Soft matter: rubber and networks

When the Poisson Ratio of Polymer Networks and Gels Is Larger Than 0.5?

We use coarse-grained molecular dynamics simulations to study deformation of networks and gels of linear and brush strands in both linear and nonlinear deformation regimes under constant pressure conditions. The simulations show that the Poisson ratio of networks and gels could exceed 0.5 in the nonlinear deformation regime. This behavior is due to the ability of the network and gel strands to sustain large reversible deformation, which, in combination with the finite strand extensibility results in strand alignment and monomer density, increases with increasing strand elongation. We developed a nonlinear network and gel deformation model which defines conditions for the Poisson ratio to exceed 0.5. The model predictions are in good agreement with the simulation results.

Forensics of polymer networks

Our lives cannot be imagined without polymer networks, which range widely, from synthetic rubber to biological tissues. Their properties-elasticity, strain-stiffening and stretchability-are controlled by a convolution of chemical composition, strand conformation and network topology. Yet, since the discovery of rubber vulcanization by Charles Goodyear in 1839, the internal organization of networks has remained a sealed 'black box'. While many studies show how network properties respond to topology variation, no method currently exists that would allow the decoding of the network structure from its properties. We address this problem by analysing networks' nonlinear responses to deformation to quantify their crosslink density, strand flexibility and fraction of stress-supporting strands. The decoded structural information enables the quality control of network synthesis, comparison of targeted to actual architecture and network classification according to the effectiveness of stress distribution. The developed forensic approach is a vital step in future implementation of artificial intelligence principles for soft matter design.

Vision-Based Contact Adhesion Measurement Method on Soft Matter Surfaces

Adhesion property measurements contribute to a comprehensive understanding of the mechanical properties of soft matters. Indentation tests are a common method for measuring the adhesion force. However, indenters generally have a large volume and a small sensing angle and, thus, are not conducive to local detection in high-precision environments. Here, we propose a vision-based contact adhesion measurement (VisCAM) method to achieve the contact image and adhesion force on soft matter surfaces from the perspective of indentation direction. The coupling of the 7.6 mm diameter probe and a flexible fiber makes the system similar to a miniaturized endoscope. Classical contact theories and finite element models are used for the contact mechanics analysis of silicone rubber. The image grayscale-load mathematical model is constructed based on the change in contact light spot. Finally, the uncertainty of the system is less than 4%, and the measurement error is 0.04 N. In-vitro kidney indentation experiments showed that the local adhesion force measurement of soft tissues can be completed. Our method provides better solutions for understanding the adhesion properties of soft matters.

Hydrogels with ultra-highly additive adjustable toughness under quasi-isochoric conditions

Bioinspired smart hydrogels with additive-switchable mechanical properties have been attracting increasing attention in recent years. However, most existing hydrogel systems suffer from limited stiffening amplitude and dramatic volume change upon response to environmental triggers. Herein, we propose a novel strategy to prepare additive-responsive hydrogels with ultra-highly adjustable toughness under quasi-isochoric conditions. The key point lies in tuning the softening transition temperature of the hydrogels with non-covalent interactions between the polymer networks and additives, shifting the hydrogels from glassy to rubbery states. As a proof of concept, a variety of glassy hydrogels are prepared and exposed to additives to trigger responsive performances. Young's modulus of the same hydrogel demonstrates up to 36 000 times ultra-broad-range tunability, ranging from 0.0042 to 150 MPa in response to different additives. Meanwhile, negligible volume changes occur, keeping the hydrogels in quasi-isochoric conditions. Interestingly, the mechanical behaviors of the hydrogels manifest remarkable dependence on the additive type and concentration since both the Hofmeister effect and hydrophobicity of the additives play pivotal roles according to mechanism investigations. Furthermore, the regulation with additives reveals satisfactory reversibility and universality. Taken together, this simple and effective approach provides a novel strategy to fabricate hydrogels with highly tunable toughness for versatile applications, including spatially patterned conductive gels and anti-icing coatings.

Mechanically Diverse Gels with Equal Solvent Content

Mechanically diverse polymer gels are commonly integrated into biomedical devices, soft robots, and tissue engineering scaffolds to perform distinct yet coordinated functions in wet environments. Such multigel systems are prone to volume fluctuations and shape distortions due to differential swelling driven by osmotic solvent redistribution. Living systems evade these issues by varying proximal tissue stiffness at nearly equal water concentration. However, this feature is challenging to replicate with synthetic gels: any alteration of cross-link density affects both the gel's swellability and mechanical properties. In contrast to the conventional coupling of physical properties, we report a strategy to tune the gel modulus independent of swelling ratio by regulating network strand flexibility with brushlike polymers. Chemically identical gels were constructed with a broad elastic modulus range at a constant solvent fraction by utilizing multidimensional network architectures. The general design-by-architecture framework is universally applicable to both organogels and hydrogels and can be further adapted to different practical applications.

Negative tension controls stability and structure of intermediate filament networks

Networks, whose junctions are free to move along the edges, such as two-dimensional soap froths and membrane tubular networks of endoplasmic reticulum are intrinsically unstable. This instability is a result of a positive tension applied to the network elements. A paradigm of networks exhibiting stable polygonal configurations in spite of the junction mobility, are networks formed by bundles of Keratin Intermediate Filaments (KIFs) in live cells. A unique feature of KIF networks is a, hypothetically, negative tension generated in the network bundles due to an exchange of material between the network and an effective reservoir of unbundled filaments. Here we analyze the structure and stability of two-dimensional networks with mobile three-way junctions subject to negative tension. First, we analytically examine a simplified case of hexagonal networks with symmetric junctions and demonstrate that, indeed, a negative tension is mandatory for the network stability. Another factor contributing to the network stability is the junction elastic resistance to deviations from the symmetric state. We derive an equation for the optimal density of such networks resulting from an interplay between the tension and the junction energy. We describe a configurational degeneration of the optimal energy state of the network. Further, we analyze by numerical simulations the energy of randomly generated networks with, generally, asymmetric junctions, and demonstrate that the global minimum of the network energy corresponds to the irregular configurations.

Softness mapping of the concentration dependence of the dynamics in model soft colloidal systems

The dynamics of a series of soft colloids comprised of polystyrene cores with poly(N-isopropylacrylamide) (PNIPAM) coronas was investigated by diffusing wave spectroscopy (DWS). The modulus of the coronas was varied by changing the cross-link density and we were able to interpret the results within a hard-soft mapping framework. The soft, swellable particle properties were modeled using an extended Flory-Rehner theory and a Hertzian pair potential. Following volume fraction jumps, softness effects on the concentration dependence of dynamics were determined, with a 'soft colloids make strong glass-forming liquid'-type of behavior observed close to the nominal glass transition volume fraction, φ_g. Such behavior from the current systems cannot be fully explained by the osmotic deswelling model alone. However, inspired by the soft-hard mapping from Schmiedeberg et al, [Europhys. Lett. 2011, 96(3), 36010] we estimated effective hard-sphere diameters and achieved a successful mapping of the α-relaxation times to a master curve below φ_g. Above φ_g, the curves no longer collapse but show strong deviations from a Vogel-Fulcher type of divergence onto soft jamming plateaux. Our results provide evidence that osmotic deswelling itself cannot fully explain the observed dynamics. Softness also plays an important role in the dynamics of soft, concentrated colloids.

Topics in the mathematical design of materials

We present a perspective on several current research directions relevant to the mathematical design of new materials. We discuss: (i) design problems for phase-transforming and shape-morphing materials, (ii) epitaxy as an approach of central importance in the design of advanced semiconductor materials, (iii) selected design problems in soft matter, (iv) mathematical problems in magnetic materials, (v) some open problems in liquid crystals and soft materials and (vi) mathematical problems on liquid crystal colloids. The presentation combines topics from soft and hard condensed matter, with specific focus on those design themes where mathematical approaches could possibly lead to exciting progress. This article is part of the theme issue 'Topics in mathematical design of complex materials'.

Biomimetic structure of chitosan reinforced epoxy natural rubber with self-healed, recyclable and antimicrobial ability

Inspired by biomaterials with hard and soft structures, we reported a type of self-healed, recyclable and antimicrobial elastomers material (ECTS) which exhibited both strong mechanical strength and high toughness. ECTS was designed by furfuryl amine modified epoxy natural rubber (ENR-FA) and furaldehyde modified chitosan (CTS-FUR) through Diels-Alder (D-A) reaction. The dynamic loading capacity of the chitosan skeleton, the stress ductility of the matrix and the dynamic cross-linking between the hard and soft components gave the elastomer excellent mechanical strength, toughness and self-healing ability. The tensile strength and the elongation at break could reach up to 7.55 MPa and 487%, respectively. In addition, due to the reversibility of the covalent bond between chitosan framework and rubber matrix, the crosslinking network destroyed by external force could be reestablished under high temperature stimulation. The mechanical properties of the sample could be restored to more than 90% of the original sample, whether it was complete fracture, cyclic damage or recyclable. ECTS exhibited excellent antibacterial activity against both gram-positive bacteria (Staphylococcus aureus) and gram-negative bacteria (Pseudomonas aeruginosa), with antibacterial efficiency more than 99%. So, ECTS might has a promising application prospect in medical materials, intelligent devices, 4D-printing, etc.

Optimizing the heterogeneous network structure to achieve polymer nanocomposites with excellent mechanical properties

Designing and optimizing the polymer network structure at the molecular level to manipulate its mechanical properties are of great scientific significance. Although heterogeneous multi-network structures have been extensively investigated, little effort has been devoted to investigating heterogeneous single-networks with a well-defined interface. Herein, through coarse-grained molecular dynamics simulation, we successfully fabricated a heterogeneous single-network, which was divided into several regions with different crosslink densities. Firstly, we found that there is an optimal crosslink density ratio between high and low crosslink density regions to obtain the best stress-strain behavior. Secondly, the effect of the regularity of the network topology (by changing the distribution of two-phase regions) on mechanical properties was also studied. It was clearly observed that the polymer network showed better elastic response and mechanical properties as the distribution of two-phase regions became uniform. Finally, we investigated the effect of the selective distribution of nanoparticles (NPs) on mechanical properties by introducing NPs into a pre-designed multiphase network. Results showed that the selective distribution of NPs in the high crosslink density region had a more significant effect on the mechanical reinforcement. Generally, our simulated results may provide some guidelines to design polymer network structures to achieve high-performance polymer nanocomposites with excellent mechanical properties.