Tribological and Rheological Study of Thixotropic Gels of 2D Nanoparticles.

A thixotropic colloidal gel constituting an aqueous dispersion of synthetic clay Laponite with varying concentrations of salt has been studied for its rheological and tribological performance as a lubricant. We observed that the incorporation of NaCl induces notable enhancements in the colloidal gel's relaxation time, elastic modulus, and yield stress. Although an increase in NaCl concentration decreases the material's relaxation time dependence on waiting time (), overall, the strength of its thixotropic character has been observed to increase with an increase in salt concentration.

Thermal Conductivity of Bottle-Brush Polymers.

Using molecular dynamics (MD) simulations of a generic model, we investigated heat propagation in bottle-brush polymers (BBP). An architecture is referred to as a BBP when a linear (backbone) polymer is grafted with the side chains of different length and grafting density ρ, which control the bending stiffness of a backbone. Investigating κ-behavior in BBP is of particular interest due to two competing mechanics: increased backbone stiffness, via and ρ, increases the thermal transport coefficient κ, while the presence of side chains provides additional pathways for heat leakage.

Free Standing Dry and Stable Nanoporous Polymer Films Made through Mechanical Deformation.

A new straight forward approach to create nanoporous polymer membranes with well defined average pore diameters is presented. The method is based on fast mechanical deformation of highly entangled polymer films at high temperatures and a subsequent quench far below the glass transition temperature T . The process is first designed generally by simulation and then verified for the example of polystyrene films.

Computational indentation in highly cross-linked polymer networks.

Indentation is a common experimental technique to study the mechanics of polymeric materials. The main advantage of using indentation is this provides a direct correlation between the microstructure and the small-scale mechanical response, which is otherwise difficult within the standard tensile testing. The majority of studies have investigated hydrogels, microgels, elastomers, and even soft biomaterials.

Thermal Conductivity of Semicrystalline Polymer Networks: Crystallinity or Cross-Linking?

Understanding the heat flow in polymers is at the onset of many developments in designing advanced functional materials. Here, however, amorphous linear polymers usually exhibit a very low thermal conductivity κ, often hindering their broad applications. In this context, two common routes to increase κ are via semicrystallinity and cross-linking.

Polymer cyclization for the emergence of hierarchical nanostructures.

The creation of synthetic polymer nanoobjects with well-defined hierarchical structures is important for a wide range of applications such as nanomaterial synthesis, catalysis, and therapeutics. Inspired by the programmability and precise three-dimensional architectures of biomolecules, here we demonstrate the strategy of fabricating controlled hierarchical structures through self-assembly of folded synthetic polymers. Linear poly(2-hydroxyethyl methacrylate) of different lengths are folded into cyclic polymers and their self-assembly into hierarchical structures is elucidated by various experimental techniques and molecular dynamics simulations.

Glass Transition of Disentangled and Entangled Polymer Melts: Single-Chain-Nanoparticles Approach.

We study the effect of entanglements on the glass transition of high molecular weight polymers, by the comparison of single-chain nanoparticles (SCNPs) and equilibrated melts of high-molecular weight polystyrene of identical molecular weight. SCNPs were prepared by electrospraying technique and characterized using scanning electron microscopy and atomic force microscopy techniques. Differential scanning calorimetry, Brillouin light spectroscopy, and rheological experiments around the glass transition were compared.

Why Do Elastin-Like Polypeptides Possibly Have Different Solvation Behaviors in Water-Ethanol and Water-Urea Mixtures?

The solvent quality determines the collapsed or the expanded state of a polymer. For example, a polymer dissolved in a poor solvent collapses, whereas in a good solvent it opens up. While this standard understanding is generally valid, there are examples when a polymer collapses even in a mixture of two good solvents.

Combined Experimental and Simulation Studies of Cross-Linked Polymer Brushes under Shear.

We have studied the effect of cross-linking on the tribological behavior of polymer brushes using a combined experimental and theoretical approach. Tribological and indentation measurements on poly(glycidyl methacrylate) brushes and gels in the presence of dimethylformamide solvent were obtained by means of atomic force microscopy. To complement experiments, we have performed corresponding molecular dynamics (MD) simulations of a generic bead-spring model in the presence of explicit solvent and cross-linkers.

Influence of Chain Stiffness, Grafting Density and Normal Load on the Tribological and Structural Behavior of Polymer Brushes: A Nonequilibrium-Molecular-Dynamics Study.

We have performed coarse-grained molecular-dynamics simulations on both flexible and semiflexible multi-bead-spring model polymer brushes in the presence of explicit solvent particles, to explore their tribological and structural behaviors. The effect of stiffness and tethering density on equilibrium-brush height is seen to be well reproduced within a Flory-type theory. After discussing the equilibrium behavior of the model brushes, we first study the shearing behavior of flexible chains at different grafting densities covering brush and mushroom regimes.

Polymer Brushes under Shear: Molecular Dynamics Simulations Compared to Experiments.

Surfaces coated with polymer brushes in a good solvent are known to exhibit excellent tribological properties. We have performed coarse-grained equilibrium and nonequilibrium molecular dynamics (MD) simulations to investigate dextran polymer brushes in an aqueous environment in molecular detail. In a first step, we determined simulation parameters and units by matching experimental results for a single dextran chain.