Block Copolymers: Thermodynamics, Dynamics, and Small-Angle Scattering

This article explores the dynamics, thermodynamics, and small-angle scattering of block copolymers. The goal is to determine what drives the applications of block copolymers, i.e. how block copolymers behave and how they are characterized. The article begins with a summary of the experimental data and various theories that comprise our understanding of block copolymer thermodynamics, with particular emphasis on phase behavior and especially the theory of microphase separation. It then considers topics related to block copolymer dynamics, including diffusion, viscoelasticity and rheology, shear-processing, and the kinetics of self-assembly. It also discusses small-angle scattering techniques as applied to block copolymer characterization, including scattering from ordered block copolymer melts.

Elastomers and Rubberlike Elasticity

This article focuses on the rubberlike elasticity of elastomers, with particular emphasis on rubberlike materials that exhibit high deformability and recoverability. It begins with a discussion of the variety of practical ways to form and characterize a rubber-elastic network, including random chemical crosslinking, highly specific chemical end-linking, polymerizations with multi-functional monomers, physical aggregation, and crosslinking in solution and in the deformed state. It then considers the effects of network structure on elastomeric properties, along with the results of elasticity experiments regarding the mechanical properties of elastomeric materials. It also examines the evolution of theories of rubber elasticity describes the specific properties of swollen polymer gels where the possibility of solvent exchange leads to some dramatic transformations in the system. Finally, it evaluates new emerging classes of rubber-elastic materials, such as liquid crystalline elastomers, where the internal microstructure added to the random network leads to some unique mechanical properties.

Biological Fluid Interfaces and Membranes

This article examines the properties of biological fluid interfaces and membranes, with particular emphasis on monolayers and bilayers. There are several examples of interfaces between biological fluids; the most relevant to human physiology are probably the liquid/air interfaces in the lungs and on the surface of the eyes. Both of these feature films with remarkable properties of compressibility and self-healing. After providing an overview of the constituent molecules of biological interfaces, this article reviews the current knowledge on surface films and membranes, giving context for their role in biology, but paying special attention to the basic physical ideas that underpin fundamental studies of in-vitro model systems. It also considers isolated membranes, characterized by tension, elasticity and viscous damping, as well as closed vesicles and cells where the membrane separates the cytoskeleton from the extracellular matrix.

Liquid Crystals

This article discusses modern directions of research in liquid crystals (LCs). LCs represent one of the best studied classes of soft matter, along with colloids, polymer solutions and melts, gels, and foams. Phenomena observed in LCs and approaches developed for their description become of heuristic value in other branches of science. This article considers the basic properties of low-molecular weight thermotropic LCs, with an additional emphasis on the developments of the last decade, such as LC colloids. It begins with an overview of thermotropic and lyotropic systems, followed by a review of the concept of order parameter, elasticity, surface anchoring, and topological defects. It also evaluates hybrid systems combining LCs with polymers or colloids, along with new ways of creating mesomorphic systems and the dynamics of LCs, including anisotropic viscosity, director dynamics and the Frederiks transition, and flow induced by thermal expansion. Finally, it describes various applications of LCs.

Dynamics of Entangled Polymers

This article explores the dynamics of entangled polymers, with particular emphasis on how the unusual and often dramatic mechanical properties of concentrated polymer systems are determined by the physics of entanglements. It begins with an overview of the foundations of entangled polymer dynamics, organized around tubes and slip links used in modeling entanglements, the packing length and concentration effects, the results of computer simulations on entanglements, topological contacts, and the effects of large deformations. The focus is on the nature of ‘entanglement’, both from a bottom-up molecular view, and from a phenomenological one. The discussion then turns to the linear viscoelasticity of entangled polymer solutions and melts, along with nonlinear viscoelasticity. Models of polymer dynamics in the linear regime are also described, including the ‘standard tube model’. The article concludes with suggestions for future work.

Order and Disorder in the Extracellular Matrix

This article examines order and disorder in the extracellular matrix (ECM). The mechanical and structural properties of the tissues in the human body are largely determined by the ECM, the fibrillar network of proteins surrounding the cells. The main component of the ECM is collagen, which self-assembles in vivo into fibrils and fibers that supply rigidity and tensile strength to the ECM. The protein is often found together with elastin, another self-assembling ECM scleroprotein, whose properties of elasticity, extensibility, and elastic recoil supplement the mechanical properties of collagen. This article first provides an overview of protein synthesis and structure, with particular emphasis on tropocollagen, tropoelastin, and recombinantly-made polypeptides. It then considers the hierarchical supramolecular organization of the ECM, collagen and elastin before concluding with an assessment of structure-function relations at the matrix level.

Physics of Granular Systems

This article discusses the fundamental physics of granular systems. It begins with an overview of the science of granular matter, followed by a description of the ‘micro’-structure on the granular level. It then considers stress transmission in mechanically equilibrated granular assemblies, focusing on conditions for marginal rigidity, isostaticity theory, and limitations of linear stress theories. It also examines the use of statistical mechanics to analyse and classify granular materials, taking into account the micro-canonical volume ensemble, structural degrees of freedom, the canonical volume ensemble and the quasi-particles of the volume ensemble, the stress ensemble, and the relationship between the volume and stress ensembles. The article concludes with an assessment of recent advances in the ongoing attempt to construct a statistical mechanical theory of granular systems.

Polyelectrolyte Solutions and Gels

This article examines the physics of polyelectrolytes, with particular emphasis on the properties of individual charged chains in solution, in more dense environments and in crosslinked gels. Polyelectrolyte solutions and gels are multicomponent systems comprised of solvent molecules, charged polymer chains, counterions, and salt ions. The properties of these systems depend on polymer and salt concentrations, the electrostatic interactions between charges, monomer-solvent interactions controlling the solvent affinity for the polymer backbone, and the interactions between charges and solvent molecules determining solvation of the ions by a solvent. This article first considers dilute salt-free polyelectrolyte solutions, focusing on polyelectrolyte chains, the condensation of counterions, and ionization equilibrium. The discussion then turns to a polyelectrolyte chain in salt solutions, semidilute polyelectrolyte solutions, and phase separation in polyelectrolyte solutions. The article concludes with an analysis of swelling in polyelectrolyte gels.

Cell Cytoskeleton

This article examines the ‘active’ part of the cell cytoskeleton — which mostly corresponds to actin and tubulin polymers and associated molecular motors — using theoretical tools derived from a soft matter physics coarse-grained approach. It begins with an overview of the cytoskeleton and its components, which include actin filaments and gels, microtubules and specialized microtubule-based organelles, molecular motors, intermediate filaments, the plasma membrane and glycocalix, the cell wall, and the extracellular matrix. It then describes coarse-grained models of the cytoskeleton and gives two examples of models for important cellular functions, namely cell migration and cell polarity. It also proposes a new kind of soft matter model providing a coarse-grained description of cytoskeletal polymers and associated molecular motors.

Fluid Transport in Gels

This article examines fluid transport and solvent dynamics in polymer gels, in equilibrium and under mechanical stress, and the effect of fluids on gel deformation. It also introduces a continuum model that describes the coupled phenomena of electric current, solvent flux, and deformation of gel network. This model is a generalization of the diffusio-mechanical coupling model of non-ionic gels. The discussion begins with an overview of the equilibrium state of non-ionic gels under the action of mechanical forces. This is followed by an analysis of the dynamics of non-ionic gels, especially the relaxation of mechanical responses caused by solvent flow. The article concludes with an assessment of the dynamics of ionic gels as well as the effect of electric field on solvent flow and the gel deformation.