Polymer Relaxation Research Articles

Coincident Correlation between Vibrational Dynamics and Primary Relaxation of Polymers with Strong or Weak Johari-Goldstein Relaxation.

The correlation between the vibrational dynamics, as sensed by the Debye-Waller factor, and the primary relaxation in the presence of secondary Johari-Goldstein (JG) relaxation, has been investigated through molecular dynamics simulations. Two melts of polymer chains with different bond length, resulting in rather different strength of the JG relaxation are studied. We focus on the bond-orientation correlation function, exhibiting higher JG sensitivity with respect to alternatives provided by torsional autocorrelation function and intermediate scattering function. We find that, even if changing the bond length alters both the strength and the relaxation time of the JG relaxation, it leaves unaffected the correlation between the vibrational dynamics and the primary relaxation. The finding is in harmony with previous studies reporting that numerical models not showing secondary relaxations exhibit striking agreement with experimental data of polymers also where the presence of JG relaxation is known.

Postextrusion Heating in Three-Dimensional Printing

Abstract Stresses result when polymer feed stock is extruded through the nozzle of a three-dimensional (3D) printer, causing undesirable surface roughness called “sharkskin,” which hinders effective bonding to the substrate. A promising method to remove the sharkskin is to reheat the polymer after extrusion. However, questions remain about the appropriate design parameters to guarantee success. A mathematical model is presented for this system, and both amorphous and crystalline polymers are examined. The former is a heat transfer problem; the latter a Stefan problem. Several effectiveness conditions are considered, including exit temperature and a duration condition related to the polymer relaxation time. Our results provide guidance on designing effective postextrusion heaters.

Analyzing the thermal and hygral behavior of wool and its impact on fabric dimensional stability for wool processing and garment manufacturing

Wool is one of the most moisture sensitive natural fibers. This paper investigated changes of wool fiber diameter, fabric dimensions and fabric dimensional properties, as a function of moisture regain, temperature and pH. Experiments were conducted on fabrics with different weave structures as well as on fabrics with and without a permanent set. Results showed that the fabrics tended to contract when they were subjected to increased temperature at saturated regain. The degree of contraction appeared to depend on the weave structure of the fabrics and permanent setting treatments. Dimensions of the wool fabrics were also found to be dependent on the pH. Greater fabric dimensions were observed at pH 7.2 than at pH 2.1. The contraction effect was almost reversible when unset fabric samples were measured in pH 2.1. The reasons for the changes of dimensional property were analyzed in terms of changes in wool fiber swelling, yarn crimp and polymer relaxation phenomena with changes in regain, temperature and pH. Industrial implications from outcomes of this research to practical wool processing are discussed in the paper.

Relaxation time of a polymer glass stretched at very large strains

The polymer relaxation dynamic of a sample, stretched up to the stress hardening regime, is measured, at room temperature, as a function of the strain $\lambda$ for a wide range of the strain rate $\dot\gamma$, by an original dielectric spectroscopy set up. The mechanical stress modies the shape of the dielectric spectra mainly because it affects the dominant polymer relaxation time $\tau$, which depends on $\lambda$ and is a decreasing function of $\dot\gamma$. The fastest dynamics is not reached at yield but in the softening regime. The dynamics slows down during the hardening, with a progressive increase of $\tau$. A small inuence of $\dot\gamma$ and $\lambda$ on the dielectric strength cannot be excluded.

Bistability in the unstable flow of polymer solutions through pore constriction arrays

Polymer solutions are often injected in porous media for applications such as oil recovery and groundwater remediation. As the fluid navigates the tortuous pore space, elastic stresses build up, causing the flow to become unstable at sufficiently large injection rates. However, it is poorly understood how the spatial and temporal characteristics of this unstable flow depend on pore space geometry, which can vary widely between different porous media. We investigate this dependence by systematically varying the spacing between pore constrictions in a one-dimensional ordered array. We find that when the pore spacing is large, unstable eddies form upstream of each constriction, similar to observations of an isolated constriction. By contrast, when the pore spacing is sufficiently small, the flow in the different pores exhibits a surprising bistability, stochastically switching between two distinct unstable flow states. We hypothesize that this unusual behaviour arises from the interplay between elongation and relaxation of polymers as they are advected through the pore space. Consistent with this idea, we find that the flow state in a given pore persists for long times; moreover, flow states are strongly correlated between neighbouring pores. Thus, the characteristics of unstable flow are not determined just by injection conditions and the geometry of the individual pores, but also depend on the spacing between pores. Ultimately, these results help to elucidate the rich array of behaviours that can arise in polymer solution flow through porous media.

Slip-spring simulations of different constraint release environments for linear polymer chains.

The constraint release (CR) mechanism has important effects on polymer relaxation and the chains will show different relaxation behaviour in conditions of monodisperse, bidisperse and other topological environments. By comparing relaxation data of linear polyisoprene (PI) chains dissolved in very long matrix and monodisperse melts, Matsumiya et al. showed that CR mechanism accelerates both dielectric and viscoelastic relaxation (Matsumiya et al. 2013 Macromolecules 46, 6067. (doi:10.1021/ma400606n)). In this work, the experimental data reported by Matsumiya et al. are reproduced using the single slip-spring (SSp) model and the CR accelerating effects on both dielectric and viscoelastic relaxation are validated by simulations. This effect on viscoelastic relaxation is more pronounced. The coincidence for end-to-end relaxation and the viscoelastic relaxation has also been checked using probe version SSp model. A variant of SSp with each entanglement assigning a characteristic lifetime is also proposed to simulate various CR environment flexibly. Using this lifetime version SSp model, the correct relaxation function can be obtained with equal numbers of entanglement destructions by CR and reptation/contour length fluctuation (CLF) for monodisperse melts. Good agreement with published experiment data is also obtained for bidisperse melts, which validates the ability to correctly describe the CR environment of the lifetime version model.

Heat transfer enhancement and reduction in low-Rayleigh number natural convection flow with polymer additives

The effects of viscoelasticity, here caused by polymer additives, on Rayleigh Bénard convection flows are investigated via direct numerical simulations at a marginally turbulent Rayleigh number. Simulations with a range of polymer length and relaxation time scales show heat transfer enhancement (HTE) and reduction (HTR). The selection of HTE and HTR depends strongly on the maximum extensional viscosity of the solution, whereas the magnitude of heat transfer modification is a function of both the maximum extensional viscosity and relaxation time of the polymer solution. The underlying physics of HTE and HTR are explored, and a mechanism of the interaction between convection cells and polymers is proposed. The findings are extrapolated to high Ra to shed some new light onto experimental observations of HTR.

Active matter in a viscoelastic environment

Active matter systems such as eukaryotic cells and bacteria continuously transform chemical energy to motion. Hence living systems exert active stresses on the complex environments in which they reside. One recurring aspect of this complexity is the viscoelasticity of the medium surrounding living systems: bacteria secrete their own viscoelastic extracellular matrix, and cells constantly deform, proliferate, and self-propel within viscoelastic networks of collagen. It is therefore imperative to understand how active matter modifies, and gets modified by, viscoelastic fluids. Here, we present a two-phase model of active nematic matter that dynamically interacts with a passive viscoelastic polymeric phase and perform numerical simulations in two dimensions to illustrate its applicability. Motivated by recent experiments we first study the suppression of cell division by a viscoelastic medium surrounding the cell. We further show that the self-propulsion of a model keratocyte cell is modified by the polymer relaxation of the surrounding viscoelastic fluid in a non-uniform manner and find that increasing polymer viscosity effectively suppresses the cell motility. Lastly, we explore the hampering impact of the viscoelastic medium on the generic hydrodynamic instabilities of active nematics by simulating the dynamics of an active stripe within a polymeric fluid. The model presented here can provide a framework for investigating more complex dynamics such as the interaction of multicellular growing systems with viscoelastic environments.

Dendrite Suppression by a Polymer Coating: A Coarse‐Grained Molecular Study

AbstractA major hurdle to the successful deployment of high‐energy‐density lithium metal based batteries is dendrite growth during battery cycling, which raises safety and cycle life concerns. Coating the Li metal anode with a soft polymer layer has been previously shown to be effective in suppressing dendrite growth, leading to uniform lithium deposition even at high current densities. A 3D coarse‐grained molecular model to study the mechanism of dendrite suppression is presented. It is found that the most effective coatings delay or even prevent dendrites from penetrating the polymer layer during deposition. The optimal deposition can be achieved by jointly tuning the polymer stiffness and relaxation time. Higher polymer dielectric permittivity and coating thickness are also effective, but the deposition rate and, therefore, the charging current density is reduced. These findings provide the basis for rational design of soft polymer coatings for stable lithium deposition.

Effects of seasoning on the vibrational properties of wood for the soundboards of string instruments.

The vibrational properties of green spruce wood samples were measured intermittently during drying and subsequent conditioning under ambient conditions to clarify the effects of seasoning. After drying, the equilibration of mass and the sound velocity of wood continued to increase, and its internal friction significantly decreased during six months of seasoning. However, those seasoning effects disappeared once the wood was moistened at 100% RH. Physical aging and stress relaxation of wood polymers was assumed to be responsible for this effect. This coincides with the empirical knowledge of violin makers: seasoning for a few years is more important than long-term aging over centuries.

PH-responsive double network alginate/kappa-carrageenan hydrogel beads for controlled protein release: Effect of pH and crosslinking agent

Bovine Serum Albumin (BSA) was encapsulated into pH-responsive alginate/kappa(κ)-carrageenan double network hydrogel beads at different pHs and CaCl2/KCl ratios. Controlled release of BSA was performed in the simulated gastric fluid (SGF) and then in simulated intestinal fluid (SIF). Alginate reinforced the mechanical properties while double network with kappa-carrageenan made slow down the release of BSA into SIF. Sulfate groups on kappa-carrageenan improved the resistivity against phosphate buffer solution. Encapsulation and BSA release was manipulated by varying pH and crosslinking concentration. The highest encapsulation efficiency (83–89%) was obtained with the hydrogel beads prepared at the lower pH than isoelectric point of BSA, then 67–72% of them released in SIF in 4–5 h while lower than 10% of BSA released in SGF, hence, protein delivery targeted intestinal was achieved. BSA release exhibited a coupling of diffusion and polymer relaxation mechanism without burst release.

Comparing the sorption kinetics of poly-tetrafluoroethylene processed either by extrusion or spark plasma sintering

The key goal of this study was to compare the sorption kinetics properties of poly-tetrafluoroethylene (PTFE) processed either by spark plasma sintering (SPS-PTFE) or extrusion methods (Ext-PTFE). A gravimetric-sorption technique was used to obtain kinetic and equilibrium adsorption data at room temperature in several liquids (toluene, n-hexane, tetrahydrofuran and chloroform). Sorption kinetics was quantified using the Berens-Hopfenberg empirical model. The results are discussed in terms of liquid diffusion and polymer relaxation. An attempt was made to correlate the crystallization rate, apparent diffusion coefficient and sorption rate for the series of samples. Additionally, the effect of sorption kinetics on the structure and mechanical properties of PTFE samples was investigated. Swelling of SPS-PTFE reduces slightly the crystallite thickness, the Young's modulus and yield stress. In contrast, the crystalline phase is less impacted by swelling for Ext-PTFE samples for witch Young's modulus is increased under the combined effect of swelling and tensile stress.

Experimental amelioration of flexural behavior under cryogenic conditioning through inter-ply fiber hybridization in FRP composites

Inter-ply fiber hybridization seems to be a very propitious technique to improve the mechanical properties of FRP composites. However, the mechanical response of these hybrid composites under cryogenic conditions has not yet fully explored experimentally. In this research paper, a systematic examination of the flexural behavior of hybrid samples conditioned at ambient temperature, zero, as well as cryogenic conditions has been carried out. In this study, hybrid polymer composites have been pondered by carbon and glass fiber as reinforcement. Fractography of the fractured samples was done by using scanning electron microscopy for featuring failure mechanisms and to validate most of the reasons mentioned in the literature regarding alteration in the properties at each tested condition. These composite materials performance might be governed by polymer relaxation at lower temperatures, matrix cracking, cryogenic hardening, and compressive residual stresses due to the difference in the coefficient of thermal expansion of the fibers and matrix. Fiber hybridization method significantly alleviated the limitations of carbon fiber such as low stain to failure, catastrophic failure, and its cost for the application to cryogenic fuel tanks for space vehicles, support structures of superconducting magnets, etc.

Deterministic polishing of micro geometries

Ultraprecision polishing of miniature components and features with small curvature radii is limited by surface accessibility, availability of suitable tools and the capability to control material removal with sufficient spatial resolution. This paper presents the development of an approach for the application of nearly deterministic micro polishing using miniature tools with 400 μm radius. To enable implementation of the proposed approach, the effect of tool wear and polymer relaxation on the control of local contact pressure is characterized and compensation of tool wear along the tool path is introduced. The process is experimentally validated by application to features in nickel phosphor with lateral dimensions down to 2 mm, yielding roughness down to less than Sa 1 nm and profile accuracy better than 10 nm P-V.

Effect of in-process laser interface heating on strength isotropy of extrusion-based additively manufactured PEEK

Fused Filament Fabrication (FFF) 3D printing provides an effective solution for fabrication of custom components at affordable cost and in short time. Nevertheless, relative to the conventional methods, the anisotropic properties exhibited by FFF 3D printed parts cause the mechanical strength to be less satisfactory. In order to resolve the problem, we propose an in-process laser local pre-deposition heating (LLPH) method, which is capable of enhancing thermal relaxation at inter-layer interface and reinforcing inter-layer adhesion, that ultimately resulted in an increase in tensile strength along the build direction. This improvement in tensile strength of the build parts with in-process laser power has been demonstrated in this manuscript. Besides, the subsequent relevance with polymer relaxation, reptation and entanglement has been discussed. As verified by mechanical testing, tensile strength was improved by 350% compared to control sample and 99.5% isotropy was achieved (build-direction strength divided by in-plane strength) with the application of laser. Scanning Electron Microscope was used to make observation of isotropic behavior at inter-layer interface.

Tough, Additively Manufactured Structures Fabricated with Dual‐Thermoplastic Filaments

Fused filament fabrication (FFF) is the most common additive manufacturing technology, but parts fabricated using FFF lack sufficient mechanical integrity for most engineering applications. Herein, a dual material (DM) filament comprising acrylonitrile butadiene styrene (ABS) with a star‐shaped polycarbonate (PC) core is fabricated using a novel thermal draw process. This DM filament is then used as feedstock in a conventional FFF printer to create 3D solid bodies with a composite ABS/PC meso‐structure. Subjecting these parts to annealing temperatures intermediate between the glass‐transition temperatures of ABS and PC produces a solid body with ductility comparable to injection‐molded ABS and fracture toughness values 15 × higher than comparable as‐printed ABS structures. The PC skeleton of specimens fabricated using the DM filament resists creep and polymer relaxation to maintain accurate part geometry during annealing. This novel DM filament can revolutionize additive manufacturing allowing low‐cost printers to produce parts with mechanical properties competitive with injection‐molded plastics.

Anomalous, Multistage Liquid Water Diffusion and Ionomer Swelling Kinetics in Nafion and Nafion Nanocomposites

The advent of commercially viable vanadium redox flow batteries has generated interest in improving upon existing membrane materials utilized in this grid-scale energy storage technology. Elucidating structure–transport relationships in perfluorosulfonated ionomer nanocomposites is of particular importance for the development of next-generation membranes, as there is a limited understanding of how nanoparticles alter transport in these membranes. In the present study, we attempt to resolve the impact of silica nanoparticles (SiNPs) on liquid water transport in Nafion–SiNP membranes. Specifically, liquid water sorption and ionomer swelling kinetics in a series of Nafion–SiNP membranes were evaluated using time-resolved attenuated total reflectance–Fourier transform infrared spectroscopy. Anomalous, multistage water uptake and swelling kinetics were observed for both Nafion and Nafion–SiNP membranes at all SiNP loadings. The first stage of the kinetic data was regressed to a diffusion–relaxation model, as water diffusion and resulting diffusion-induced polymer relaxation kinetics during this first stage were found to be highly coupled. Suppressed water transport and swelling kinetics were observed with the introduction of SiNPs, though this trend did not hold true for higher SiNP loadings. Thermal annealing of the membranes was also observed to impact the transport and swelling properties of the Nafion and Nafion–SiNP membranes, exhibiting a synergistic effect with the SiNPs at low nanoparticle loadings. Finally, the multistage water uptake mechanism in dry Nafion and Nafion–SiNP nanocomposites was understood to be governed by the time-dependent local water activity in the membrane. Overall, this study presents a clear framework that can be employed to characterize and tune aqueous transport through Nafion nanocomposite membranes.

Electrophoresis in dilute polymer solutions

We analyse the electrophoresis of a weakly charged particle with a thin double layer in a dilute polymer solution. The particle velocity in polymer solutions modelled with different constitutive equations is calculated using a regular perturbation in the polymer concentration and the generalized reciprocal theorem. The analysis shows that the polymer is strongly stretched in two regions, the birefringent strand and the high-shear region inside the double layer. The electrophoretic velocity of the particle always decreases with the addition of polymers due to both increased viscosity and fluid elasticity. At a small Weissenberg number ( ), which is the product of the polymer relaxation time and the shear rate, the polymers inside the double layer contribute to most of the velocity reduction by increasing the fluid viscosity. With increasing , viscoelasticity decreases and shear thinning increases the particle velocity. Polymer elasticity alters the fluid velocity disturbance outside the double layer from that of a neutral squirmer to a puller-type squirmer. At high , the strong extensional stress inside the birefringent strand downstream of the particle dominates the velocity reduction. The scaling of the birefringent strand is used to estimate the particle velocity.

Hydrodynamic correlations of viscoelastic fluids by multiparticle collision dynamics simulations.

The emergent fluctuating hydrodynamics of a viscoelastic fluid modeled by the multiparticle collision dynamics (MPC) approach is studied. The fluid is composed of flexible, Gaussian phantom polymers that interact by local momentum-conserving stochastic MPCs. For comparison, the analytical solution of the linearized Navier-Stokes equation is calculated, where viscoelasticity is taken into account by a time-dependent shear relaxation modulus. The fluid properties are characterized by the transverse velocity autocorrelation function in Fourier space as well as in real space. Various polymer lengths are considered-from dumbbells to (near-)continuous polymers. Viscoelasticity affects the fluid properties and leads to strong correlations, which overall decay exponentially in Fourier space. In real space, the center-of-mass velocity autocorrelation function of individual polymers exhibits a long-time tail, independent of the polymer length, which decays as t-3/2, similar to a Newtonian fluid, in the asymptotic limit t → ∞. Moreover, for long polymers, an additional power-law decay appears at time scales shorter than the longest polymer relaxation time with the same time dependence, but negative correlations, and the polymer length dependence L-1/2. Good agreement is found between the analytical and simulation results.

Viscoelastic behaviour of composites with epoxy matrix filled by cork powder

Due to the viscoelastic properties of cork and polymeric matrices, stress relaxation and creep behaviour of multiphase composites incorporating cork are important properties that should be considered at the design stage. For this purpose, composites with the same lay-up but with different fibres were manufactured with neat epoxy resin and epoxy resin filled with cork powder. It was concluded that higher values of relaxation and creep displacement occur when Kevlar fibres are used in place of carbon fibres and, for both, when cork powder was introduced into the resin. Considering the strain correspondent to 150 MPa for Kevlar composites, the difference observed between the initial and final bending stress is around 19.1 MPa for laminates with neat resin and 13.5% higher for laminates with matrix filled by cork powder. In terms of creep and comparatively to the neat resin, the displacement increases 24.6% when the resin incorporates cork powder. Regardless of whether the resin was pure or filled with cork powder, the same trend was observed for carbon composites, but with less expressive values for the same comparisons previously established. Finally, the maximum error obtained between the theoretical and experimental results of 0.6% for all conditions tested shows that the Kohlrausch-Williams-Watts (KWW) function can be used to accurately predict the relaxation stress response. In terms of creep response, this function was compared with the Findley model and, regardless of the success obtained with both, the best predictions are reached with the KWW model.