Plastic Deformation Mechanisms of Semicrystalline and Amorphous Polymers

Unveiling the Nanoconfinement Effect on Crystallization of Semicrystalline Polymers Using Coarse-Grained Molecular Dynamics Simulations

Semicrystalline polymers under nanoconfinement show distinct structural and thermomechanical properties compared to their bulk counterparts. Despite extensive research on semicrystalline polymers under nanoconfinement, the nanoconfinement effect on the local crystallization process and the unique structural evolution of such polymers have not been fully understood. In this study, we unveil such effects by using coarse-grained molecular dynamics simulations to study the crystallization process of a model semicrystalline polymer-polyvinyl alcohol (PVA)-under different levels of nanoconfinement induced by nanoparticles that are represented implicitly. We quantify in detail the evolution of the degree of crystallinity (XC) of PVA and examine distinct crystalline regions from simulation results. The results show that nanoconfinement can promote the crystallization process, especially at the early stage, and the interfaces between nanoparticles and polymer can function as crystallite nucleation sites. In general, the final XC of PVA increases with the levels of nanoconfinement. Further, nanoconfined cases show region-dependent XC with higher and earlier increase of XC in regions closer to the interfaces. By tracking region-dependent XC evolution, our results indicate that nanoconfinement can lead to a heterogenous crystallization process with a second-stage crystallite nucleation in regions further away from the interfaces. In addition, our results show that even under very high cooling rates, the nanoconfinement still promotes the crystallization of PVA. This study provides important insights into the underlying mechanisms for the intricate interplay between nanoconfinement and the crystallization behaviors of semicrystalline polymer, with the potential to guide the design and characterization of semicrystalline polymer-based nanocomposites.

Coarse-Grained Simulations on Polyethylene Crystal Network Formation and Microstructure Analysis

Understanding and characterizing semi-crystalline models with crystalline and amorphous segments is crucial for industrial applications. A coarse-grained molecular dynamics (CGMD) simulations study probed the crystal network formation in high-density polyethylene (HDPE) from melt, and shed light on tensile properties for microstructure analysis. Modified Paul-Yoon-Smith (PYS/R) forcefield parameters are used to compute the interatomic forces among the PE chains. The isothermal crystallization at 300 K and 1 atm predicts the multi-nucleus crystal growth; moreover, the lamellar crystal stems and amorphous region are alternatively oriented. A one-dimensional density distribution along the alternative lamellar stems further confirms the ordering of the lamellar-stack orientation. Using this plastic model preparation approach, the semi-crystalline model density (ρcr) of ca. 0.913 g·cm-3 and amorphous model density (ρam) of ca. 0.856 g·cm-3 are obtained. Furthermore, the ratio of ρcr/ρam ≈ 1.06 is in good agreement with computational (≈1.096) and experimental (≈1.14) data, ensuring the reliability of the simulations. The degree of crystallinity (χc) of the model is ca. 52% at 300 K. Nevertheless, there is a gradual increase in crystallinity over the specified time, indicating the alignment of the lamellar stems during crystallization. The characteristic stress-strain curve mimicking tensile tests along the z-axis orientation exhibits a reversible sharp elastic regime, tensile strength at yield ca. 100 MPa, and a non-reversible tensile strength at break of 350%. The cavitation mechanism embraces the alignment of lamellar stems along the deformation axis. The study highlights an explanatory model of crystal network formation for the PE model using a PYS/R forcefield, and it produces a microstructure with ordered lamellar and amorphous segments with robust mechanical properties, which aids in predicting the microstructure-mechanical property relationships in plastics under applied forces.

Highly stretchable polymer semiconductor thin films with multi-modal energy dissipation and high relative stretchability

Stretchable polymer semiconductors (PSCs) have seen great advancements alongside the development of soft electronics. But it remains a challenge to simultaneously achieve high charge carrier mobility and stretchability. Herein, we report the finding that stretchable PSC thin films (<100-nm-thick) with high stretchability tend to exhibit multi-modal energy dissipation mechanisms and have a large relative stretchability (rS) defined by the ratio of the entropic energy dissipation to the enthalpic energy dissipation under strain. They effectively recovered the original molecular ordering, as well as electrical performance, after strain was released. The highest rS value with a model polymer (P4) exhibited an average charge carrier mobility of 0.2 cm2V-1s-1 under 100% biaxial strain, while PSCs with low rS values showed irreversible morphology changes and rapid degradation of electrical performance under strain. These results suggest rS can be used as a parameter to compare the reliability and reversibility of stretchable PSC thin films.

High-Saline-Enabled Hydrophobic Homogeneous Cross-Linking for Extremely Soft, Tough, and Stretchable Conductive Hydrogels as High-Sensitive Strain Sensors

Design of admirable conductive hydrogels combining robust toughness, soft flexibility, desirable conductivity, and freezing resistance remains daunting challenges for meeting the customized and critical demands of flexible and wearable electronics. Herein, a promising and facile strategy to prepare hydrogels tailored to these anticipated demands is proposed, which is prepared in one step by homogeneous cross-linking of acrylamide using hydrophobic divinylbenzene stabilized by micelles under saturated high-saline solutions. The influence of high-saline environments on the hydrogel topology and mechanical performance is investigated. The high-saline environments suppress the size of hydrophobic cross-linkers in micelles during hydrogel polymerization, which weaken the dynamic hydrophobic associations to soften the hydrogels. Nevertheless, the homogeneous cross-linked networks ensure antifracture during ultralarge deformations. The obtained hydrogels show special mechanical performance combining extremely soft deformability and antifracture features (Young's modulus, 5 kPa; stretchability, 10200%; toughness, 134 kJ m-2; and excellent anticrack propagation). The saturated-saline environments also endow the hydrogels with desirable ion conductivity (106 mS cm-1) and freezing resistance (<20 °C). These comprehensive properties of the obtained hydrogels are quite suitable for flexible electronic applications, which is demonstrated by the high sensitivity and durability of the derived strain sensors.

Understanding Lithium Dendrite Suppression by Hybrid Composite Separators: Indentation Measurements Informed by Operando X-ray Computed Tomography

This study investigates the significance of the mechanics of hybrid particle-polymer separators in the stabilization of lithium metal interfaces by probing these properties in realistic conditions informed by X-ray microcomputed tomography (micro-CT). Elastic properties and viscoelastic behavior of inorganic microparticle-filled poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP) films are characterized using a nanoindentation experiment whose displacement simulates the interfacial response seen in operando micro-CT. It is determined that the dominating mechanical behavior in this hybrid separator relevant to lithium metal cell conditions is comprised of viscoelasticity. Consistent with this finding, along with correlations across other physicochemical properties, a mechanism describing the improvement of lithium metal cycling performance according to inorganic filler type and content is proposed.

Unbreakable Hydrogels with Self-Recoverable 10 200% Stretchability

Design of tough hydrogels maintaining structural integrity under multivariable mechanical loads remains hugely challenging because the anticipated characteristics such as stretchability, strength, toughness, and fracture resistance can hardly be compatible. Herein, a simple but robust hydrogel network formed by copolymerization of divinyl benzene with acrylamide in micellar solutions for ultra-high fracture resistance and self-recoverable stretchability is proposed. The network provides dynamic association of hydrophobic domains and homogeneous crosslinking of hydrophilic chains, which shows step-by-step deformation process. The dynamic associations allow recoverable small deformations, then the homogeneous crosslinking ensures reversible unfolding and alignment of polymer chains to self-strengthen for ultra-large deformations without crack propagations. The resultant hydrogels exhibit comprehensive unbreakable feature with self-recoverable ultra-high stretchability (100% recovery from 10 200% strain), superior fracture resistance (toughness > 26 kJ m^-2 ), and anticrack propagation and fatigue (fatigue threshold: ≈2.5 kJ m^-2 ). Even the prenotched hydrogels can undergo tens cyclic loads at 10 200% strain and thousands cyclic loads at 200% strain without noticeable changes in mechanical performance. The robust network prepared from homogeneous hydrophobic crosslinking provides a facile approach and a new mechanism to explore tough hydrogels with superior antifracture and extreme self-recoverable deformability for diverse applications.

Effects of Mechanical Stress on Insulation Structure and Performance of HV Cable

Mechanical stresses generated during manufacturing and laying process of high voltage cables can result in degradation of insulation properties, affecting the stable operation of the transmission system. Traditional test methods for testing the effect of mechanical stress on the insulation properties of polyethylene still have some shortcomings to be explored and it is able to explain the changes of the insulation properties of polyethylene under mechanical stress from a microscopic perspective. In order to further study the effect of stress on the insulation properties of polyethylene, microstructural changes, the breakdown field strength, conductivity and charge distribution of polyethylene at different elongation rates are investigated by a combination of experimental and molecular dynamics simulations. The results show that the increase in stress leads to a decrease in crystallinity and microcrystalline size of the material decrease. The untwisting and orientation of the polyethylene molecular chains during the stretching process can create cavities, resulting in an uneven sample distribution and thickness reduction, leading to a reduction in the breakdown field strength. Meanwhile, some crystal regions are transformed into amorphous regions. The loose amorphous regions facilitate the directional migration of carriers, resulting in the increase of conductivity. When the elongation ratio is smaller, the distance between the molecular chains increases and the trap depth of the specimen becomes shallower. This facilitates the migration of ions and electrons and increases the rate of decay of the surface potential. When the stretch is further increased, new traps are created by broken molecular chains to limit the movement of charges, decreasing the decay rate of the surface potential and reducing the insulation properties of the polyethylene. Meanwhile, the molecular dynamics model of semi-crystalline polyethylene was developed to observe the microstructure and energy changes during the stretching process. The conclusions in terms of tensile tests were verified from a microscopic perspective.

Strain induced crystallization of polymers at and above the crystallization temperature by coarse-grained simulations

We examine the behavior of short and long polymers by means of coarse-grained computer simulations of a by-polyvinyl alcohol inspired model. In particular, we focus on the structural changes in the monomer and polymer scales during cooling and the application of uni-axial true strain. The straining of long polymers results in the formation of a semi-crystalline system at temperatures well above the crystallization temperature, which allows for the study of strain induced crystallization.

Coarse-grained molecular dynamics simulations of void generation and growth processes in the fracture of the lamellar structure of polyethylene

We investigate the void generation and growth process in semicrystalline polymers by large-scale coarse-grained molecular dynamics simulations. Voids are generated in the amorphous layers and grow spherically and then cylindrically, consistent with the results of previous experiments. Interestingly, the fusion of voids is indicated to enlarge the voids in the direction perpendicular to the stretching direction, but not beyond the crystalline layers. The orientational order along the stretching direction increased sharply before void generation, but the increase leveled off afterward. The simulations also clearly reveal that the crystalline layers break but do not bend in the fragmentation process. The dependence of the void growth process on stretching velocity and the stability levels of voids at constant strain are also discussed.

Direct Correlation of Single-Particle Motion to Amorphous Microstructural Components of Semicrystalline Poly(ethylene oxide) Electrolytic Films

Semicrystalline polymers constitute some of the most widely used materials in the world, and their functional properties are intimately connected to their structure on a range of length scales. Many of these properties depend on the micro- and nanoscale heterogeneous distribution of crystalline and amorphous phases, but this renders the interpretation of ensemble averaged measurements challenging. We use superlocalized widefield single-particle tracking in conjunction with AFM phase imaging to correlate the crystalline morphology of lithium-triflate-doped poly(ethylene oxide) thin films to the motion of individual fluorescent probes at the nanoscale. The results demonstrate that probe motion is intrinsically isotropic in amorphous regions and that, without altering this intrinsic diffusivity, closely spaced, often parallel, crystallite fibers anisotropically constrain probe motion along intercalating amorphous channels. This constraint is emphasized by the agreement between crystallite and anisotropic probe trajectory orientations. This constraint is also emphasized by the extent of the trajectory confinement correlated to the width of the measured gaps between adjacent crystallites. This study illustrates with direct nanoscale correlations how controlled and periodic arrangement of crystalline domains is a promising design principle for mass transport in semicrystalline polymer materials without compromising their mechanical stability.

Compression-induced anti-nematic order in glassy and semicrystalline polymers

We provide new insights into the molecular origin of the asymmetry between uniaxial tensile and compressive deformation of glassy and semicrystalline polymers using molecular dynamics simulations. The difference between the two responses strongly depends on the chain length and is the largest at intermediate chain lengths. Irrespective of chain length, the intra- and interchain organization of polymers under extension and compression are remarkably distinct. The chains align along the tensile axis leading to a global nematic order of the bonds and end-to-end vectors, whereas compression reorganizes polymers to lie in planes perpendicular to the compressive axis resulting in the emergence of an anti-nematic order and destruction of crystallinity. Regardless of the initial glassy or semicrystalline structure, the deformed state of polymers at large strains converge towards the same kind of structure that only depends on the deformation mode.

Disentangling and Lamellar Thickening of Linear Polymers during Crystallization: Simulation of Bimodal and Unimodal Molecular Weight Distribution Systems

We have performed coarse-grained molecular dynamics simulations to study the isothermal crystallization of bimodal and unimodal molecular weight distribution (MWD) polymers with equivalent average molecular weight (M_w). By using primitive path analysis, we can monitor the entanglement evolution during the process of crystallization. We have discovered a quantitative correlation between the degree of disentanglement and crystallinity, indicating that chain disentanglement permits the process of crystallization. In addition, the crystalline stem length also displays a linear relation with the degree of disentanglement at different temperatures. Based on the observation in our simulations, we can build a scenario of the whole process of chain disentangling and lamellar thickening on the basis of chain sliding diffusion. Furthermore, we have enough evidence to infer that the temperature dependence of crystalline stem length is basically a result of temperature dependence of chain sliding diffusion. Our observations are also in agreement with Hikosaka's sliding diffusion theory. Compared to the unimodal system, the disentanglement degree of the bimodal system is more delayed than its crystallinity due to the slower chain sliding of the long-chain component; the bimodal system reaches a larger crystalline stem length at all temperatures due to the promotion of higher chain sliding mobility of the short-chain component.

The Continuum Approach to the Description of Semi-Crystalline Polymers Deformation Regimes: The Role of Dynamic and Translational Defects

This paper presents a new approach to describe the mechanical behavior of semi-crystalline polymers, the plastic deformation of which is determined by their two-phase structure. To describe the plastic behavior of semi-crystalline polymers, a two-phase model is used. In the framework of this model, one phase is in a hard (crystalline) state, and the other in a soft (amorphous) state. The two-phase material is modeled by a single-phase homogeneous continuum based on the approximation of the effective medium. It is assumed that two infinitely close material points of the continuum are connected in series by elastic and viscous bonds, which corresponds to the Maxwell model. It is shown that, in this case, the Maxwell continuum is a pseudo-Euclidean space. Generalizing the definition of defects from a three-dimensional space to a four-dimensional pseudo-Euclidean space, we obtained a dynamic system of nonlinear, interrelated equations to describe the behavior of translational-type defects in the solid phase and dynamic defects in the amorphous phase. As an example of an application for these equations, the phenomenon of creep under uniaxial loading is considered. It is shown that the formalism of the proposed two-phase model makes it possible to describe creep phenomenon regularities, which correspond to both the aging theory and the flow theory.

Scaling Behavior of Anisotropy Relaxation in Deformed Polymers

Drawing an analogy to the paradigm of quasielastic neutron scattering, we present a general approach for quantitatively investigating the spatiotemporal dependence of structural anisotropy relaxation in deformed polymers by using small-angle neutron scattering. Experiments and nonequilibrium molecular dynamics simulations on polymer melts over a wide range of molecular weights reveal that their conformational relaxation at relatively high momentum transfer Q and short time can be described by a simple scaling law, with the relaxation rate proportional to Q. This peculiar scaling behavior, which cannot be derived from the classical Rouse and tube models, is indicative of a surprisingly weak direct influence of entanglement on the microscopic mechanism of single-chain anisotropy relaxation.

Calculations of free energy of surface interactions in crystalline polyethylene

The surface free energy of the crystalline polyethylene (PE) is an important property related with wettability, adhesion, and crystal growth. We investigated the profiles of free energy of surface interactions in the fully thermalized crystalline PE during debonding and shearing with atomistic molecular dynamics simulations using steered molecular dynamics and umbrella sampling techniques. The stress profiles during debonding and shearing processes were also estimated and compared with those obtained from analogous deformation simulations. We estimated the vacuum surface free energies of two different crystallographic surfaces (100) and (010) of the crystalline PE from the free energy changes during the debonding process. The estimated surface free energies were insensitive to the choice of simulation protocols after combining estimates from both forward and backward processes and were in excellent agreement with those obtained from an experiment on PE single crystal aggregates, which underscores the importance of the inclusion of the entropic contribution in the free energy calculated with the fully flexible interface adopted in this study.

Molecular Dynamics Investigation of the Relaxation Mechanism of Entangled Polymers after a Large Step Deformation

The chain retraction hypothesis of the tube model for nonlinear polymer rheology has been challenged by the recent small-angle neutron scattering (SANS) experiment (Wang, Z.; Lam, C. N.; Chen, W.-R.; Wang, W.; Liu, J.; Liu, Y.; Porcar, L.; Stanley, C. B.; Zhao, Z.; Hong, K.; Wang, Y., Fingerprinting Molecular Relaxation in Deformed Polymers. Phys. Rev. X 2017, 7, 031003). In this work, we further examine the microscopic relaxation mechanism of entangled polymer melts after a large step uniaxial extension by using large-scale molecular dynamics simulation. We show that the unique structural features associated with the chain retraction mechanism of the tube model are absent in our simulations, in agreement with the previous experimental results. In contrast to SANS experiments, molecular dynamics simulations allow us to accurately and unambiguously determine the evolution of the radius of gyration tensor of a long polymer chain after a large step deformation. Contrary to the prediction of the tube model, our simulations reveal that the radius of gyration in the perpendicular direction to stretching increases monotonically toward its equilibrium value throughout the stress relaxation. These results provide a critical step in improving our understanding of nonlinear rheology of entangled polymers.

Crystallization of finite-extensible nonlinear elastic Lennard-Jones coarse-grained polymers

The ability of a simple coarse-grained finite-extensible nonlinear elastic (FENE) Lennard-Jones (LJ) polymer model to be crystallized is investigated by molecular dynamics simulations. The optimal FENE Lennard-Jones parameter combinations (ratio between FENE and LJ equilibrium distances) and the optimal lattice parameters are calculated for five different perfect crystallite structures: simple tetragonal, body-centered tetragonal, body-centered orthorhombic, hexagonal primitive, and hexagonal close packed. It was found that the most energetically favorable structure is the body-centered orthorhombic. Starting with an equilibrated polymer liquid and with the optimal parameters found for the body-centered orthorhombic, an isothermal treatment led to the formation of large lamellar crystallites with a typical chain topology: folded, loop, and tie chains, and with a crystallinity of about 60%-70%, similar to real semicrystalline polymers. This simple coarse-grained Lennard-Jones model provides a qualitative tool to study semicrystalline microstructures for polymers.

Role of the Intercrystalline Tie Chains Network in the Mechanical Response of Semicrystalline Polymers

We examine the microscopic origin of the tensile response in semicrystalline polymers by performing large-scale molecular dynamics simulations of various chain lengths. We investigate the microscopic rearrangements of the polymers during tensile deformation and show that the intercrystalline chain connections known as tie chains contribute significantly to the elastic and plastic response. These results suggest that the mechanical behavior of semicrystalline polymers is controlled by two interpenetrated networks of entanglements and tie chains.

Plasticity resulted from phase transformation for monolayer molybdenum disulfide film during nanoindentation simulations

Molecular dynamics simulations on nanoindentation of circular monolayer molybdenum disulfide (MoS₂) film are carried out to elucidate the deformation and failure mechanisms. Typical force-deflection curves are obtained, and in-plane stiffness of MoS₂ is extracted according to a continuum mechanics model. The measured in-plane stiffness of monolayer MoS₂ is about 182 ± 14 N m^-1, corresponding to an effective Young's modulus of 280 ± 21 GPa. More interestingly, at a critical indentation depth, the loading force decreases sharply and then increases again. The loading-unloading-reloading processes at different initial unloading deflections are also conducted to explain the phenomenon. It is found that prior to the critical depth, the monolayer MoS₂ film can return to the original state after completely unloading, while there is hysteresis when unloading after the critical depth and residual deformation exists after indenter fully retracted, indicating plasticity. This residual deformation is found to be caused by the changed lattice structure of the MoS₂, i.e. a phase transformation. The critical pressure to induce the phase transformation is then calculated to be 36 ± 2 GPa, consistent with other studies. Finally, the influences of temperature, the diameter and indentation rate of MoS₂ monolayer on the mechanical properties are also investigated.