Deformation Of Polymers Research Articles

Computational modeling of the large deformation and flow of viscoelastic polymers

Deformation of soft polymeric materials often involves complex nonlinear or transient mechanical behaviors. This is due to the dynamic behaviors of polymer chains at the molecular level within the polymer network. In this paper, we present a computational formulation to describe the transient behavior (e.g., viscoelasticity) of soft polymer networks with dynamic bonds undergoing large to extreme deformation. This formulation is based on an Eulerian description of kinematics and a theoretical framework that directly connects the molecular-level kinetics of dynamic bonds to the macroscopic mechanical behavior of the material. An extended finite element method is used to discretize the field variables and the governing equations in an axisymmetric domain. In addition to validating the framework, this model is used to study how the chain dynamics affect the macroscopic response of material as they undergo a combination of flow and elasticity. The problems of cavitation rheology and polymer indentation under extreme deformation are investigated in this context.

Active Brownian Filamentous Polymers under Shear Flow.

The conformational and rheological properties of active filaments/polymers exposed to shear flow are studied analytically. Using the continuous Gaussian semiflexible polymer model extended by the activity, we derive analytical expressions for the dependence of the deformation, orientation, relaxation times, and viscosity on the persistence length, shear rate, and activity. The model yields a Weissenberg-number dependent shear-induced deformation, alignment, and shear thinning behavior, similarly to the passive counterpart. Thereby, the model shows an intimate coupling between activity and shear flow. As a consequence, activity enhances the shear-induced polymer deformation for flexible polymers. For semiflexible polymers/filaments, a nonmonotonic deformation is obtained because of the activity-induced shrinkage at moderate and swelling at large activities. Independent of stiffness, activity-induced swelling facilitates and enhances alignment and shear thinning compared to a passive polymer. In the asymptotic limit of large activities, a polymer length- and stiffness-independent behavior is obtained, with universal shear-rate dependencies for the conformations, dynamics, and rheology.

Individual circular polyelectrolytes under shear flow

Individual circular polyelectrolytes in simple shear flow are studied by means of mesoscale hydrodynamic simulations, revealing the complex coupling effects of shear rate, electrostatic interaction, and circular architecture on their conformational and dynamical properties. Shear flow deforms the polyelectrolyte and strips condensed counterions from its backbone. A decrease in condensed counterions alters electrostatic interactions among charged particles, affecting shear-induced polymer deformation and orientation. Circular architecture determines the features of deformation and orientation. At weak electrostatic interaction strengths, the polyelectrolyte changes its shape from an oblate ring at small shear rates to a prolate ring at large shear rates, whereas strong electrostatic interaction strengths are associated with a transition from a prolate coil to a prolate ring. Circular polyelectrolytes exhibit tumbling and tank-treading motions in the range of large shear rates. Further study reveals a similarity between the roles of intramolecular electrostatic repulsion and chain rigidity in shear-induced dynamics.

Influence of Template Modulus of Elasticity on Polymer Filling, Deformation and Stress Distribution

Influence of Template Modulus of Elasticity on Polymer Filling, Deformation and Stress Distribution

Fracture processes of crystalline polymers using coarse-grained molecular dynamics simulations

Molecular simulations are powerful tools for revealing the properties of polymers at the molecular level. In particular, coarse-grained molecular dynamics simulations are useful for elucidating the deformation and fracture processes of polymers. However, in the case of crystalline polymers, it is difficult to reproduce experimentally observed structures and mechanical properties using these models. This review describes our recent investigations into the deformation and fracture processes of crystalline polymers using coarse-grained molecular dynamics simulations. We were able to successfully reproduce the lamellar structure of polyethylene, which is a fundamental structural feature of this polymer, and obtain a stress–strain curve that exhibited good consistency with that observed experimentally. The molecular dynamics simulations revealed that void generation in the amorphous layers was caused by the movement of the chain ends, which is difficult to observe through experiments. The conditions required to reproduce the experimentally observed structure and mechanical properties using molecular simulations are also discussed. In this focus review, our recent investigations into the deformation and fracture processes of crystalline polymers using coarse-grained molecular dynamics simulations are described. The lamellar structure of polyethylene, a fundamental structural feature of this polymer, is successfully reproduced. Then, a stress– strain curve that exhibited good consistency with that observed experimentally is obtained. Molecular simulations are a powerful tool for elucidating the mechanisms of the deformation and fracture processes of crystalline polymers at the molecular level and this review will contribute to the development of this field of research.

Microdefect identification in polymers by mapping depth-resolved phase-difference distributions using optical coherence tomography

A novel method for the identification of microdefects in transparent or semi-transparent polymers and polymer composites is established, which maps the depth-resolved phase-difference distribution during polymer deformation using optical coherence tomography (OCT), and detects internal microdefects using the defect-induced phase-difference-distribution anomalies. For validation, epoxy resin, silicone rubber and glass fiber-reinforced resin matrix composite samples with internal defects with different dimensions were measured by using a line-field spectral-domain OCT system. Small defects, which could not be observed from the cross-sectional images, were identified from the measured phase-difference distributions; fibers and microdefects of a similar size in the polymer composites were distinguished using the proposed novel method. The proposed method can be employed, not only for polymers and polymer composites, but also for other transparent and semi-transparent materials such as glass and ceramic.

Filament instability under constant loads

Buckling of semi-flexible filaments appears in different systems and scales. Some examples are: fibers in geophysical applications, microtubules in the cytoplasm of eukaryotic cells and deformation of polymers freely suspended in a flow. In these examples, instabilities arise when a system’s parameter exceeds a critical value, being the Euler force the most known. However, the complete time evolution and wavelength of buckling processes are not fully understood. In this work we solve analytically the time evolution of a filament under a constant compressive force in the small amplitude approximation. This gives an insight into the variable force scenario in terms of normal modes. The evolution is highly sensitive to the initial configuration and to the magnitude of the compressive load. This model can be a suitable approach to many different real situations.

Strain Invariant Failure Theory – Part 1: An extensible framework for predicting the mechanical performance of fibre reinforced polymer composites

Failure prediction for composite materials is still a major area of interest within both the academic literature and the composite industry. This paper presents a modelling and experimental framework which has been developed for the application of Strain Invariant Failure Theory (SIFT), but which can be adapted to other failure prediction methods if required. SIFT is set of physics-based failure criteria developed to predict the onset of irreversible deformation of glassy polymers. The paper provides an overview of the available evidence for SIFT and provides a complete methodology for applying SIFT to predict the onset of failure in a composite component.Fundamental to the method presented is a modular and hierarchical dehomogenisation procedure; allowing for the evaluation of stress and strain state variables within the constituent components of the composite. Due to the constant evolution of composite characterisation techniques, the method presented in this paper is designed to be extensible (i.e. accommodating of new materials and product forms) and modular (i.e. subsections of the method can be easily substituted or refined). Known limitations of the current method are clearly presented and discussed. A companion paper will present validation of the approach for simple uniaxial and biaxial coupon experiments and will discuss how the method can be applied to a range of existing composite characterisation testing standards.

Influence of liquid media on the craze initiation in amorphous polylactide

The influence of various liquid media (aliphatic alcohols, water-alcohol solutions, saturated hydrocarbons, and organosilicon liquids) on the development of uniaxial deformation and nucleation of crazes in amorphous polylactide (PLA) films has been studied. It has been found that in the presence of the liquids the tensile stress during the polymer deformation decreases by 3–4 times. According to the Griffith's theory, linear dependences (with an accuracy of R2 = 0.96–0.97) have been revealed between the yield point or the flow stress of PLA and the square root of product of the elasticity modulus of the polymer in a liquid and the interfacial surface energy at the polymer-liquid medium interface calculated using the Owens-Wendt equation. The data obtained may be used to predict the critical mechanical stress at which crazes will be initiated in PLA.

Geometric decomposition of the conformation tensor in viscoelastic turbulence

This work introduces a mathematical approach to analysing the polymer dynamics in turbulent viscoelastic flows that uses a new geometric decomposition of the conformation tensor, along with associated scalar measures of the polymer fluctuations. The approach circumvents an inherent difficulty in traditional Reynolds decompositions of the conformation tensor: the fluctuating tensor fields are not positive definite and so do not retain the physical meaning of the tensor. The geometric decomposition of the conformation tensor yields both mean and fluctuating tensor fields that are positive definite. The fluctuating tensor in the present decomposition has a clear physical interpretation as a polymer deformation relative to the mean configuration. Scalar measures of this fluctuating conformation tensor are developed based on the non-Euclidean geometry of the set of positive definite tensors. Drag-reduced viscoelastic turbulent channel flow is then used an example case study. The conformation tensor field, obtained using direct numerical simulations, is analysed using the proposed framework.

Friction spot welding of carbon fiber-reinforced polyetherimide laminate

Here, carbon fiber-reinforced polyetherimide (PEI) laminate was joined via friction spot welding (FSpW). The feasibility and fracture mechanism of the FSpW joints were investigated. The sound joint with smooth surface and without hook defect was achieved. The strong bonding formed at the sleeve stirring zone and thermo-mechanically affected zone, resulting from the macromolecular interdiffusion and interlocking of the smashed carbon fiber at the bonding interface by thermo-mechanical behavior. Increasing rotational velocity enhanced the mixing degree of the carbon fiber at the interface and strengthened the bonding interface, which improved tensile shear properties. The maximum tensile shear load of the FSpW joint with a joining area of 66.4 mm2 reached 1600 N, which was comparable to the strength of state-of-the-art welding. The fracture surface morphologies revealed a typical ductile fracture containing the deformation of polymer and the pull-out of the carbon fiber. The FSpW has feasible and potential to join carbon fiber-reinforced thermoplastic composites laminate.

Flexible Electronics Technologies for the Fabrication of Surface Dielectric Barrier Discharge Devices

Flexible electronics have been extensively studied for the development of flexible displays and semiconductor devices. Heterojunction and patterning techniques between metal and polymer have been studied to realize complicated electrode wiring.[1] Flexible electronics technologies can also be used to fabricate devices for surface dielectric barrier discharge (SDBD). Recently, SDBD were developed as a flexible device based on polymer dielectric substrates.[2] The flexible devices using polymer dielectrics have technical issues such as bonding between metal electrodes and polymer substrates, patterning of metal electrodes, and suppression of polymer deformation by plasma generated radicals. In this presentation, we report the result of SDBD device fabricated by printed electronics technologies. For example, the SDBD devices based on the polyimide substrate with the patterned electrode by printing method showed stable discharge characteristics with a dielectric thickness of 100 micrometers or less, a sinusoidal frequency of 5-10 kHz, and a discharge voltage of 2 kV. Download : Download high-res image (222KB) Download : Download full-size image Figure 1. SDBD device fabricated on polyimide substrates (left), and discharge image (right)

Deformation behavior and energy absorption capability of polymer and ceramic-polymer composite microlattices under cyclic loading

Abstract

A new bound on polymer quantization via an opto-mechanical setup

The existence of a minimal measurable length as a characteristic length in the Planck scale is one of the main features of quantum gravity and has been widely explored in the context. Various different deformations of spacetime have been employed successfully for the purpose. However, polymer quantization approach is a relatively new and dynamic field towards the quantum gravity phenomenology, which emerges from the symmetric sector of the loop quantum gravity. In this article, we extend the standard ideas of polymer quantization to find a new and tighter bound on the polymer deformation parameter. Our protocol relies on an opto-mechanical experimental setup that was originally proposed to explore some interesting phenomena by embedding the minimal length into the standard canonical commutation relation. We extend this scheme to probe the polymer length deformed canonical commutation relation of the center of mass mode of a mechanical oscillator with a mass around the Planck scale. The method utilizes the novelty of exchanging the relevant mechanical information with a high intensity optical pulse inside an optical cavity. We also demonstrate that our proposal is within the reach of the current technologies and, thus, it could uncover a decent realization of quantum gravitational phenomena thorough a simple table-top experiment.

On the Molecular Origin of Charge Separation at the Donor–Acceptor Interface

AbstractFullerene‐based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor–acceptor (D–A) interface. Model systems comprised of polythiophene‐based donor and rylene diimide‐based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct‐contact intermolecular polarization play in establishing a driving force (energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules.

Evidence of Deformation-Dependent Heat Capacity and Energetic Elasticity in a Cross-Linked Elastomer Subjected to Uniaxial Elongation

We present a novel infrared thermography technique to investigate the dependence of heat capacity on deformation in cross-linked polymers. This phenomenon is directly related with the longstanding question of whether or not there is an energetic contribution to the stress in deformed polymers and, in general, to the nonisothermal, viscoelastic behavior of polymer materials. By tracking the temperature evolution of samples heated by a laser, we are able to measure heat capacity changes relative to the equilibrium value in an elastomer subjected to uniaxial extension. We find that the heat capacity increases with elongation in lightly cross-linked cis-1,4-polyisoprene. Remarkably, the onset of heat capacity dependence on deformation is observed at strains similar to those required to achieve finite extensibility. The deviation from the equilibrium value of heat capacity is consistent with an independent set of experiments comparing anisotropy in thermal diffusivity from forced Rayleigh scattering and therma...

Plasticity Mechanism for Glassy Polymers: Computer Simulation Picture

This review summarizes the data published over the past two and a half decades on the mechanism of plastic deformation of bulk isotropic linear glassy polymers in uniaxial tension, compression, and shear at low deformation temperatures (Тdef < 0.6Тg) and moderate loading rates. The main attention is focused on studies concerning the numerical computer simulations of plasticity of organic polymer glasses. The plastic behavior of glassy polymers at nano-, micro-, and macrolevels in the range of macroscopic strains up to ≈100% is discussed. Plasticity mechanisms are compared for organic, inorganic, metallic, polymer, and nonpolymer glasses with different chemical structures and types of interparticle interactions. The general common mechanism of plasticity of glassy compounds, the nucleation of plasticity carriers in them, and the structure of such carriers and their dynamics are covered. Within the framework of the common plasticity mechanism, the specific features of deformation in glassy polymers are analyzed. Specifically, the participation of conformational transformations in macromolecules in the deformation response of polymer glasses, change in intensity of the yield peak with the thermomechanical prehistory of the sample, and the role of van der Waals interactions in the accumulation of excess potential energy by the sample under loading are considered. The role of free volume and structural and dynamic heterogeneities in the plasticity of glasses is also discussed.

Long-Term Multipliers and Deformability of Fiber-Reinforced Polymer Prestressed Concrete

Long-Term Multipliers and Deformability of Fiber-Reinforced Polymer Prestressed Concrete

Probing relationship between structure and viscosity of unentangled polymers in steady shear flow

Structural responses of unentangled polymers of various architectures, including linear, ring, star, asymmetric star, and H-shaped polymers, in steady shear flow are investigated via nonequilibrium molecular dynamics simulations. As observed in our previous simulations, these polymers, when having an identical equilibrium mean-square radius of gyration exhibit the same viscosity curve in both the linear and nonlinear regimes. In this article, the polymer orientation and deformation and the rate of deformation are calculated as a function of shear rate. The differences between different architectures in most of these properties are found to be related to the polymer extensibility, the number of free ends and the ability of folding. The unique architecture-independent quantity is the gradient component of the radius of gyration tensor. We find that a simple relation η=C η (3 / )3/2 gives a near-perfect fit to shear viscosity η data, where C η is a constant. The results suggest that this relation between chain thickness and viscosity is universal for unentangled polymers of various architectures.

Tensile deformation of semi-crystalline polymers by molecular dynamics simulation

In this work, the microstructure evolution of semi-crystalline polymers during tensile deformation is analyzed by molecular dynamics simulation. A perfect semi-crystalline lamellar structure with crystalline/amorphous interface perpendicular to tensile direction is created with the help of coarse-grained (CG) model of poly(vinyl alcohol) (PVA). During the tensile test, two kinds of strain rates are applied to the lamellar stack to determine the stress–strain curves, yield stresses, and crystallinities. Consistent with experimental findings, two yield points were observed in the semi-crystalline sample which was corresponded to the fine and coarse crystallographic slips in the lamellar structure, where the crystal stems gradually rotated into the direction of applied stress during chain slips. After the second yielding point when the crystal stems had been rotated fully into the direction of applied stress, the lamellar structure was destroyed and it resulted in a decrease of crystallinity. In addition, the increase of the strain rate led to the acceleration of destruction of crystal structures. It is worth noting that the stress induced crystallization was observed in the interfacial region, and newly crystallized beads were belonged to the same microcrystalline domain as crystalline region due to memory effects. This work provides direct comparison of structure evolution between crystalline and amorphous region in semi-crystalline polymers during tensile deformation, and it is helpful for the design and mechanical property analysis of semi-crystalline polymers.