Plastic Deformation of Semicrystalline Polyethylene by Molecular Simulation

Dependence of Thermally Activated Relaxation of Crystalline Stems on the Molecular Topology at Crystalline/Amorphous Interfaces in Polyethylene

We investigate the relaxation dynamics of crystalline stems in relation to the molecular topology of the crystalline/amorphous interface, employing coarse-grained molecular dynamics. To efficiently generate model semicrystalline systems of linear polyethylene with a realistic interphase morphology, we simplified the Monte Carlo method by introducing molecular dynamics for faster relaxation. The structural properties of the generated systems are validated against experimental measurements, theoretical predictions, and existing simulation data. The models suggest that the probability distribution of loop-entry sites on the lamellar surface can be described by a power law in terms of the distance between the entry sites. By considering realistic interphase morphology, we are able to improve the prediction of the overall activation energy for the relaxation of crystalline stems, aligning it closely with experimental measurements. The largest model predicts that crystalline stems connected via large loops, i.e., those that exceed the entanglement length, and long tails are associated with increased activation energy; whereas stems connected to shorter tails show the lowest activation energy. These predictions can guide the future development of tougher semicrystalline polymers by providing insights into how amorphous chain morphology contributes to the activation energy and the relaxation dynamics of crystalline chains.

Interrelation of macroscopic mechanical properties and molecular scale thermal relaxation of biodegradable and non-biodegradable polymers

Comparative analysis of macroscopic mechanical properties of a biodegradable polymer polypropylene carbonate (PPC) is carried out concerning two most commonly used, non-biodegradable synthetic polymers-high-density polyethylene (HDPE) and linear-low density polyethylene (LLDPE). Responses of the films of these polymers when subjected to mechanical and thermal stresses are analyzed. Response to tensile stress reveals the highest elongation at break (EB) in PPC films (396 ± 104 mm), compared to HDPE (26 ± 0.5 mm) and LLDPE (301 ± 143 mm), although the elastic modulus (YM) of PPC is around 50 ± 6 MPa, 3-fold lesser than LLDPE (YM = 153 ± 7 MPa) and 6-fold lesser than HDPE (YM = 305 ± 32 MPa). The plastic deformation response of PPC is intermediate to that of HDPE and LLDPE; initial strain softening is followed by strain hardening in LLDPE, a plateau region in PPC, and prolonged strain softening in HDPE. Crystalline domains in HDPE produce restriction on molecular motion. Crystallinity abruptly decreases by 70% in HDPE following a thermal quench, showing the possibility of free chain molecular mobility during plastic deformation. High correlation among Raman modes for all polymers reveals cooperative relaxation processes after thermal quench; C-C stretching modes and C-H bending, CH2wagging modes have Pearson's correlation coefficient 0.9. The integrated peak intensity and width of the C-C stretching Raman mode is 3-fold higher in PPC than HDPE after a thermal quench, showing enhanced molecular mobility and contributing modes in PPC. The peak width of this mode shows a strong negative correlation of -0.7 with the YM and a strong positive correlation of 0.6 with EB, showing that higher amorphicity leads to enhanced molecular mobility and EB at the cost of YM. This study reveals importance of molecular-scale response in governing the macroscopic properties of polymers.

Preparation and Characterization of High-Density Polyethylene with Alternating Lamellar Stems Using Molecular Dynamics Simulations

Mechanical recycling is the most efficient way to reduce plastic pollution due to its ability to maintain the intrinsic properties of plastics as well as provide economic benefits involved in other types of recycling. On the other hand, molecular dynamics (MD) simulations provide key insights into structural deformation, lamellar crystalline axis (c-axis) orientations, and reorganization, which are essential for understanding plastic behavior during structural deformations. To simulate the influence of structural deformations in high-density polyethylene (HDPE) during mechanical recycling while paying attention to obtaining an alternate lamellar orientation, the authors examine a specific way of preparing stacked lamella-oriented HDPE united atom (UA) models, starting from a single 1000 UA (C1000) chain of crystalline conformations and then packing such chain conformations into 2-chain, 10-chain, 15-chain, and 20-chain semi-crystalline models. The 2-chain, 10-chain, and 15-chain models yielded HDPE microstructures with the desired alternating lamellar orientations and entangled amorphous segments. On the other hand, the 20-chain model displayed multi-nucleus crystal growth instead of the lamellar-stack orientation. Structural characterization using a one-dimensional density profile and local order parameter {P2(r)} analyses demonstrated lamellar-stack orientation formation. All semi-crystalline models displayed the total density (ρ) and degree of crystallinity (χ) range of 0.90-0.94 g/cm-3 and ≥42-45%, respectively. A notable stress yield (σ_yield) ≈ 100-120 MPa and a superior elongation at break (ε_break) ~250% was observed under uniaxial strain deformation along the lamellar-stack orientation. Similarly, during the MD simulations, the microstructure phase change represented the average number of entanglements per chain (). From the present study, it can be recommended that the 10-chain alternate lamellar-stack orientation model is the most reliable miniature model for HDPE that can mimic industrially relevant plastic behavior in various conditions.

Insights into nanomechanical behavior of B. mori silk fibroin-hydroxyapatite bio-nanocomposite using MD simulations: Role of varying hydroxyapatite content

Nanocomposite material composed of Bombyx mori Silk Fibroin and hydroxyapatite (B. mori SF-HA) is a potential biomaterial for bone tissue engineering. Here, Bombyx mori Silk Fibroin (B. mori SF) is a flexible and tough organic, polymer phase, and hydroxyapatite (HA) is hard and stiff mineral phase. Knowledge about mechanical deformation behavior together with governing mechanisms, and the role of the two phases (SF and HA phase) and interfacial interactions between them, in B. mori SF-HA biomaterial, at fundamental level is an important factor to consider while developing the tissue grafts. Such nanometer scale behavior is often preferably investigated using molecular dynamics method. Present study aims at understanding the mechanical deformation behavior and associated physical mechanisms in B. mori SF-HA bio-nanocomposite, at nanoscale. For this purpose, computational atomistic models of B. mori SF-HA bio-nanocomposite are developed with varying HA content. Mechanical behavior analysis of these composite models under tensile loading were performed using Molecular Dynamics (MD) simulations. Elastic modulus and tensile strength values in the range of 7-20 GPa and 200-700 MPa, respectively, are obtained for B. mori SF-HA composite, in case of different HA contents, wherein, increased mechanical properties are observed with increase in HA content. Analyses of the deformation trajectories show that the deformation flow behavior in B. mori SF-HA bio-nanocomposites is mainly defined by the soft SF phase. However, energetics analyses show that, the HA phase and SF-HA interfacial interactions also play a considerable role in mechanical performance of B. mori SF-HA bio-nanocomposite. Additionally, interfacial shear strength values in B. mori SF-HA bio-nanocomposite, for different HA contents, have also been obtained. The observations made and insights gained in present work has contribution and impact in gaining an insight into the mechanistic interactions occurring at nanoscale between SF and HA phases in B. mori SF-HA bio-composite.

Mechanisms of Shock Dissipation in Semicrystalline Polyethylene

Semicrystalline polymers are lightweight, multiphase materials that exhibit attractive shock dissipation characteristics and have potential applications as protective armor for people and equipment. For shocks of 10 GPa or less, we analyzed various mechanisms for the storage and dissipation of shock wave energy in a realistic, united atom (UA) model of semicrystalline polyethylene. Systems characterized by different levels of crystallinity were simulated using equilibrium molecular dynamics with a Hugoniostat to ensure that the resulting states conform to the Rankine-Hugoniot conditions. To determine the role of structural rearrangements, order parameters and configuration time series were collected during the course of the shock simulations. We conclude that the major mechanisms responsible for the storage and dissipation of shock energy in semicrystalline polyethylene are those associated with plastic deformation and melting of the crystalline domain. For this UA model, plastic deformation occurs primarily through fine crystallographic slip and the formation of kink bands, whose long period decreases with increasing shock pressure.

Unraveling Tribochemistry and Self-Lubrication Mechanism of Polytetrafluoroethylene by Reactive Coarse-Grained Molecular Dynamics Simulations

Lubrication of polymeric materials generally involves processes of atomic-scale chemical bond forming/breaking at the interface and mesoscale chain reorientation, disentanglement, and so forth. However, it is difficult to describe the important aspects of tribochemical reactions by conventional coarse-grained molecular dynamics (CGMD) simulations. Here, reactive CGMD simulations were conducted based on the ReaxFF force field to study the tribochemical interactions between polytetrafluoroethylene (PTFE) and iron. The chemical bond forming/breaking between the molecular chain and countersurface was fitted through the bond dissociation energies of specific reaction sites from all-atom ReaxFF-MD simulations. This enabled a quantitative description of tribochemical reactions in a macromolecule system. First, the number of anchoring bonds between PTFE molecules and the countersurface showed a strong correlation with the friction coefficient. The shearing process induced breaking of the interfacial anchoring bonds as well as chain disentanglement in the matrix, which consequently led to ordering reorientation of molecular chains toward sliding direction and hence decrease of friction. Second, two competitive factors were clarified to affect polymer friction with varying temperatures. The decrease of interfacial anchoring reactivity and molecular chain mobility at low temperature prohibited reorientation of molecular chains and increased the friction coefficient. On the other hand, the hardening of PTFE and the reduction in effective contact area at low temperatures decreased the friction coefficient. This led to a turning point with a maximum friction coefficient around 100 K. These results shed light on the essential role of tribochemical reactions on polymer lubrication, especially under low temperatures, which provides design guidance of polymeric lubrication systems for engineering applications.

Molecular Insights into the Wall Slip Behavior of Pseudoplastic Polymer Melt in Nanochannels during Micro Injection Molding

Wall slip directly affects the molding quality of plastic parts by influencing the stability of the filling flow field during micro injection molding. The accurate modeling of wall slip in nanochannels has been a great challenge for pseudoplastic polymer melts. Here, an effective modeling method for polymer melt flow in nanochannels based on united-atom molecular dynamics simulations is presented. The effects of driving forces and wall–fluid interactions on the behavior of polyethylene melt under Poiseuille flow conditions were investigated by characterizing the slip velocity, dynamics information of the flow process, and spatial configuration parameters of molecular chains. The results indicated that the united-atom molecular dynamics model could better describe the pseudoplastic behavior in nanochannels than the commonly used finitely extensible nonlinear elastic (FENE) model. It was found that the slip velocity could be increased with increasing driving force and show completely opposite variation trends under different orders of magnitude of the wall–fluid interactions. The influence mechanism was interpreted by the density distribution and molecular chain structure parameters, including disentanglement and orientation, which also coincides with the change in the radius of gyration.

Nascent structure memory erased in polymer stretching

Stretching of semicrystalline polymer materials is fundamentally important in their mechanical performance and industrial processing. By means of dynamic Monte Carlo simulations, we compared the parallel stretching processes between the initially bulk amorphous and semicrystalline polymers at various temperatures. In the early stage of stretching, semicrystalline polymers perform local and global melting-recrystallization behaviors at low and high temperatures, while the memory effects occur upon global melting-recrystallization at middle temperatures. However, the final crystallinities, crystalline bond orientations, chain-folding probabilities, residual stresses, and crystallite morphologies at high enough strains appear as the same at each temperature, irrelevant to the initially amorphous and semicrystalline polymers, indicating that the common post-growth melting-reorganization processes determine the final products. In addition, both final products harvest the highest crystallinities in the middle temperature region because the postgrowth stage yields the vast nuclei followed with less extent of crystal growth in the low temperature region and few nuclei followed with large extent of crystal growth in the high temperature region. Our observations imply that a large enough strain can effectively remove the thermal history of polymers, similar to the thermal treatment at a high enough temperature; therefore, the fracture strength of semicrystalline polymers depends upon their final structures in stretching, not related to their nascent semicrystalline structures.

A surprisingly gentle approach to cavity containing spherocylindrical microparticles from ordinary polymer dispersions in flow

Herein, we demonstrate a facile approach to fully transform spherical polymeric microparticles to elongated spherocylinders containing an internal cavity under ambient and mild stirring conditions. Critical to the process is to deform the amorphous and non-crosslinked particles under glassy conditions for an unusually long time; 120 hours for the poly(styrene-co-glycidyl methacrylate) microparticles discussed in greatest detail. Larger particles in the 5 micron and greater range were markedly more susceptible to the shear imposed by stirring the aqueous dispersion. The resulting morphology is robust and kinetically frozen yet reverts to the original spherical shape if annealed above the glass transition temperature with suitable temperature or plasticizer. The volume fraction of the internal void can be modulated by particle composition and process conditions and is irregular in shape we believe as a result of a cavitation event during plastic deformation.

Super Stretchable and Compressible Hydrogels Inspired by Hook-and-Loop Fasteners

Inspired by hook-and-loop fasteners, we designed a hydrogel network containing α-zirconium phosphate (ZrP) two-dimensional nanosheets with a high density of surface hydroxyl groups serving as nanopatches with numerous "hooks," while polymer chains with plentiful amine functional groups serve as "loops." Our multiscale molecular simulations confirm that both the high density of hydroxyl groups on nanosheets and the large number of amine functional groups on polymer chains are essential to achieve reversible interactions at the molecular scale, functioning as nano hook-and-loop fasteners to dissipate energy. As a result, the synthesized hydrogel possesses superior stretchability (>2100% strain), resilience to compression (>90% strain), and durability. Remarkably, the hydrogel can sustain >5000 cycles of compression with torsion in a solution mimicking synovial fluid, thus promising for potential biomedical applications such as artificial articular cartilage. This hook-and-loop model can be adopted and generalized to design a wide range of multifunctional materials with exceptional mechanical properties.

Visualization of the Necking Initiation and Propagation Processes during Uniaxial Tensile Deformation of Crystalline Polymer Films via the Generation of Fluorescent Radicals

To visualize and simultaneously quantify the necking behavior of crystalline polymer films during uniaxial stretching, tetraarylsuccinonitrile (TASN) moieties were introduced into polymers at the center of the main chain. TASN can produce a relatively stable radical that emits yellow fluorescence in response to mechanical stress. During the uniaxial elongation test of the TASN-centered crystalline polymers, the yellow fluorescence derived from the dissociated TASN radicals was used for microscale observations that showed the orientation of the polymer chains in the stretching direction. Furthermore, by comparing the radical generation in linear and star-shaped TASN-centered crystalline polymers during their tensile deformation, we found that the TASN dissociation ratio is higher in the star-shaped polymer, which has more chains connected to the lamellar crystal. Thus, the microforces generated in the amorphous region during uniaxial stretching were probed via the use of TASN, which allowed a direct visualization of the necking initiation and propagation processes as well as a quantification via electron paramagnetic resonance spectroscopy.

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.

Progressive Molecular Rearrangement and Heat Generation of Amorphous Polyethene Under Sliding Friction: Insight from the United-Atom Molecular Dynamics Simulations

Frictional heat has been widely used in various polymer-based advanced manufacturing. The fundamental understanding of the thermodynamics of the interfacial friction of polymer bulk materials can help to avoid compromising the process controllability. In this work, we have performed united-atom molecular dynamics (MD) simulations to reveal the interfacial friction heating mechanism of amorphous polyethene (PE) in both the single sliding friction (SSF) and reciprocating sliding friction (RSF) modes. Different from the traditional view that the plastic deformation was the primary source of heat generation, the RSF process with no apparent plastic deformation in this work shows a better heat generation performance than SSF, where plastic deformation dominated the friction process. Our analysis uncovers that the mechanism of the interfacial friction heating enhancement in RSF can be attributed to the concentrated high-frequency chain motion related to molecular rearrangement, which is not clearly related to the deformation degree.

Dynamic Responsive Formation of Nanostructured Fibers in a Hydrogel Network: A Molecular Dynamics Study

In an effort to study natural fiber formation, such as, e.g., spider silk, we present a model, which is capable of forming biomimetic fibrillar nanostructure from a hydrogel micellar network. The latter consists of interacting atomic groups which form cores of micelles, and of flexible chains forming the shells of the micelles. Micelles are connected in a compact network by linearly stretched chains. The structural elements of the network can be transformed during deformation from micellar into fibrillary type and their evolution is found to depend significantly on strain rate. Our model suggests a set of conditions suitable for the formation of nanostructured fibrillar network. It demonstrates that a fibrillar structure is only formed upon sufficiently fast stretching while, in contrast, the micellar gel structure is preserved, if the material is pulled slowly. We illustrate this key aspect by a minimalistic model of only four chains as part of the whole network, which provides a detailed view on the mechanism of fibril formation. We conclude that such a simplified structure has similar functionality and is probably responsible for the formation of nano-structured molecular fibrils in natural materials.

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.

Measuring Tie Chains and Trapped Entanglements in Semicrystalline Polymers

A label-free method for quantifying stress-transmitter (or elastically effective molecule) content (p) in semicrystalline polymers, including both tie molecules and bridging entanglements, is developed and demonstrated based on swelling with deuterated vapor and characterization with small angle neutron scattering. The p results are compared with the predictions of recent semi empirical, statistical values for tie molecule content and structural characterization parameters, including strain hardening modulus and an infrared spectroscopy derived parameter ( β ) that describes the degree of difficulty for the amorphous content to align and reshape over a distance with applied load. A strong correspondence is observed, suggesting that the initial network of elastically active molecules, dictated by the molecular architecture and crystallization conditions, can be directly correlated to the post yield tensile values irrespective of the subsequent morphological changes that result during the tensile deformation. These comparisons are also consistent with simulations indicating that polyethylene homopolymers have more bridging entanglements than copolymers and that the average tie molecule has a larger impact on mechanical properties than the average bridging entanglement. Contrary to high temperature bulk swelling measurements, it is found that the Michaels-Hausslein vapor-swelling theory cannot fit the experimental data while our modified Flory-Rehner theory can fit the data.

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.

Molecular dynamics simulation of shish-kebab crystallization of polyethylene: Unraveling the effects of molecular weight distribution

By means of molecular dynamics simulations, extensional flow was performed on five polyethylene models with different molecular weight distributions (MWDs) precisely designed in view of Grubbs, metallocene, Ziegler-Natta, and chromium-based catalysts, while ignoring the sequence distributions of short branches to shed light on the molecular mechanism of MWD on shish-kebab formation. The formation of shish-kebab crystallites can be divided into three stages: the emergence of precursors, evolution from precursors to shish nuclei, and the formation of lamellar crystallites. The results demonstrated that the precursors initiated from trans-rich segments with local order and minor crystallinity grew into large shish nuclei and eventually evolved into lamellae. There were more inconsecutively trans-state bonds occurring in long chains rather than in short chains, which promoted an easier transformation from precursors to shish nuclei. Therefore, broader MWDs make positive contributions to the formation of shish nuclei, increase the crystallization speed, and the generation of a more regular, compact, and thicker lamella with less tie molecule fractions, while the final crystallinity is independent of MWD.

The evolution of crystalline structures during gel spinning of ultra-high molecular weight polyethylene fibers

Most studies are focused on the final mechanical properties of the fiber and the processing window required to achieve high moduli and tensile strength. Several studies have alluded to the fact that the crystalline morphologies developed during gel spinning and post-drawing are very important in determining the final mechanical properties. However, it is surprising to know that no clear correlation exists between the crystalline structure and initial, evolving, and final mechanical properties. In an attempt to define structure-property relationships, we have developed novel tools to quantify the effect of processing on crystalline structure evolution. We examine through controlled gel-spinning and SAXS analysis the effect of flow kinematics on the development of crystalline structures. Direct correlations are made between polymer solution relaxation time, extension rates, crystallization time and gel-spun crystalline morphologies. We report direct evidence of flow induced crystallization, which approaches an asymptotic crystallization rate at high Weissenberg numbers. For Wi < 1, the crystalline structure is only slightly affected by equilibrium. For Wi > 1, the crystalline structure is highly anisotropic due to chain orientation/stretch during spinning. Fibers spun at different Weissenberg numbers are drawn to low draw ratios at constant temperature to measure the initial structure evolution. A qualitative SAXS analysis clearly shows similar evolution of different starting structures with the formation of more straight chain crystals upon drawing. However, there remain quantitative differences between the length of straight chain crystals and the size and distribution of lamellar domains depending on the starting structure.

Crystal Growth in Polyethylene by Molecular Dynamics: The Crystal Edge and Lamellar Thickness

We carried out large-scale atomistic molecular dynamics simulations to study the growth of twin lamellar crystals of polyethylene initiated by small crystal seeds. By examining the size distribution of the stems-straight crystalline polymer segments-we show that the crystal edge has a parabolic profile. At the growth front, there is a layer of stems too short to be stable, and new stable stems are formed within this layer, leading to crystal growth. Away from the edge, the lengthening of the stems is limited by a lack of available slack length in the chains. This frustration can be relieved by mobile crystal defects that allow topological relaxation by traversing through the crystal. The results shed light on the process of polymer crystal growth and help explain initial thickness selection and lamellar thickening.

Mechanisms of reinforcement in polymer nanocomposites

Qualitatively different stress–strain responses of polymer nanocomposites are shown to result from the dynamical evolution of three principal molecular structural motifs in the polymer–filler network.

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.