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Frictional properties of cartilage resurfacing implants should be sufficiently low to limit damaging of the opposing cartilage during articulation. The present study determines if native lubricious molecule proteoglycan 4 (PRG4) can adsorb onto a layer-by-layer bioinspired coating composed of poly-l-lysine (PLL) and dopamine modified hyaluronic acid (HADN) and thereby can reduce the friction between implant and articular cartilage. An ELISA was developed to quantify the amount of immobilized human recombinant (rh)PRG4 after exposure to the PLL-HADN coating. The effect on lubrication was evaluated by comparing the coefficient of friction (CoF) of bare polycaprolactone (PCL) disks to that of PLL-HADN coated PCL disks while articulated against cartilage using a ring-on-disk geometry and a lubricant solution consisting of native synovial fluid components including rhPRG4. The PLL-HADN coating effectively immobilized rhPRG4. The surface roughness of PCL disks significantly increased while the water contact angle significantly decreased after application of the coating. The average CoF measured during the first minute of bare PCL against cartilage exceeded twice the CoF of the PLL-HADN coated PCL against cartilage. After 60 min, the CoF reached equilibrium values which were still significantly higher for bare PCL compared to coated PCL. The present study demonstrated that PCL can effectively be coated with PLL-HADN. Additionally, this coating reduces the friction between PCL and cartilage when a PRG4-rich lubricant is used, similar to the lubricating surface of native cartilage. This makes PLL-HADN coating a promising application to improve the clinical success of PCL-based cartilage resurfacing implants. ispartof: Journal Of Biomedical Materials Research Part B-Applied Biomaterials vol:111 issue:8 ispartof: location:United States status: published

HypothesisThe micellization of block copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) is driven by the dehydration of PPO at elevated temperatures. At low concentrations, a viscous solution of isolated micelles is obtained, whereas at higher concentrations, crowding of micelles results in an elastic gel. Alternating PEO-PPO multiblock copolymers are expected to exhibit different phase behavior, with altered phase boundaries and thermodynamics, as compared to PEO-PPO-PEO triblock copolymers (Pluronics®) with equal hydrophobicity, thereby proving the pivotal role of copolymer architecture and molecular weight.ExperimentsMultiple characterization techniques were used to map the phase behavior as a function of temperature and concentration of PEO-PPO multiblock copolymers (ExpertGel®) in aqueous solution. These techniques include shear rheology, differential and adiabatic scanning calorimetry, isothermal titration calorimetry and light transmittance. The micellar size and topology were studied by dynamic light scattering.FindingsMultiblocks have lower transition temperatures and higher thermodynamic driving forces for micellization as compared to triblocks due to the presence of more than one PPO block per chain. With increasing concentration, the multiblock copolymers in solution gradually evolve into a viscoelastic network formed by soluble bridges in between micellar nodes, whereas hairy triblock micelles jam into liquid crystalline phases resembling an elastic colloidal crystal.

Spin-coated blend films with complex phase-separated morphology find broad applications while precise tailoring of the morphology is still challenging. In this study, graphene oxide (GO)-based Janus nanosheets were synthesized by interfacial polymerization in a GO nanosheet stabilized Pickering emulsion with polystyrene (PS) and poly(hydroxyethyl methacrylate) synchronously being grafted to the GO nanosheet from the oil and water sides. The Janus nanosheets make the morphology of spin-coated PS/poly(methyl methacrylate)(PMMA) blend films tunable over the full height of the film until the substrate as their preferential assembly at the PS/PMMA interface and attachment on the glass substrate drive the top PS phase to migrate towards the substrate and the bottom PMMA phase to dewet from the substrate towards the air. By varying blend composition and Janus nanosheet loading, morphologies are readily transformed from a PS network on top of PMMA to PS droplets in the PMMA matrix and from PS encapsulated by PMMA to PMMA cavities in the PS network, etc.. This enables generating thin films with various morphologies ranging from a flat surface to cavity-network structures, droplet-matrix structures or bi-continuous structures, etc. at will. Moreover, the morphologies trapped by jammed nanosheets at the interface are super stable against further evolution upon annealing.

Azobenzene liquid crystal polymers offer the potential to fabricate autonomously operated actuators remotely controlled by light. Usually, in fully polymerized actuators, only three different states can be obtained: the initial state, the metastable light-actuated state, and the recovered state, where the first and last states are identical and stable. Here, we show that conventional azobenzene liquid crystal polymers that retain a small amount of unpolymerized monomers can exhibit multistable deformation states after manual bending and upon irradiation. This nonbonded fraction of monomers migrates under the influence of local stress gradients, such as those resulting from bending, enabling the actuator to adapt its shape to counteract the stresses induced manually or upon irradiation, resulting in stable recovery states that differ from the initial shape. Oscillatory movement of the azobenzenes and photosoftening facilitate monomer migration and thus allow multiple stable shapes using only lower-energy blue and green lights. Such materials have the potential for biomedical and microfluidic applications where light-induced, multistable states are desired and harmful UV-light needs to be avoided.

External gear pumps are used in fluid transport systems because of their tight clearances and accurate flow control. These tight clearances are a challenge for numerical studies in terms of spatial discretization. In earlier work, the flow of a viscous fluid in an external gear pump is computed using the finite element method (FEM). An element size based on the respective distance between boundaries is proposed. In this study, results based on the earlier work are compared to extrusion experiments of EPDM. The aim of this study is not only to validate the numerical simulations, but also to determine what material characteristics need to be taken into account for an accurate output prediction of the external gear pump. Especially the introduction of shear-thinning behavior results in an improvement of the amplitude of the pressure difference fluctuation. Taking into account compressibility, alters the torque fluctuation in such a way that it mimics the experiments. Unfortunately, the fluctuation in torque still has a too high amplitude. Eventually, simulations are performed including shear-thinning behavior, a temperature- and pressure-dependent viscosity, and compressibility. The effect of measuring the material behavior using oscillatory or shear experiments is shown. Furthermore, the simulations are applied to a second EPDM. Finally, different processing conditions are tested. For the simulations, only qualitative agreement is found, possibly as a result of the no slip boundary condition.

Researchers have long sought to predict the mechanical behavior of polyethylene from its microstructure. In particular, the yield strength and yield kinetics have been reported to be dependent on crystallinity and crystal thickness, but the relative importance of these two microstructural attributes has not been shown. In the present work, a series of microstructures was obtained through a combination of controlled quench rates from the melt and inclusion of various amounts of hexene comonomer. The yield strength for a wide range of strain-rates was linearly dependent on the crystallinity, and independent of crystal thickness (chain stem length), both measured by Raman spectroscopy. Similarly, yield kinetics described by a Ree-Eyring two-process stress activated model showed linear dependence on crystallinity and no dependence on crystal thickness. The results of the present work call into question models of yield kinetics dependent on screw dislocation nucleation, which depend on crystal thickness.

Construction with cementitious materials via 3D printing requires a highly thixotropic behavior, which is a challenge for construction industry. In this work a facile and low-cost gelatin-modified thermal responsive smart cement paste formulation was developed for extrusion-based 3D printing in cold environments The paste's temperature-dependent rheological properties were characterised, and the underlying mechanism was explored. At room temperature, the modified paste presents similar rheological properties as the reference cement paste. After resting for 10 min at 5 °C, the modified cement paste has a yield stress of 1900 Pa which is 10 times that of the reference paste. Moreover, the mechanical properties of the modified paste are comparable to or higher than that of the reference paste at later age. Our approach may inspire a facile manipulation of paste rheology through temperature, which facilitates smartly controlled 3D concrete printings.

In this paper, effects of pectin varying in overall charge (OCH) and local charge density (LCD) on the structure and thermal stability of Lysozyme (lys) are studied. The secondary and local tertiary structures of lys undergo at pH 5.1 a two-step change upon binding with increasing amounts of pectin, in which the peak of fluorescence first increases and shows a blue shift and the amount of helix conformation drops at a pectin/lys mass ratio (q) below or equal to that corresponding to maximal complexation qmax. With increasing amount of pectin, the native like state is recovered at q > qmax. The decrease in helix content depends mainly on the OCH of pectin. Analysis of the thermodenaturation behavior of lys reveals that affinity of pectin with denatured lys is higher than for the native state of lys. At the same OCH of pectin, unfolding of lys is more significant in case of a high LCD (blockwise distribution of charges) of the former. At q = qmax (first stage of binding) the key factors stabilizing lys within its complexes with pectin against thermo-aggregation are a high LCH of pectin and, to a lesser degree, a low OCH. At q > > qmax (second stage of binding), a high LCH of pectin is a key factor inhibiting thermo-aggregation of lys.

In paper degradation studies, the viscosity-average degree of polymerisation (DPv) is often used as a key indicator of the extent of degradation of cellulosic paper. DPv can be deduced from the viscosity of dilute paper solutions, as typically measured through glass capillary viscometry. The current study proposes an efficient, alternative method to evaluate DPv of cellulosic paper, which is based on rotational rheometry. The proposed methodology relies on the application of a shear flow in a thin film of cellulose solution to measure its dynamic viscosity, from which DPv can be subsequently derived in a straightforward fashion. Rheometry allows to measure the viscosity for a range of shear rates, which results in multiple DPv evaluations per sample, and thus in statistically representative data from an individual test. Further, rheometry typically requires considerably less paper mass per test than glass capillary viscometry, which makes the method attractive for paper degradation studies with limited sample availability. Also, rheometry measurements are less work-intensive than glass capillary viscometry measurements. The rheometry method has been applied to 4 hygrothermally aged cellulose paper samples and the unaged counterpart. The measurement results regarding the age-dependency of DPv and the number of cellulose chain scissions are compared to those obtained by glass capillary viscometry, showing a very good agreement. At a longer ageing time, both experimental methods reveal a non-linear decrease in time of DPv, and a non-linear increase in time of the number of cellulose chain scissions, which indicate that the cellulose ageing process is realistically captured. The agreement in measurement results further demonstrates that rheometry is an easy-to-use, accurate and efficient alternative for DPv measurements by glass capillary viscometry.

The present study focuses on the influence of the molecular weight distribution (MWD) on the crack-growth kinetics of fatigue-crack propagation in high-density polyethylene (HDPE) homopolymers. Compact-tension specimens of HDPE homopolymer grades, with polydispersities ranging from 2 to 45 and weight-averaged molar mass ranging from 49 to 450 kg/mol, are tested in cyclic loading at temperatures ranging between 23 and 92 °C. Through a variation of sample thickness, linear elastic-fracture mechanics is shown to apply for the chosen geometry (compact tension). It was found that the crack-propagation kinetics obey the Paris–Erdogan law, for which the Paris–Erdogan exponent m is (highly) similar for all grades tested (m = 3.9), implying that the Paris–Erdogan prefactor A is the governing parameter for the crack-growth kinetics. Relatively poor correlations are observed when the prefactor A is plotted as a function of both the tie-molecule fraction derived from the theoretical model by Huang and Brown, J. Mater. Sci.1988,23, 3648, and the average number of effective physical cross-links per chain as derived by Tervoort et al., Macromolecules2002,35, 8467. A far better correlation is observed between prefactor A and the weight-average molecular weight (Mw), which improved further when Mw is corrected for the width of the MWD, taking into account the z-average molecular weight Mz, through the ratio Mz/Mw. A power-law correlation of prefactor A with Mw and the width-corrected Mw reveals slopes of −3.4 and −3.3, respectively. Because a molecular slip within the fibrils would require chain transport through the crystalline blocks, the temperature dependence of the fatigue-crack-growth kinetics is investigated to identify the underlying molecular processes. This investigation reveals the existence of a high-temperature and a low-temperature deformation process, both of which can be related to chain-slip mechanisms through their respective activation energies (125 and 50 kJ/mole), as their activation energies are considerably lower than that required for chain scission (430 kJ/mol). This, combined with the power-law exponent of −3.4, would suggest a possible connection between the underlying failure mechanisms of craze fibrils and reptation-like dynamics. Furthermore, experiments at elevated temperatures on a selection of homopolymer grades suggest that the MWD has no influence on the temperature dependence of fatigue-crack propagation for HDPE homopolymers., Macromolecules, 54 (24), ISSN:1520-5835, ISSN:0024-9297

[Abstract] The use of supramolecular polymers to construct functional biomaterials is gaining more attention due to the tunable dynamic behavior and fibrous structures of supramolecular polymers, which resemble those found in natural systems, such as the extracellular matrix. Nevertheless, to obtain a biomaterial capable of mimicking native systems, complex biomolecules should be incorporated, as they allow one to achieve essential biological processes. In this study, supramolecular polymers based on water-soluble benzene-1,3,5-tricarboxamides (BTAs) were assembled in the presence of hyaluronic acid (HA) both in solution and hydrogel states. The coassembly of BTAs bearing tetra(ethylene glycol) at the periphery (BTA-OEG4) and HA at different ratios showed strong interactions between the two components that led to the formation of short fibers and heterogeneous hydrogels. BTAs were further covalently linked to HA (HA-BTA), resulting in a polymer that was unable to assemble into fibers or form hydrogels due to the high hydrophilicity of HA. However, coassembly of HA-BTA with BTA-OEG4 resulted in the formation of long fibers, similar to those formed by BTA-OEG4 alone, and hydrogels were produced with tunable stiffness ranging from 250 to 700 Pa, which is 10-fold higher than that of hydrogels assembled with only BTA-OEG4. Further coassembly of BTA-OEG4 fibers with other polysaccharides showed that except for dextran, all polysaccharides studied interacted with BTA-OEG4 fibers. The possibility of incorporating polysaccharides into BTA-based materials paves the way for the creation of dynamic complex biomaterials. The authors acknowledge the ICMS Animation Studio for providing the artwork. S.V.-A. and G.M. acknowledge the funding received by Gravitation Program “Materials Driven Regeneration,” funded by the Netherlands Organization for Scientific Research (024.003.013). J.M. acknowledges a Marie Skłodowska-Curie postdoctoral fellowship (794016) for financial support. G.M. acknowledges the funding received by the Swiss National Science Foundation (SNSF “Early PostDoc Mobility” P2EZP2-178435). R.C. acknowledges TA Instruments for providing the DHR-3 rheometer under the Young Distinguished Rheologist Award instrument grant. S.S. and E.W.M acknowledge the European Research Council (H2020-EU.1.1., SYNMAT project, ID 788618). Netherlands Organisation for Scientific Research; 024.003.013 Swiss National Science Foundation; P2EZP2-178435

Filament stretching rheometry is a prominent experimental method to determine rheological properties in extensional flow whereby the separating plates determine the extension rate. In literature, several correction factors that can compensate for the errors introduced by the shear contribution near the plates have been introduced and validated in the linear viscoelastic regime. In this work, a systematic analysis is conducted to determine if a material-independent correction factor can be found for non-linear viscoelastic polymers. To this end, a finite element model is presented to describe the flow and resulting stresses in the filament stretching rheometer. The model incorporates non-linear viscoelasticity and a radius-based controller for the plate speed is added to mimic the typical extensional flow in filament stretching rheometry. The model is validated by comparing force simulations with analytical solutions. The effects of the end-plates on the extensional flow and resulting force measurements are investigated, and a modification of the shear correction factor is proposed for the non-linear viscoelastic flow regime. This shows good agreement with simulations performed at multiple initial aspect ratios and strain rates and is shown to be valid for a range of polymers with non-linear rheological behaviour.

The migration of non-Brownian particles inside a droplet subjected to shear flow is investigated. The viscosity ratio, particle concentration, particle to droplet size ratio and capillary number are systematically varied. It was observed that particles migrate in the vorticity direction either towards or away from the equatorial plane. The main factor determining the migration direction is the viscosity ratio. The particle concentration and applied capillary number have no influence on the equilibrium distribution and only affect the time necessary to reach this distribution. In order to identify the flow profile, both velocity and shear rate magnitudes to which the particles inside the droplet are subjected, are numerically simulated using the Volume-of-Fluid method in a particle-free droplet/matrix system. The numerical predictions of the flow are used to evaluate the presence of possible driving mechanisms for cross-streamline particle motion, such as particle inertia and shear rate gradients. Depending on the conditions, the simulations reveal significant differences in the streamline profiles and shear rate gradients within the droplet. The observed steady state particle distributions can partially be explained by shear-induced migration towards zones of low shear rates and/or low streamline curvature. However, in complex flows, such as inside a deformed droplet, different migration mechanisms can occur simultaneously. Hence, confinement inside the droplet and potential secondary flows may also play a role.

The existence of epitaxy in polybutene-1 form II-on-form I cross-nucleation was investigated by nanofocused synchrotron X-ray diffraction. To this aim, form I hedrites, which show a lamellar stacking with a uniform crystal lattice orientation, were adopted as the substrate. The form II crystals develop a spherulitic transcrystalline layer on the lateral surface of the form I substrate, due to cross-nucleation. Through mapping both the intensity and azimuthal orientation of characteristic planes of the two polymorphs, a clearly defined nucleation area between form II (daughter) and form I (parent) could be identified. Comparing the two-dimensional diffraction patterns in that area, a preferred mutual orientation of the two structure is revealed. In particular, the (200)II plane of form II and the (110)I plane of form I are reciprocally oriented at a fixed angle of ∼8.5°. This orientation results in almost parallel (110) planes between the two structures. It is calculated that the mismatch between interchain distances within the common (110) planes is about 4%, while that along the chain axes is less than 10%, both well below the accepted mismatch criterion for epitaxial crystallization. These results provide solid evidence for the existence of an epitaxial relationship in the cross-nucleation between polybutene-1 form II and form I. We note that the (110) contact plane between the two structures in cross-nucleation is the same as the one involved in the well-studied solid-state phase transformation from form II to form I. Moreover, the X-ray nanofocus approach and the proposed data analysis could be effectively applied to other cross-nucleating systems to shed light on the role of epitaxy in this peculiar phenomenon of nucleation between polymorphs.

Abstract Thermoplastic semi-crystalline polymers are excellent candidates for tissue engineering scaffolds thanks to facile processing and tunable properties, employed in melt-based additive manufacturing. Control of crystallization and ultimate crystallinity during processing affect properties like surface stiffness and roughness. These in turn influence cell attachment, proliferation and differentiation. Surface stiffness and roughness are intertwined via crystallinity, but never studied independently. The targeted stiffness range is besides difficult to realize for a single thermoplastic. Via correlation of thermal history, crystallization and ultimate crystallinity of vitamin E plasticized poly(lactide), surface stiffness and roughness are decoupled, disclosing a range of surface mechanics of biological interest. In osteogenic environment, human mesenchymal stromal cells were more responsive to surface roughness than to surface stiffness. Cells were particularly influenced by overall crystal size distribution, not by average roughness. Absence of mold-imposed boundary constrains makes additive manufacturing ideal to spatially control crystallization and henceforward surface roughness of semi-crystalline thermoplastics. Graphic abstract

Biochar is an excellent, but less-used candidate to serve as an alternative filler in poly(3-hydroxybutyrate) (PHB)-based composites. Increasing amounts of biochar between 20 and 50 wt% were incorporated in PHB/char and PHB/thermoplastic starch (TPS)/char composites and its effects on the microstructure, crystallization and thermal properties were investigated. PHB shows a significant reduction in molecular weight after processing and the increasing amounts of biochar decreases this even stronger. From thermogravimetric analysis, it was clear that the onset degradation temperature of the PHB/char composites (255 °C) is only slightly influenced by the biochar particles up to 40 wt%. Contrastingly, this temperature reduces to 245 °C when 50 wt% of biochar is added. Additional data confirm that morphology and crystallization kinetics are enhanced up to 40 wt% of biochar, while even higher percentages of filler clearly have an opposite effect. Finally, this work reveals the ability of TPS to work as an excellent intermediator between biochar and PHB at biochar concentrations up to 20 wt%, where degradation and resulting reduction in molecular weight remains limited as compared to that of the PHB/char sample. Furthermore, like biochar, TPS acts as a nucleation agent in the composites and removes the influence of the biochar on the thermal stability of the composites. This work was supported by Vlaams Agentschap Innoveren en Ondernemen (VLAIO) and European Institute of Innovation and Technology (EIT, BM20160604). This authors would like to acknowledge MartineVanhamel for DSC, FT-IR and TGA measurements.

In this work we describe the production of spherical polymer particles from thermoplastic polymers. A simple method is developed to transform rough powders into spherical particles. The polymer particles are molten in heated oil whereafter interfacial tension driven reduction of curvature and shape retraction induce a transformation to the equilibrium spherical shape. The sphericity of the particles is validated by determining the aspect ratio, roundness, as well as circularity. Furthermore, batches of narrow particle size distribution are attained by sieving on different mesh sizes. The effects of the material characteristics and processing conditions are investigated and rationalized based on the governing physical processes and their scaling relations. As a result, the processing conditions can a priori be tailored depending on the material characteristics. The versatility of the method is confirmed by generating spherical particles from various thermoplastic polymers including PS, LDPE, PP, PA12 and PA12 filled with carbon nanotubes.

The effects of temperature, pressure, and imposed strain on the structural transition pathways of glassy atactic polystyrene (aPS) are studied for a wide range of conditions. By employing an atomistic description of the system, we systematically explore its free energy landscape, emphasizing connections between local free energy minima. A triplet of two minima connected to each other via a first-order saddle point provides the full description of each elementary structural relaxation event. The basis of the analysis is the potential energy landscape (PEL), where efficient methods for finding saddle points and exploring transition pathways have been developed. We then translate the stationary points of the PEL to stationary points of the proper free energy landscape that obeys the macroscopically imposed constraints (either stress- or strain-controlled). By changing the temperature under isobaric conditions (i.e., Gibbs energy landscape), we probe the temperature dependence of the transition rates of the subglass relaxations of aPS, thus obtaining their activation energies by fitting to the Arrhenius equation. The imposition of different strain levels under isothermic conditions allows us to estimate the apparent activation volume of every elementary transition. Our findings are in good agreement with experimental observations for the same system, indicating that both length- and time-scales of the structural transitions of glassy aPS can be obtained by proper free energy minimization of atomistically detailed configurations.

In an extrusion process, an external gear pump can be used to control the flow rate of the system. When extruding polymers, the viscosity is quite high, resulting in negligible inertia and thus laminar flow. The external gear pump contains two gears, one driven by a motor and one driven by means of contact with the other gear. In our previous work, the flow of a viscous fluid through an external gear pump was studied using the finite element method. Local mesh refinement was applied based on the respective distance between boundaries. Furthermore, the rotation of both gears was imposed. In this work, the rotation of one gear is imposed, whereas the other gear is freely rotating. However, the minimum distance between the gears is limited to a minimum value. When this value is reached, contact is assumed and also the rotation of second gear is imposed. A reversion of the torque on this gear results in a release of contact. In this manner, a quasi driver/driven situation is created in the numerical simulations. It is observed that contact is released periodically, and thus cannot be assumed present continuously, as is often prescribed. Non-Newtonian material properties, such as shear thinning and the pressure dependence of the density or the viscosity, alter how long contact is released during a tooth rotation.

The mechanical properties and network structure of photocurable polymers are strongly dependent on processing conditions. Here it is reported that highly crosslinked acrylate systems undergo unexpected additional post-curing during DMTA measurements, resulting in an increase in glass-transition temperature (Tg). A detailed study of the conditions under which this increase in Tg takes place unequivocally shows that a small (0.1%) oscillatory strain applied above Tg is responsible for additional cross-linking reactions. The effect of strain-induced post-curing is confirmed by applying post-curing treatments under oscillatory shear strain in rheological tests. Different acrylate systems were characterized and the results show that the strain induced post-curing depends on the network structure of the polymer. In polymer networks with an initial high crosslink density the effect is pronounced while in polymers with an initial lower crosslink density no shift in Tg is observed.

OBJECTIVE: The clinical success of focal metallic resurfacing implants depends largely on the friction between implant and opposing cartilage. Therefore, the present study determines the lubricating ability of the synovial fluid components hyaluronic acid (HA), proteoglycan 4 (PRG4) and a surface-active phospholipid (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, POPC), on the articulation between cartilage and a Cobalt Chromium Molybdenum (CoCrMo) implant surface, compared with two cartilage surfaces. METHODS: A ring-on-disk geometry was used to perform repeated friction measurements at physiologically relevant velocities (6 and 60 mm/s) using lubricants with an increasing number of components present. Shear measurements were performed in order to evaluate the viscosity. To ensure that it is clinically relevant to explore the effect of these components, the presence of PRG4 in synovial fluid obtained from primary and revision knee and hip implant surgeries was examined. RESULTS: PRG4 in the presence of HA was found to significantly reduce the coefficient of friction for both cartilage-cartilage and cartilage-CoCrMo interface. This is relevant, as it was also demonstrated that PRG4 is still present at the time of revision surgeries. The addition of POPC had no effect for either configurations. HA increased the viscosity of the lubricating fluid by one order of magnitude, while PRG4 and POPC had no effect. CONCLUSION: The present study demonstrates the importance of selecting the appropriate lubrication solution to evaluate implant materials with biotribology tests. Because PRG4 is a key component for reducing friction between cartilage and an opposing surface, developing coatings which bind PRG4 is recommended for cartilage resurfacing implants. ispartof: Osteoarthritis And Cartilage vol:29 issue:6 pages:894-904 ispartof: location:England status: Published online

The structure formation of particles with induced dipoles dispersed in a viscous fluid, under a spatially and temporarily uniform external electric or magnetic field, is investigated by means of Brownian Dynamics simulations. Dipole–dipole interactions forces, excluded volume forces and thermal fluctuations are accounted for. The resulting structures are characterized in terms of average orientation of their inter-particle vectors (second Legendre polynomial), network structure, size of particle clusters, anisotropy of the gyration tensor of every cluster and existence of (cluster) percolation. The magnitude of the strength of the external field and the volume fraction of particles are varied and the structural evolution of the system is followed in time. The results show that the characteristic timescale calculated from the interaction of only two dipoles is also valid for the collective dynamics of many-particle simulations. In addition, the magnitude of the strength of the external field in the range of values we investigate influences only the magnitude of the deviations around the average behavior. The main characteristics (number density of branch-points and thickness of branches) of the structure are mainly affected by the volume fraction. The possibility of 3D printing these systems is explored. While the paper provides the details about the case of an electric field, all results presented here can be translated directly into the case of a magnetic field and paramagnetic particles.

Abstract For suspensions with electromagnetic particles exposed to an external field, we examine the effect of the solidification of the suspending medium on the formation of particle structures, representative of the curing of a photo-reactive resin during stereolithography. To that end, Brownian Dynamics (BD) simulations are examined in which the solidification of the suspending medium can be incorporated by increasing its viscosity in the course of time. For illustrative purposes, it is assumed that the viscosity function is known apriori in explicit and parametrized form, however, experimental data can be used as well. It is demonstrated that one can study the effects of the viscosity increase by a transformation of time, akin to the time–temperature superposition principle, but here also in the presence of thermal noise on the suspended particles. Therefore, instead of performing BD simulations with a continuously increasing viscosity (computationally inefficient), we advocate performing simulations at a constant (low) viscosity and subsequently transform time (nonlinearly) for re-interpretation of the simulation results. So doing, one can predict the formation of particle structures during on-going solidification of the suspending medium. In practice, the viscosity increase is so drastic that further evolution of the particle structure can be considered as arrested after the characteristic transition-time of the viscosity is reached. Semi-quantitative rules of thumb are formulated for the 3D-printing practitioner. Graphical Abstract

In our previous study, we demonstrated that the time-dependent failure of transversely loaded UD (unidirectional) glass/iPP is fully plasticity-controlled and proposed a lifetime prediction method based on plasticity to predict transverse failure. In the present work, extending our previous study to other off-axis angles, we aim to investigate the effect of the off-axis angle on the time-dependent failure of UD glass/iPP, and to propose a lifetime prediction method for the off-axis failure. Glass/iPP specimens with different off-axis angles are tested at various strain rates, creep, and fatigue loads to characterize the anisotropic, time-dependent mechanical response. It is demonstrated that the influences of strain rate and fiber orientation angle on tensile strength are multiplicatively separable; also referred to as factorizable, enabling one to characterize the angle dependence at a single strain rate and the strain rate dependence at a single angle. Moreover, similar to transverse loading, off-axis failure is also observed to be plasticity-controlled. Based on these observations, the lifetime prediction method for the plasticity-controlled transverse failure is extended to off-axis loading using the aforementioned factorazibility, which resulted in lifetime predictions in agreement with the experimental creep and fatigue data.

The blend morphology model developed by Wong et al., based on Peters et al., is used to investigate the development of the disperse polymer blend morphology in twin-screw extruder flow. First, the model is written in a point-wise form suitable for using in conjunction with particle tracking. Particle tracking methods are used to generate trajectories along the flow field. Macroscopic droplet populations are placed along these trajectories and the velocity gradient tensor is extracted and applied on the point-wise blend morphology model. Very large morphology differences arise between trajectories that pass through the middle gap and those that do not. In the global distribution of (macroscopically averaged, monodisperse) droplet sizes, two distinct peaks appear due to these different trajectories. Given enough number of screw rotations, a droplet population can reach almost every position in the twin-screw extruder and travel along both types of particle trajectories. The effect of varying the gap size is that the largest droplets are unaffected, but the smallest droplets are smaller for a smaller gap size due to the higher maximum shear rate. The effect of varying the viscosity ratio on the global droplet size distribution is found to be nonlinear and is strongly determined by the Grace curve. The effect on polydisperse droplet populations is found to be that trajectories that do not pass through the gap evolve toward a single peak, whereas trajectories that do pass through the gap lead to a split into two peaks that ultimately rejoin as one peak. It is concluded that the initial position of a population in the twin-screw extruder has a very large effect on the developing transient blend morphology, though future work should be done on the importance of the initial position on the steady-state blend morphology after a very large number of screw rotations.

Investigating and understanding the intrinsic material properties of biogenic materials, which have evolved over millions of years into admirable structures with difficult to mimic hierarchical levels, holds the potential of replacing trial-and-error-based materials optimization in our efforts to make synthetic materials of similarly advanced complexity and properties. An excellent example is biogenic silica which is found in the exoskeleton of unicellular photosynthetic algae termed diatoms. Because of the complex micro- and nanostructures found in their exoskeleton, determining the intrinsic mechanical properties of biosilica in diatoms has only partly been accomplished. Here, a general method is presented in which a combination of in situ deformation tests inside an SEM with a realistic 3D model of the frustule of diatom Craspedostauros sp. (C. sp.) obtained by electron tomography, alongside finite element method (FEM) simulations, enables quantification of the Young’s modulus (E = 2.3 ± 0.1 GPa) of this biogenic hierarchical silica. The workflow presented can be readily extended to other diatom species, biominerals, or even synthetic hierarchical materials.

Binding of lambda carrageenan (lCG) to bovine serum albumin (BSA) and the effect of the formation path, namely mixing (M) at specific pH versus titration (T) from a neutral mixture, on complexation have been studied using turbidimetry, phase analysis, dynamic light scattering, confocal laser scanning and bright-field optical microscopy, circular dichroism and fluorescence measurements. The sizes and shape of the complex particles formed under T-conditions differ significantly from those obtained under M-conditions, and at maximal complexation T-complexes contain significantly less lCG as compared to M-complexes. At a pH below pIBSA phase analysis reveals two domains of q corresponding to an approximately constant lCG/BSA weight ratio in the complex phase q* (domain I), and a decreasing q* (domain II). Binding stoichiometries at pH 4.2 and 4.7 are 1:77 and 1:119 mole lCG/mole BSA. The yield of BSA in the complex phase is higher than 98% at pH 4.2 and 4.7, and the complexes form and are stable in the presence of 1.0M–1.5M NaCl. The interaction did not lead to protein denaturation or significant changes in the tertiary and secondary structure and illustrates an enhancement of the hydrophobicity of the protein within the complex particles. At pH 5.2 (pH on the wrong side of PIBSA) up to 37.6% BSA was bound to lCG, and the weight ratio of lCG/BSA in the complex particles (q*) was weakly dependent on q in the complete q range studied. The molecular origin of the unusually strong interaction between lCG and BSA is discussed.

The blend morphology model developed by Wong et al. (Rheologica Acta, 2019), based on Peters et al. (J Rheol 45(3):659–689, 2001), is used to investigate the development of the polydispersity of the disperse polymer blend morphology in complex flow. First, the model is extended with additional morphological states. The extended model is tested for simple shear flow, where it is found that the droplet size distribution does not simply scale with the shear rate, because this scaling does not hold for coalescing droplets. Subsequently, the model is applied to Poiseuille flow, showing formation of distinct layers, which occurs in realistic pressure-driven flows. Finally, the model is applied on an eccentric cylinder flow, where histograms are made of the average droplet size throughout the domain. It is observed that outer cylinder rotation results in narrow distributions where the small droplets are relatively large, whereas inner cylinder rotation results in broad distributions where the small droplets are significantly smaller than in the case of outer cylinder rotation. Eccentricity seems to only have a minor effect if the maximum shear rate is held constant. The flow profile and history in combination with the maximum shear rate strongly determine how the polydisperse droplet size distribution develops.

Determination of mechanical properties of (silica) thin films is important for the development of new applications. However, a versatile approach for small-scale sample fabrication and in situ mechanical testing of these materials is currently lacking which can overcome the existing difficulties in conventional testing approaches. Here, it is shown that by combining focused ion beam scanning electron microscopy (FIB-SEM) with micromanipulators free-standing silica beams of the desired dimensions can be fabricated. Using in situ bending tests on such beams inside the SEM and finite element method simulations, the Young's modulus of thin-film silica and the effects of gallium ion beam radiation on Young's modulus are determined. Furthermore, the effects of controlled relative humidity on the mechanical behavior of silica beams are investigated. The demonstrated FIB-SEM approach can be extended to other materials and preparation conditions to measure, model, and optimize mechanical properties of thin films for a wide range of (coating) applications.

The microstructure of products manufactured by selective laser sintering (SLS) is known to be highly dependent on various process and material parameters. The latter thus also affect the final part properties. While most work has focused on ex-situ characterization of the printed parts, little is known about the time-dependent microstructure development during sintering. In this work, we present direct observations of the microstructural evolution during laser sintering of polyamide 12 (PA12) particle doublets by in-situ synchrotron wide angle X-ray diffraction (WAXD), using our in-house developed laser sintering setup. Simultaneously, the neck growth between the particles and the temperature are captured via optical and infrared microscopy. We show that isothermal crystallization experiments under quiescent conditions are not sufficient to describe crystallization in a non-isothermal process like SLS. The enhanced crystallization kinetics in small particles suggests that both temperature and flow play a role. This finding was corroborated by the critical Weissenberg numbers estimated from rheological reptation and Rouse time scales. Furthermore, a microstructure survey has been carried out by microtoming thin slices of the sintered doublets. Both optical and atomic force microscopy reveal significant differences in the crystalline structure of the laser-affected zone as compared to the un-sintered region.

A framework is presented for the formulation of a class of continuum constitutive models for sharp interfaces with non-linear viscoelastic behaviour due to a considerable isotropic interfacial microstructure. For the formulation of a thermodynamically consistent elastoviscoplastic interface constitutive model we adapt an approach successful in describing the behaviour of bulk polymer glasses. The model has a clear separation between dilatation and shear, and is used to predict phenomena related to the plasticity of interfaces observed in the experimental literature, which is relevant for many applications. Stress–strain predictions in standard interfacial rheological flows, i.e. shear and dilatation, are investigated numerically. A predominantly elastic response is obtained at small deformations, with a transition to primarily plastic flow at high stress levels. In interfacial shear flow, strain softening and eventually a plastic plateau occur upon further deformation beyond the yield point. The yield stress and strain and (the relative strength of) the stress overshoot in interfacial shear flow are shown to be controlled by two dimensionless groups of parameters in the model. In interfacial dilatation, the model predicts elastoviscoplastic behaviour with a stress maximum and a decreasing stress without a plateau at even larger deformations. These phenomena are studied for various choices for the parameters in the model.

Magnetic microbeads have been widely used for the capturing of biomarkers, as well as for microfluidic mixing for point-of-care diagnostics. In magnetic micromixing, microbead motion is generated by external electromagnets, inducing fluid kinetics, and consequently mixing. Here, we utilize an in-plane rotating magnetic field to induce magnetic bead mixing in a circular microfluidic chamber that allows better access with (optical) readout than for existing micromixing approaches. We analyze the magnetic bead dynamics, the induced fluid profiles and we quantify the mixing performance of the system. The rotating field causes the combination of (1) a global rotating flow counter to the external field rotation induced by magnetic particles moving along the chamber side wall, with (2) local flow perturbations induced by rotating magnetic bead clusters in the central area of the chamber, rotating in the same direction as the external field. This combination leads to efficient mixing performance within 2 min of actuated magnetic field. We integrate magnetic mushroom-shaped features around the circumference of the chamber to generate significantly higher global fluid velocities compared with the no-mushroom configuration, but this results in less efficient mixing due to the absence of the central rotating bead clusters. To validate and understand the experimental results and to predict further enhancement of mixing, we carry out numerical simulations of induced fluid profiles and their corresponding mixing indices, and we explore the additional effect of integrating geometrical structures. The micromixing method we introduce here is particularly suitable for microfluidic devices in which the biochemical assay happens in a microfluidic chamber under no-flow conditions, i.e., with initially stagnant fluids, and for which the time-to-result is critical, such as in point-of-care diagnostics.

Commodity polymers are produced in large volumes, providing robust mechanical properties at relatively low costs. The products made from these commodity polymers typically offer only static functionalities. Over the past decade, however, in the scientific literature, stimuli-responsive additives and/or polymer coatings have been introduced to commodity polymers, yielding composites and bilayers that change shape in response to light, temperature, and/or humidity. These stimuli responsive commodity polymers allow the marketing and sales of these otherwise bulk products as "high-end" smart materials for applications spanning from soft actuators to adaptive textiles. This Spotlight on Applications presents an overview of recent intriguing works on how shape changing commodity polymer composite and bilayer actuators based on polyamide 6, poly(ethylene terephthalate), polyethylene, and polypropylene have been fabricated that respond to environmental stimuli and discusses their potential applications.

Producing lightweight polymeric actuators able to generate high stresses typical of hard metals and/or ceramics remains challenging. The photo-mechanical responses of ultra-drawn ultra-high molecular weight polyethylene (UHMWPE) actuators containing azobenzene photo-switches with symmetrically attached polyethylene (PE) side chains are reported. Long PE side chains promote dispersion within the apolar UHMWPE matrix, and the ultra-drawn films are highly aligned. The ultra-drawn azobenzene-doped UHMWPE films have high Young's moduli (∼100 GPa) and are viscoelastic at room temperature at strains below 1%. The photo-mechanical response of the films is fast (6 × 104 Pa (kg m−3)−1) to UV or visible light at a low strain (∼0.06%). The actuator responds to rotating linearly polarized light, causing a photo-induced stress wave response. Such rapid, high-stress, low-strain, photo-mechanical responses are unique in soft polymer systems with physical values approaching hard metals/ceramics.

Anisotropic thermal transport induced by deformation and the linear relation between the thermal conductivity and stress tensors, also known as the stress-thermal rule (STR), are tested via molecular dynamics simulations in well-entangled linear polyethylene (PE) and polystyrene (PS) melts subjected to extensional flow. We propose a method to determine the stress in deformed molecular melts, a key component missing in prior simulation studies on thermal transport in polymers that prevented verification of the STR. We compare our results with available data from previous experimental and simulation studies. Thermal conductivity (TC) is found to increase (decrease) in the direction parallel (perpendicular) to the imposed stretch. We find that the STR is valid for both PE and PS over a wide range of deformation rates and stress levels. In direct agreement with experimental evidence and the STR, we observe that for a given strain, the anisotropy in TC increases with the strain rate. Surprisingly, our results for PE question the universal behavior with respect to polymer chemistry suggested by experiments by showing a significantly higher proportionality constant (the stress-thermal coefficient) between stress and anisotropy in TC. We argue that this discrepancy can be explained by the high degree of entanglement interactions in PE affecting the transport of energy at the molecular level. Our conjecture is tested by studying an entangled linear PS melt, a polymer with a much lower entanglement plateau, for which thermal transport experimental results are available. For PS, the normalized stress-thermal coefficient is found to be commensurate with the experimental value. Finally, we test the fundamental molecular hypothesis of preferential energy transport along the backbone of polymer chains used to formulate the STR, which was prompted by early experimental evidence showing an increase in TC with chain length. We are able to establish that the increase in TC with chain length in PE melts fades as the system becomes entangled (i.e., TC remains constant beyond the critical entanglement chain length that marks the transition to entanglement-dominated rheological behavior). Our findings are of key importance in developing robust molecular-to-continuum methodologies for the study of nonisothermal macroscopic flows that are extremely relevant to polymer manufacturing processes., MTCIATTP 750985

Non-covalently crosslinked polymeric networks are promising materials towards sustainable and recyclable plastics. Here, we present the post-functionalization of poly(dimethyl siloxane) (PDMS) with supramolecular moieties, attached as grafts to the PDMS backbone, to obtain recyclable PDMS networks. We select three different supramolecular motifs that differ in interaction strength and investigate how these differences affect the dynamic behavior of the networks. The introduction of dinitrohydrazones (hydz), which afford weak supramolecular interactions by π-stacking, resulted in a viscous material at room temperature. Stronger self-association was achieved by the introduction of benzene-1,3,5-carboxamides (BTAs) and ureidopyrimidinones (UPys), which self-assemble via triple and quadruple hydrogen bonding, respectively. This resulted in a thermoplastic elastomeric material for BTA-based PDMS and brittle materials for Upy-based PDMS. Time- and temperature-dependent mechanical measurements reveal that the dynamic nature of the supramolecular bonds becomes slower upon increasing the interaction strength. The polymers are fully recyclable by solvation or compression molding without the loss of material properties. Thereby, by using one linear PDMS backbone, we demonstrate how fundamentally different material properties are obtained by changing the supramolecular interaction strength and type of non-covalent crosslinks. These molecular insights broaden the scope and application of PDMS-based sustainable materials.

By spraying liquid crystal mixtures onto stretched polyamide 6 (PA6) substrates, dual-responsive heat/humidity bilayer actuators are generated. The oriented PA6 guides the self-organization of the liquid crystal monomers into well-aligned, anisotropic liquid crystal networks. The bilayer responds to changes in the environmental relative humidity, resulting in bending of the actuator with the liquid crystal network inside the curvature. In contrast, in conditions of constant high humidity (80%RH), increasing the temperature triggers the liquid crystal network coating to bend the bilayer in the opposing direction. The dual-responsivity to changes in environmental humidity and temperature is examined in detail and discussed theoretically to elucidate the humidity-gated, temperature responsive properties revealing guidelines for fabricating anisotropic bilayer actuators.

Abstract The effect of confinement on the conformation of N dumbbells in D dimensions close to a non-interacting and rigid flat wall is examined. Using statistical mechanics and numerical calculations, the partition coefficient and the confinement-induced change in the configurational entropy are calculated as a function of the conformation tensor $${\varvec{c}}$$ c and of the distance of the dumbbells from the wall. Analytical predictions and numerical results for $$D=1$$ D = 1 concerning the behavior close to the limiting cases (onset of and saturation of confinement) agree favorably; in one case where an analytical prediction has not been achieved, a thorough numerical study establishes the limiting behavior nevertheless. Beyond these limiting cases, the overall behavior of the partition coefficient and the configurational entropy has been examined as well in detail, for various choices of the parameters. Furthermore, it is shown that the effect of confinement for $$D>1$$ D > 1 is captured entirely by the partition coefficient determined for $$D=1$$ D = 1 . In general, the average extension of the dumbbells in the direction perpendicular to the wall is decreased the closer the dumbbells are to the wall. Also, the decay of the partition coefficient with increasing extension of the dumbbells becomes steeper, i.e., more localized, the higher the number of dumbbells N. Finally, it is discussed under what conditions these results can be used also for the case of slab- (i.e., slit-) confinement. Graphical abstract

We discuss the connection between high-temperature mechanics, block structure, and composition of a model series of industrially relevant, soft, thermoplastic elastomers (TPEs) containing polydisperse hard blocks (HBs). The high-strain deformation behavior of these materials results from the combination of multiple dynamics in the system, i.e., the HB associations and the mobile and entangled amorphous phase. Many soft-TPEs show a reduction in toughness with increasing temperature. Molecular weight (Mw) has been shown to improve the temperature-dependent mechanics by increasing network connectivity. In this work, we investigate the possibility to increase the network connectivity by tuning block length at constant Mw and composition. The average number of HBs per chain can be used to quantify network connectivity; however, by using block statistics, we show how increasing this value is not enough to increase the high-temperature mechanics, especially in the case of polydisperse HBs. Since temperature affects the HB ability to associate with each other, only the number of associated HBs per chain determines network connectivity. The experimental results are consistent with modeling predictions, revealing how decreasing the average block length influences the crystal stability, which ultimately controls network connectivity, and how this relationship is affected by temperature.

In the present study, the influence of electron-beam irradiation on plasticity-controlled and crack-growth-controlled failure in high-density polyethylene (HDPE) is investigated and the effect of both molecular weight distribution (MWD) and short chain branching (SCB) content are taken into account. Size exclusion chromatography (SEC) is used to study the evolution of the MWD of the sol fraction as a function of irradiation dose. Here, it is seen that chains shorter than the percolation threshold (5 kDa) are largely unaffected by electron beam radiation, while the fraction of longest chains (M > 300 kDa) is nearly entirely incorporated into the cross-linked network. Both yield stress and Young's modulus increased with irradiation dose, where the magnitude of the increase appears to be connected to the gel fraction. The (fatigue) crack growth kinetics of the grades changed relatively little with irradiation dose, which is unexpected. Furthermore, convergence of the crack growth kinetics parameter to a narrow range of values could be observed for the investigated grades at relatively high gel fractions. This would imply that the crack growth kinetics become increasingly independent of the MWD upon irradiation cross-linking, which could be attributed to a shift in the underlying crack growth mechanism from chain slip to chain scission., Journal of Polymer Science, 60 (4), ISSN:2642-4169, ISSN:2642-4150

An external gear pump is a relatively simple and inexpensive pump, that is used in a variety of production systems. Numerous works have studied the external gear pump using numerical simulations; however, typically low-viscosity fluids and turbulent flow conditions are considered. Previous work of the authors focused on predicting the output fluctuation and the volumetric efficiency of an external gear pump processing high-viscosity fluids using a 2D representation. For certain conditions, backflow through all clearances could occur, resulting in a drop in volumetric efficiency. This calls for a full 3D model. Furthermore, high residence time zones are observed in the inflow channel of the pump. The 3D shape of these zones is still unknown. The aim of this work is to investigate the effect of the axial clearances on the performance of the external gear pump. A 3D mesh is generated by extruding the 2D mesh in the third direction, resulting in prism elements. This reduces the required number of elements and therewith makes the simulations computationally feasible. Introducing the axial clearances results in a lower efficiency compared to the 2D simulations. With particle tracking, the high residence time zones in the inflow channel are visualized in the 3D simulations.

In this contribution, we unravel the transient network topology and elasticity of micellar networks formed by hydrophobically associating multiblock copolymers in aqueous solutions. Unlike studies on conventional triblock copolymers bearing hydrophobic blocks as end groups, our research focuses on alternating poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) multiblock copolymers having multiple hydrophobic PPO blocks along the chain. We adopt a combinatorics approach to extend and generalize the mechano-statistical transient network model developed by Annable et al. for telechelic triblock copolymers [ Journal of Rheology, 1993, 37, 695] to multiblock copolymers. The model allows one to predict the concentration dependent elasticity of networks formed by multiblock copolymers with known molecular characteristics by using knowledge of the micellar network microstructure. The spatial distribution of the hydrophobic nodes is inferred from Small-Angle X-ray Scattering (SAXS) by converting the structure factor to the radial distribution function. The number of closely neighboring micellar cores between which an elastic bridge can be formed (nm) is calculated by spherical integration of the radial distribution function up to a distance of the radius of gyration of an intermediate soluble PEO block. Using the evolution of nm with concentration as an input for the model, the predictions show good agreement with experimental elasticity data, as inferred from the plateau modulus in linear shear rheology. The network evolves from loop-dominated, poorly elastic with cross-linking nodes with low functionality at low concentrations to bridge-dominated, highly elastic with higher node functionalities at more elevated concentrations. It is anticipated that our generalized mechano-statistical transient network model can also be used for equally spaced, multisticker associating polymers forming networks by multifunctional interactions other than micellar aggregation.

In this study, the effect of different process conditions on the material properties of a single UV-cured layer of methacrylate resin, typically used in the stereolithography (SLA) process, is assessed. This simplified approach of the SLA process gives the opportunity to study the link between process conditions and mechanical properties without complicated interactions between different layers. Fourier-transform infrared analysis is performed to study the effect of light intensity, curing time, and initiator concentration on the monomer conversion. A model is developed based on the reaction kinetics of photopolymerization that describes and predicts the experimental data. The effect of curing time and light intensity on the glass-transition temperature is studied. A unique relation exists between conversion and glass-transition temperature, independent of the light intensity and curing time. Tensile tests on UV-cured resin show an increase in yield stress with increasing curing time and a linear relation between glass-transition temperature and yield stress. However, a lower light intensity leads to a different network structure characterized by a lower yield stress and glass-transition temperature. The correlations between process conditions and the mechanical properties of UV-cured methacrylate systems are established to better understand the role of the processing parameters involved in the SLA process. ispartof: Macromolecules vol:52 issue:23 pages:9220-9231 ispartof: location:United States status: published

In this work, we study different rheological and thermal phenomena present during laser sintering of two polystyrene (PS) particles using fully resolved numerical simulations. In our analysis, we varied the laser power, the initial temperature and the thermal convection coefficient, used different rheological descriptions for the flow behavior and in addition studied the effect of the substrate (used in the experiments) on the temperature distribution of the system. Although we are not able to fully describe the results of the experiments with our simulations for the given parameter set, we obtained important insights in the significance of thermal initial and boundary conditions by systematically studying the sintering process.

The effect of time-dependent external fields on the structures formed by particles with induced dipoles dispersed in a viscous fluid is investigated by means of Brownian Dynamics simulations. The physical effects accounted for are thermal fluctuations, dipole-dipole and excluded volume interactions. The emerging structures are characterised in terms of particle clusters (orientation, size, anisotropy and percolation) and network structure. The strength of the external field is increased in one direction and then kept constant for a certain amount of time, with the structure formation being influenced by the slope of the field-strength increase. This effect can be partially rationalized by inhomogeneous time re-scaling with respect to the field strength, however, the presence of thermal fluctuations makes the scaling at low field strength inappropriate. After the re-scaling, one can observe that the lower the slope of the field increase, the more network-like and the thicker the structure is. In the second part of the study the field is also rotated instantaneously by a certain angle, and the effect of this transition on the structure is studied. For small rotation angles ($$\theta \le 20^{{\circ }}$$ θ ≤ 20 ∘ ) the clusters rotate but stay largely intact, while for large rotation angles ($$\theta \ge 80^{{\circ }}$$ θ ≥ 80 ∘ ) the structure disintegrates and then reforms, due to the nature of the interactions (parallel dipoles with perpendicular inter-particle vector repel each other). For intermediate angles ($$20 20 < θ < 80 ∘ ), it seems that, during rotation, the structure is altered towards a more network-like state, as a result of cluster fusion (larger clusters). The details provided in this paper concern an electric field, however, all results can be projected into the case of a magnetic field and paramagnetic particles.

An elasto-viscoplastic constitutive model was previously developed and successfully used to describe the anisotropic time-dependent response of short fiber reinforced thermoplastics. It is demonstrated that when it is applied to systems with a higher level of anisotropy, the apparent anisotropy of the rate-dependent yield stress deviates from the Hill parameters that are used to define it. This becomes more apparent if the level of anisotropy is increased. The origin of this discrepancy is mathematically investigated and shown to be related to the viscoplastic multiplier of the associated flow rule employed. A solution is proposed that completely restores consistency and extends the applicability of the model to systems with high anisotropy.

In this paper we study the numerical stability of four positive (semi-)definite formulations for viscoelastic fluid flows: the LCR formulation of Fattal and Kupferman (2004), the Cholesky-log formulation of Vaithianathan and Collins (2003), the b-symmetric formulation of Balci et al. (2011) and the CDT formulation of Carrozza et al. (2019). The numerical stability is tested by evaluating the critical time step in four different flow problems: the flow around a cylinder confined between two walls, planar extrudate swell, an axisymmetric swell problem and the extension of the axisymmetric swell problem to three dimensions. Although LCR is a good performer throughout, the CDT formulation turns out to be the most stable/robust one. The numerical stability of the Cholesky-log and the b-symmetric formulation turns out to be much lower in at least one benchmark. This supports our hypothesis that a vector-like formulation increases numerical stability at least as much as the LCR formulation and that positive definiteness is not the main contribution to numerical stability.

Electrolytes play an integral role in the successful operation of any battery chemistry. The reemergence of the sodium-ion battery (SIB) chemistry has therefore rejuvenated the search for optimized SIB salts and solvents. Recent experiments have found that 1 M NaPF 6 in ethylene carbonate (EC) and propylene carbonate (PC), EC 0.5 : PC 0.5 (w/w) is the best binary electrolyte for SIBs. However, mathematical models, to elucidate these experimental findings, have so far been lacking. Furthermore, no attempts to understand the effect of EC composition on the conductivity and electrolyte stability have been performed. Herein, the viscosity and conductivity profiles of NaPF 6 in EC 0.5 : PC 0.5 electrolyte are unraveled, using experimental and modeling approaches at different temperatures and salt concentrations. The viscosity is measured in a double-wall Couette cell and for the first time, the ionic conductivity is determined using two Pt blocking electrodes in a PAT-Cell electrochemical setup. Modeling is performed using the Advanced Electrolyte Model (AEM), a statistical mechanics software. It is shown that the conductivity and viscosity relationship follows a simple Stokes' law even at a low temperatures and high concentrations. In addition, the stability of binary and ternary electrolytes on hard carbon is shown to correlate with the preferential ion solvation of EC.