Isotopic Effect on the Gel and Glass Formation of a Charged Colloidal Clay: Laponite

Engineering Protein-Clay Nanosheets Composite Hydrogels with Designed Arginine-Rich Proteins

Clay nanosheets (CNSs) have been widely used in the design of nanocomposite biomaterials. CNSs display a disk-like morphology with strong negatively charged surfaces. It has been shown that guanidinium-containing molecules can bind CNSs through noncovalent salt-bridge interactions and thus serve as "molecular glues" for CNSs. Making use of the guanidinium side chain in arginine, here, we designed novel arginine-rich elastomeric proteins to engineer protein-CNS nanocomposite hydrogels. Our results showed that these arginine-rich proteins can interact with CNSs effectively and can cross-link CNSs into hydrogels. Rheological measurements showed that mechanical properties of the resultant hydrogels depended on the arginine content in the arginine-rich proteins as well as CNS/protein concentration. Compared with hydrogels constructed from CNSs or proteins alone, the novel protein-CNS nanocomposite hydrogels show much improved mechanical properties. Our work opens up a new avenue to engineer functional protein hydrogels for various applications.

Direct link between mechanical stability in gels and percolation of isostatic particles

Colloidal gels have unique mechanical and transport properties that stem from their bicontinuous nature, in which a colloidal network is intertwined with a viscous solvent, and have found numerous applications in foods, cosmetics, and construction materials and for medical applications, such as cartilage replacements. So far, our understanding of the process of colloidal gelation is limited to long-time dynamical effects, where gelation is viewed as a phase separation process interrupted by the glass transition. However, this purely out-of-equilibrium thermodynamic picture does not address the emergence of mechanical stability. With confocal microscopy experiments, we reveal that mechanical metastability is reached only after isotropic percolation of locally isostatic environments, establishing a direct link between the load-bearing ability of gels and the isostaticity condition. Our work suggests an operative description of gels based on mechanical equilibrium and isostaticity, providing the physical basis for the stability and rheology of these materials.

Microstructure and Soft Glassy Dynamics of an Aqueous Laponite Dispersion

Synthetic hectorite clay Laponite RD/XLG is composed of disk-shaped nanoparticles that acquire dissimilar charges when suspended in an aqueous medium. Owing to their property to spontaneously self-assemble, Laponite is used as a rheology modifier in a variety of commercial water-based products. In particular, an aqueous dispersion of Laponite undergoes a liquid-to-solid transition at about 1 vol % concentration. The evolution of the physical properties as the dispersion transforms to the solid state is reminiscent of physical aging in molecular as well as colloidal glasses. The corresponding soft glassy dynamics of an aqueous Laponite dispersion, including the rheological behavior, has been extensively studied in the literature. In this feature article, we take an overview of recent advances in understanding soft glassy dynamics and various efforts taken to understand the peculiar rheological behavior. Furthermore, the continuously developing microstructure that is responsible for the eventual formation of a soft solid state that supports its own weight against gravity has also been a topic of intense debate and discussion. In particularly, extensive experimental and theoretical studies lead to two types of microstructures for this system: an attractive gel-like or a repulsive glass-like structure. We carefully examine and critically analyze the literature and propose a state (phase) diagram that suggests an aqueous Laponite dispersion to be present in an attractive gel state.

Surface chemistry, rheology and microstructure of purified natural and synthetic hectorite suspensions

Natural (N-) and synthetic (S-) hectorite suspensions were found to display significant time-dependent rheology or ageing behaviour and shear thinning flow behaviour. The ageing behaviour was characterised by an increasing yield stress with rest time. The yield stress continued to increase even after a week of rest, a reflection of a long process. An open sponge-like cellular microstructure formed by platelet particles interacting attractively in the overlapping edge-face configuration was captured by cryo-SEM of gel samples prepared at high pressure (∼2000 bar) and subjected to rapid cryo-freezing, for both N- and S-hectorite gels. Even nano-discotic S-hectorite particles formed platelet particles hundreds of nanometres in length in the overlapping coin configuration. This structure, displaying a cell size ranging from tens to several hundred nanometres, is formed by strong attractive and repulsive forces. The platelets showed deformations such as bending and curling of the edges in response to these forces. The S-hectorite platelets are smaller and more rigid. During ageing the particles in the structure experience a net force. These particles will move in response causing force imbalance to be experienced by neighbouring particles and they will move in response. This action and reaction percolate through the network structure causing a high concentration of particles to respond. As a consequence the ageing process takes a long time to reach equilibrium. Various ageing models were used to fit the ageing data. The N-hectorite gels displayed a maximum yield stress at pH ∼ 8 and a particle zeta potential of -35 mV. This suggests the presence of critical positive and negative charge density is needed to form a structure with maximum strength. The zeta potential is negative and quite insensitive to pH from pH 4 to 12.

Dynamical behavior of microgels of interpenetrated polymer networks

Microgel suspensions of an interpenetrated Polymer Network (IPN) of PNIPAM and PAAc in D2O have been investigated through dynamic light scattering as a function of temperature, pH and concentration across the Volume Phase Transition (VPT). The dynamics of the system is slowed down under H/D isotopic substitution due to different balance states between polymer/polymer and polymer/solvent interactions suggesting the crucial role played by H-bonding. The swelling behavior, reduced with respect to PNIPAM and water, has been described by the Flory-Rehner theory, tested for PNIPAM microgel and successfully expanded to higher order for IPN microgels. Moreover the concentration dependence of the relaxation time at neutral pH has highlighted two different routes to approach the glass transition: Arrhenius and super-Arrhenius (Vogel Fulcher Tammann) respectively below and above the VPT and a fragility plot has been derived. Fragility can be tuned by changing temperature: across the VPT particles undergo a transition from soft-strong to stiff-fragile.