A Study of the Pull‐Out Performance of PPTA Fibres in Composites of Ethylene‐Type Ionomer Matrices/PPTA Fibres

The mean frictional shear stresses of six ionomer resins and sized Kevlar fibre were determined from fibre pull‐out tests. A study of the failure mechanisms occurring during pull‐out revealed that fibre delamination and fibre resin adhesion were factors which increased the measured frictional shear stresses and that there was a definite grouping of high and low frictional shear stress values. The low frictional shear stress values were used to calculate the mean frictional shear stress values, τ_B, because these were uncomplicated by fibre delamination and fibre resin adhesion, since these factors (delamination and adhesion) are certainly not unexpected in an ionomer/Kevlar composite. From these shear stress values, it was determined that critical fibre lengths should be between 35 and 72 mm for the high tensile strength Kevlar fibres within an ionomer matrix, for the composite to be used effectively. The ratio of the debonding force (F_B) to the frictional shear force (F_F), θ, did not vary significantly with the lengths of the embedded reinforcing fibres. Both debonding and frictional forces indicate increasing trends with the interfacial contact areas. The ratio of the interfacial bonding strength (τ_B) to the frictional shear stress (τ_F), ϕ, for the resin PEA‐6 compared to the surface modified poly(p‐phenylene terephthalamide) (PPTA) fibre ranged from 2 to 24. These ratios were grouped into two, viz: those where ϕ > 11 and those with ϕ < 7. Using only the τ_F where ϕ > 11 provided a mean frictional shear stress of 0.94 MPa and a standard deviation, s, of 0.23 MPa (the number of test samples, n, was 9). This value is little different from the frictional shear stresses measured for sized PPTA (0.84 MPa). The decrease in the values of ϕ is attributed to the decrease in τ_B, due to the surface modification reaction, without necessarily affecting the frictional shear stress, τ_F.     

Rheology of aging aqueous muscovite clay dispersions

In hydrometallurgical mineral processing, time and pH dependent rheology of dispersions comprising reactive particles can exert a striking influence on material processability and handleability. Rheological studies of 0.32 solid volume fraction (57 wt% solid) aqueous dispersions of muscovite clay mineral aging at pH 7 and 1 for 4 h were undertaken to investigate the temporal, pulp chemistry-mediated particle interactions. In particular, the dispersions’ flow and deformation behaviour in tandem with viscoelastic structure changes over time were probed. Dispersions at both pH 7 and 1 displayed non-Newtonian, Bingham plastic behaviour and strong time-dependent viscosities and yield stresses. Fresh dispersions showed a weak rheology at pH 7 initially, accompanied by thixotropy which transformed to anti-thixotropic and rheopectic behaviour upon aging. Furthermore, the viscosities, yield stresses and both the viscous (G″) and elastic (G′) moduli accentuated dramatically with time, reflecting an elastic to a more sol-like viscoelastic gel transformation. In contrast to pH 7, the particle interactions at pH 1 initially led to the formation of a highly networked gel structure which displayed thixotropic and predominantly elastic behaviour. Upon aging however, systematic attenuation of thixotropy, suspension viscosity, shear yield stress and viscoelasticity occurred. Above certain shear stresses, viscoelastic gel to sol structure transformation occurred, the creep behaviour being distinctly pH and time dependent. Differing pH-mediated pulp chemistries which prevailed at pH 7 and 1 upon aging were responsible for the contrasting, temporal particle interactions and microstructure evolution.     

Rheological characterization of carbon black/polystyrene solution systems

Steady‐shear measurements of suspensions of carbon blacks (CB) in polystyrene (PS)/di‐(butyl phthalate) (DBP) solution were investigated as a function of volume fraction (?) of CB to clarify the effect of the primary particle size and the structure of CB aggregates on the rheological properties. The suspensions show a typical shear‐thinning behavior in the range of a shear rate studied. The Casson model was applied to evaluate the viscosity at infinite of shear rate η_∞ and the yield stress σ_y for the suspensions. Relative viscosity η_∞/η_m, (η_m: medium viscosity) thus obtained was compared to the high‐frequency viscosity for the ideal hard‐sphere silica suspensions to evaluate the effective volume fraction ?_eff of CB aggregates. The ?_eff value was larger for the higher‐structure CB with higher DBP absorption value, irrespective of the primary particle size. The yield stress σ_y had almost the same ?_eff dependence for neutral furnace CB/(PS/DBP) suspensions, although it was larger for acetylene black (AcB)/(PS/DBP) suspensions. These results demonstrated that the effective volume fraction is the most important quantity to characterize the CB aggregates on the rheological properties. It was also found that the correction of the medium viscosity changes due to polymer adsorption on the CB surface is important since neutral furnace CB adsorbs PS polymers but AcB hardly adsorbs PS polymers in the solution. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Pure shear deformation of physical and chemical gels of poly(vinyl alcohol)

Pure shear deformation reveals the significant differences in elastic properties of the poly(vinyl alcohol) (PVA) gels with almost identical initial modulus, but with different types of crosslinks, physical crosslinks formed by microcrystallites and chemical crosslinks made of covalent bonds. The ratio of the two principal stresses steeply increases with elongation in the physical gels, while that remains almost constant independently of stretching in the chemical gels. The marked growth of the stress ratio with elongation in the physical gels leads to the negative values of the derivative of the elastic free energy (W₂) with respect to the second invariant of the deformation tensor in the whole range of deformation, which is firstly observed for elastomeric materials. By contrast, the chemical gels exhibit the positive values of W₂ like most chemically crosslinked rubbers. Among the existing theories of rubber elasticity, the classical non-Gaussian three-chain model considering the effect of finite chain-length is qualitatively successful in accounting for the steep increase of the stress ratio and the negative values of W₂ in the physical gels, although it fails to reproduce the large difference in the stress-strain behavior among uniaxial, pure shear and equi-biaxial deformations. These features of the physical gels are expected to stem from the structural characteristics such as fewer amounts of slippery-trapped entanglement along network strands compared to the chemical PVA gels.     

Rheological properties of PDMS filled with CaCo3: The effect of filler particle size and concentration

The effects of filler particle size and concentration on the rheological properties of hydroxyl terminated polydimethylsiloxane (HO‐PDMS) filled with calcium carbonate (CaCO₃) were investigated by an advanced rheometric expansion system (ARES). The Casson model was used to describe the relationship between shear stress and shear rate for steady‐state measurement. Micron‐CaCO₃ could not afford the CaCO₃/HO‐PDMS suspensions obvious shear thinning behavior and a yield stress high enough, whereas nano‐CaCO₃ could provide the suspensions with remarkable shear thinning behavior and high yield stress. Incorporation of nano‐CaCO₃ into HO‐PDMS resulted in the transformation of HO‐PDMS from a mainly viscous material to a mainly elastic material. With increasing nano‐CaCO₃ content, shear thinning behavior of nano‐CaCO₃/HO‐PDMS suspensions became more obvious. Remarkable yield stress was observed in nano‐CaCO₃/HO‐PDMS suspensions with high filler content, and increased with increasing nano‐CaCO₃ content. The degree of thixotropy was quantitatively determined using a thixotropic loop method. It was found that nano‐CaCO₃ favored more the buildup of filler network structure in the suspensions than micron‐CaCO₃ at the same weight fraction. Furthermore, increasing nano‐CaCO₃ content accelerated the establishment of filler network structure in the nano‐CaCO₃/HO‐PDMS suspensions. An overshoot phenomenon was observed in the nano‐CaCO₃/HO‐PDMS suspensions at high shear rates. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 3395–3401, 2006     

A friction bond model for the effects of sinusoidal vibrations upon shear stress in particulate systems

A theoretical model has been derived which describes the effects of vibration upon Critical State shear stress for a bed of particles failing in shear and subject to vibrations. The model is based upon the concept of hypothetical friction bonds acting across a shear plane and relies predominantly upon mechanistic arguments. One solution of the resultant differential equation relates the shear stress τ_sf at frequency f to vibration velocity U in the following manner: τ_sf=τ^∞_s + (τ_s−τ^∞_s) exp(−AU) The model is compared with the experimental data of Roberts and is of similar mathematical form.     

Aggregate of nanoparticles: rheological and mechanical properties

The understanding of the rheological and mechanical properties of nanoparticle aggregates is important for the application of nanofillers in nanocompoistes. In this work, we report a rheological study on the rheological and mechanical properties of nano-silica agglomerates in the form of gel network mainly constructed by hydrogen bonds. The elastic model for rubber is modified to analyze the elastic behavior of the agglomerates. By this modified elastic model, the size of the network mesh can be estimated by the elastic modulus of the network which can be easily obtained by rheology. The stress to destroy the aggregates, i.e., the yield stress (σ _y ), and the elastic modulus (G') of the network are found to be depended on the concentration of nano-silica (ϕ, wt.%) with the power of 4.02 and 3.83, respectively. Via this concentration dependent behavior, we can extrapolate two important mechanical parameters for the agglomerates in a dense packing state (ϕ = 1): the shear modulus and the yield stress. Under large deformation (continuous shear flow), the network structure of the aggregates will experience destruction and reconstruction, which gives rise to fluctuations in the viscosity and a shear-thinning behavior.     

Designing crosslinked hyaluronic acid hydrogels with tunable mechanical properties for biomedical applications

This study develops a simple copolymerization/crosslinking technique to control the swelling and mechanical properties of hyaluronic acid‐based hydrogels. Because of the widespread acceptance of poly(ethylene glycol) in biomedical applications, functionalized oligomers of ethylene glycol (EG) were used as comonomers to crosslink methacrylated hyaluronic acid (MHA). The swelling degree, shear and elastic moduli, and fracture properties (stress and strain) of the gels were investigated as a function of the crosslinking oligomer length and reactive group(s). It was hypothesized that acrylated oligomers would increase the crosslink density of the gels through formation of kinetic chains by reducing the steric hindrances that otherwise may limit efficient crosslinking of hyaluronic acid into gels. Specifically, after crosslinking 13 wt % MHA (47% degree of methacrylation) with 0.06 mol % of (EG)_n‐diacrylate, the swelling ratio of the MHA gel decreased from 27 to 15 g/g and the shear modulus increased from 140 to 270 kPa as n increased from 1 to 13 units. The length and functionality (i.e., acrylate vs. methacrylate) of the oligomer controlled the crosslink density of the gels. The significant changes in the gel properties obtained with the addition of low levels of the PEG comonomer show that this method allows precise tuning of the physical properties of hyaluronic acid (HA) gels to achieve desired target values for biomedical applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42009.     

Correlations of the first normal stress difference with shear stress and of the storage modulus with loss modulus for homopolymers

The effects of temperature, molecular weight and its distribution, side chain branching, and the structure of polymers on the elastic behavior of bulk homopolymers were investigated, by using logarithmic plots of first normal stress difference (N₁) against shear stress (σ₁₂) and logarithmic plots of storage modulus (G′) against loss modulus (G″). For the investigation, we have used data from the literature as well as our recent experimental results, covering a very wide range of temperature and shear stress or loss modulus. It has been found that such plots are very weakly sensitive to (or virtually independent of) temperature and to the molecular weight of high molecular weight polymers, but strongly dependent upon the molecular weight distribution and the degree of side chain branching. A theoretical interpretation of the observed correlations is presented, using molecular theories.     

Effects of sewage sludge on rheological characteristics of coal-water slurry

The coal-sludge slurry (CSS) containing coal, sewage sludge and water was prepared to study the effects of sewage sludge on rheological characteristics of the CSS. The yield stress, thixotropy and rheological type of CSS were investigated and compared with those of coal-water slurry (CWS). The results showed that the yield stress of CSS appears at the shear rate range from 0.05 to 0.14 s⁻¹. For CSS with the naphthalene sulfonate sodium formaldehyde condensate as dispersant and the sludge/coal mass ratio of 10:100, the yield stress can reach to 22.9 Pa. The thixotropy was quantitatively described by the thixotropy loop area, and sewage sludge can obviously improve the CWS thixotropy. The non-Newtonian behavior of CSS was characterized by a progressive decrease in viscosity with increasing shear rate at the shear rate range from 5 to 180 s⁻¹. By the analysis of FTIR, SEM and optical microscope, hydrophilic functional groups and colloidal structure of sewage sludge play the key roles on the different rheological characteristics of CSS and CWS.     

Chain structure,phase morphology,and toughness relationships in polymers and blends

We present chain structure, phase morphology, and toughness relationships in thermoplastic polymers and polymer/rubber blends. In neat polymers, molecular aspects of craze/yield behavior are controlled by two chain parameters: entanglement density ν_e and characteristic ratio C_∞. The crazing stress is proportional to ν, and the yield stress is proportional to C_∞. The dispersed rubber toughens a polymer/rubber blend mainly by promoting energy dissipation of the matrix. The toughening efficiency correlates with the rubber phase morphology and the chain structure of the matrix.     

Extrusion die swell of carbon black-filled natural rubber

Extrusion die swell of natural rubber compounded with a wide variety of carbon blacks has been determined in a capillary rheometer using a long circular die. The range of variation of carbon black loading, surface area, and structure are, respectively, 10 to 60 phr, 44 to 124 m²/g, and 78 to 120 cc/100 g. The effective carbon black volume fraction φ_e not participating in the strain recovery leading to die swell is assumed to be the sum of the actual filler volume fraction and the fraction of unextractable rubber determined experimentally for each compound. Bagley and Duffey's analysis of extrusion die swell of unfilled polymers as unconstrained elastic recovery was adopted for a filled elastomeric system whose relative shear modulus (G/G₀) is assumed to vary as (1 ? φ_e)^?N. The matrix shear modulus G₀, originally introduced by Nakazima and Shida on the basis of a linearized approximation, will depend on the shear stress level because of nonlinear deformation. The power N will vary with shear stress which changes the orientation of carbon black aggregates. Except for these features, die swell data for a wide range of carbon black compounds fall on a single curve when plotted in the manner of the predicted relation between the wall shear stress, die swell, and φ_e. Replacing φ_e by Medalia's φ′ based on an equivalent sphere concept introduces a larger scatter around the mean curve.     

Dynamic swelling behavior of pH‐sensitive anionic hydrogels used for protein delivery

There have been many attempts to use anionic hydrogels as oral protein delivery carriers because of their pH‐responsive swelling behavior. The dynamic swelling behavior of poly(methacrylic acid‐co‐methacryloxyethyl glucoside) and poly(methacrylic acid‐g‐ethylene glycol) hydrogels was investigated to determine the mechanism of water transport through these anionic hydrogels. The exponential relation M_t/M_∞ = ktⁿ (where M_t is the mass of water absorbed at time t and M_∞ is the mass of water absorbed at equilibrium) was used to calculate the exponent (n) describing the Fickian or non‐Fickian behavior of swelling polymer networks. The mechanism of water transport through these gels was significantly affected by the pH of the swelling medium. The mechanism of water transport became more relaxation‐controlled in a swelling medium of pH 7.0, which was higher than pK_a of the gels. The experimental results of the time‐dependent swelling behaviors of the gels were analyzed with several mathematical models. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 89: 1606–1613, 2003     

Shear modulus in closely packed gel suspensions

Rheological properties and swelling were examined in a series of concentrations of particles of crosslinked polyacrylate gels in water or salt solutions. The modulus during steady shear G_s = 2τ²/P₁₁-P₂₂ was determined from shear stress τ and primary normal force difference P₁₁-P₂₂ in a cone-and-plate rheometer. G_s was nearly constant with shear rate for the gel particles in the closely packed condition. The dynamic storage modulus G′ determined by ecentric rotating disc rheometry increased with increasing frequency for all concentrations. The apparent equilibrium shear modulus G_e determined by stress relaxation agreed closely at all concentrations and ionic strengths with the corresponding values of G_s, and hence G_s is considered a good estimate of equilibrium shear modulus for this gel material.     

Rheological Characterization of Metalized and Non‐Metalized Ethanol Gel Propellants

The gellation of metalized and non‐metalized ethanol with a methylcellulose gelling agent and its effect on the rheological properties (flow and dynamic study) of these gels is reported herein. The rheological study shows that increasing the shear rate reduces the apparent viscosity for a given yield stress (for a shear rate range of 1 to 12 s⁻¹) for both shear rate ranges (1 to 12 and 1 to 1000 s⁻¹) covered in present experiment. The gellant and metal particle concentrations significantly influence the gel apparent viscosity. Distinct changes in thixotropic behavior were observed, while decreasing the concentration of MC gellant and Al metal particles in the ethanol gels. The dynamic study showed that all of the linear viscoelastic regions (LVE) of the gel samples were independent of strain percentage (1 to 10). The G′ values depended on the frequency and exceeded the G′′ values, which indicated a gel‐like highly structured material. The tanδ values showed that all of the ethanol gels were elastic and weak physical gels with a high degree of cross‐linking.     

Effect of spatial gel inhomogeneity on the elastic modulus of strong polyelectrolyte hydrogels

Summary The effect of spatial inhomogeneity on the elastic modulus of ionic poly(acrylamide) (PAAm) hydrogels has been investigated with the static light scattering measurements. The gels were prepared by free-radical crosslinking copolymerization of acrylamide and 2-acrylamido-2-methylpropane sulfonic acid sodium salt (AMPS) monomers and N,N’-methylenebis(acrylamide) crosslinker. The crosslinker concentration of the hydrogels was fixed in our experiments while the mole fraction of AMPS in the comonomer feed (x_i) was varied over a wide range. Elasticity measurements show that the modulus of elasticity of gels increases with increasing x_i from 0 to 0.2. The excess scattering of gels, that is, the degree of spatial gel inhomogeneity rapidly decreases with increasing x_i and approaches to zero at x_i=0.05. Debye-Bueche analysis of the light scattering data indicates frozen concentration fluctuations in gel; the size of the static structures increases while the extent of concentration fluctuations reduces with increasing x_i. It was shown that the macroscopic elastic properties of PAAm gels are mainly controlled by the microscopic gel structure determined by the scattering measurements.     

On the Relaxation Rate in the Primary Transition Region of Amorphous Polymers

Abstract

It is known that the stress relaxation behaviour of solids is largely independent of their structure. This similarity is expressed as F ≈ 0.1σ₀? where F is the maximum slope of the relaxation curve in a stress In(time)-diagram and σ₀? the initial effective stress of the experiment. This relation is found to be valid in temperature regions where no transitions occur; it is not expected to hold in the primary transition region of amorphous polymers. With reference to the theory of a damped Debye lattice, it is shown that the maximum value of F/σ₀? is ca. 0.28 in this region. It is further pointed out that Fσ₀? varies with temperature in a way reminiscent of that of the mechanical loss factor. The predictions are in agreement with experimental facts.     

Viscoelasticity of concentrated polyacrylonitrile solutions: effects of solution composition and temperature

The effects of solution composition and temperature on the viscoelasticity of concentrated polyacrylonitrile (PAN) solutions were studied using a variety of rheological measurements, such as steady‐state shearing, dynamic stress sweep and transient rheological tests. The first normal stress difference N₁ and the shear stress τ were found to increase with decreasing temperature and increasing PAN concentration and water content in the solutions. The crossover point of N₁ and τ, denoting the equal contribution of viscosity and elasticity to the viscoelasticity of the solutions, moved to lower shear rates at lower temperature, higher PAN concentration and higher water content. The values of the relaxation time (λ) were larger at 70 °C than at 40 °C. In addition, the changes of λ with PAN concentration and water content were different at the two temperatures, ascribed to the different states of the solutions. The PAN solutions were in the linear viscoelastic regime in the temperature range 40–70 °C when the shear stress was below 300 Pa. The creep compliance recovery rate decreased with increasing temperature, but increased with increasing PAN concentration and water content. Thixotropic tests showed that the thixotropy of the solutions was also affected by the solution composition and temperature. Gelation was found to influence the way the solution composition and temperature affected the viscoelastic properties of the PAN solutions. Copyright © 2011 Society of Chemical Industry     

Effect of vulcanization on the low-strain dynamic properties of filled rubbers

A change in the degree of vulcanization in a carbon black-filled rubber is shown not to affect the change of in-phase shear modulus with strain amplitude, provided the amount of filler is kept constant. Similar results are found with the change of out-of-phase modulus and phase angle with strain amplitude. A similar value of change of in-phase shear modulus with strain amplitude (G′₀ – G′_∞) is found for a vulcanized and unvulcanized tire tread rubber; this behavior is attributed for the breakdown of the secondary three-dimensional carbon black aggregated network. Detailed consideration is given to other factors such as hydrodynamic, shape factor, and relaxation effects which contribute to the difference between the shear modulus of the gum rubber and the filled rubber when all the carbon black structure has been broken down. The consequence of the breakdown of the secondary aggregated structure of carbon black are considered in terms of factors affecting heat build-up in tires.     

Gelation mechanism and rheological properties of polyacrylamide crosslinking with polyethyleneimine and its plugging performance in air‐foam displacement

Gels based on polyacrylamide crosslinking with polyethyleneimine have attracted attention because of their resulting high strength and good thermal stability. This study investigated the gelation mechanism of the polymeric gel and its plugging performance in air‐foam flooding. An optic microrheology analyzer was used to monitor the gelation process. The crosslinking reaction occurred in two steps, as determined from the elasticity factor curves, and the polymeric gels adopted a semisolid state from solution, as determined from the solid liquid balance curves. The elastic modulus values were higher than the viscous modulus values, indicating that mature gels were elastic‐based materials. The yield stress increased gradually with increasing polymer dosage, which was consistent with the breakthrough pressure and the trend of displacement pressure. The mature gels showed significant thixotropy. In the core displacement test, the preferred injection volume of the gel was 0.1 pore volume, and the stable pressure of the foam flooding was increased by about three times after the core was plugged. The blocking effect for cores with small original permeability was better than that with large permeability. The best blocking resulted from simultaneous treatment of both ends of the cores, followed by front‐end treatment and rear‐end treatment. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45778.