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The behaviour of ring-shaped polymers is one of the last big mysteries in polymer physics. Davide Michieletto and Matthew S Turner illustrate just why they are so hard to understand – using some delicious home-cooked pasta that they dub "anelloni".

Glassy-state structural relaxation is a physical phenomenon that occurs in most amorphous and semicrystalline polymers after their melt-processing. As such, after these industrial conformation processes, polymers are usually found in a thermodynamic non-equilibrium state. Therefore, polymer chains undergo thorough conformational changes on their way towards a thermodynamically stable state. These molecular-level events involve macroscopic-level modifications, which are manifested as changes in physical properties. Here we propose a simple approach to monitor the physical aging in polymers by differential scanning calorimetry (DSC). Accordingly, the β H enthalpy relaxation rate is directly extracted from the DSC curves taking into account the enthalpy loss at different aging times. This method allows a simple quantification of the bulk aging dynamics in polymers, and can be extrapolated to systems containing reinforcing particles; i.e. composites and nanocomposites. This safe, low cost and simple experiment designed for undergraduate students of physical, chemical and engineering specialities serves as a guide to easily determine the physical aging suffered by polymeric materials during their storage and to understand its implications at macroscopic level, which is a relevant field in condensed-matter physics. Given the paramount relevance of physical aging in glassy materials, this experiment may also serve to raise awareness of the importance of the thermal history of materials on their resulting properties, which is overlooked too often.

Paper is a material known to everybody. It has a network structure consisting of wood fibres that can be mimicked by cooking a portion of spaghetti and pouring it on a plate, to form a planar assembly of fibres that lie roughly horizontal. Real paper also contains other constituents added for technical purposes.This review has two main lines of thought. First, in the introductory part, we consider the physics that one encounters when 'using' paper, an everyday material that exhibits the presence of disorder. Questions arise, for instance, as to why some papers are opaque and others translucent, some are sturdy and others sloppy, some readily absorb drops of liquid while others resist the penetration of water. The mechanical and rheological properties of paper and paperboard are also interesting. They are inherently dependent on moisture content. In humid conditions paper is ductile and soft, in dry conditions brittle and hard.In the second part we explain in more detail research problems concerned with paper. We start with paper structure. Paper is made by dewatering a suspension of fibres starting from very low content of solids. The processes of aggregation, sedimentation and clustering are familiar from statistical mechanics. Statistical growth models or packing models can simulate paper formation well and teach a lot about its structure.The second research area that we consider is the elastic and viscoelastic properties and fracture of paper and paperboard. This has traditionally been the strongest area of paper physics. There are many similarities to, but also important differences from, composite materials. Paper has proved to be convenient test material for new theories in statistical fracture mechanics. Polymer physics and memory effects are encountered when studying creep and stress relaxation in paper. Water is a 'softener' of paper. In humid conditions, the creep rate of paper is much higher than in dry conditions.The third among our topics is the interaction of paper with water. The penetration of water into paper is an interesting transport problem because wood fibres are hygroscopic and swell with water intake. The porous fibre network medium changes as the water first penetrates into the pore space between the fibres and then into the fibres. This is an area where relatively little systematic research has been done. Finally, we summarize our thoughts on paper physics.

Advancements in single-molecule force spectroscopy techniques such as atomic force microscopy and magnetic tweezers allow investigation of how domain folding under force can play a physiological role. Combining these techniques with protein engineering and HaloTag covalent attachment, we investigate similarities and differences between four model proteins: I10 and I91-two immunoglobulin-like domains from the muscle protein titin, and two α + β fold proteins-ubiquitin and protein L. These proteins show a different mechanical response and have unique extensions under force. Remarkably, when normalized to their contour length, the size of the unfolding and refolding steps as a function of force reduces to a single master curve. This curve can be described using standard models of polymer elasticity, explaining the entropic nature of the measured steps. We further validate our measurements with a simple energy landscape model, which combines protein folding with polymer physics and accounts for the complex nature of tandem domains under force. This model can become a useful tool to help in deciphering the complexity of multidomain proteins operating under force.

Understanding protein folding and the functional properties of intrinsically disordered proteins (IDPs) requires detailed knowledge of the forces that act in polypeptide chains. These forces determine the dimensions and dynamics of unfolded and disordered proteins and have been suggested to impact processes such as the coupled binding and folding of IDPs, or the rate of protein folding reactions. Much of the progress in understanding the physical and chemical properties of unfolded and intrinsically disordered polypeptide chains has been made possible by the recent developments in single-molecule fluorescence techniques. However, the interpretation of the experimental results requires concepts from polymer physics in order to be understood. Here, I review some of the theories used to describe the dimensions of unfolded polypeptide chains under varying solvent conditions together with their more recent application to experimental data.

The interplay of topological constraints, excluded volume interactions, persistence length and dynamical entanglement length in solutions and melts of linear chains and ring polymers is investigated by means of kinetic Monte Carlo simulations of a three dimensional lattice model. In unknotted and unconcatenated rings, topological constraints manifest themselves in the static properties above a typical length scale $dt \sim 1/\sqrt{l\phi}$ ($\phi$ being the volume fraction, $l$ the mean bond length). Although one might expect that the same topological length will play a role in the dynamics of entangled polymers, we show that this is not the case. Instead, a different intrinsic length de, which scales like excluded volume blob size $\xi$, governs the scaling of the dynamical properties of both linear chains and rings., Comment: 7 pages. 4 figures

The role of screening in the vortex-loop phase transition in the anisotropic 3D X-Y model (describing HTc superconductors, etc.) is considered. RG analysis shows that effects of screening plays a decisive role in the 3D X-Y model contrary to the 2D one. As a result, a Kosterlitz-Thouless-like criterion of the phase transition, related with the appearance of a single vortex thread, which is disclosed on the system boundaries and has a smooth contour, cannot be formulated. Coulomb-type interactions between elements of the entangled loops deform and reclose these loops in order to screen the interactions. As a result one can consider vortex configurations corresponding to the effective medium with Debye-type screening. The polymer physics approach is used to show that parts of the contour of the entangled vortex loops undergo the tangle-globule transition. Due to the approach used a Kosterlitz-Thouless-like criterion for the phase transition may be formulated. As a result an estimation of the critical temperature of the phase transition is obtained for the 3D X-Y model with arbitrary anisotropy.

Macromolecular materials are everywhere – in all kinds of plastics, in food, in the polymer glasses used in compact discs and in the high-tech organic composites that are replacing steel in cars. Many crucial biological materials such as DNA and proteins are also macromolecules. Clearly it is important to understand the physical and chemical mechanisms responsible for the many desirable properties of these materials.

We find and discuss the scaling dimensions of the branch 0 manifold of the Nienhuis O(n) loop model on the square lattice, concentrating on the surface dimensions. The results are extracted from a Bethe ansatz calculation of the finite-size corrections to the eigenspectrum of the six-vertex model with free boundary conditions. These results are especially interesting for polymer physics at two values of the crossing parameter lambda . Interacting self-avoiding walks on the Manhattan lattice at the collapse temperature ( lambda = pi /3) and Hamiltonian walks on the Manhattan lattice ( lambda = pi /2) are discussed in detail. Our calculations illustrate the importance of examining both odd and even strip widths when performing finite-size correction calculations to obtain scaling dimensions.

Soft materials are materials with a low shear modulus relative to their bulk modulus and where elastic restoring forces are mainly of entropic origin. A sparse population of strong bonds connects molecules together and prevents macroscopic flow. In this review we discuss the current state of the art on how these soft materials break and detach from solid surfaces. We focus on how stresses and strains are localized near the fracture plane and how elastic energy can flow from the bulk of the material to the crack tip. Adhesion of pressure-sensitive-adhesives, fracture of gels and rubbers are specifically addressed and the key concepts are pointed out. We define the important length scales in the problem and in particular the elasto-adhesive length Γ/E where Γ is the fracture energy and E is the elastic modulus, and how the ratio between sample size and Γ/E controls the fracture mechanisms. Theoretical concepts bridging solid mechanics and polymer physics are rationalized and illustrated by micromechanical experiments and mechanisms of fracture are described in detail. Open questions and emerging concepts are discussed at the end of the review.

Living cells are organized by soft polymeric scaffolds which, from the perspective of condensed matter physics, are extraordinarily complex fluids. Cells are active, highly dynamic and far from equilibrium states. This dynamics is reflected in processes such as cell motility, mechanosensitivity, and cell division. The complexity of biological cells involves activation patterns of many genes and a vast set of molecular interactions of their products. By identifying cellular subunits which can act as independent functional modules, the complexity of biological cells becomes amenable to systematic study, both experimental and theoretical. Furthermore, the study of cellular systems can lead to new ideas to create artificial devices and machines on scales from nanometres to micrometres. An important example for such a module is the filament network known as the cytoskeleton together with sets of specific associated molecules. The cytoskeleton consists mainly of long elastic filaments, namely actin filaments, intermediate filaments and microtubules. These filaments are semiflexible polymers, i.e. polymers which behave as soft rods that bend under thermal and nonthermal forces in the cell. The transiently gel-like network of filaments in the cell is driven in a state far from thermodynamic equilibrium and is therefore an inherently active material. The cytoskeleton generates cellular motion sufficiently strong to push rigid AFM cantilevers out of the way. The active forces are generated by molecular motor-based nano-structures and by the assembly of filament networks via polymerization and cross-linking. Further, the nano-sized motors overcome the inherently slow, often glass-like Brownian polymer dynamics, resulting for cytoskeletal polymeric scaffolds in novel self-organization, rapid switching between fluid and solid states, and transitions between ordered and unordered states. The physics of the cytoskeleton combines polymer physics, statistical mechanics and cell biophysics to address a class of soft matter with novel and highly dynamic properties. We think that the present Focus Issue in New Journal of Physics can reflect the richness and excitement of this interdisciplinary field which is currently evolving rapidly. It combines theoretical and experimental approaches as well as the investigation of physical model systems and biophysical studies of living cells. Focus on Physics of the Cytoskeleton Contents Pattern formation in active cytoskeletal networks R Peter, V Schaller, F Ziebert and W Zimmermann Kinetics of stress fibers Matthew R Stachowiak and Ben O'Shaughnessy Actin-based propulsion of functionalized hard versus fluid spherical objects Vincent Delatour, Shashank Shekhar, Anne-Cecile Reymann, Dominique Didry, Kim Ho Diep Le, Guillaume Romet-Lemonne, Emmanuele Helfer and Marie-France Carlier Probing friction in actin-based motility Yann Marcy, Jean-Francois Joanny, Jacques Prost and Cecile Sykes Filament networks attached to membranes: cytoskeletal pressure and local bilayer deformation Thorsten Auth, S A Safran and Nir S Gov Less is more: removing membrane attachments stiffens the RBC cytoskeleton Nir S Gov Local and global deformations in a strain-stiffening fibrin gel Qi Wen, Anindita Basu, Jessamine P Winer, Arjun Yodh and Paul A Janmey Collective Langevin dynamics of flexible cytoskeletal fibers Francois Nedelec and Dietrich Foethke Statistical analysis of neuronal growth: edge dynamics and the effect of a focused laser on growth cone motility T Betz, D Koch, B Stuhrmann, A Ehrlicher and J Kas Coupling biochemistry and mechanics in cell adhesion: a model for inhomogeneous stress fiber contraction Achim Besser and Ulrich S Schwarz Programming protein self assembly with coiled coils Hendrik Dietz, Thomas Bornschlogl, Roland Heym, Frauke Konig and Matthias Rief Active-filament hydrodynamics: instabilities, boundary conditions and rheology Sriram Ramaswamy and Madan Rao Hydrodynamic theory for multi-component active polar gels J F Joanny, F Julicher, K Kruse and J Prost Effects of cross-links on motor-mediated filament organization Falko Ziebert, Igor S Aranson and Lev S Tsimring Actin network architecture and elasticity in lamellipodia of melanoma cells Frank Fleischer, Revathi Ananthakrishnan, Stefanie Eckel, Hendrik Schmidt, Josef Kas, Tatyana Svitkina, Volker Schmidt and Michael Beil Shear rheology of a cell monolayer Pablo Fernandez, Lutz Heymann, Albrecht Ott, Nuri Aksel and Pramod A Pullarkat Disassembly of actin networks by filament severing A E Carlsson Spontaneous waves in muscle fibres Stefan Gunther and Karsten Kruse The glassy wormlike chain Klaus Kroy and Jens Glaser Frank Julicher, Max Planck Institut fur Physik komplexer Systeme, Dresden, Germany Josef Kas, Abteilung fur die Physik weicher Materie, Universitat Leipzig, Germany

Upon temperature changes, we observe a systematic shift of creep compliance curves J(t) for single living breast epithelial cells. We use a dual-beam laser trap (optical stretcher) to induce temperature jumps within milliseconds, while simultaneously measuring the mechanical response of whole cells to optical force. The cellular mechanical response was found to differ between sudden temperature changes compared to slow, long-term changes implying adaptation of cytoskeletal structure. Interpreting optically induced cell deformation as a thermorheological experiment allows us to consistently explain data on the basis of time-temperature superposition, well known from classical polymer physics. Measured time shift factors give access to the activation energy of the viscous flow of MCF-10A breast cells, which was determined to be 80kJmol 1 . The presented measurements highlight the fundamental role that temperature plays for the deformability of cellular matter. We propose thermorheology as a powerful concept to assess the inherent material properties of living cells and to investigate cell regulatory responses upon environmental changes.

We obtain measurements of the elastic energy of short (18–30 bp) molecules of ds DNA constrained into a sharply bent conformation, using a thermodynamic method with the DNA in solution. We consider the case where there is one nick in the ds DNA, and find that the system develops a kink at a critical torque τc≈27 pN×nm. In this regime the elastic energy is linear in the end-to-end distance (EED). For smaller torques the DNA is smoothly bent and described by the worm-like–chain energy, which is also approximately linear in the EED, but with a different slope. Thus we access both the high and low elastic energy regimes, and the transition between the two.

Biological physics is in its heyday: we are witnessing the great success of single molecule techniques, new possibilities for probing and manipulating biological systems, and new theoretical concepts. For instance, optical tweezers allow one to manipulate individual biopolymers and obtain previously inaccessible results. These include measurements of the interaction between single stranded DNA binding proteins and the DNA molecule itself, and confirmation of 1D motion along the DNA chain. New optical methods allow the motion of individual particles within a cell to be tracked in space and time. Such experimental advances also inspire new directions for theory. For instance, the exploration of fluctuation theorems has added a completely new perspective to statistical mechanics. This focus issue brings together a number of studies that exemplify this development. The papers range from conceptual statistical mechanical approaches to biopolymer physics to cell mechanics. From a physics perspective, cells are complex systems combining numerous processes that can either be driven by thermal fluctuations (such as the search processes of DNA binding proteins for their binding sites on the DNA) or by active motion fuelled by biochemical energy (such as molecular motors). A complete description of all these processes remains elusive. Moreover they cannot be simulated. The individual puzzles are beginning to be unraveled, but many questions remain. For example, how are intracellular transport and regulation affected by the high degree of molecular crowding? How is the arrangement of individual genes on the DNA connected with the specific spatial configuration of the DNA in the cell? It will be interesting to see what a similar survey produces in ten years' time. We would like to thank the Editorial Board of Physical Biology for the opportunity to organize this focus issue. Thanks also to IOP Publishing, and to Andrew Malloy in particular, for the professional handling of the issue. And of course we would like to thank all those who accepted our invitation and contributed interesting reports on the state-of-the-art in their respective fields.

The dynamics of macromolecules are characterized by the presence of several length scales and related timescales in which relevant phenomena take place. This defines the complex nature of the liquid and renders its theoretical treatment a difficult matter. The necessity of developing theoretical approaches that can describe in a comprehensive manner properties observed at many different length scales is a fundamental challenge in polymer physics. This review paper summarizes some key problems arising from this challenge and different approaches taken so far in attempting to solve them. Theoretical models play a pivotal role in building the infrastructure that allows one to model these multiscale properties. We present methods for coarse-graining the structure of soft-matter systems, which provide effective potentials that are input to multiscale simulations. We also present methods for coarse-graining the dynamics of macromolecules in dilute solutions and in the melt state. Although much progress has already been made, obtaining comprehensive theoretical tools that are efficient and reliable in predicting complex fluid dynamics across many timescales of interest still remains an open challenge.

A cornerstone of modern polymer physics is the `Flory ideality hypothesis' which states that a chain in a polymer melt adopts `ideal' random-walk-like conformations. Here we revisit theoretically and numerically this pivotal assumption and demonstrate that there are noticeable deviations from ideality. The deviations come from the interplay of chain connectivity and the incompressibility of the melt, leading to an effective repulsion between chain segments of all sizes $s$. The amplitude of this repulsion increases with decreasing $s$ where chain segments become more and more swollen. We illustrate this swelling by an analysis of the form factor $F(q)$, i.e. the scattered intensity at wavevector $q$ resulting from intramolecular interferences of a chain. A `Kratky plot' of $q^2F(q)$ {\em vs.} $q$ does not exhibit the plateau for intermediate wavevectors characteristic of ideal chains. One rather finds a conspicuous depression of the plateau, $\delta(F^{-1}(q)) = |q|^3/32\rho$, which increases with $q$ and only depends on the monomer density $\rho$., Comment: 4 pages, 4 figures, EPL, accepted January 2007

The dynamic development of optical devices for the future blue laser compact disk (CD-) technology calls for a projection of the usability of the appropriate substrate polymers. As the new hardware is not yet available, a direct examination of the substrate polymers is not possible. Computer simulations may present a useful tool to predict substrate properties parallel to the development of the hardware components and prior to the chemical synthesis of polymers. With methods of polymer physics and computer simulations, important open questions are worked out. These are, the birefringence of CDs for different polymer materials, their mechanical strength and melt flow, the influence of the pit replication on the optical disk readout-signals and the influence of water absorption on the dishing of the disk. An outlook on some recent results on structurally modified polycarbonates with lower birefringence levels, that might become important in the near future is given.

The many-parameter Gaussian theory of a polymer chain is in the asymptotic region N to infinity (N is the number of monomer units) equivalent to a one-parameter scaled Gaussian theory. Such theories fulfil the necessary condition of minimum free energy only in a polymer chain without remote interactions along the chain. Therefore they cannot be used as a reliable basis in SCF calculations in polymers.

The current status of the physics of liquid-crystalline polymers is reviewed. Major attention is paid to the theory of the nematic state of solutions and melts of polymers of varying architecture, and also of polyelectrolytes. The conditions for phase equilibrium in solutions and melts and the elastic properties of systems are studied, as well as light scattering in the region of a nematic phase transition and the dynamic properties of rigid-chain polymers.

A review is given of the use of various methods of the laser light scattering to determine parameters of polymers in dissolved, gel, and molten states. Systems comprising a spectrometer and a correlator, designed specially for such studies, are described. A brief account is given of the theory of determination of different types of motion in a polymer from the correlation function of the light scattered by a sample.

The authors show how a random walk in a plane, constrained to enclose a given area, can be used to approximately represent the properties of an entangled polymer molecule. The statistical mechanical properties of the loop are calculated exactly and the distribution function for the enclosed areas is found. For the case of a random walk with free ends joined by a straight line segment, the distribution function is given by the Cauchy distribution. This implies that the area has statistical fractal properties but does not have a mean. For a genuinely closed random walk, a mean exists but the distribution of areas is not fractal. The spatial and mechanical properties of the constrained configurations have also been calculated analytically. If the unrestricted coil can be regarded as an entropic spring of zero natural length, then the area-constrained configurations behave qualitatively like springs with a finite natural length. The deformation behaviour also shows both softening and hardening dependent on the area imposed.

For pt.V see Phys. Rev. & vol.25, no.5, p.2801-11 (1982). The Gell-Mann-Low style conformational space renormalisation method for polymers is generalised to describe the crossover between the random walk and self-avoiding walk limits, i.e. to describe the excluded volume dependence. Explicit calculations are provided to order epsilon =4-d (d the spatial dimensionality) for the full end-to-end vector distribution function, the coherent elastic scattering function, the second virial coefficients and (R2) and (S2). The crossover functions are required therefore to exhibit the correct asymptotic limits of both the random and self-avoiding walks. The theory demonstrates that the latter choice implies that the expansion factors, alpha 2 and alpha s2, for the mean square end-to-end vector (R2) and radius of gyration (S2), respectively, are not universal functions of the single scaling variable describing the strength of the excluded volume interactions. Nevertheless, much of the available experimental data on long chain polymers appears to involve small renormalised dimensionless excluded volume, and therefore alpha 2 and alpha s2 are approximately universal quantities. Comparisons between our theoretical predictions and experimental data on the second virial coefficient and alpha s2 show good agreement.

The authors study the Edwards model of polymer physics using techniques based on stochastic quantisation. The approximating theory with dimensionless coupling constant is conjectured to have end-to-end distance given exactly by the Flory formula.

A new lattice spin model for many self-avoiding polymers is introduced in which the chain length distribution is fully controllable with a single generating ('magnetic') field. The model utilises spins with additional internal symmetry degrees of freedom to impose a causal connectivity of the polymer bonds on the lattice. Use of the method of random fields then produces an equivalent n to 0 limit field theory. The Flory-Huggins theory for a polymer solution emerges simply from this field theory in the mean field approximation. Polymer-polymer interactions between polymer segments on nearest-neighbour lattice are introduced into the field theory, and the low polymer volume fraction limit of the theory reduces to the Edwards type field theory for dilute through semidilute polymer solutions. A sketch is provided towards the treatment of branched polymers with fully controllable chain and branch length distributions and branching probabilities as well as a kinetic polymerisation system governed by specified propagation and termination probabilities.

Using a path integral approach an explicit expression is obtained for the two-particle probability of a polymer chain for which both the elastic energy of stretching and the elastic energy of bending are taken into account. The end-to-end distribution function is extracted from this result. Points out that because of the elastic energy of bending the probability is non-Markoffian and, thus, in this case the two-particle probability is not identical in form to the end-to-end distribution. Moreover, when calculating averages over the length of the polymer it is necessary to use the two-particle probability rather than the end-to-end distribution. As an example the authors calculate the particle scattering factor for a dilute solution of polymers.

This article surveys theoretical and experimental work on piezo- and pyroelectricity of polymers in the 1970's with special emphasis on the origins of these properties. The origins are classified into three types: (A) internal strain (§2), (B) strain- and temperature-dependences of spontaneous polarization (§3), and (C) elastic and/or dielectric heterogeneity of a system with embedded charges (§4). The origin of piezo- and pyroelectricity of poly(vinylidene fluoride) is discussed as a typical example of electret (§5). Piezoelectric relaxations of polymers are discussed in some detail (§6). Methods of measurements of piezo- and pyroelectric constants of polymer films (§7) and applications of polymer films as new transducer materials (§8) are briefly reviewed.

In recent years polymer physics has developed rapidly, and it now embraces the structure and the mechanical, optical, electrical and magnetic properties of solid polymers, melts and polymer solutions. In this review we will discuss only the structure and mechanical properties of solid polymers.

Many aspects of our physical understanding of the electrical properties of polymers are very limited, with consequent disagreement amongst research workers about their interpretation. Critical discussion of these differences can only be helpful, and a suitable forum was provided at the Polymer Physics Group's two day conference at UWIST in Cardiff on 18–19 September.

The 11th Europhysics macromolecular physics conference, held at Leeds University on 7–10 April 1981 and attended by some 80 delegates, reflected the increasing diversity of polymer physics. There is still, of course, considerable interest in the mechanical properties of oriented polymers and their relation to structure, and about 40% of the 35 papers presented dealt with such topics.

Three different papers on methods of preparing new phases in polyethylene and polystyrene were the highlights of the biennial polymer physics conference held at Shrivenham in September. Recent experiments on high pressure crystallization of polyethyllene, reported by Dr Bassett (Reading), have revealed the existence of the intermediate phase which he had predicted at the previous biennial conference.

Physical aging is the tendency of polymers to change their properties, in particular their stiffness and creep behaviour, during a period of storage. This was the subject of a one-day meeting, organised jointly by the Polymer Physics Group of The Institute of Physics and the Macro UK group and held in London on 3 March 1981.