Molecular confinement accelerates deformation of entangled polymers during squeeze flow

The squeezing of polymers in narrow gaps is important for the dynamics of nanostructure fabrication by nanoimprint embossing and the operation of polymer boundary lubricants. We measured stress versus strain behavior while squeezing entangled polystyrene films to large strains. In confined conditions where films were prepared to a thickness less than the size of the bulk macromolecule, resistance to deformation was markedly reduced for both solid-glass forging and liquid-melt molding. For melt flow, we further observed a complete inversion of conventional polymer viscosity scaling with molecular weight. Our results show that squeeze flow is accelerated at small scales by an unexpected influence of film thickness in polymer materials.     

Linking models of polymerization and dynamics to predict branched polymer structure and flow

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.     

Scale-free intermittent flow in crystal plasticity

Under stress, crystals irreversibly deform through complex dislocation processes that intermittently change the microscopic material shape through isolated slip events. These underlying processes can be revealed in the statistics of the discrete changes. Through ultraprecise nanoscale measurements on nickel microcrystals, we directly determined the size of discrete slip events. The sizes ranged over nearly three orders of magnitude and exhibited a shock-and-aftershock, earthquake-like behavior over time. Analysis of the events reveals power-law scaling between the number of events and their magnitude, or scale-free flow. We show that dislocated crystals are a model system for studying scale-free behavior as observed in many macroscopic systems. In analogy to plate tectonics, smooth macroscopic-scale crystalline glide arises from the spatial and time averages of disruptive earthquake-like events at the nanometer scale.     

Influence of solid friction on polymer relaxation in gel electrophoresis

Solid friction between a charged polymer and fixed gel points can dramatically affect polymer mobility in gel electrophoresis. The effect is present when a polymer chain is entangled over many gel points along a portion of its length, leading to significantly different behavior than predicted by conventional theory: the mobility of the chain decreases and exhibits a stronger length dependence, which separates long linear charged polymers of different molecular weights.     

Pattern formation in homogeneous polymer solutions induced by a continuous-wave visible laser

We report an unexpected nonphotothermal material organization induced by continuous-wave visible laser light at low power levels. This effect is observed along the laser beam propagation direction in fully transparent entangled solutions of common homopolymers featuring sufficiently high molecular mass and optical anisotropy along the chain backbone. The resulting formation of long-lived stringlike or dotlike patterns on the micrometer scale, probed by dark-field coherent imaging, depends on the molecular mass, architecture, solvent nature, and polymer concentration. Electrostrictive and alignment forces as well as chain cooperativity are responsible for the osmotic compression of the polymer solute. Subsequent waveguiding effects induce autoamplification and "pattern writing" upon prolonged illumination. This wave-medium coupling could potentially lead to photorefractive, microoptics, and nanotechnology applications.     

Materials science. Polymers go with the flow

The flow of polymer melts and solutions differs substantially from that of other liquids, because the long polymer chains can become entangled and cannot cross each other freely in their motion. In his Perspective, Marrucci provides an overview of theoretical attempts to explain polymer flow. He highlights the report by Bent et al., who have performed sophisticated experiments on linear polymers that prove key elements of the latest theory. Furthermore, the approach used by the authors will become an important experimental tool for investigating more complex polymer systems.     

Detection of molecular alignment in confined films

Optical second harmonic generation was used to study the in-plane alignment of self-assembled silane monolayers attached to a glass surface under mechanical loading. The measurements allow correlation of the macroscopic forces acting on the monolayer with the average orientation and the azimuthal molecular alignment of the terminal molecular entity. Compression and shear forces lead to an alignment of the initially randomly oriented molecules on a macroscopic length scale. The change in azimuthal alignment of molecules under mechanical stress was found to be irreversible on the time scale of 12 hours, whereas changes of the molecular tilt angle were reversible.     

振动力场作用下高分子熔体第一法向应力差的计算

基于连续介质力学、高分子微观结构动力学和流变测量学的基础理论，推导出了平行叠加振动条件下高分子熔体第一法向应力差的计算公式；相应地，建立了在自行研制的恒速型毛细管动态流变装置上计算高分子熔体第一法向应力差的步骤和方法．以低密度聚乙烯(LDPE)为原材料，实验测量一定频率和振幅下毛细管的瞬态挤出胀大值、毛细管瞬时入口压力、活塞杆瞬时振动位移、瞬时入口压力与振动位移的相位差等参数值，将这些参数值代入上述计算公式即可求得LDPE熔体在毛细管壁处的第一法向应力差．     

Response of flexible polymers to a sudden elongational flow

Individual polymers at thermal equilibrium were exposed to an elongational flow producing a high strain rate, and their dynamics were recorded with video fluorescence microscopy. The flow was turned on suddenly so that the entire evolution of molecular conformation could be observed without initial perturbations. The rate of stretching of individual molecules is highly variable and depends on the molecular conformation that develops during stretching. This variability is due to a dependence of the dynamics on the initial, random equilibrium conformation of the polymer coil. The increasing appearance at high strain rates of slowly unraveling hairpin folds is an example of nonergodic dynamics, which can occur when a statistical mechanical system is subjected to nonadiabatic, or "sudden," external forces.     

Single-polymer dynamics in steady shear flow

The conformational dynamics of individual, flexible polymers in steady shear flow were directly observed by the use of video fluorescence microscopy. The probability distribution for the molecular extension was determined as a function of shear rate, gamma;, for two different polymer relaxation times, tau. In contrast to the behavior in pure elongational flow, the average polymer extension in shear flow does not display a sharp coil-stretch transition. Large, aperiodic temporal fluctuations were observed, consistent with end-over-end tumbling of the molecule. The rate of these fluctuations (relative to the relaxation rate) increased as the Weissenberg number, gamma;tau, was increased.     

Magma flow instability and cyclic activity at soufriere hills volcano, montserrat, british west indies

Dome growth at the Soufriere Hills volcano (1996 to 1998) was frequently accompanied by repetitive cycles of earthquakes, ground deformation, degassing, and explosive eruptions. The cycles reflected unsteady conduit flow of volatile-charged magma resulting from gas exsolution, rheological stiffening, and pressurization. The cycles, over hours to days, initiated when degassed stiff magma retarded flow in the upper conduit. Conduit pressure built with gas exsolution, causing shallow seismicity and edifice inflation. Magma and gas were then expelled and the edifice deflated. The repeat time-scale is controlled by magma ascent rates, degassing, and microlite crystallization kinetics. Cyclic behavior allows short-term forecasting of timing, and of eruption style related to explosivity potential.     

Rheology and microscopic topology of entangled polymeric liquids

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 phenomenological 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.     

Dislocation mean free paths and strain hardening of crystals

Predicting the strain hardening properties of crystals constitutes a long-standing challenge for dislocation theory. The main difficulty resides in the integration of dislocation processes through a wide range of time and length scales, up to macroscopic dimensions. In the present multiscale approach, dislocation dynamics simulations are used to establish a dislocation-based continuum model incorporating discrete and intermittent aspects of plastic flow. This is performed through the modeling of a key quantity, the mean free path of dislocations. The model is then integrated at the scale of bulk crystals, which allows for the detailed reproduction of the complex deformation curves of face-centered cubic crystals. Because of its predictive ability, the proposed framework has a large potential for further applications.     

Chain pullout and mobility effects in friction and lubrication

The interfacial shear stress that occurs when a network of a polymer that is highly mobile at the segment level (an elastomer) is slid over a smooth surface of an immobile (glassy) polymer has been measured. The glassy material is covered by a thin layer of end-attached chains of the mobile material. The experiment was designed so that there were no free chains at the interface; the slip occurred between network chains on the one side and rigid material plus end-attached mobile chains on the other side. Two main results were obtained. (i) The interfacial shear stress is strongly affected by the segment mobility of the materials on both sides of the slip plane, and considerably lower stress is observed when the materials on both sides of the interface are highly mobile. (ii) Very thin layers of tethered chains can increase the interfacial friction. Both results are relevant to the understanding of a number of practical situations that range from the operation of thin layers of lubricants, such as those found in magnetic storage devices, to the problem of wall slip and melt fracture in polymer processing.     

Molecular ordering of organic molten salts triggered by single-walled carbon nanotubes

When mixed with imidazolium ion-based room-temperature ionic liquid, pristine single-walled carbon nanotubes formed gels after being ground. The heavily entangled nanotube bundles were found to untangle within the gel to form much finer bundles. Phase transition and rheological properties suggest that the gels are formed by physical cross-linking of the nanotube bundles, mediated by local molecular ordering of the ionic liquids rather than by entanglement of the nanotubes. The gels were thermally stable and did not shrivel, even under reduced pressure resulting from the nonvolatility of the ionic liquids, but they would readily undergo a gel-to-solid transition on absorbent materials. The use of a polymerizable ionic liquid as the gelling medium allows for the fabrication of a highly electroconductive polymer/nanotube composite material, which showed a substantial enhancement in dynamic hardness.     

Quantum phase transition of a magnet in a spin bath

The excitation spectrum of a model magnetic system, LiHoF4, was studied with the use of neutron spectroscopy as the system was tuned to its quantum critical point by an applied magnetic field. The electronic mode softening expected for a quantum phase transition was forestalled by hyperfine coupling to the nuclear spins. We found that interactions with the nuclear spin bath controlled the length scale over which the excitations could be entangled. This generic result places a limit on our ability to observe intrinsic electronic quantum criticality.     

Numerical modeling of the flow of organic fertilizers in land application equipment

The knowledge of the flow behaviours of organic fertilizers in land application equipment as well as the machine–product interactions is very sparse and empirical and can hardly contribute to the design of innovative high-performance systems or to the optimization of the operational parameters of available machines. To assess the potential of using numerical modeling as a tool to optimize the design and operation of land application equipment, the specific objectives of the work reported herein were to model the flow of organic fertilizers in such machines and to validate the models against various parameters measured during field experiments using commercial spreaders.The discrete element method (DEM) was used to simulate the flow of compost while computational fluid dynamics (CFD) was applied to sludge spreading. Input parameters for both modeling methods were derived from physical and rheological properties of composts and sludges.The operation of a spreader featuring dual vertical beaters was first simulated and the ground distributions and power requirements obtained were consistent with published results for that type of machine. Two types of composts were modeled in simulations aimed at studying the effect of a flow-control gate on the discharge flow and energy requirements of the discharge conveyor. Dimensionless parameters were developed to allow for comparisons to be made between the scaled simulations and the experimental results obtained with a full size spreader. Simulated results for the discharge flow in the open gate configuration were in good agreement with measured data. The mass efficiency values for dry compost, which represent a dimensionless measure of the discharge rate, were 1.21 and 1.22 for the measured and simulated cases, respectively. The effect of the gate on the power requirements of the discharge conveyor were replicated by the models. Fracture behaviours observed in the bulk of product during field experiments were also replicated by the model.Two types of sludge exhibiting different rheological properties were modeled using a CFD code. Field experiments were carried out with a sludge spreader to measure the discharge rate of the spreader for these types of sludges. The simulated flow rate curves closely replicated the experimental ones, for both sludge types. The simulated streamlines during the unloading of the spreader were also well correlated to observations made during field experiments. The flow of sludge on a spinning disc was studied using high-speed photography and a scaled spreader physical model. The velocity of the sludge on the disc could be calculated using two different parameters measured on the images. It was found that the viscosity of the sludge influenced the spreading pattern. The flow of sludge on the disc was also numerically simulated with CFD. The simulated and measured residence time of the sludge on the disc were influenced by the viscosity of the sludge and were in close agreement. The measured and simulated residence times for fluid pasty sludge were 0.021 and 0.025 s, respectively. The measured and simulated values for plastic pasty sludge were 0.037 and 0.033 s, respectively.The DEM was successfully applied to the flow of compost and CFD was effectively used to model the flow of sludge in a land application machine. Both numerical modeling techniques showed promising results and have the potential to become more accurate through more detailed simulations and improvements in the products constitutive models.     

The microscopic basis of the glass transition in polymers from neutron scattering studies

Recent neutron scattering experiments on the microscopic dynamics of polymers below and above the glass transition temperature T(g) are reviewed. The results presented cover different dynamic processes appearing in glasses: local motions, vibrations, and different relaxation processes such as alpha- and beta-relaxation. For the alpha-relaxation, which occurs above T(g), it is possible to extend the time-temperature superposition principle, which is valid for polymers on a macroscopic scale, to the microscopic time scale. However, this principle is not applicable for temperatures approaching T(g). Below T(g), an inelastic excitation at a frequency of some hundred gigahertz (on the order of several wave numbers), the "boson peak," survives from a quasi-elastic overdamped scattering law at high temperatures. The connection between this boson peak and the fast dynamic process appearing near T(g) is discussed.     

孤立原子和双模腔内原子之间的纠缠突然死亡研究

通过计算并发度和线性熵研究了初始处于纠缠态的两个两能级原子与双模场相互作用系统的纠缠动力学特性,讨论了原子初始纠缠度和腔场初始纠缠度对并发度的影响。结果表明,两原子之间的纠缠出现纠缠突然死亡(ESD)现象,纠缠死亡持续的时间长度和原子初始的纠缠度无关,然而依赖于腔场初始纠缠度;在整个时间演化过程中,两原子和腔场之间一直保持着纠缠状态。