Polymer Relaxation Research Articles

Fatigue-induced stress-softening in cross-linked multi-network elastomers: Effect of damage accumulation

Cross-linked elastomers are reinforced elastomers with high stretchability and toughness. However, most elastomeric materials lose their toughness due to the accumulation of damage under cyclic loadings during their lifetime. While recent advances in the process and characterization of the multi-network elastomers have led to significant improvements in their properties, our understandings of the accumulated damage mechanisms within the material remain sparse and inconclusive. Here, a physically motivated constitutive model is presented for multi-network elastomers subjected to a high number of cyclic deformations. The model can be particularly used to elucidate the inelastic features, such as permanent damage during the deformation of each cycle. The observed damage may be induced from the chain scission, chain slippage, polymer relaxation, or fatigue detachment of polymer chains. Therefore, irreversible chain detachment and decomposition of the network due to its highly cross-linked structure are explored as the underlying reasons for the nonlinear progressive stress softening phenomenon. The model is validated against the uni-axial tension cyclic experimental tests. The results show that progressive breakage of the chains is responsible for damage accumulation in the material. The model with a few physically motivated material constants is expected to provide a tool to understand the complex nature of constitutive behavior in the multi-network elastomers.

A new approach for modeling viscoelastic thin film lubrication

Lubricants can exhibit significant viscoelastic effects due to the addition of high molecular weight polymers. The overall behavior of the mixture is vastly different from a simpler Newtonian fluid. Therefore, understating the influence of viscoelasticity on the load carrying capacity of the film is essential for lubricated contacts. A new modeling technique based on lubrication theory is proposed to take into account viscoelastic effects. As a result, we obtain a modified equation for the pressure, i.e. the viscoelastic Reynolds (VR) equation. We have first examined a parabolic slider to mimic a roller bearing configuration. An increase of the load carrying capacity is observed when polymers are added to the lubricant. Furthermore, our results are compared with existing models based on the lubrication approximation and direct numerical simulations (DNS). For small Weissenberg number (Wi), i.e. the ratio between the polymer relaxation time and the residence time scale, VR predicts the same pressure of the linearized model, in which ϵWi is the perturbation parameter (ϵ is the ratio between the vertical length scale and the horizontal length scale). However, the difference grows rapidly as viscoelastic effects become stronger. Excellent quantitative and qualitative agreement is observed between DNS and our model over small to moderate Weissenberg number. While DNS is numerically unstable at high values of the Weissenberg number, VR does not have the same issue allowing to capture the evolution of the stress and pressure also when the viscoelastic effects are strong. It is shown that even in high shear flows, normal stresses have the largest impact on load carrying capacity and thus cannot be neglected. Furthermore, the additional pressure due to viscoelasticity comprises two components, the first one due to the normal stress and the second one due to the shear stress. Afterwards, the methodology used for the parabolic slider is extended to a plane slider where, instead, the load decreases by adding polymers to the fluid. In particular, under the effect of the polymers surface slopes enhance the rate at which pressure gradients increase, whereas curvature opposes this along the contact. Therefore, the increase of the load carrying capacity observed for viscoelastic lubricants is due to its shape close to the inlet, which is steeper than the plane slider.

The effect of fluid viscoelasticity in lubricated contacts in the presence of cavitation

In this work we study the influence of fluid viscoelasticity on the performance of lubricated contacts in the presence of cavitation. Several studies of viscoelastic lubricants have been carried out, but none of them have considered the possibility of the presence of cavitation. To describe the effect of viscoelasticity, we use the Oldroyd-B model. By assuming that the product between ϵ, i.e. the ratio between vertical and horizontal length scales, and the Weissenberg number (Wi), i.e. the ratio between polymer relaxation time and flow time scale, is small, we can linearise the viscoelastic thin film equations, following the approach pioneered by "Tichy, J., 1996, Non-Newtonian lubrication with the convected Maxwell model." Consequently, the zeroth-order in ϵWi corresponds to a Reynolds equation modified to describe also the film cavitation through the mass-conserving Elrod-Adams model. We consider the flow of viscoelastic lubricants using: (i) a cosine profile representing a journal bearing unwrapped geometry, and (ii) a pocketed profile to model a textured surface in lubricated contacts. The introduction of viscoelasticity decreases the length of cavitated region in the cosine profile due to the increasing pressure distribution within the film. Consequently, the load carrying capacity increases with Wi by up to 50% in the most favorable condition, confirming the beneficial influence of the polymers in bearings. On the other hand for the pocketed profile, results show that the load can increase or decrease at higher Wi depending on the texture position in the contact. The squeeze flow problem between two plates is also modeled for viscoelastic lubricants considering an oscillating top surface. For this configuration a load reduction is observed with increasing Wi due to the additional time needed to reform the film at high Wi. Furthermore, if viscoelastic effects increase, the cavitation region widens until reaching a value of Wi for which a full-film reformation does not occur after the initial film rupture.

Intramolecular relaxation of ring polymers in dilute solutions

The intramolecular relaxation dynamics of unconcatenated ring polymers in dilute solutions is theoretically investigated within the framework of the Rouse–Zimm theory. The excluded volume interactions (EVIs) between the nonbonded monomers are modeled by a harmonic potential, where the interaction parameter is evaluated from Flory’s mean-field approach. The hydrodynamic interactions (HIs) between the pairs of monomers are approximated by a preaveraged Oseen tensor. The mechanical moduli are dominated by the smaller relaxation rates corresponding to the collective relaxation modes in the low frequency regime, while they are governed by the higher relaxation rates corresponding to the local relaxation modes in the high frequency regime. EVI decreases the relaxation rates of the normal modes where the decrease for the collective modes is larger than that for the local modes, which consequently expands the width of the relaxation spectrum. The characteristic overall relaxation time is evaluated from the inverse of the crossover frequency, which is the same for rings of various sizes with HI and with both HI and EVI, while it shifts to lower frequencies with increasing ring size for the Rouse rings.

Swelling and Drug Release in Polymers through the Theory of Poisson-Kac Stochastic Processes.

Experiments on swelling and solute transport in polymeric systems clearly indicate that the classical parabolic models fail to predict typical non-Fickian features of sorption kinetics. The formulation of moving-boundary transport models for solvent penetration and drug release in swelling polymeric systems is addressed hereby employing the theory of Poisson–Kac stochastic processes possessing finite propagation velocity. The hyperbolic continuous equations deriving from Poisson–Kac processes are extended to include the description of the temporal evolution of both the Glass–Gel and the Gel–Solvent interfaces. The influence of polymer relaxation time on sorption curves and drug release kinetics is addressed in detail.

Reduction of birefringence by dynamic asymmetry in miscible blends of dissimilar polycarbonates

We found that the birefringence of miscible blends of dissimilar polycarbonates was lower than those of the neat component polymers though the sign of the birefringence was same. The modulus and birefringence of the blends decreased more gently in the glass transition region but more steeply in the rubbery plateau region than those of the neat component polymers. The modulus and birefringence relaxation curve fitting with the KWW and modified Rouse equations revealed wider distortion relaxation and orientation distributions in the blends than in the neat component polymers. The wide distortion relaxation might be attributed to the dynamic asymmetry of the blends. The orientation relaxed faster in the blend than in the component polymers, indicating accelerated orientation relaxation, which could be explained by assuming cooperative relaxation of the component polymers strongly affected by fast relaxation component of the low-Tg one due to the dynamic asymmetry with a large glass transition temperature difference between the component polymers. Owing to the accelerated relaxation, the elongated blends exhibited lower birefringence than the component polymers.

Numerical Simulation of Tensile Residual Stresses in SWCNT-Reinforced Polymer Composites

Abstract The residual stresses play a significant role in the mechanical properties and strengthening capability of nanocomposites. The present research aims to numerically investigate the residual stress relaxation in nanotube-reinforced polymers in response to mechanical tensile loading. The systems under study consist of the armchair and zigzag single-walled carbon nanotubes (SWCNT) embedded in a polymer matrix. The nanotubes and polymer matrix are assumed to be bonded by van der Waals interactions based on the Lennard-Jones (L-J) potential at the interface. The interactions between carbon atoms in the nanotube and nodes in the polymer matrix are modelled by equivalent springs. In order to evaluate the analysis of elastic-perfectly plastic using finite element (FE) modelling, first, relaxation of the plastic residual stresses on steel hemisphere in contact with a rigid flat surface was examined in a loading-unloading cycle and verified with available data. Afterwards, the residual stress relaxation in nanotubes with different space-frame structures was computed due to displacement-controlled loading. Finally, the stress state and the plastic residual stresses in the nanocomposite for different carbon nanotube content were analyzed and discussed during loading and unloading. Regarding the effect of tensile stress, it was revealed that nanotube structures have significant effects on the residual stresses created in the nanocomposite.

An orientation-stretch coupled model for entangled comb polymers

A semi-empirical constitutive equation is suggested for the model comb polymer with the purpose to simultaneously describe the intrinsic medium amplitude oscillatory shear (MAOS) behaviors and transient behaviors in start-up elongational experiments. The molecular stress and hierarchical relaxation of comb polymers are discussed on the basis of the orientation-stretch coupled conformation tensor of both backbone and arm, which are governed by the same evolution equation with different relaxation times. The discrete Baumgaertel–Schausberger–Winter spectrum is adopted for the backbone relaxation. The model has four parameters that are fully determined from the linear spectrum. The equation set is solved analytically by the perturbation method under MAOS and solved numerically under start-up extensional flows. The predictions on the nonlinear rheological behaviors are well consistent with the experiments.

Polymer scission in turbulent flows

Abstract

Slow magnetic relaxation in mixed-valence coordination polymer, containing Co(III) cluster and Co(II) nodes

Single-molecule magnets in cobalt clusters is much less well known than it is in manganese(II)/(III)/(IV) clusters. To explore new types of cobalt clusters and their potential applications, a new mixed valence coordination polymer ([Co(III)3Co(II)2(mba)6(Hdtba) (H2O)4]n, H2dtba=2,2’-dithiodibenzoic acid, H2mba=2-mercaptobenzoic acid) (1) is constructed by employing cobalt chloride hexahydrate and H2mba as starting reactants under hydrothermal conditions. Single-crystal XRD studies indicate that 1 consists of a trinuclear cobalt(III)-sulfur cluster moiety and two discrete cobalt(II) nodes, which result in the formation of a covalent three-dimensional complex network. Similarly, the cobalt(III) cluster moiety acts as a building unit in this infinite coordination polymer 1 instead of the discrete and tiara-like clusters previously reported in the literature. Magnetic studies show its slow magnetic relaxation in the absence of an applied dc field, which is characteristic behavior of single-ion magnets, caused by the individual Co(II) ions.

Effect of interaction enhancement on rheological response of polypropylene /polybutadiene blend composites

Enhancement interaction between polybutadiene (PB) nanoparticle and polypropylene (PP) matrix by grafts via hydrogen bonding was verified and characterized by dynamic rheological analysis. The introduction of acrylamide (AM) facilitated PB uniform distribution in PP and strengthened interfacial interaction. The correlated structure based on H-bonding interaction leading to the viscoelastic nonlinearity, which restricted polymer relaxation and exhibited different rheological responses. The formation of correlated structure is an energetically favorable process driven by enthalpy, while the dynamics of construction is controlled by diffusion of the binding groups correspondingly, thus, the reconstruction and recoverability depend on the thermodynamic equilibrium distorted by either the shearing or the temperature. Therefore, the recovery ratio increases from 77.31%, 96.53%, to 98.87%, for the blend with identical composition, when increment of temperature from 180, 200–220 °C, is attributed to relaxation enhancement for both the decrement of local viscosity and the acceleration of diffusion.

Electrospun Nanofibers Withstandable to High-Temperature Reactions: Synergistic Effect of Polymer Relaxation and Solvent Removal

Fiber breakage is found to be a ubiquitous phenomenon during the thermal treatments for electrospun nanofibers, because of the presence of solvent molecules and unrelaxed assembly of polymer chains. Here a strengthening strategy is designed by introducing a pre-heating stage for the as-spun nanofibers. At a temperature above the polymer’s glass transition temperature, the chains can get sufficiently relaxed by the aid of hot solvent, and at the same time, the solvent confined in the entangled chains can fully and steadily evaporate, but not erupt, off the nanofiber, when the temperature is just as high as the boiling point. Therefore, the nanofibers become more uniform in structure and can withstand the subsequent thermal reactions. For polyacrylonitrile-based electrospinning, such strategy can improve the final tensile strength from 42 to 112 MPa for the nanofiber films.

Polymeric Films Containing Tenoxicam as Prospective Transdermal Drug Delivery Systems: Design and Characterization

The administration of drugs via transdermal therapeutic systems has become an attractive form of therapeutic approach, considering its advantages and the high patient compliance achieved, making them a viable alternative, especially in the treatment of chronic diseases. The purpose of our study was the development of polymer-based films containing tenoxicam (TX) and the analysis of dissolution kinetics. Auxiliary substances represent an important part of pharmaceutical forms, so during the first stage, TX and excipient compatibility were verified. Fourier Transform Infrared Spectroscopy (FT-IR) and Differential Scanning Calorimetry (DSC) analyses were performed on TX and on physical mixtures of TX-HPMCE5 and TX-HPMC15kcP. Three polymeric films of TX (TX1, TX2, and TX3) were prepared using a solvent evaporation technique. Release studies were done at 32 °C ± 1 °C with a Franz diffusion cell. The results of the DSC and FT-IR analyses demonstrated the compatibility of the active substance with the two matrix-forming polymers. The results obtained in the release studies of TX from the proposed polymeric films suggested a pH-dependent behavior in all three polymeric films. At pH 5.5, flux values were between 8.058 ± 0.125 μg·cm−2·h−1 and 10.850 ± 0.380 μg·cm−2·h−1; and at pH 7.4, between 10.990 ± 0.2.490 μg·cm−2·h−1 and 53.140 ± 0.196 μg·cm−2·h−1. The Korsmeyer–Peppas model described a non-Fickian transport mechanism. The n values varied between 0.63–0.7 at pH 5.5 and 0.73–0.86 at pH 7.4, which suggested a diffusion depending on the matrix hydration and polymer relaxation.

On the Size Effect of Additives in Amorphous Shape Memory Polymers.

Small additive molecules often enhance structural relaxation in polymers. We explore this effect in a thermoplastic shape memory polymer via molecular dynamics simulations. The additive-to-monomer size ratio is shown to play a key role here. While the effect of additive-concentration on the rate of shape recovery is found to be monotonic in the investigated range, a non-monotonic dependence on the size-ratio emerges at temperatures close to the glass transition. This work thus identifies the additives’ size to be a qualitatively novel parameter for controlling the recovery process in polymer-based shape memory materials.

Dielectric Performance of Silica-Filled Nanocomposites Based on Miscible (PP/PP-HI) and Immiscible (PP/EOC) Polymer Blends

This study compares different polymer-nanofiller blends concerning their suitability for application as insulating thermoplastic composites for High Voltage Direct Current (HVDC) cable application. Two polymer blends, PP/EOC (polypropylene/ethylene-octene copolymer) and PP/PP-HI (polypropylene/ propylene - ethylene copolymer) and their nanocomposites filled with 2 wt.% of fumed silica modified with 3-aminopropyltriethoxysilane were studied. Morphology, thermal stability, crystallization behavior dynamic relaxation, conductivity, charge trap distribution and space charge behavior were studied respectively. The results showed that the comprehensive performance of the PP/PP-HI composite is better than the one of the PP/EOC composite due to better polymer miscibility and flexibility, as well as lower charging current density and space charge accumulation. Nanosilica addition improves the thermal stability and dielectric properties of both polymer blends. The filler acts as nucleating agent increasing the crystallization temperature, but decreasing the degree of crystallinity. Dynamic mechanical analysis results revealed three polymer relaxation transitions: PP glass transition ( $\beta$ ), weak crystal reorientation ( $\alpha 1$ ) and melting ( $\alpha 2$ ). The nanosilica introduced deep traps in the polymer blends and suppressed space charge accumulation, but slightly increased the conductivity. A hypothesis for the correlation of charge trap distribution and polymer chain transition peaks is developed: In unfilled PP/EOC and PP/PP-HI matrices, charges are mostly located at the crystalline-amorphous interface, whereas in the filled PP/EOC/silica and PP/PP-HI /silica composites, charges are mostly located at the nanosilica-polymer interface. Overall, the PP/PP-HI (55/45) nanocomposite with 2 wt.% modified silica and 0.3 wt.% of antioxidants making it $a$ promising material for PP based HVDC cable insulation application with $a$ reduced space charge accumulation and good mechanical properties.

Effects of Asymmetric Molecular Architecture on Chain Stretching and Dynamics in Miktoarm Star Copolymers

We use broad-band dielectric spectroscopy (BDS) and small-angle X-ray scattering (SAXS) to investigate the impact of architectural asymmetry in the miktoarm star copolymer on the dielectric polymer relaxations. The miktoarm copolymers studied contain one or two polystyrene (PS) chains of constant molecular weight and two identical poly(cis-1,4-isoprene) (PI) chains with varied molecular weights. Using the chains in the PI block as dielectric probes, we find that the architecturally asymmetric miktoarm star copolymer systems (PSPI2) feature distributions in chain relaxation times and dielectric relaxation strengths that are not dependent on molecular weight or morphology, in stark contrast to the effects of morphological confinement observed for symmetric diblock systems (PS-b-PI or PS2PI2). Along with evidence from the SAXS measurements regarding phase separation, these results are attributed to influences from chain stretching within the framework of the Gaussian chain model for block copolymer systems. As such, the molecular architecture in block copolymers happens to be a versatile handle to control polymer chain dynamics and, ultimately, the macroscopic physicochemical properties in architecturally complex polymers.

Green biocomposites of chitosan dispersed in natural rubber matrices, and their urea controlled-release application

A green biocomposite comprising chitosan dispersed in partially cross-linked natural rubber matricesd was prepared. The starting materials and biocomposites were characterized by means of Fourier transform–infrared spectroscopy, thermogravimetric analysis, and scanning electron microscopy. Chitosan was found well dispersed in partially cross-linked natural rubber matrices indicating that natural rubber latex is successfully stabilized by sodium dodecyl sulfate. The swelling study of the biocomposites was carried out in several solvents. The biocomposites were used for the controlled-release of urea. The release behavior was found consistent, controlled, and prolonged over a period of 72 h. The Korsmeyer–Peppas model was applied to the experimental data and the possible release mechanism was found Fickian diffusion, which indicates that the polymer relaxation time is much greater than the characteristic solvent diffusion time.

Nonmonotonic dependence of comb polymer relaxation on branch density in semidilute solutions of linear polymers

The relaxation of comb polymers in semidilute solutions is studied using single-molecule fluorescence microscopy and Brownian dynamics simulations with long-range hydrodynamic interactions (HI). Unexpectedly, comb polymer relaxation is found to depend nonmonotonically on branching density, such that lightly branched chains relax faster than linear chains in semidilute solutions. These results challenge commonly held notions in the field and show that the dynamics of branched polymers in nondilute solutions is governed by a coupling between polymer architecture, long-range HI, and excluded volume interactions.

Techniques of Mucilage and Gum Modification and their Effect on Hydrophilicity and Drug Release.

The manuscript aims to describe the techniques of modification of gums and mucilages and their effect on hydrophilicity and drug release. The interest is increased in the fields of polymers which are obtained from natural origin and used in the preparation of pharmaceuticals. Mucilage and gum are natural materials widely used in the preparation of novel dosage and conventional dosage forms. They are used in the pharmaceutical industry for various purposes like suspending, emulsifying, bio-adhesive, binding, matrix-forming, extended release and controlled release agent. Gum and mucilage are biodegradable, less toxic, cheap and easily available. Moreover, mucilage and gum can be changed to acquire tailored materials for the delivery of drugs and allow them to compete with commercially available synthetic products. These polysaccharides have unique swellability in an aqueous medium that can exert a retardant effect on drug release or act as a super disintegrant, depending on the concentration utilized in the preparation. Drug release mechanism from hydrophilic matrices consisting of gums and mucilages is based on solvent penetration-induced polymer relaxation, diffusion of entrapped drug followed by degradation or erosion of the matrix. The present manuscript highlights the advantages, modifications of gum and mucilage, their effects on hydrophilicity and drug release as well as aspects of the natural gums which can be assumed to be bifunctional excipient because of their concentration-dependent effect on drug release and their high degree of swellability.

A lattice Boltzmann modeling of viscoelastic drops’ deformation and breakup in simple shear flows

The deformation and breakup of viscoelastic drops in simple shear flows of Newtonian liquids are studied numerically. Our three-dimensional numerical scheme, extended from our previous two-dimensional algorithm, employs a diffusive-interface lattice Boltzmann method together with a lattice advection–diffusion scheme, the former to model the macroscopic hydrodynamic equations for multiphase fluids and the latter to describe the polymer dynamics modeled by the Oldroyd-B constitutive model. A block-structured adaptive mesh refinement technique is implemented to reduce the computational cost. The multiphase model is validated by a simulation of Newtonian drop deformation and breakup under an unconfined steady shear, while the coupled algorithm is validated by simulating viscoelastic drop deformation in the shear flow of a Newtonian matrix. The results agree with the available numerical and experimental results from the literature. We quantify the drop response by changing the polymer relaxation time λ and the concentration of the polymer c. The viscoelasticity in the drop phase suppresses the drop deformation, and the steady-state drop deformation parameter D exhibits a non-monotonic behavior with the increase in Deborah number De (increase in λ) at a fixed capillary number Ca. This is explained by the two distribution modes of the polymeric elastic stresses that depend on the polymer relaxation time. As the concentration of the polymer c increases, the degree of suppression of deformation becomes stronger and the transient result of D displays an overshoot. The critical capillary number for unconfined drop breakup increases due to the inhibitive effects of viscoelasticity. Different distribution modes of elastic stresses are reported for different De.