Constrained Swelling Research Articles

Hygro-mechanical long-term behaviour of spruce, pine and lime wood: parameter identification and model validation

Lime wood, spruce and pine are investigated with regard to its hygro-mechanical long-term behaviour. Experiments are conducted for an identification of model parameters and for model validation. Swelling and shrinkage coefficients, dry density, sorption characteristics and parameters for visco-elasticity, visco-plasticity and mechano-sorption are determined for the main material directions. Supplemented by literature values, a complete set of parameters for long-term hygro-mechanical modelling of wood species is found. Constrained swelling and shrinkage are analysed and the origin of the stress development is investigated. It is demonstrated, that creep phenomena lead to significant stress reduction by relaxation, in case of moisture changes especially due to mechano-sorption. The influence of different model parts is investigated. A numerical parameter study shows the influence of several material parameters on the stress evolution. Experimental material investigations such as those presented here are essential for the application of numerical simulation methods for the prediction of material behaviour and for the assessment of deformations, stresses and damage potential of climatically loaded timber structures.

Physicochemical properties of amorphous granular starch (AGS) prepared by non-thermal gelatinization by high hydrostatic pressure (HHP) and spray drying

Corn starch was gelatinized by high hydrostatic pressure (HHP) and spray drying to make amorphous granular starch (AGS), and their physicochemical properties were compared with the conventionally prepared (heat-gelatinized and spray dried) AGS to devise a novel AGS preparation methodology. Pressure-induced (PAGS) and heat-induced AGS (HAGS) maintained their granular shape but lost their birefringence indicating that both methods could prepare AGS. DSC (differential scanning calorimeter) and XRD (X-ray diffraction) analysis confirmed the complete loss of amylopectin double helices and crystallinity of both PAGS and HAGS. However, their swelling power, solubility, RVA pasting properties, acid/shear stability, gel forming ability and textural properties were completely different. PAGS exhibited constrained swelling, suppressed amylose leaching, and reduced viscosity. Notably, HAGS formed a gel without heating, whereas PAGS yielded a viscous paste with water-soluble attributes. Even after reheating, PAGS maintained its granular structure with comparably less swelling and weaker gel strength than HAGS. Consequently, newly developed PAGS exhibited distinctive characteristics compared to the conventional HAGS, such as lower solubility and swelling power, viscosity, textural properties, and high acid and shear stabilities, rendering it a viable option for various applications within the food industry.

Finite Deformation of Swollen pH-Sensitive Hydrogel Cylinder Under Extension and Torsion and its Poynting Effect: Analytical Solution and Numerical Verification

According to the rise of technological interest in hydrogels, there is a growing necessity for precise mechanical and thermodynamic experimental characterizations. One of the common experimental protocols is applying combined extension-torsion loading on a cylinder. Therefore, we have proposed an analytical solution for the problem of torsion superimposed extension coupling with pH variation on a cylinder composed of the pH-sensitive hydrogel. Additionally, the Poynting effect as an inherently nonlinear phenomenon is studied for a hydrogel cylinder under extension-torsion loading. The finite deformation of a swollen cylinder is investigated using a constitutive model that captures network stretching, mixing of polymeric network and solvent, mixing of mobile ions with the solvent, and acidic groups’ dissociation. To evaluate the analytical solution accuracy, 3D finite element analysis (FEA) has been carried out for the same problem. In this regard, it is required to develop a user-defined subroutine, UHYPER, for pH-sensitive hydrogels; the developed subroutine is validated via free-swelling and constrained swelling problems. Compared to the FEA, the presented analytical solution is more interesting regarding its low computational cost. It is perceived that the computational cost of the analytic solution is much lower than the one required for FE analysis (around 1%). Thus, the proposed solution can be utilized as aneffective toolto examine each factor on the swelling behavior of hydrogels, including mechanical properties as well as loading parameters. Moreover, the developed analytical solution can be employed for calibrating material parameters. Finally, Poynting effects are examined for various axial stretches and twists, pH values, and cross-linked densities.

An analytic model for effective mechanical properties and local contact stresses in lithium-ion porous electrodes

Explicit expressions for stress–strain relationships and swelling strains are derived for porous lithium-ion electrodes. Local contact stresses between neighboring electrode particles are also predicted. The analytical model is based on similar microstructural averaging techniques that previously have been applied to simulations of powder compaction. Both direct particle–particle and particle–binder–particlecontacts are considered. The model gives explicit dependencies of constituent material properties and parameters describing the microstructure geometry. Examples are presented for electrodes that have particle–particle contacts below the percolation limit. The prediction of the E-modulus shows good agreement with experimental results. Constrained swelling resulting from intercalation of lithium-ions has also been simulated. The resulting electrode stress state compares well with numerical predictions by the finite element and the discrete element method. Local particle–particle contact stresses of the order 1–6 GPa have as well been predicted. A simplified model using a model for rigid-plastic deformation of particles have shown that the stress–strain behavior during the first charging cycle may substantially differ from subsequent cycles, a phenomenon that also has been experimentally observed.

Modeling of the Photo-Thermal-pH Triple-Responsive Hydrogels Considering the Coupled Effect of Photothermal Conversion and Electrochemistry

Abstract Photo-thermal-pH triple-responsive hydrogels are special kinds of smart materials which can respond to changes in ambient temperature, pH, and light intensity simultaneously. In this paper, a field theory is developed for the photo-thermal-pH triple-responsive hydrogels by incorporating effects of dissociation of acidic groups and photo-thermal conversion into the thermodynamic theory of temperature-sensitive hydrogels. Constitutive equations are derived by means of the free energy imbalance inequality. The feasibility of the presented model is tested and validated by comparing the simulation results for stress-free swelling with the available experimental data. Moreover, the sensitivities of the presented model to changes in temperature, pH, and light intensity are further studied through the biaxial constrained swelling case, and the factors affecting deformation is investigated numerically.

Transient swelling behaviour of dual stimuli sensitive nanocomposite hydrogels

Smart nanocomposite hydrogels can undergo large deformation under external stimuli. Several coupled physical processes simultaneously affect the transient swelling behaviour of pH and temperature-sensitive hydrogels. Besides, the hydrogel properties change in response to the external stimuli in the presence of nanoparticles, which affect its transient deformation and swelling behaviour. This work focused on transient swelling modeling of pH and temperature-sensitive polymer/clay nanocomposite hydrogels. The time-dependent processes were considered simultaneously with material properties updating in each time-step during the finite element procedure. A set of experiments was designed to evaluate the numerical solving results in the free and constrained swelling conditions. The results illustrate that the proposed developed model successfully predicts the transient swelling behaviour of pH and temperature-sensitive either for hydrogels or nanocomposite hydrogel counterparts, which can offer a guideline to the abundant usage of these soft materials.

Finite deformation swelling of a temperature-sensitive hydrogel cylinder under combined extension-torsion

The swelling behavior of a temperature-sensitive poly-N-isopropylacrylamide (PNIPAM) hydrogel circular cylinder is studied subjected to combined extension-torsion and varied temperature. In this regard, a semi-analytical solution is proposed for general combined loading. A finite element (FE) analysis is conducted, subjecting a hydrogel cylinder to the combined extension-torsion and the varied temperature to evaluate the validity and accuracy of the solution. A user-defined UHYPER subroutine is developed and verified under free and constrained swelling conditions. The FE results illustrate excellent agreement with the semi-analytical solution. Due to the complexity of the problem, some compositions and applied loading factors are analyzed. It is revealed that for larger cross-linked density and larger ending temperature, the cylinder yields higher stresses and smaller radial swelling deformation. Besides, the radial and hoop stresses increase by applying larger twist and axial stretch. The hoop stresses intersect at approximately R/Rout = 0.58, where the hoop stress vanishes. Besides, the axial force has direct and inverse relationships with the axial stretch and the twist, respectively. However, the resultant torsional moment behavior is complex, and the position of the maximum point varies significantly by altering the axial stretch and the twist.

Mechanical response and equilibrium swelling of temperature-responsive gels

A model is developed for the mechanical behavior of thermo-responsive gels that accounts for an increase in concentration of physical cross-links between chains driven by phase separation. Governing equations are derived for equilibrium water uptake under unconstrained and constrained swelling, as well as for the elastic response of swollen gels. These relations are applied to the analysis of swelling diagrams on NIPA, VCL, PEG, and NIPA-DMIAA gels. The ability of the model to predict the growth of elastic moduli above the volume phase transition temperature is confirmed by numerical simulation.

Swelling of glucose-responsive gels functionalized with boronic acid

A model is developed for the elastic response of a glucose-sensitive gel functionalized with boronic acid under swelling in aqueous solutions of glucose with various pH. A gel is treated as a three-phase medium composed of a solid phase (partially ionized polymer network), solvent (water), and solute (mobile glucose molecules and ions). Constitutive equations are derived by means of the free energy imbalance inequality for three-dimensional deformation with finite strains. Numerical analysis demonstrates the ability of the model to describe the effects of pH, molar fraction of glucose, and concentration of functional groups on equilibrium water uptake diagrams under unconstrained and constrained swelling.

Coupling behavior of the pH/temperature sensitive hydrogels for the inhomogeneous and homogeneous swelling

In this work, a model is developed to continuously predict homogeneous and inhomogeneous swelling behavior of pH/temperature sensitive PNIPAM hydrogels. Employing the model, homogeneous swelling of the pH/temperature sensitive hydrogel is investigated for free and biaxial constrained swelling cases. Comparing the model results with the experimental data available in the literature, the validity of the model is confirmed. The model is then employed to investigate inhomogeneous swelling of a spherical shell on a hard core both analytically and numerically for pH or temperature variations. In this regard, numerical tools are developed via preparing a user defined subroutine in ABAQUS software. Then, the complicated problem of contact between the hydrogel shell and a micro-channel with rigid walls is also investigated. Considering the results, we can say that the model is applicable for solving engineering boundary value problem of pH/temperature sensitive hydrogels.

Modeling the effects of pH and ionic strength on swelling of polyelectrolyte gels.

A model is developed for the elastic response of a polyelectrolyte gel under unconstrained and constrained swelling in a water bath with an arbitrary pH, where a monovalent salt is dissolved. A gel is treated as a three-phase medium consisting of an equivalent polymer network, solvent (water), and solute (mobile ions). Transport of solvent and solute is thought of as their diffusion through the network accelerated by an electric field formed by mobile and fixed ions and accompanied by chemical reactions (self-ionization of water molecules, dissociation of functional groups attached to polymer chains, and formation of ion pairs between bound charges and mobile counter-ions). Constitutive equations are derived by means of the free energy imbalance inequality for an arbitrary three-dimensional deformation with finite strains. Adjustable parameters in the governing relations are found by fitting equilibrium swelling diagrams on several hydrogels. The effects of pH, ionic strength of solution, and constraints on equilibrium water uptake are studied numerically.

A nonlinear, transient finite element method for coupled solvent diffusion and large deformation of hydrogels

Hydrogels are capable of coupled mass transport and large deformation in response to external stimuli. In this paper, a nonlinear, transient finite element formulation is presented for initial boundary value problems associated with swelling and deformation of hydrogels, based on a nonlinear continuum theory that is consistent with classical theory of linear poroelasticity. A mixed finite element method is implemented with implicit time integration. The incompressible or nearly incompressible behavior at the initial stage imposes a constraint to the finite element discretization in order to satisfy the Ladyzhenskaya–Babuska–Brezzi (LBB) condition for stability of the mixed method, similar to linear poroelasticity as well as incompressible elasticity and Stokes flow; failure to choose an appropriate discretization would result in locking and numerical oscillations in transient analysis. To demonstrate the numerical method, two problems of practical interests are considered: constrained swelling and flat-punch indentation of hydrogel layers. Constrained swelling may lead to instantaneous surface instability for a soft hydrogel in a good solvent, which can be regulated by assuming a stiff surface layer. Indentation relaxation of hydrogels is simulated beyond the linear regime under plane strain conditions, in comparison with two elastic limits for the instantaneous and equilibrium states. The effects of Poisson’s ratio and loading rate are discussed. It is concluded that the present finite element method is robust and can be extended to study other transient phenomena in hydrogels.

Inhomogeneous and homogeneous swelling behavior of temperature-sensitive poly-(N-isopropylacrylamide) hydrogels

The aim of this work is to develop a model to continuously predict inhomogeneous and homogeneous swelling behavior of temperature-sensitive poly-(N-isopropylacrylamide) hydrogels. Employing this model, some benchmark homogeneous problems such as free, unidirectional constrained and biaxial constrained swelling as well as swelling of core-shell structures are investigated. The main advantage of the model is its ability to solve inhomogeneous deformations due to a stable behavior in the vicinity of the phase transition temperature. Therefore, inhomogeneous swelling of a spherical shell on a hard core with application to microfluidics is analytically and numerically investigated for various thicknesses of the shell. Based on the solved examples, it is shown that the model possesses continuity and stability in the vicinity of the phase transition temperature.

Swelling of pH-sensitive hydrogels.

A model is derived for the elastic response of polyelectrolyte gels subjected to unconstrained and constrained swelling. A gel is treated as a three-phase medium consisting of a solid phase (polymer network), solvent (water), and solutes (mobile ions). Transport of solvent and solutes is modeled as their diffusion through the network accelerated by an electric field formed by ions and accompanied by chemical reactions (dissociation of functional groups attached to the chains). Constitutive equations (including the van't Hoff law for ionic pressure and the Henderson-Hasselbach equation for ionization of chains) are derived by means of the free energy imbalance inequality. Good agreement is demonstrated between equilibrium swelling diagrams on several pH-sensitive gels and results of simulation. It is revealed that swelling of polyelectrolyte gels is driven by electrostatic repulsion of bound charges, whereas the effect of ionic pressure is of secondary importance.

Volume phase transition in thermo-responsive hydrogels: constitutive modeling and structure–property relations

A model is developed for the elastic response of a thermo-responsive hydrogel subjected to swelling under an arbitrary deformation with finite strains. The constitutive equations involve the stress–strain relation, the nonlinear diffusion equation for water molecules, the heat conduction equation, and the Allen–Cahn equation for a scalar order parameter (proportional to the concentration of hydrophilic segments in polymer chains). Material constants are found by fitting equilibrium mass uptake diagrams for macro- and microgels under unconstrained and constrained swelling in the vicinity of the volume phase transition temperature. Quantitative agreement is demonstrated between the experimental data and the results of simulation. The effect of composition of hydrogels (concentrations of monomers and cross-linker) on adjustable parameters is analyzed numerically.

Time-dependent response of hydrogels under constrained swelling

Constitutive equations are developed for the viscoplastic behavior of covalently cross-linked hydrogels subjected to swelling. The ability of the model to describe the time-dependent response is confirmed by comparison of results of simulation with observations on partially swollen poly(2-hydroxyethyl methacrylate) gel specimens in uniaxial tensile tests with a constant strain rate and tensile relaxation tests. The stress–strain relations are applied to study the kinetics of unconstrained and constrained swelling. The following conclusions are drawn from numerical analysis: (i) maximum water uptake under constrained swelling a viscoplastic hydrogel is lower than that for unconstrained swelling of its elastic counterpart and exceeds maximum water uptake under constrained swelling of the elastic gel, (ii) when the rate of water diffusion exceeds the rate of plastic flow in a polymer network, swelling curves (mass uptake versus time) for viscoplastic gels under constraints demonstrate characteristic features of non-Fickian diffusion.

Constrained Swelling of Polymer Networks: Characterization of Vapor-Deposited Cross-Linked Polymer Thin Films

In certain real situations, polymer networks cannot swell freely but are subjected to constraints by action of external forces or geometric restrictions. In this contribution, the restriction to swelling by adhesion of a cross-linked polymer film deposited on a rigid substrate is considered. Such constraints prevent swelling in plane, and the osmotic force concentrates on extension of network chains in the direction normal to the surface. We analyze existing rubber elasticity models taking into account finite extensibility of network chains and derive the elastic contribution to the chemical potential of the solvent in swollen networks. For the mixing contribution to the change of the Gibbs energy, it is more appropriate to consider the network containing cross-links, but with undeformed network chains than an un-cross-linked polymer of infinite degree of polymerization, because continuing formation of bonds within an infinite molecule further decreases the entropy. The additional term is proportional to ...

Constrained swelling and instability of a temperature-sensitive hydrogel ring

AbstractWe analyze the constrained swelling of a temperature-sensitive hydrogel ring, a key component of a previously developed adaptive liquid microlens. Hard constraints make the deformation and stress distribution within the gel nonuniform. The compressive stress may even cause the instability of the ring. We code a material model for poly(N-isopropylacrylamide) hydrogel and predict wrinkling instability at the inner periphery of the ring by using a commercial finite element method software. The presented model and the codes can be utilized to aid the design of soft devices based on temperature-sensitive hydrogels.

Class III Phosphoinositide 3‐Kinase/VPS34 and Dynamin are Critical for Apical Endocytic Recycling

Recycling is a limiting step for receptor-mediated endocytosis. We first report three in vitro or in vivo evidences that class III PI3K/VPS34 is the key PI3K isoform regulating apical recycling. A substractive approach, comparing in Opossum Kidney (OK) cells a pan-class I/II/III PI3K inhibitor (LY294002) with a class I/II PI3K inhibitor (ZSTK474), suggested that class III PI3K/VPS34 inhibition induced selective apical endosome swelling and sequestration of the endocytic receptor, megalin/LRP-2, causing surface down-regulation. GFP-(FYVE)x2 overexpression to sequester PI(3)P caused undistinguishable apical endosome swelling. In mouse kidney proximal tubular cells, conditional Vps34 inactivation also led to vacuolation and intracellular megalin redistribution. We next report that removal of LY294002 from LY294002-treated OK cells induced a spectacular burst of recycling tubules and restoration of megalin surface pool. Acute triggering of recycling tubules revealed recruitment of dynamin-GFP and dependence of dynamin-GTPase, guidance directionality by microtubules, and suggested that a microfilamentous net constrained endosomal swelling. We conclude that (i) besides its role in endosome fusion, PI3K-III is essential for endosome fission/recycling; and (ii) besides its role in endocytic entry, dynamin also supports tubulation of recycling endosomes. The unleashing of recycling upon acute reversal of PI3K inhibition may help study its dynamics and associated machineries.

INHOMOGENEOUS LARGE DEFORMATION KINETICS OF POLYMERIC GELS

This work examines the dynamics of nonlinear large deformation of polymeric gels, and the kinetics of gel deformation is carried out through the coupling of existing hyperelastic theory for gels with kinetic laws for diffusion of small molecules. As finite element (FE) models for the transient swelling process is not available in commercial FE software, we develop a customized FE model/methodology which can be used to simulate the transient swelling process of hydrogels. The method is based on the similarity between diffusion and heat transfer laws by determining the equivalent thermal properties for gel kinetics. Several numerical examples are investigated to explore the capabilities of the present FE model, namely: a cube to study free swelling; one-dimensional constrained swelling; a rectangular block fixed to a rigid substrate to study swelling under external constraints; and a thin annulus fixed at the inner core to study buckling phenomena. The simulation results for the constrained block and one-dimensional constrained swelling are compared with available experimental data, and these comparisons show a good degree of similarity. In addition to this work providing a valuable tool to researchers for the study of gel kinetic deformation in the various applications of soft matter, we also hope to inspire works to adopt this simplified approach, in particular to kinetic studies of diffusion-driven mechanisms.