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Self-mending rubber When a rubber-band breaks, that's it: time to get another one. But a remarkable new material described in this issue behaves rather differently. Consisting of molecules containing three different functional groups that form multiple hydrogen bonds, the molecules associate to form a 'supramolecular rubber' containing both chains and cross-links. The system shows rubber-like behaviour, that is, recoverable extensibility when stretched to several times its original length. In contrast to conventional rubbers made of macromolecules, these systems when broken or cut can self-heal when the fractured surfaces are brought together at room temperature. The new material can be synthesized from simple ingredients — fatty acids and urea — and once synthesized it is readily reprocessed. In its current form supramolecular rubber has slow strain recovery and it 'creeps' under stress, but by adjusting the starting ingredients, a spectrum of properties is attainable. Molecules that associate together to form both chains and cross-links via hydrogen bonds are described; the system shows rubber-like behaviour and when broken or cut can be mended by bringing together fractured surfaces to self-heal at room temperature. Rubbers exhibit enormous extensibility up to several hundred per cent, compared with a few per cent for ordinary solids, and have the ability to recover their original shape and dimensions on release of stress 1 , 2 . Rubber elasticity is a property of macromolecules that are either covalently cross-linked 1 , 2 or connected in a network by physical associations such as small glassy or crystalline domains 3 , 4 , 5 , ionic aggregates 6 or multiple hydrogen bonds 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 . Covalent cross-links or strong physical associations prevent flow and creep. Here we design and synthesize molecules that associate together to form both chains and cross-links via hydrogen bonds. The system shows recoverable extensibility up to several hundred per cent and little creep under load. In striking contrast to conventional cross-linked or thermoreversible rubbers made of macromolecules, these systems, when broken or cut, can be simply repaired by bringing together fractured surfaces to self-heal at room temperature. Repaired samples recuperate their enormous extensibility. The process of breaking and healing can be repeated many times. These materials can be easily processed, re-used and recycled. Their unique self-repairing properties, the simplicity of their synthesis, their availability from renewable resources and the low cost of raw ingredients (fatty acids and urea) bode well for future applications.

We present a predictive scheme connecting the topological structure of highly branched entangled polymers, with industrial-level complexity, to the emergent viscoelasticity of the polymer melt. The scheme is able to calculate the linear and nonlinear viscoelasticity of a stochastically branched \"high-pressure free radical\" polymer melt as a function of the chemical kinetics of its formation. The method combines numerical simulation of polymerization with the tube/ entanglement physics of polymer dynamics extended to fully nonlinear response. We compare calculations for a series of low-density polyethylenes with experiments on structural and viscoelastic properties. The method provides a window onto the molecular processes responsible for the optimized rheology of these melts, connecting fundamental science to process in complex flow, and opens up the in silico design of new materials.

The viscoelastic properties of high molecular weight polymeric liquids are dominated by topological constraints on a molecular scale. In a manner similar to that of entangled ropes, polymer chains can slide past but not through each other. Tube models of polymer dynamics and rheology are based on the idea that entanglements confine a chain to small fluctuations around a primitive path that follows the coarse-grained chain contour. Here we provide a microscopic foundation for these highly successful phenom-enological models. We analyze the topological state of polymeric liquids in terms of primitive paths and obtain parameter-free, quantitative predictions for the plateau modulus, which agree with experiment for all major classes of synthetic polymers.

The use of fully bio-based and biodegradable materials for massive applications, such as food packaging, is an emerging tendency in polymer research. But the formulations proposed in this way should preserve or even increase the functional properties of conventional polymers, such as transparency, homogeneity, mechanical properties and low migration of their components to foodstuff. This is not always trivial, in particular when brittle biopolymers, such as poly(lactic acid) (PLA), are considered. In this work the formulation of innovative materials based on PLA modified with highly compatible plasticizers, i.e. oligomers of lactic acid (OLAs) is proposed. Three different synthesis conditions for OLAs were tested and the resulting additives were further blended with commercial PLA obtaining transparent and ductile materials, able for films manufacturing. These materials were tested in their structural, thermal and tensile properties and the best formulation among the three materials was selected. OLA with molar mass (M n ) around 1,000 Da is proposed as an innovative and fully compatible and biodegradable plasticizer for PLA, able to replace conventional plasticizers (phthalates, adipates or citrates) currently used for films manufacturing in food packaging applications.

Measurement of the thermoviscoelastic behavior of glass-forming liquids in the nanometer size range offers the possibility of increased understanding of the fundamental nature of the glass-transition phenomenon itself. We present results from use of a previously unknown method for characterizing the rheological response of nanometer-thick polymer films. The method relies on the imaging capabilities of the atomic force microscope and the reduction in size of the classical bubble inflation method of measuring the biaxial creep response of ultrathin polymer films. Creep compliance as a function of time and temperature was measured in the linear viscoelastic regime for films of poly(vinyl acetate) at a thickness of 27.5 nanometers. Although little evidence for a change in the glass temperature is found, the material exhibits previously unobserved stiffening in the rubbery response regime.

Described is a detachable fixture for a rotational rheometer, the Sentmanat Extensional Rheometer Universal Testing Platform, which incorporates dual wind-up drums to ensure a truly uniform extensional deformation during uniaxial extension experiments on polymers melts and elastomers. Although originally developed as an extensional rheometer, this highly versatile miniature test platform is capable of converting a conventional rotational rheometer host system into a single universal testing station able to characterize a host of physical properties on a variety of polymer melts and solid-state materials over a very wide range of temperatures and kinematic deformations and rates. Experimental results demonstrating these various testing capabilities are presented for a series of polymers of varying macrostructure and physical states.

A mechanical model is presented, in which viscoelastic response is described by the action of time-dependent latch elements. The model represents viscoelastic changes occurring through incremental jumps as opposed to continuous motion. This is supported by the observation that polymeric creep, recovery and stress relaxation can be correlated with stretched exponential functions, i.e. Weibull and Kohlrausch-Williams-Watts, since (i) the former is also used in reliability engineering to represent the failure of discrete elements and (ii) there is evidence of the latter being an approximation to the Eyring potential energy barrier relationship, which describes motion in terms of molecular jumps.

The diffusion process in the molten state at a polymer/polymer interface of symmetrical and model bilayers has been investigated using a small-amplitude oscillatory shear measurement. The polymers employed in this study were poly (vinylidene fluoride) (PVDF) and poly (methyl methacrylate) (PMMA) of varying molecular weights and polydispersities. The measurements were conducted in the linear viscoelastic regime (small deformations) so as to decouple the effect of flow from the diffusion. The focus of this paper has been to investigate the effects of healing time, angular frequency ( ω ), temperature, and molecular weight on the inter-diffusion and the triggered interphase between the neighboring layers. The kinetics of diffusion, based on the evolution of the apparent diffusion coefficient ( D a ) versus the healing time, was experimentally obtained. The transition from the non-Fickian to the normal Fickian region for the inter-diffusion at the interface was clearly observed, qualitatively consistent with the reptation model, but it occurred at a critical time greater than the reptation time ( τ rep ). In non-Fickian region, effects of frequency and temperature were studied with regard to the ratio of the apparent diffusion coefficient to the self-diffusion coefficient ( D a / D s ). The D s determined in the Fickian region was found to be consistent with Graessley’s model as well as with the literatures. And the dependence of the D s on the frequency agreed well with the Doi–Edwards theory, in particular, scaling as at ω  > 1/ τ e and at ω  < 1/ τ rep . Our experimental results also confirmed that the dependence of the D s on the temperature for PMMA and PVDF can be well described by the Arrhenius law. Moreover, blends of PMMAs have been proposed in order to be able to change the . The rheological investigations of these corresponding bilayers rendered it possible to monitor the effect of on the diffusion process. The obtained results gave , thus corroborating some earlier studies and some experimental results recently reported by Time-Resolved Neutron Reflectivity Measurements. Lastly, the thickness of the interphase and its corresponding viscoelastic properties could be theoretically determined as a function of the healing time.

This study focuses on the influence of molecular weight on the rheological, thermal, and mechanical behavior of poly(ether‐ether‐ketone) (PEEK), a semicrystalline high‐performance polymer. The results show that the molecular weight of PEEK has significant influence on its rheological, thermal, and mechanical behavior. It was found that PEEK has the unique characteristic of two shear‐thinning regions. The shear viscosity and the stress relaxation time of PEEK increase significantly as molecular weight increases. In general, the Cox‐Merz rule is valid for all grades of PEEK. As molecular weight increases, the melting temperature of PEEK decreases slightly, but its isothermal and nonisothermal crystallization temperatures drop dramatically. As molecular weight increases, the crystallinity, the crystallization rate, and the magnitude of crystallization activation energy decrease. The crystallization kinetics study indicates that PEEK tends to form spherical crystalline structures, regardless of its molecular weight. As molecular weight increases, the tensile strength at yield, the tensile modulus, and the flexural modulus of PEEK decrease slightly, whereas the tensile strength at break, the tensile strain at break, the modulus of toughness, and the impact strength of PEEK increase significantly. POLYM. ENG. SCI., 2011. © 2010 Society of Plastics Engineers

A slight rearrangement of the classical Cox and Merz rule suggests that the shear stress value of steady shear flow, , and complex modulus value of small amplitude oscillatory shear, G  ∗  ( ω ) = (G ′2 + G ″2 ) 1/2 , are equivalent in many respects. Small changes of material structure, which express themselves most sensitively in the steady shear stress, τ , show equally pronounced in linear viscoelastic data when plotting these with G  ∗  as one of the variables. An example is given to demonstrate this phenomenon: viscosity data that cover about three decades in frequency get stretched out over about nine decades in G  ∗  while maintaining steep gradients in a transition region. This suggests a more effective way of exploiting the Cox–Merz rule when it is valid and exploring reasons for lack of validity when it is not. The τ  −G  ∗  equivalence could also further the understanding of the steady shear normal stress function as proposed by Laun.