Stress–Strain Relationship of Highly Stretchable Dual Cross-Link Gels: Separability of Strain and Time Effect

We studied the stress–strain relation of model dual cross-link gels having permanent cross-links and transient cross-links over an unusually wide range of extension ratios λ and strain rates ϵ̇ (or time <i>t</i> = (λ – 1)/ϵ̇). We propose a new analysis method and separate the stress into strain- and time-dependent terms. The strain-dependent term is derived from rubber elasticity, while the time-dependent term is due to the failure of transient cross-links and can be represented as a time-dependent shear modulus which shows the same relaxation as in small strain. The separability is applicable except for the strain stiffening regimes resulting from the finite extensibility of polymer chains. This new analysis method should have a wide applicability not only for hydrogels but also for other highly viscoelastic soft solids such as soft adhesives or living tissues

Tuning Structure and Rheological Properties of Polyelectrolyte-Based Hydrogels through Counterion-Specific Effects

Tuning at will the properties of gel-forming systems is of key relevance for many biotechnological, agricultural, and biomedical applications. For polyelectrolyte-based gels, ion-specific effects can be an attractive way for this purpose. This study investigates the counterion-specific effect on the microscopic structure and the rheological properties of a physical hydrogel formed of ionene-type cationic polyelectrolytes. The focus is on two monovalent halide counterions (F– and Cl–) and a divalent counterion (SO42–). A strong counterion-specific effect appears within ionene-based gels. In the case of halide counterions, gelation is more effective for more weakly hydrated counterions. Indeed, strongly hydrated counterions maintain electrostatic repulsions between the chains and as a consequence gel formation is shifted toward higher concentrations (higher critical gelation concentration, CGC). The combination of the complementary small-angle X-ray and neutron scattering (SAXS and SANS) techniques reveals a strong contribution of ion–ion correlations in the structure of the gel network. Contrary to chloride gels, which present a single correlation length characterizing the distance between the cross-linking nodes, fluoride gels present an additional network of nodes. This is accompanied by a very rapid increase of the elastic modulus of fluoride gels, once CGC is reached. With divalent counterions, the gelation is even more remarkable with a lower CGC and a higher elastic modulus at equivalent polyelectrolyte concentrations. The presence of divalent counterions favors the association of chains, probably by a bridging effect. This evokes the “egg-box” model, and the characteristic scaling of the elastic modulus with reduced gel concentration confirms this. However, only a narrow concentration window for gel-forming exists for divalent counterions before precipitation takes over due to too strong attractive chain–chain interactions

Anions as Efficient Chain Stoppers for Hydrogen-Bonded Supramolecular Polymers

The chain length of hydrogen-bonded supramolecular polymers and thus their rheological properties can be controlled by the presence of so-called chain stoppers: these monofunctional monomers are able to interact with the monomers and break the polymer chains. In this letter, we show that the use of anions, strong hydrogen bond competitors, instead of precisely designed complementary units is a very simple approach to tuning the rheology of a bisurea-based hydrogen-bonded supramolecular polymer. All of the anions tested were able to break the supramolecular chains, resulting in a dramatic drop in the viscosity of the solutions and were found to be more efficient than a previously described organic stopper. A careful study of the rheological properties of bisurea solutions in the presence of H2PO4,N(C4H9)4 showed that the presence of this ion does not modify the nature of the bisurea supramolecular assembly. For a molar fraction of stopper of only 10−5, the viscosity of bisurea solutions decreases by a factor of 10 as a result of the formation of shorter supramolecular assemblies

Double Networks: Hybrid Hydrogels with Clustered Silica

Model hybrid hydrogels reinforced by silica nanoparticles were designed by polymerizing and cross-linking the gels in situ. The polymer–particle interactions were tuned by using either poly(dimethylacrylamide) (PDMA), which adsorbs on silica, or poly(acrylamide) (PAAm), which does not. Besides, the dispersion state of silica nanoparticles was tuned from well-dispersed to aggregated by changing the pH from 9, which ensures repulsive interactions between nanoparticles and good dispersion state, to about 6, which affects the surface chemistry of silica and promotes aggregation. The dispersion states were characterized by small-angle X-ray scattering (SAXS). The mechanical behavior of hybrid gels with aggregated nanoparticles is markedly different from those where silica is well-dispersed within the matrix. PDMA-based hybrid gels display pronounced nonlinear behavior, somehow similar to those observed in filled elastomers. The nonlinearities are even more pronounced in gels with aggregated particles, with strong strain stiffening along with large dissipation. For those samples, reinforcement can be attributed to the combination of both reversible interactions between PDMA and silica nanoparticles, which provide strain stiffening and recovery, and the response of the silica network. Recovery processes observed in hybrid gels with dispersed particles are preserved when silica particles are aggregated, but the characteristic time needed to fully recover the mechanical response is extended from a few seconds to several hours. In PAAm-based hybrid gels with aggregated silica nanoparticles, no recovery processes are observed. This implies that the properties, namely, the very high linear tensile modulus and high dissipated energy, are driven by the rigid network formed by nanoparticle aggregation, which provides high dissipative capabilities, especially when compared to PAAm-based hybrid gels with dispersed silica, that remain soft and fragile. These gels exhibit a quite inhomogeneous structure, with permanent damage under elongation. The nonlinear dynamical behavior of hybrid gels was investigated by large amplitude oscillatory shear (LAOS) experiments. While unfilled gels show no nonlinearity up to very large strain amplitude, marked nonlinear effects combining a drop in modulus (similar to the Payne effect) and strain stiffening for increasing strain amplitude are observed in PDMA-based hybrid gels, certainly due to polymer adsorption onto nanoparticles. PAAm-based hybrid gels also show nonlinearity, with a drop in modulus for increasing strain but no strain stiffening, indicating that the presence of fillers alone can induce nonlinearity in the absence of strong, reversible polymer–particle interactions. PAAm-based hybrid gels with aggregated silica show very high stiffness and high dissipative properties at the expense of stretchability, though. Also, the structure seems to be permanently damaged under stress, revealing the importance of silica–polymer interactions for permanent mechanical reinforcement. Altogether, the analysis of the nonlinear behavior indicates the importance of combining dynamic adsorption of polymer chains on silica nanoparticles with mechanical reinforcement provided by the silica network

Supramolecular Crosslinked Hydrogels: Similarities and Differences with Chemically Crosslinked Hydrogels

The specific design of a water-soluble supramolecular cross-linker based on a terpyridine–iron(II) bis-complex is reported. Copolymerization of this cross-linker with acrylamide monomers in water allows a novel one-step synthesis of metallo-supramolecular hydrogels. The synthesized hydrogels were characterized by rheology, dynamic light scattering, and 1H double-quantum nuclear magnetic resonance experiments. They reveal great similarities with the rheological behavior of a chemically crosslinked acrylamide network but differences in the structure at low length scales. Characterization also shows that the supramolecular cross-linker behaves similarly to a permanent bond at the observed time scales (from 10–6 to almost 1000 s), thanks to its relatively high binding energy. However, unlike their chemical counterparts, supramolecular gels show polyelectrolyte swelling behavior and stimulus responsiveness when put in contact with an oxidant. A controlled tuning of the physical–chemical properties of the final gel, ranging from the initial supramolecular gel properties to those of a polymer solution, is then achievable

Combined Effect of Chain Extension and Supramolecular Interactions on Rheological and Adhesive Properties of Acrylic Pressure-Sensitive Adhesives

A new approach for the elaboration of low molecular weight pressure-sensitive adhesives based on supramolecular chemistry is explored. The synthesis of model systems coupled with probe-tack tests and rheological experiments highlights the influence of the transient network formed by supramolecular bonds on the adhesion energy. The first step of our approach consists of synthesizing poly­(butyl acrylate-<i>co</i>-glycidyl methacrylate) copolymers from a difunctional initiator able to self-associate by four hydrogen bonds between urea groups. Linear copolymers with a low dispersity (<i>M</i>_n = 10 kg/mol, Ip < 1.4) have been synthesized via atom transfer radical polymerization. Films of the copolymers were then partially cross-linked through reaction of the epoxy functions with a diamine. The systematic variation of the average ratio of glycidyl methacrylate and diamine per copolymer shed light on the respective role played by the supramolecular interactions (between bis-urea groups and with the side chains) and by the chain extension and branching induced by the diamine/epoxy reaction. In this strategy, the adhesive performance can be optimized by modifying the strength of “stickers” (via the structure of the supramolecular initiator, for instance) and the polymer network (e.g., via the length and level of branching of the copolymer chains) in order to approach commercial PSA-like properties (high debonding energy and clean removal)

Microstructure and Self-Assembly of Supramolecular Polymers Center-Functionalized with Strong Stickers

This manuscript describes the microstructure of a series of nearly monodisperse poly­(<i>n</i>-butyl) acrylate (PnBA) chains center-functionalized with a triurea interacting moiety, able to self-associate by six hydrogen bonds. Different molecular weights have been investigated, from 5000 g·mol^–1 up to 80 000 g·mol^–1. For molecular weights (<i>M</i>_n) below 40 000 g·mol^–1, X-ray scattering experiments and atomic force microscopy at ambient temperature clearly show that the systems organize as nanofibers hexagonally packed in oriented domains. This supramolecular structure explains the solid-like gel behavior of these polymers, which is suppressed at high temperature (at an order–disorder transition temperature). For higher molecular weights, nanofibers still form at ambient temperature but their concentration is too low to self-assemble in oriented domains. This is consistent with the reported viscoelastic behavior of these systems described in the companion paper