Viscoelastic Exponent Research Articles

'Reverse' Hofmeister effects on the sol-gel transition rates for an α-helical peptide-PEG bioconjugate.

We examine the dynamics of the sol-gel transition for end-functionalized linear- and 4-arm-peptides bioconjugated to poly-ethylene glycol (PEG) in aqueous environments with increasingly chaotropic (Cl- < Br- < I-) anions. A 23-amino acid peptide sequence is rationally designed to self-assemble upon folding into the ordered α-helical conformation due to the hydrophobic effect. We use Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy (ATR-FTIR) to quantify the ensemble average reversible secondary structure transitions as a function of electrolyte concentration and specific ion effects along the Hofmeister series. Subsequently, microrheology is used to quantify the kinetics of the gelation process, as it relates to folding and specific ion interactions. Our key findings were non-intuitive. We observe the faster evolution of the gel transitions in systems with more chaotropic anions. For our peptides in aqueous solution, "water-structuring" ions yield faster assembly behavior with a viscoelastic exponent, n, closer to unity representing self-assemblies that are Rouse-like. In contrast, ions that are "water-breaking" resulted in smaller viscoelastic exponents where self-assembly dynamics result in a viscoelastic exponent that suggests polymer entanglements.

Gelation behavior of polysaccharide-based interpenetrating polymer network (IPN) hydrogels

We report the preparation and rheological characterization of interpenetrating polymer network (IPN) hydrogels made from alginate and hydrophobically modified ethyl hydroxyl ethyl cellulose (HMEHEC). To our knowledge, there have been no studies of the gelation behavior of IPNs. We found that the rheology of these systems can be easily tuned, with the elastic modulus of the IPN strongly dependent on the relative ratio of HMEHEC to alginate. The sol–gel transition of these systems was found to satisfy the Winter–Chambon criterion for gelation at various crosslinker densities. From the power law relationship of the dynamic moduli (G ′ ~G ″ ~ω n), the exponent n appears to be dependent on both the crosslinker density and relative amount of two polymers. The value of n was found to be ~0.5 for all samples for stoichiometric amounts of crosslinker. The effect of molecular weight of HMEHEC on the gel point and viscoelastic exponent has also been reported. Alginate seems to dominate the kinetics of the process but the effect of high molecular weight HMEHEC on the gel point, especially at lower proportion was also evident.

Characteristic rheological features of high concentration PVA solutions in water with different degrees of polymerization

AbstractThis work aims to study the impact of the degree of polymerization (Pn) on the rheological behavior of PVA solutions (15 wt %) under steady and oscillatory shear conditions. The values of Pn range from 1300 to 2400. The results show that PVA‐13 solution exhibits almost Newtonian flow behavior, whereas high Pn PVA solutions exhibit shear thinning behavior, for which the complex viscosity increases with Pn. However, the shear viscosity of PVA‐24 is lower than that of PVA‐20 at low shear rate (&lt;1 s−1). In addition, the storage modulus of PVA solutions increases with increasing degree of polymerization from 1300 to 2000. There is a sharp increase at degrees of polymerization higher than 1300. However, the storage modulus of PVA‐24 is lower than that of PVA‐17 and PVA‐20 at low shear rate (&lt;1 rad/s). The viscoelastic exponent of the PVA solutions shows a minimum value at degree of polymerization 2000. Thixotropy is detected by a hysteresis loop of the flow curves. The creep response of PVA solutions has also been investigated, with PVA‐24 showing negligible compliance recovery. These facts indicate that increasing Pn produces a corresponding effect in varying the molecular structure and types of hydrogen bonding, including intra‐ and inter‐chain, and polymer–water hydrogen bonding. Rheological parameters (G′, n, tanδ, J, and so on) can be used to evaluate the change in the molecular structure and type of hydrogen bonding. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010

Synthesis and characterization of diblock copolymers of styrene and N-tert-butylacrylamide and gelation evidenced by rheology

The self-assembly of block copolymers in melt and in solution states is of current interest for the design of hierarchical materials. In this contribution, we report the synthesis of amphiphilic block copolymers of styrene and N-tert-butylacrylamide (SmNtBAMn) with narrow molecular weight distributions by reversible addition–fragmentation chain transfer (RAFT) technique. Block copolymers were characterized by gel permeation chromatography (for distribution width), 1H NMR spectroscopy (for molar mass and composition), thermal analysis and transmission electron microscopy (morphology investigation). These diblock copolymers in 1-octanol, a selective solvent for the N-tert-butylacrylamide block, self assemble to form micelles. At higher concentrations, these diblock copolymers in 1-octanol form glass-clear gels. Dynamic viscoelastic properties of a diblock copolymer, S278NtBAM517, in 1-octanol have been studied as a function of copolymer concentration. Scaling relations for the micelles and gels have been established in the terminal frequency range. Critical gelation concentration (cg) was determined by frequency independence of loss tangent in the vicinity of gel point. The cg, viscoelastic exponent, n and front factor, S were determined to be 11.3 wt%, 0.677 and 0.55 Pa-s0.677, respectively.

The chaotic response of the viscoelastic traveling string: an integral constitutive law

This paper investigates chaotic behaviors of an axially traveling viscoelastic string with geometric nonlinearity. The stress and the strain of the viscoelastic string obey the Boltzmann superposition principle. The Galerkin method is applied to truncate a nonlinear partial-differential–integral equation governing transverse motion into a set of ordinary differential–integral equations. For the string modeled as a standard linear solid, new auxiliary variables are introduced to transform those equations into ordinary differential equations. By use of the Poincare maps, the chaotic behaviors are presented based on the numerical solutions of the ordinary differential equations. The bifurcation diagrams are presented for varying one of the following parameter: the axial traveling speed, the amplitude of tension fluctuation, the viscoelastic exponent and coefficient of the string, while other parameters are fixed.

Textural properties of low-fat cookies containing carbohydrate- or protein-based fat replacers

Carbohydrate- or protein-based fat mimetics were used to replace up to 50% of fat in cookies. The effect of the type of fat mimetic and of the percentage of fat replacement on textural behaviour of the products was studied by compression tests. The stress–strain curves obtained were fitted by an exponential equation containing as parameters the maximum stress ( σ max), the maximum strain ( ε max) and a viscoelastic exponent ( f). A simple mathematical model for σ max and the ratio σ max/ ε max, indicative of hardness and brittleness of the cookies, respectively, was developed. Both these values depended on the percentage of fat replacement and the type of fat replacer used. Hardness and brittleness of the cookies generally increased with fat replacement, but a moderate increase was obtained by some of the fat mimetics, resulting in products with better textural characteristics than their low-fat, no mimetic-added counterparts. The viscoelastic exponent increased also with fat replacement and the model parameters depended on the type of the fat mimetic.

Effect of pre-treatment on viscoelastic behaviour of potato strips

The viscoelastic behaviour of french fries during deep fat frying after two different types of drying pre-treatment (air drying and osmotic dehydration) was examined under uniaxial compression tests. The stress-strain data of the compression test were modelled using a simple mathematical model, containing parameters such as the maximum stress ( σ max), the maximum strain ( ε max), the elasticity parameter ( E) and the viscoelastic exponent ( p). The effect of air drying duration and the type of osmotic solution used on the compression behaviour of fried potatoes was introduced through the corresponding effects upon the model parameters. Pre-fry drying as well as osmotic pre-treatment increase the maximum stress and maximum strain of french fries during frying. Air drying pre-treatment increases the crispness of potato strips while osmotic pre-treatment does not affect it, with the exception of sugar solutions.

Viscoelastic behaviour of potato strips during deep fat frying

The viscoelastic behaviour of french fries during deep fat frying was examined under uniaxial compression tests, as a function of the main process variables (oil temperatures, oil type) for various frying times. The stress–strain data of the compression test were modelled using a simple mathematical model, containing parameters such as the maximum stress ( σ max), the maximum strain ( ε max), the elasticity parameter ( E) and the viscoelastic exponent ( p). The effect of oil content, oil temperature and oil type on the compression behaviour of fried potatoes was introduced through the corresponding effects upon the model parameters. The results showed that maximum stress and maximum strain increase during frying. Crispness of potato strips is higher for hydrogenated oil, while the use of refined oil decreases potato crispness.

Rheological properties of syndiotacticity-rich ultrahigh molecular weight poly(vinyl alcohol) dilute solution

Viscoelastic behavior of the dilute solution of ultrahigh molecular weight syndiotactic poly(vinyl alcohol) (UHMW s-PVA)/dimethyl sulfoxide (DMSO)/water was investigated through rheological response. Below a critical temperature, the dynamic storage modulus (G′) of the solution became greater than the dynamic loss modulus (G″) and the viscoelastic exponent for G′ became smaller than that for G″ before macroscopic gelation, which indicates the evolution of viscoelastic solid properties at the sol state. Also, the loss tangent (tan δ) of the solution below the critical temperature increased with increasing frequency. Consequently, the dilute solution of UHMW s-PVA/DMSO/water showed the rheological behaviors as can be observed in general chemical or physical gel systems below the critical temperature. These results suggest that solid-like heterogeneity prevailed in the solution before macroscopic gelation with decreasing temperature. Such heterogeneity was considered as phase-separated domains attributed to spinodal decomposition. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 569–576, 2001

The effect of drying methods on viscoelastic behaviour of dehydrated fruits and vegetables

The effect of drying methods on the rheological behaviour of dehydrated apple, banana, carrot and potato was examined by uniaxial compression tests on cylindrical specimens. Samples were dehydrated using four different drying methods: osmotic, conventional, vacuum and freeze drying. After drying samples at varying final moisture, the contents were tested for compressibility using pressures ranging from 0.02 to 8 kg kg - db. A simple mathematical model was used to describe the viscoelastic behaviour of the examined materials. Four parameters with physical meaning were incorporated in the model: the maximum stress (σ max ), the maximum strain (∈ max ), the elastic parameter (E) and the viscoelastic exponent (p). It was shown that all the parameters were dependent on both the drying method and the material. Osmotic dehydration seems to increase the viscous nature of fruits and decrease their elasticity, while freeze-dried materials seem to have the most elastic structure.

The effect of drying methods on viscoelastic behaviour of dehydrated fruits and vegetables

SummaryThe effect of drying methods on the theological behaviour of dehydrated apple, banana, carrot and potato was examined by uniaxial compression tests on cylindrical specimens. Samples were dehydrated using four different drying methods: osmotic, conventional, vacuum and freeze drying. After drying samples at varying final moisture, the contents were tested for compressibility using pressures ranging from 0.02 to 8 kg kg‐1 db. A simple mathematical model was used to describe the viscoelastic behaviour of the examined materials. Four parameters with physical meaning were incorporated in the model: the maximum stress (γmax), the maximum strain (εmax), the elastic parameter (E) and the viscoelastic exponent (p). It was shown that all the parameters were dependent on both the drying method and the material. Osmotic dehydration seems to increase the viscous nature of fruits and decrease their elasticity, while freeze‐dried materials seem to have the most elastic structure.

EFFECT OF OSMOTIC DEHYDRATION ON VISCOELASTIC PROPERTIES OF APPLE AND BANANA

ABSTRACT The rheological behavior of osmotically dehydrated apple and banana was examined under uniaxial compression and relaxation tests of cylindrical specimens. Compression and relaxation tests were performed, following air drying of osmotically pre-treated samples, at various moisture contents ranging from 0.1 to 1.5 kg/kg dry basis. The maximum stress and the corresponding strain were correlated to the moisture content using simple mathematical equations. It was shown that both parameters increase as water was removed and their values are significantly higher than the corresponding values for untreated air dried samples. The effect of moisture content on compressive behavior of osmotically dried materials was introduced through its effect on the four model parameters: the maximum stress (σmax) the maximum strain (ϵmax), the elastic parameter (E) and the viscoelastic exponent (p). The stress relaxation data of osmotically treated samples were modeled using a two-term Maxwell model. It was shown that osmotic pre-treatment increased the remaining force and it decreased the relaxation time of dehydrated samples. The osmotic dehydration and therefore the sugar gain tend to increase the viscous nature of fruits and decrease their elasticity for both materials, causing plasticity of the structure.

COMPRESSION ANALYSIS OF DEHYDRATED AGRICULTURAL PRODUCTS

ABSTRACT The textural properties of apple, banana, carrot and potato were experimentally determined by uniaxial compressive tests of cylindrical specimens at a constant deformation rate of 5mm/min. Compression tests were performed, following air drying, at various moisture contents ranging from 0.2 to 6 kg/kg db. The tests were performed using a universal texture testing machine and simple mathematical equations were used to correlate the maximum experimental stress and the corresponding strain to the moisture content. It was shown that the maximum stress decreases as the moisture content decreases, until a critical moisture content of 1.8 kg water/kg dry solids. Further removal of water tends to increase the maximum stress. The maximum experimental strain was found to increase as water was removed. The stress-strain data of compression test were modelled using a simple mathematical model, containing parameters such as the maximum stress (ow), the maximum strain (dim), the elastic parameter (E) and the viscoelastic exponent (p). The effect of the moisture content on the compressive behavior of dried materials was introduced through its effect on the model parameters. The shift in compression behavior at 1.8 kg water/kg solids leads to the conclusion that there is an important change of structure at this moisture content.

Viscoelastic behaviour of dehydrated products during rehydration

Viscoelastic behaviour of dehydrated products during rehydration of apple, banana, carrot and potato was examined under uniaxial compression tests. Samples were dehydrated with four different drying methods: conventional, vacuum, freeze and osmotic-freeze drying and after that they were rehydrated in an air dryer at 50°C and 80% air humidity. Compression tests were performed during rehydration for various moisture contents, ranging from 0.01 to 1.5 kg/kg dry basis. The viscoelastic behaviour of dehydrated products during rehydration were examined comparing the values of the four parameters incorporated into the stress–strain model, during rehydration with those of dehydrated products. The four examined parameters: maximum stress, maximum strain, elastic parameter and viscoelastic exponent seem to show a hysteresis phenomenon. It can be concluded that dehydrated product do not keep their viscoelastic behaviour after rehydration due to structural damages that occur during drying. More specifically, freeze dried materials present the highest hysteresis after rehydration, losing their elasticity and becoming more viscous. Osmotic pretreatment seems to help freeze dried materials to keep their elastic nature, probably due to solids gain. Air and vacuum dried materials showed the smallest hysteresis tendency, keeping their viscoelastic characteristics during rehydration close to those of dried materials.

Viscoelastic scaling in polymer gels

New scaling laws for chain networks are derived to describe the fundamental relationships between the viscosity exponent (k), viscoelastic exponent (m), stretched exponent (β), spatial dimension (d). fractal dimension (df), and a universal constant (γ). The scaling of the total number of monomers and the radius of gyration is defined by df. We have discovered γ = m/β to be a universal constant which relates the shear modulus of a polymer gel melt to the shear modulus near the glass transition. Analyzing the size-dependent shear viscosity, we have determined γ = 3dfcd/(7d−5dfc) = 2.647 for d = 3 where dfc is the fractal dimension of critical clusters at the gel point. By using γ, the present theory extends previous work pertaining to systems near the sol-gel transition, and shows how properties far from the critical point can be explained. The theoretical prediction is in good agreement with viscoelastic measurements.

Dynamic Viscoelasticity of Gelling and Nongelling Aqueous Mixtures of Ethyl(hydroxyethyl)cellulose and an Ionic Surfactant

Oscillatory shear experiments have been carried out over an extended temperature range (10−45 °C) on thermoreversible gelling and nongelling aqueous systems of ethyl(hydroxyethyl)cellulose (EHEC) of different polymer concentrations in the presence of various amounts of sodium dodecyl sulfate (SDS). At moderate ratios (r) of SDS/EHEC, the semidilute systems form gels at elevated temperatures, whereas no temperature-induced gels are formed at high values of r in this temperature region. The gel point temperature depends on the polymer/surfactant composition, similar to the cloud point. It is shown that phase separation and gelation are phenomena that are closely related. At the gel temperature, a power law frequency dependence of the dynamic moduli (G‘ ∼ G‘‘ ∼ ωn) was constantly observed. Depending on the composition of the system, the viscoelastic exponent assumes values in the approximate range 0.1−0.7. This finding indicates that the structure of the incipient gel is strongly influenced by the concentrat...

Phase separation of aqueous solution of methylcellulose

As we determined visually by the temperature cloud point method, the coexistence phase curve of methylcellulose in aqueous solution belongs to the LCST (low critical solution temperature) type. Rheological dynamic measurements reveal the existence of three gel domains. The gel (I) localized in the homogeneous phase at low concentration and low temperature, is a very weak gel, where the cross-links are attributed to pairwise hydrophobic interactions between the most hydrophobic zones of the backbone: the trimethyl blocks. The second gel (II) was revealed in the high concentration regime and below the LCST, it may be attributed to the formation of crystallites which play the role of cross-linking points. The third gel was concomitant to the micro-phase separation. In these turbid gels, syneresis develops slowly with time: the higher the temperature and the lower the concentration, the faster the syneresis. Near the three sol-gel transitions, a power law frequency dependence of the loss and storage moduli was observed and the viscoelastic exponent Δ(G′ G″ ω Δ) was found to be 0.76 and 0.8 and to reach 1 at high concentration.

Effects of Temperature, Surfactant, and Salt on the Rheological Behavior in Semidilute Aqueous Systems of a Nonionic Cellulose Ether

Oscillatory shear experiments have been carried out on thermoreversible gelling and nongelling semidilute aqueous systems of ethyl(hydroxyethyl)cellulose (EHEC) (at a constant polymer concentration of 1 wt %) in the presence of various amounts of sodium dodecyl sulfate (SDS) and at some different levels of NaCl addition. Depending on the concentrations of surfactant and salt, a temperature-induced sol-gel transition or only a viscosification of the solution was observed. For the gelling systems, the value of the gel temperature, determined by the observation of a frequency independent loss tangent, was found to be dependent on the composition of the system. At the gel temperature, a power law frequency dependence of the dynamic storage modulus (G‘ ∼ ωn‘) and loss modulus (G‘‘ ∼ ωn‘‘) was constantly observed with n‘ = n‘‘ = n. Values of the viscoelastic exponent n in the range 0.3−0.4 were reported. The value of n, as well as the gel strength parameter S, was dependent on the composition of the system. The...

Viscoelastic and SAXS investigation of fractal structure near the gel point in alginate aqueous systems

The sol-gel transition and the fractal structure of the gel have been studied for aqueous systems of alginates with different mannuronate/guluronate (M/G) ratios. The power law for the dynamic moduli, G'(ω) ∼ G''(ω) ∼ ω n , can be applied near the gel point, and the viscoelastic exponent n depends on the concentration and the M/G ratio of the alginate. The fractal dimension d f can be obtained from the SAXS data. The value of D f also depends on the concentration and the M/G ratio. The d f in the G component-rich system is larger than that in the M component-rich system

Viscoelasticity of crosslinked gels.

The damping characteristics of a crosslinked polymer are determined by its viscoelastic properties. Yet the loss mechanisms of networks have not been fully understood. It is shown that insight into network behavior can be gained from studies of how viscoelasticity evolves during the curing process. In addition, the peculiar viscoelasticity of the incipient gel (the polymer exactly at the gel point) is shown to offer unique damping characteristics. Experimental studies on an epoxy system show that the evolution of structure prior to the gel point follows the predictions of percolation and that the divergence of the bulk viscosity and the power law behavior of the dynamic shear moduli, G’∼G‘∼ωΔ, at the gel point follow the predictions of a dynamic scaling theory. A new superposition principle (time-cure superposition) holds for the linear viscoelastic responses at different extents of reaction in the critical regime both before and after the gel point. From the master curve beyond the gel point, the divergence of the inverse equilibrium modulus and the terminal relaxation time are shown to obey the theoretical predictions as well. In addition, an intriguing relationship exists between the relaxation time spectra of a network at the gel point and in the fully cured state. Measurements of the dynamic shear moduli of several epoxy and siloxane networks, with equilibrium moduli varying by four orders of magnitude, show G‘∼ωΩ at all extents of reaction past the gel point, where the network viscoelastic exponent Ω remains at the incipient gel value Δ. This surprising correspondence suggests that chemical networks possess some remnant of the fractal structure of the incipient gel. [Work supported by the U.S. DOE under Contract No. DE-AC-04-76DP00789.]