Elastomeric Gel Research Articles

Stretchable, self-healing, transparent macromolecular elastomeric gel and PAM/carrageenan hydrogel for self-powered touch sensors

The booming of next-generation soft electronics has aroused great demand for the flexible touch sensors, and their key problem is to ensure good transparency and durability. Herein, glycerin and hydroxyethyl cellulose elastomer (GHEC), and PAM/carrageenan double network (DN) hydrogel are prepared by a simple one-pot polymerization method. The as-prepare DN hydrogel are sandwiched between two GHEC layers to act as a self-powered and self-healing triboelectric nanogenerator (SS-TENG), with the average peak value of VOC and ISC reaching 347.5 V and 0.98 μA respectively. When the SS-TENG is damaged, it can complete self-healing without degradation of the electrical output performance and mechanical property, guaranteeing good durability. The prepared SS-TENG exhibits excellent flexibility and high transmittance up to 93%. Further, a 3 × 3 sef-powered transparent touch sensor array based on the SS-TENG is developed. Our SS-TENG paves the way for the development of new flexible self-powered touch screen sensors.

Photoswitchable Covalent Adaptive Networks Based on Thiol-Ene Elastomers.

Covalent adaptive networks combine the advantages of cross-linked elastomers and dynamic bonding in a single system. In this work, we demonstrate a simple one-pot method to prepare thiol-ene elastomers that exhibit reversible photoinduced switching from an elastomeric gel to fluid state. This behavior can be generalized to thiol-ene cross-linked elastomers composed of different backbone chemistries (e.g., polydimethylsiloxane, polyethylene glycol, and polyurethane) and vinyl groups (e.g., allyl, vinyl ether, and acrylate). Photoswitching from the gel to fluid state occurs in seconds upon exposure to UV light and can be repeated over at least 180 cycles. These thiol-ene elastomers also exhibit the ability to heal, remold, and serve as reversible adhesives.

Effect of Concentration of Ion in the Solution on the Deformation Behavior of Gel under Simple Tension

Elastomeric gels are soft-elastic materials consisting of a three-dimensional cross-linked polymer network and liquid filling the space among such network. To take advantage of the swelling-deformation behavior of elastomeric gel and explore its possibility for applications in engineering as a structural member, the effect of concentration of ion in the solution on the deformation behavior of gel should be clarified, especially when the gel is loaded inside the solution. In this study, we at first proposed a nonaffine model for the elastomeric gel to account for the change of the entangling structure of polymer chains during the swelling-deformation process, in which the change of the number of polymer chains per unit volume N is depending on the first invariant of right Cauchy-Green deformation tensor I1. And then, the simple tensional deformation behavior of gel is calculated when the gel is loaded inside the solution with different concentration of ion. The results show that the nonaffine model can reproduce the experimental results very well. Moreover, the high concentration of ion leads to the high deformation resistance.

Deformation of Molecular Chain's Network Structure of Spherical Gel under Quasi-static Compressive Loading Condition

Elastomeric gels are soft-elastic materials consisting of a three-dimensional crosslinked polymer network and liquid filling the space among such network. To take advantage of the swelling-deformation behavior of elastomeric gel and explore its possibility for applications in engineering as a structural member, investigations on the swelling-deformation behavior under various loading conditions are indispensable. Therefore, in this study, we at first proposed a nonaffine model for the elastomeric gel to account for the change of the entangling structure of polymer chains during the swelling-deformation process, in which the change of the number of polymer chains per unit volume N is depending on the first invariant of right Cauchy-Green deformation tensor I1. And then, to investigate the effect of the nonaffine movement of the polymer chain on the mechanical behavior of the elastomeric gel, we performed a FEM simulation for a spherical gel under quasi-static compressive loading condition. The results show that the swelling-deformation behavior of the spherical gel is quite different at the positions such as the polar, the equator or the center of the sphere.

Effect of Swelling Ratio on Mechanical Property of Elastomeric Gel

Effect of Swelling Ratio on Mechanical Property of Elastomeric Gel

In situ crosslinked PMMA gel electrolyte from a low viscosity precursor solution for cost-effective, long lasting and sustainable lithium-ion batteries

To address the shortcomings of current gel electrolytes such as cost, processability, safety and sustainability, we report here on the in situ preparation of a crosslinked gel polymer electrolyte by polymerization of methyl methacrylate with poly(ethylene glycol) dimethacrylate within a liquid electrolyte composed of an ethylene carbonate/propylene carbonate low volatility solvent mixture. Lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) is used as main lithium salt since it is stable thermally and vs. hydrolysis, which is advantageous for gel formation and recycling. Aluminum current collector corrosion and the efficient operation of the graphite anode are addressed via the use of lithium bis(oxalato)borate (LiBOB), vinylene carbonate and 1-propene 1,3-sultone as additives. The self-standing elastomeric gel electrolyte allows reaching conductivities close to those of the liquid fraction with a conductivity of 3.9 mS cm−1 at 20 °C probably due to a high mobility as suggested by the glass transition temperature that is almost unaffected by the presence of the polymer. From a processing point of view, the use of methyl methacrylate (MMA) allows decreasing the viscosity of the precursor solution for efficient wetting of the cell components prior to thermal polymerization. The good contact allows cycling graphite/LiMn0.33Ni0.33Co0.33O2 (NMC111) cells with a 2.1 mAh cm−2 areal capacity steadily for hundreds of cycles.

Stretchable, self-healing, transient macromolecular elastomeric gel for wearable electronics

Flexible and stretchable electronics are emerging in mainstream technologies and represent promising directions for future lifestyles. Multifunctional stretchable materials with a self-healing ability to resist mechanical damage are highly desirable but remain challenging to create. Here, we report a stretchable macromolecular elastomeric gel with the unique abilities of not only self-healing but also transient properties at room temperature. By inserting small molecule glycerol into hydroxyethylcellulose (HEC), forming a glycerol/hydroxyethylcellulose (GHEC) macromolecular elastomeric gel, dynamic hydrogen bonds occur between the HEC chain and the guest small glycerol molecules, which endows the GHEC with an excellent stretchability (304%) and a self-healing ability under ambient conditions. Additionally, the GHEC elastomeric gel is completely water-soluble, and its degradation rate can be tuned by adjusting the HEC molecular weight and the ratio of the HEC to glycerol. We demonstrate several flexible and stretchable electronics devices, such as self-healing conductors, transient transistors, and electronic skins for robots based on the GHEC elastomeric gel to illustrate its multiple functions.

Bending a gel beam with consideration of entanglements and finite extensibility

A hybrid beam containing a gel layer attached on the top of an elastic non-swellable substrate has been normally used to manufacture diverse sensors and actuators. Generally, when the system uses different gels, it is necessary to develop differentiated models for the hybrid beam. Recently, on the basis of the generalized ideal elastomeric gel model, a unified relationship between the gels' swelling and the elastic beam's bending curvature is formulated by Cai, which is separated from some particular swelling mechanisms of gels. He plugged neo-Hookean model and Gent model of the stretching polymer network to the generalized equations. However, the polymer networks usually have many entanglements due to the uncrossability of the network chains as well as a finite extensibility resulting from the full stretching of the network chains. The effect of entanglements cannot be captured by these two models. In this letter, the nonaffine model proposed by Davidson and Goulbourne is adopted to the generalized equations derived by Cai. The results show that entanglements as well as chain extensibility have important influences on the mechanical bending behaviors of the gels' beam.

A Nonaffine Molecular Chain Network Model for Elastomeric Gel

Submerged in a solvent-containing environment and subject to applied forces, a nonaffine molecular chain network absorbs the solvent and deforms, forming an elastomeric gel. The Helmholtz free energy of the gel is separated into the contribution due to stretching the nonaffine molecular chain network and that due to mixing the polymer and the solvent. The effect of the nonaffine movement of the molecular chain on the deformation behavior of the gel is discussed.

Quasi‐Solid‐State Dye‐Sensitized Solar Cells Containing a Charged Thermoplastic Elastomeric Gel Electrolyte and Hydrophilic/phobic Photosensitizers

Quasi‐Solid‐State Dye‐Sensitized Solar Cells Containing a Charged Thermoplastic Elastomeric Gel Electrolyte and Hydrophilic/phobic Photosensitizers

Quasi‐Solid‐State Dye‐Sensitized Solar Cells Containing a Charged Thermoplastic Elastomeric Gel Electrolyte and Hydrophilic/phobic Photosensitizers

As the threat of global climate change due to combustion emissions becomes increasingly alarming, the search for clean and sustainable alternative energy is of paramount importance. With this objective in mind, harnessing solar energy is particularly attractive due to ongoing improvements in silicon‐, perovskite‐, and organic‐based solar cells. Of these, dye‐sensitized solar cells (DSSCs) constitute a promising technology due to their relatively low cost, moderate conversion efficiency, and robust mechanical properties. An important breakthrough in the development of DSSCs is the use of polymer gel electrolytes. In this work, we employ amphi­philic thermoplastic elastomers (TPEs) in the form of sulfonated block ionomer (SBI) homologs for this purpose. Since the midblock of each charged copolymer is hydrophilic, the resultant microphase‐separated nanostructures consist of continuous ionic channels that facilitate electron diffusion. A unique characteristic of the SBI archetype studied here is that the morphology can be solvent‐templated. We exploit this feature by introducing either hydrophilic or hydrophobic photosensitizers into the DSSCs. When the SBI exhibits a primarily lamellar morphology, a hydrophilic dye yields the highest efficiency (7.0%), whereas the opposite is observed when the nanostructure consists of a solvent‐templated nonpolar matrix. Photosensi­tizer tunability is augmented by the intrinsic mechanical and adhesive properties of a TPE.

Bending a beam by a generalized ideal elastomeric gel

A hybrid beam with a gel layer bonded on the top of an elastic non-swellable substrate has been commonly adopted to make various sensors and actuators. Usually, different models need to be developed for the hybrid beam when different gels are used in the system. In this article, based on the generalized ideal elastomeric gel model, we formulate a unified relationship between the swelling of hydrogels and the bending curvature of the elastic beam, which is independent of specific swelling mechanisms of gels. We further illustrate that the equations derived in the article can be used to validate the ideal elastomeric gel model and measure the elasticity of polymer networks of the gels.

Gel Mechanics: A Thermo-mechanically Coupled Theory for Fluid Permeation in Elastomeric Materials

An elastomeric gel is a cross-linked polymer network swollen with a solvent, and certain gels can undergo large reversible volume changes as they are cycled about a critical temperature. We have developed a continuum-level theory to describe the coupled mechanical deformation, fluid permeation, and heat transfer of such thermally-responsive gels. In discussing special constitutive equations we limit our attention to isotropic materials, and consider a model based on a Flory-Huggins model for the free energy change due to mixing of the fluid with the polymer network, coupled with a non-Gaussian statistical-mechanical model for the change in configurational entropy — a model which accounts for the limited extensibility of polymer chains. We have numerically implemented our theory in a finite element program. We have shown that our theory is capable of simulating swelling, squeezing of fluid by applied mechanical forces, and thermally-responsive swelling/deswelling of such materials.

Mechanical behavior of poly(methyl methacrylate)-based ionogels.

Ionogels are formed when a cross-linked polymer network absorbs an ionic liquid. Ionogels are ionic conductors and, as such, are being considered for use in stretchable electronics and artificial muscles or nerves. The use of ionogels in these applications is limited in part by their mechanical behavior. Here we present an ionogel prepared by swelling covalently cross-linked poly(methyl methacrylate) in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide. The resulting ionogel is compliant, stretchable, and relatively tough. We demonstrate that the swelling ratio, elastic modulus, stretchability, and fracture energy of the ionogel depend sensitively on the cross-link density of the polymer network. The behavior of the ionogel is well captured by the model of the ideal elastomeric gel combined with the Flory-Huggins model for the energy of mixing.

Force and stroke of a hydrogel actuator

Hydrogels that undergo a volume phase transition in response to an external stimulus are of great interest because of their possible use as actuator materials. The performance of an actuator material is normally characterized by its force–stroke curve, but little is known about the force–stroke behavior of hydrogels. We use the theory of the ideal elastomeric gel to predict the force–stroke curves of a temperature-sensitive hydrogel and introduce an experimental method for measuring the curve. The technique is applied to PNIPAm hydrogels with low cross-link densities. The maximum force generated by the hydrogel increases with increasing cross-link density, while the maximum stroke decreases. The force–stroke curves predicted by the theory of the ideal elastomeric gel are in very good agreement with the experimental curves.

Viscoelasticity and poroelasticity in elastomeric gels

An elastomeric gel is a mixture of a polymer network and a solvent. In response to changes in mechanical forces and in the chemical potential of the solvent in the environment, the gel evolves by two concurrent molecular processes: the conformational change of the network, and the migration of the solvent. The two processes result in viscoelasticity and poroelasticity, and are characterized by two material-specific properties: the time of viscoelastic relaxation and the effective diffusivity of the solvent through the network. The two properties define a material-specific length. The material-specific time and length enable us to discuss macroscopic observations made over different lengths and times, and identify limiting conditions in which viscoelastic and poroelastic relaxations have either completed or yet started. We formulate a model of homogeneous deformation, and use several examples to illustrate viscoelasticity-limited solvent migration, where the migration of the solvent is pronounced, but the size of the gel is so small that the rate of change is limited by viscoelasticity. We further describe a theory that evolves a gel through inhomogeneous states. Both infinitesimal and finite deformation are considered.

PH-Induced Cross-Linking of Dopamine-Containing Block Copolymers with Fe<sup>3+</sup> to Form Self-Healing Hydrogels

The unusual amino acid 3,4-dihydroxyphenylalanine with a catechol side chain is believed to impart both adhesive and cohesive characteristics of mussel adhesive proteins. Inspired by the pH-tunable coordination between catechol moiety and Fe3+, responsible for the high extensibility and self-healing properties of the cuticle of mussel byssal threads, we report the synthesis of a dopamine modified Pluronic F68 block copolymer (PluF68-Dopa) with high coupling efficiency and utilization of catechol-Fe3+interactions to yield cross-linking hydrogels with enhanced mechanical strength and self-healing properties. Ultraviolet-visible spectroscopy was used to identify PluF68-Dopa-Fe3+complex species via pH control in dilution solution. At lower pH, concentrated PluF68-Dopa-Fe3+mixture remained a fluid, whereas at higher pH an elastomeric gel immediately formed. PluF68-Dopa-Fe3+cross-linked hydrogels are not only pH-responsive but also possess self-healing properties due to the reversible bond interactions between catechol moiety and Fe3+. These new biomimetic materials are expected to have potential biomedical and engineering applications.

Development of a High Precision Method to Characterize Poisson’s Ratios of Encapsulant Gels Using a Flat Disk Configuration

A high precision test method based on the compression of thin elastomeric disks was developed and implemented in order to measure Poisson’s ratios of a silicone gel and organic gel. Test fixtures consisted of a thin disk of the elastomeric gel cast between two rigid platens with the lower platen containing a centrally installed pressure sensor. A compressive load was applied to the upper glass plate and the resulting stress through the deformed disk was measured using the attached pressure sensor. Poisson’s ratios of the gels were easily determined from the thin disk’s aspect ratio and the ratio of the measured stress from the pressure sensor to the applied compressive stress. The high degree of precision for this technique is indicated by the measured Poisson’s ratios for both materials which were obtained to at least the third significant digit. Additionally, an increase in the precision of this method was observed for lower aspect ratios of the cured disks, specifically the thickness to radius. An alternative high precision cylindrical experimental technique was implemented in this study to compare the resulting Poisson’s ratio measurements to this flat disk method. This method utilized a hollow steel cylinder fitted with matching pairs of pressure sensors located at the top and bottom. Comparable results were found between the two methods for the investigated silicone gel system.

Experimental determination of equations of state for ideal elastomeric gels

A polymer network can absorb a solvent and swell, forming an elastomeric gel. The model of ideal elastomeric gels is based on two assumptions. First, the volume of a gel is the sum of the volume of the dry network and that of the solvent. Second, the free energy of the gel is the sum of the free energy due to stretching the network and that due to mixing the polymer and the solvent. These assumptions lead to a set of equations of state, which can be tested experimentally without invoking any specific models of statistical mechanics. Here we test the model of ideal elastomeric gels by conducting experiments with polyacrylamide hydrogels, and by extracting from the literature four sets of data on polyacrylamide hydrogels and polyacrylamide–water solutions. For an ideal elastomeric gel, the effect of mixing the polymer and the solvent is represented by the osmotic pressure as a function of the swelling ratio. We show that this function obtained by several distinct experimental methods is consistent. Specifically, the function obtained from a gel under different states of applied stress is the same, the function obtained from a free-swelling gel is the same as that obtained from the constrained-swelling gel, the function is independent of the crosslink density, and the function obtained from the gels is similar to that obtained from the solutions. We further show that the Flory–Huggins model of mixing with a constant Flory–Huggins parameter does not fit the experimental data well, but does capture the trend of the data over four orders of magnitude in the osmotic pressure.

A thermo-mechanically coupled theory for fluid permeation in elastomeric materials: Application to thermally responsive gels

An elastomeric gel is a cross-linked polymer network swollen with a solvent, and certain gels can undergo large reversible volume changes as they are cycled about a critical temperature. We have developed a continuum-level theory to describe the coupled mechanical deformation, fluid permeation, and heat transfer of such thermally responsive gels. In discussing special constitutive equations we limit our attention to isotropic materials, and consider a model based on a Flory–Huggins model for the free energy change due to mixing of the fluid with the polymer network, coupled with a non-Gaussian statistical–mechanical model for the change in configurational entropy—a model which accounts for the limited extensibility of polymer chains. We have numerically implemented our theory in a finite element program. We show that our theory is capable of simulating swelling, squeezing of fluid by applied mechanical forces, and thermally responsive swelling/de-swelling of such materials.