Unexpected Fracture Behavior of Ultrasoft Associative Hydrogels Due to Strain-Induced Crystallization

Strain-induced crystallization (SIC) is a well-known toughening strategy in elastomers, but is rarely observed in hydrogels due to their high-water content and limited deformability. Here we report a phenomenon of SIC in highly swollen and associative hydrogels by introducing an extremely large deformation by indentation with a needle. Using in situ birefringence imaging, we discovered that SIC occurs close to the needle tip upon large strain, displacing the nucleation of a crack from the needle tip to a position further away from the tip. The morphology of the fracture as well as the force to induce the gel fracture with the needle can be controlled by playing with temperature and cross-linking and hence triggering or not the SIC. Our discovery points to a future direction in creating SIC in highly swollen hydrogels, with potential implications for many biological material designs, and surgical injury prediction or prevention in associative tissues

Structure of Surfaces and Interfaces of Poly(<i>N,N</i>-dimethylacrylamide) Hydrogels

We investigated the surface structure of hydrogels of poly­(<i>N,N</i>-dimethylacrylamide) (PDMA) hydrogels synthesized and cross-linked simultaneously by redox free radical polymerization. We demonstrate the existence of a less cross-linked layer at the surface of the gel at least at two different length scales characterized by shear rheology and by neutron reflectivity, suggesting the existence of a gradient in cross-linking. The composition of the layer is shown to depend on the degree of hydrophobicity of the mold surface and is weaker for more hydrophobic molds. While the macroscopic tests proved the existence of a relatively thick under-cross-linked layer, we also demonstrated by neutron reflectivity that the gel surface at the submicrometric scale (500 nm) was also affected by the surface treatment of the mold. These results should have important implications for the measurement of macroscopic surface properties of these hydrogels such as friction or adhesion

Probing pH-Responsive Interactions between Polymer Brushes and Hydrogels by Neutron Reflectivity

We investigated the effect of specific interactions on the structure of interfaces between a brush and a hydrogel on the polymer chain length scale. We used a model system for which the interactions between the brush and the gel are switchable. We synthesized weak polyelectrolyte brushes of poly­(acrylic acid) and hydrogels of polyacrylamide and poly­(<i>N</i>,<i>N</i>-dimethylacrylamide) which interact solely when the poly­(acrylic acid) is mainly in its acidic form. The monomer density profiles of the poly­(acrylic acid) brush immersed in pure deuterium oxide (D₂O) or in contact with a D₂O-swollen gel were determined by neutron reflectivity. At pH 2 when the brush is in its neutral form, it interacts with the gel by hydrogen bonds while at pH 9 when the brush is a polyelectrolyte it is not interacting with the gel. Our results show that the presence of interactions with the gel at pH 2 increases the swelling ratio of the brush relative to that in pure D₂O, meaning that the brushes exhibit conformations which are more extended from the surface than in the absence of interactions

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

Time Dependent Behavior of a Dual Cross-Link Self-Healing Gel: Theory and Experiments

Recent experiments have shown that hydrogels with enhanced toughness can be synthesized by incorporating self-healing physical cross-links in a chemically cross-linked gel network. These gels exhibit rate dependent mechanical behavior, suggesting that improved mechanical properties are closely tied to the breaking and reattaching of temporary cross-links in the gel network. In this work, the connection between rate dependent mechanical behavior and kinetics of breaking and reattachment of temporary cross-links is quantified using a three-dimensional finite strain constitutive model. The parameters of the model are fitted using relaxation and constant strain rate tests in uniaxial tension of a model dual-cross-link gel. The stress versus time curves of more complex strain histories, involving loading followed by unloading at different rates, is successfully and quantitatively predicted by our model. Such modeling strategy combining physically based kinetics and three-dimensional large strain mechanics shows great promise for quantitative modeling of soft biological tissues and synthetic counterparts containing dynamic bonds

Enhanced Adhesion of Elastic Materials to Small-Scale Wrinkles

The adhesive properties of a material can be greatly affected simply by wrinkling its surface. We show the importance of selecting the wrinkle feature sizes (amplitude, <i>b</i>; and wavelength, λ) that complement the material-defined length scale related to the adhesion energy and modulus. A rigid circular cylindrical punch patterned with aligned wrinkles ranging in amplitude from 0.5 to 5.0 μm with a fixed aspect ratio of 0.1 is used to characterize the adhesion of elastic films of smooth poly­(dimethyl siloxane) (PDMS). The cross-linker concentration used to form the PDMS layers is varied to determine the impact of material properties on wrinkled surface adhesion. The elastic films have an average thickness of 240 μm and the average probe radius is 1 mm, leading to a confined contact scenario. The separation stress and work of debonding are presented for each cross-linker concentration with testing rates ranging over 3 orders of magnitude. For stiffer films (10 wt % cross-linker, <i>E</i>' ≈ 3.00 MPa), small wrinkles (<i>b ≈ </i>0.5 μm) increase the separation stress by nearly 200% relative to a smooth interface whereas large wrinkles (<i>b</i> ≈ 5.0 μm) are shown to reduce adhesion significantly. A substantial increase in the debonding energy is also observed for these small-amplitude wrinkles contacting stiff materials. No discernible impact of wrinkled surface topography on the adhesion of softer (2 and 4 wt % cross-linker, 0.05 MPa < <i>E</i>' < 0.30 MPa) films is measured

Enhanced Adhesion of Elastic Materials to Small-Scale Wrinkles

The adhesive properties of a material can be greatly affected simply by wrinkling its surface. We show the importance of selecting the wrinkle feature sizes (amplitude, <i>b</i>; and wavelength, λ) that complement the material-defined length scale related to the adhesion energy and modulus. A rigid circular cylindrical punch patterned with aligned wrinkles ranging in amplitude from 0.5 to 5.0 μm with a fixed aspect ratio of 0.1 is used to characterize the adhesion of elastic films of smooth poly­(dimethyl siloxane) (PDMS). The cross-linker concentration used to form the PDMS layers is varied to determine the impact of material properties on wrinkled surface adhesion. The elastic films have an average thickness of 240 μm and the average probe radius is 1 mm, leading to a confined contact scenario. The separation stress and work of debonding are presented for each cross-linker concentration with testing rates ranging over 3 orders of magnitude. For stiffer films (10 wt % cross-linker, <i>E</i>' ≈ 3.00 MPa), small wrinkles (<i>b ≈ </i>0.5 μm) increase the separation stress by nearly 200% relative to a smooth interface whereas large wrinkles (<i>b</i> ≈ 5.0 μm) are shown to reduce adhesion significantly. A substantial increase in the debonding energy is also observed for these small-amplitude wrinkles contacting stiff materials. No discernible impact of wrinkled surface topography on the adhesion of softer (2 and 4 wt % cross-linker, 0.05 MPa < <i>E</i>' < 0.30 MPa) films is measured

Debonding Mechanisms of Soft Materials at Short Contact Times

A carefully controlled, custom-built adhesion testing device was developed which allows a precise, short dwell time on the order of milliseconds to be applied during a contact adhesion experiment. The dwell time dependence of the adhesive strength of crosslinked poly­(dimethylsiloxane) (PDMS) in contact with glass and uncrosslinked styrene butadiene rubber (SBR) in contact with glass and with itself was tested with a spherical probe in a confined Johnson–Kendall–Roberts (JKR) geometry. Analysis of the contact images revealed several unique separation mechanisms which are dependent on dwell time and interfacial properties. PDMS–glass interfaces show essentially no dependence of adhesion on the dwell time while the adhesive strength and separation mechanisms of SBR interfaces are shown to vary drastically for dwell times ranging from 40 to 10 000 ms. This influence of dwell time is particularly pronounced for polymer–polymer (SBR–SBR) interfaces. Observations of cavitation due to trapped air pockets in the center of the contact at very short contact times illustrate a transition between a defect-controlled debonding and an interface-controlled debonding which has not been previously reported

Debonding Mechanisms of Soft Materials at Short Contact Times

A carefully controlled, custom-built adhesion testing device was developed which allows a precise, short dwell time on the order of milliseconds to be applied during a contact adhesion experiment. The dwell time dependence of the adhesive strength of crosslinked poly­(dimethylsiloxane) (PDMS) in contact with glass and uncrosslinked styrene butadiene rubber (SBR) in contact with glass and with itself was tested with a spherical probe in a confined Johnson–Kendall–Roberts (JKR) geometry. Analysis of the contact images revealed several unique separation mechanisms which are dependent on dwell time and interfacial properties. PDMS–glass interfaces show essentially no dependence of adhesion on the dwell time while the adhesive strength and separation mechanisms of SBR interfaces are shown to vary drastically for dwell times ranging from 40 to 10 000 ms. This influence of dwell time is particularly pronounced for polymer–polymer (SBR–SBR) interfaces. Observations of cavitation due to trapped air pockets in the center of the contact at very short contact times illustrate a transition between a defect-controlled debonding and an interface-controlled debonding which has not been previously reported

Debonding Mechanisms of Soft Materials at Short Contact Times

A carefully controlled, custom-built adhesion testing device was developed which allows a precise, short dwell time on the order of milliseconds to be applied during a contact adhesion experiment. The dwell time dependence of the adhesive strength of crosslinked poly­(dimethylsiloxane) (PDMS) in contact with glass and uncrosslinked styrene butadiene rubber (SBR) in contact with glass and with itself was tested with a spherical probe in a confined Johnson–Kendall–Roberts (JKR) geometry. Analysis of the contact images revealed several unique separation mechanisms which are dependent on dwell time and interfacial properties. PDMS–glass interfaces show essentially no dependence of adhesion on the dwell time while the adhesive strength and separation mechanisms of SBR interfaces are shown to vary drastically for dwell times ranging from 40 to 10 000 ms. This influence of dwell time is particularly pronounced for polymer–polymer (SBR–SBR) interfaces. Observations of cavitation due to trapped air pockets in the center of the contact at very short contact times illustrate a transition between a defect-controlled debonding and an interface-controlled debonding which has not been previously reported