Viscoelastic Properties and Shock Response of Coarse-Grained Models of Multiblock versus Diblock Copolymers: Insights into Dissipative Properties of Polyurea

Molecular Origins of Shock Dissipation in Polymer-Based Nanocomposites

To explain the molecular origin of shock dissipation in polymer-based composites, a new method is proposed by absolutely quantifying the relative position between molecular chains, which directly reflects the shock dissipation energy between molecular chains and at the interfaces with nanofillers during mechanical deformation. The effects of nanofiller content and system temperature on energy dissipation were examined based on this newly proposed method, and the differences in energy dissipation between the thermosetting polymer (polydimethylsiloxane) and thermoplastic polymer (polyether-ether-ketone) under identical physical conditions were both studied to validate the reliability of the quantification method proposed in this paper. The present study offers a tool that provides theoretical insights to understand the molecular origins of shock dissipation in polymer composites as well as to facilitate the optimal design of composites with outstanding damping characteristics.

State-of-the-Art Polyurea Coatings: Synthesis Aspects, Structure-Properties Relationship, and Nanocomposites for Ballistic Protection Applications

This review presents polyurea (PU) synthesis, the structure-properties relationship, and characterization aspects for ballistic protection applications. The synthesis of polyurea entails step-growth polymerization through the reaction of an isocyanate monomer/prepolymer and a polyamine, each component possessing a functionality of at least two. A wide range of excellent properties such as durability and high resistance against atmospheric, chemical, and biological factors has made this polymer an outstanding option for ballistic applications. Polyureas are an extraordinary case because they contain both rigid segments, which are due to the diisocyanates used and the hydrogen points formed, and a flexible zone, which is due to the chemical structure of the polyamines. These characteristics motivate their application in ballistic protection systems. Polyurea-based coatings have also demonstrated their abilities as candidates for impulsive loading applications, affording a better response of the nanocomposite-coated metal sheet at the action of a shock wave or at the impact of a projectile, by suffering lower deformations than neat metallic plates.

Shock compression of semiflexible polymers

We employ molecular dynamics simulations to investigate the shock compression of linear semiflexible polymers. While the propagation velocity of a shock primarily depends on density, both chain rigidity and chain orientation significantly influence the shock width and the final temperature of the system. In general, the shock wave triggers molecular buckling in chains oriented perpendicular to the compression front. Following the passage of the front, the semiflexible chains buckle with a wavelength that decreases with the compression speed as λm ∼ up-0.2, and subsequent relaxation leads to a banana-like liquid crystal phase. In ordered systems with molecules oriented perpendicular to the compression front, the shock width increases by a factor of up to 10 compared to a similar isotropic system, resulting in enhanced shock energy absorption. These findings indicate that chain stiffness plays a critical role in the impact absorption properties of polymeric materials.

Shock melting of lamellae-forming block copolymers

While the propagation of shocks through monoatomic liquids and solids is now well understood, the response of macromolecular systems to shock compression remains far less studied. Here we use molecular dynamics simulations to study the shock compression of diblock copolymers assembled in a lamellae morphology, which may display outstanding ballistic performance. For the first time, we show that the morphologies observed after the passage of the shock resemble those observed at equilibrium, at a temperature dictated by the compression velocity. In copolymers, shock compression leads to a decrease in the lamellae period, favoring the mixing of the polymer blocks, such that strongly segregated initial morphologies evolve into less segregated phases after the passage of the shock, or can even melt into an isotropic phase for strong shocks.

Effect of the content and strength of hard segment on the viscoelasticity of the polyurethane elastomer: insights from molecular dynamics simulation

Due to the wide application, it is very crucial to understand the viscoelasticity of the polyurethane elastomer (PU, denoted by soft-hard block copolymer), which contains the soft segments (SS) and hard segments (HS). Thus, in this work, the effect of the content and strength of HS on the viscoelasticity of PU is explored in detail by adopting a coarse-grained model. First, the phase morphology of PU is characterized where both the single continuous phase and the bicontinuous phase are observed by varying the content of HS. Then, the viscoelasticity of PU is calculated by analyzing the storage modulus, the loss modulus, and the loss factor, which depends on the content and strength of HS. To further elucidate the mechanism for the storage modulus, the normalized interaction energy, the order parameter, and the formation probability of the HS or SS phase are characterized with the shear strain amplitude, which reflects the deformation of the phase structure. Then, the energy dissipation is quantified, which can rationalize the loss modulus well. A parameter is introduced, which considers the relative slippage and the content of HS or SS. It can explain the change in the loss factor with the content and strength of HS. In summary, this work can help to further understand how the content and strength of hard segments affect the viscoelasticity of the soft-hard block PU and structure evolution at the molecular level.

Mechanical Properties of Solution-Blended Graphene Nanoplatelets/Polyether-Ether-Ketone Nanocomposites

A lightweight composite with outstanding damping effects and impact resistance is significant for the design of functional structures in advanced equipment, such as aircraft and space vehicles. In this paper, the mechanical properties of solution-blended graphene nanoplatelets (GNPs)/polyether-ether-ketone (PEEK) nanocomposites were characterized by both experimental and numerical methods. It can be found that the layer number and packing configuration of graphene layers are critical to the efficiency of energy dissipation in the composite, while a pack of six- layer graphene and the perpendicular arrangement to the shockwave direction provide the most outstanding energy dissipation ability. The reflection of shockwave caused by graphene reinforcements in the nanocomposite was found to be the dominating reason for the enhanced energy dissipation effect. Physical mechanisms of energy dissipation are investigated by a molecular modeling method to provide insights into the cross-scale design of graphene-reinforced nanocomposites as structural materials.

Gruneisen parameters of bead-spring chains: MD simulation and theory

Molecular Dynamics (MD) simulations were carried out in a microcanonical ensemble to compute the Gruneisen parameter (denoted as γ) of a liquid of bead-spring chains having 10 beads/chain. γ was studied over a wide range of temperatures below and above the glass transition temperature. We found that the Gruneisen parameter varied in the range of 2.1-3.1 and was significantly higher than typically observed experimentally in real polymers. In the glass, a theory was developed for γ using a cell model in which the beads are harmonically bound to their respective cell centers. The resulting Gruneisen parameter is predicted to increase slightly with temperature. Above the glass transition temperature, we employed the generalized Flory dimer equation-of-state and the polymer reference interaction model theory to calculate γ. In these calculations, we found that γ decreased with temperature in the liquid. The theoretical predictions for γ were found to be in good qualitative agreement with our MD simulations, without any adjustable parameters, both above and below T_g. In experiments on real polymers, γ undergoes a sharp discontinuity at the glass transition. By contrast, in our MD simulations, γ varies smoothly over a broad transition region.

Influence of the interfacial interaction strength on the viscoelasticity of hard-soft block copolymer based nanocomposites: a molecular dynamics simulation study

The effect of the interaction strength between hard segments (soft segments) and nanofillers on the morphology and viscoelastic mechanical properties of nanocomposites based on hard-soft block copolymers was studied by adopting molecular dynamics simulations. The morphologies of the models studied in this work could be classified into three types, each with distinct mechanical properties. The Payne effect and reinforcement of G' were successfully reproduced. Two mechanisms responsible for the viscoelastic behaviours were proposed, including damage of hard domains and desorption of polymer chains from the nanofillers. Further, the evolution of the polymer-nanofiller interface and hard domains was examined by monitoring the energy, snapshots, and corresponding microstructure parameters during deformation. The damage of hard domains was in the form of transforming from the compact state to the loose state.

Effect of Polymerized Ionic Liquid Structure and Morphology on Shockwave Energy Dissipation

The ability of nanosegregated polymerized ionic liquids (PILs) to dissipate shockwave energy is investigated for a series of imidazolium-based PILs with varying alkyl spacer length. The PILs are designed to have similar glass transition temperatures but different structures. X-ray scattering analysis reveals that each of the amorphous PILs exhibit distinct nanoscale structural heterogeneity, depending on the length of the chain spacer. We find that a higher structural heterogeneity, determined from the intensity of the intercluster scattering peak, in the PILs with longer alkyl spacers results in greater shockwave energy dissipation. In addition, we observe the crystalline phase is less effective at dissipating shockwave energy than the amorphous phase due to the close packed morphology and slow kinetics.

Shock-Induced Ordering in a Nano-segregated Network-Forming Ionic Liquid

Understanding shockwave-induced physical and chemical changes of impact-absorbing materials is an important step toward the rational design of materials that mitigate the damage. In this work, we report a series of network-forming ionic liquids (NILs) that possess an intriguing shockwave absorption property upon laser-induced shockwave. Microstructure analysis by X-ray scattering suggests nano-segregation of alkyl side chains and charged head groups in NILs. Further post-shock observations indicate changes in the low-Q region, implying that the soft alkyl domain in NILs plays an important role in absorbing shockwaves. Interestingly, we observe a shock-induced ordering in the NIL with the longest (hexyl) side chain, indicating that both nano-segregated structure and shock-induced ordering contribute to NIL's shockwave absorption performance.

Thermomechanical properties and deformation of coarse-grained models of hard-soft block copolymers

In this paper, we investigate the enhancement mechanism of the mechanical properties for hard-soft block copolymers by using molecular dynamics simulations at various temperatures. A coarse-grained approach is adopted to study sufficiently generic models. Our numerical experiments demonstrate that the nonbond potential plays a more significant role in mechanical properties compared to the bond potential. This finding serves as a cornerstone to understand the hard-soft materials. To explore the effect of hard segments, four copolymers with different concentrations and energy factors that describe the interaction between hard beads are conducted. Simulation results show that the mechanical performances of the system with large attractive force and small concentration of hard segments could be improved dramatically in conjunction with a moderate increment of the glass transition temperature. In particular, the energy factor shows a substantial influence in determining the microphase separation as well as the morphology of hard domains. These observations are believed to provide design guidelines for polymeric materials in engineering practice.