets prepared by extrusion/spheronization

The influence of adding two concentrations (5 and 25%) of non-ionic surfactants, one hydrophilic and the other hydrophobic, plus mixtures of equal parts of the two, on the rheological properties of a mixture of equal parts of microcrystalline cellulose and ibuprofen with water has been assessed by capillary rheometry. The mixtures were also used to form pellets by extrusion/spheronization and their in vitro dissolution in simulated intestinal fluid was measured. As with previous rheological studies of these types of pastes, the flow was non-Newtonian (shear thinning). Other rheological parameters were determined in terms of die entry angles, extensional flow and elastic parameters of recoverable shear and compliance. By comparisons with previous studies with the model drug, it was found that it was these latter parameters that were indicative of the ability of the formulations to produce satisfactory pellets. The experiments also identified that the level of surfactant as opposed to the type of surfactant determined the rheological properties of the wet mass. All the formulations were able to produce round pellets with a narrow size distribution, whereby their median size increased with the concentration of the surfactant. The 25% level of each of the surfactant formulations provided a rapid release of the drug (100% within 30 min), but the 5% level and the mixed surfactant formulations provided lower drug release profiles.

Interfacial slip‐stick transition induced by reactive compatibilization in ethylene‐co‐butyl acrylate elastomer dispersed polyamide 6 blend subjected to high shear flow and extensional flow

AbstractThe effect of phase interaction induced by reactive compatibilization during high shear and extensional flow in polyamide (PA6) and ethylene‐co‐butyl acrylate (EBA) blends was studied using advanced dual bore capillary rheometer. The viscosity‐composition behavior of the uncompatibilized PA6/EBA blends exhibited negative deviation behavior from log‐additivity rule. The interfacial slip mechanism, operative between the matrix PA6 and dispersed EBA during shear flow was studied by the use of Lin's and Bousmina‐Palierne‐Utracki (BPU) model for viscosity for the blends under the processing conditions. On the other hand, the compatibilized PA6/EBA‐g‐MAH0.49/EBA blends with varying dispersed phase volume fraction show positive deviation behavior. The reactive compatibilizers EBA‐g‐MAH0.49 and EBA‐g‐MAH0.96 increased the phase interaction with adequate reduction in the dynamic interfacial tension, which favored the particle break‐up and stabilized the morphology in the compatibilized blends. The extensional viscosity of the blends has enhanced because of the inclusion of EBA in all the uncompatibilized and compatibilized blends. The melt elasticity and elasticity function were systematically studied from first normal stress coefficient functions (ψ1). The variation in the recoverable shear strain (γR), shear rate dependent relaxation time (λ) and shear compliance (Jc) under various shear rates were thoroughly analyzed for all the blend compositions. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

New Results on the Correlation Molecular Architecture‐Melt Elasticity‐Blowing Process‐Film Properties for Conventional and Metallocene‐Catalyzed Polyethylenes

AbstractSummary: Melt elasticity of a series of conventional and metallocene‐catalyzed PEs is investigated. The samples show very noticeable differences in their molecular architecture, including a variable degree of LCB and MWD. Four parameters (De, RSI, recoverable shear and steady state compliance), determined through dynamic viscoelastic measurements, are correlated with molecular architecture and blown film properties. RSI parameter shows a particular capacity to discern the LCB effect from the MWD effect on PE melt elasticity. Enhancing melt elasticity improves bubble stability, but provokes more haze and poorer impact properties in the film.Dart impact values vs. relaxation spectrum index.magnified imageDart impact values vs. relaxation spectrum index.

Effect of ultrasound on the viscoelasticity and rheology of polystyrene extruded through a slit die

AbstractThe rheological and processing behavior (melt fracture performance) of polystyrene extruded through a slit die is studied as a function of the ultrasound vibration intensity. The apparent viscosity reduces 29% and die pressure reduces 22% compared with that without ultrasound vibration. The viscosity of the melt decreases exponentially as ultrasound intensity increases and an Arrhenius equation fits the data well. Ultrasound vibration also leads to a decrease of the die swell ratio and a postponement of melts fracture. Characteristic relaxation times at the onset of melt fracture are calculated according to the hypothesis that the melt fracture behavior of a polymer is affected by a balance between its viscous (viscosity) and elastic properties (recoverable shear). Ultrasound vibration shortens the relaxation time of polystyrene molecular chains. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2907–2911, 2006

Quasi-Three-Dimensional Simulation of Viscoelastic Flow through a Straight Channel with a Square Cross Section

When a viscoelastic fluid flows in a straight channel with a noncircular cross section, a secondary flow is developed. We discussed the effect of flow characteristics on the secondary flow of the viscoelastic fluid through a straight channel with a square cross section by the viscoelastic flow simulation using the finite element method. The Phan-Thien Tanner (PTT) model was employed as the constitutive equation. The second normal stress difference N2 contributes to the secondary flow motion of a viscoelastic fluid through a straight channel with a square cross section. The degree of the secondary flow expressed by the maximum streamline function |ψmax| depends on the recoverable shear for the second normal stress difference −N2/2σ , where σ is shear stress. According to the distribution of N2 on the cross section, the flow direction of the secondary flow depends on the pressure drop generated by N2. The elongational flow characteristics also contribute to the secondary flow state. On the whole, the degree of the secondary flow increases with the strain-hardening. However, the contribution of elongational flow characteristics to the secondary flow is smaller than that of −N2/2σ.

Creep recovery of random ethylene-octene copolymer melts with varying comonomer content

This paper presents an investigation of the effects of varying comonomer content from 20 to 38 wt % in random ethylene-octene copolymers made with a metallocene catalyst on the melt rheology including transients in shear creep recovery from high strains. The copolymer with 20% octene appeared to be the same as that reported to be long-chain branched in the literature and was used as a reference material. The materials were characterized in oscillatory shear and shear creep at stresses ranging from 5 to 30 000 Pa. Van Gurp-Palmen plots prepared from dynamic modulus data and zero-shear viscosity values from creep tests at very low stress confirmed that the reference material was long-chain branched and established that long-chain branching was absent from the copolymer with the highest comonomer content. The steady state recoverable shear compliance values for the three copolymers were very close. However, following large shear deformations brought about at higher strain rates, strain recovery was reduced most significantly for the copolymer with the highest comonomer content where long-chain branching was absent. The reduction in recovery at short times was also most significant for this copolymer.

Rheological characterization of cellulose solutions in N‐methyl morpholine N‐oxide monohydrate

AbstractThree different modes of rheological properties were measured on 11 and 13 wt % solutions of cellulose in N‐methyl morpholine N‐oxide (NMMO) monohydrate, in which concentration range lyocell fibers of much reduced fibrillation are preferably produced. The dynamic rheological responses revealed that the Cox–Merz rule did not hold for these cellulose solutions. Both cellulose solutions showed a shear thinning behavior over the shear rate measured at 85, 95, 105, and 115°C. However, 13 wt % solution gave rise to yield behavior at 85ºC. The power law index ranged from 0.36 to 0.58. First normal stress difference (N1) was increased with lowering temperature and with increasing concentration as expected. Plotting N1 vs shear stress (τω) gave almost a master curve independent of temperature and concentration, whose slope was about 0.93 for both cellulose solutions over the shear rate range observed (τω > 500 Pa). In addition, the cellulose solutions gave high values of recoverable shear strain (SR), ranging from 60 to 100. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 216–222, 2002

Melt elasticity and flow activation energy of nylon 6/polystyrene blends

Nylon 6 (PA 6) is blended with polystyrene (PS), so as to reduce the moisture absorption and hence to impart dimensional stability to the polymer. The melt elastic properties of the blends including die swell, principal normal stress difference and recoverable shear strain have been analysed. The temperature sensitivity of the flow behaviour of the blends is studied using Arrhenius plots.