Understanding the Mechanical and Viscoelastic Properties of Graphene Reinforced Polycarbonate Nanocomposites Using Coarse-Grained Molecular Dynamics Simulations

Improved Ballistic Impact Resistance of Nanofibrillar Cellulose Films with Discontinuous Fibrous Bouligand Architecture

Natural protective materials offer unparalleled solutions for impact-resistant material designs that are simultaneously lightweight, strong, and tough. Particularly, the Bouligand structure found in the dactyl club of mantis shrimp and the staggered structure in nacre achieve excellent mechanical strength, toughness, and impact resistance. Previous studies have shown that hybrid designs by combining different bioinspired microstructures can lead to enhanced mechanical strength and energy dissipation. Nevertheless, it remains unknown whether combining Bouligand and staggered structures in nanofibrillar cellulose (NFC) films, forming a discontinuous fibrous Bouligand (DFB) architecture, can achieve enhanced impact resistance against projectile penetration. Additionally, the failure mechanisms under such dynamic loading conditions have been minimally understood. In our study, we systematically investigate the dynamic failure mechanisms and quantify the impact resistance of NFC thin films with DFB architecture by leveraging previously developed coarse-grained models and ballistic impact molecular dynamics simulations. We find that when nanofibrils achieve a critical length and form DFB architecture, the impact resistance of NFC films outperforms the counterpart films with continuous fibrils by comparing their specific ballistic limit velocities and penetration energies. We also find that the underlying mechanisms contributing to this improvement include enhanced fibril sliding, intralayer and interlayer crack bridging, and crack twisting in the thickness direction enabled by the DFB architecture. Our results show that by combining Bouligand and staggered structures in NFC films, their potential for protective applications can be further improved. Our findings can provide practical guidelines for the design of protective films made of nanofibrils.

Investigation of molecular mechanisms of polyvinylidene fluoride under the effects of temperature, electric poling, and mechanical stretching using molecular dynamics simulations

This study uses molecular dynamics (MD) simulations to investigate the molecular mechanisms of polyvinylidene fluoride (PVDF) influenced by temperature, electric poling, and mechanical stretching. The β-phase, with all-trans ⟨T⟩ planar zigzag conformation, is known to have the best potential of energy harvesting, while α-phase, with alternating trans ⟨T⟩ and gauche ⟨G⟩ linkages, is more stable in terms of potential energy. By applying an electric field and uniaxial deformation to an amorphous PVDF system, we study the transformation from α- to β-phase and corresponding molecular mechanisms by tracking the molecular chain conformation using the trans percentages (P_T). After complete relaxation of molecular chains, the chain conformations and P_T values indicate a typical distribution pattern of α-phase. Next, we observe that the dipole moment of the system increases significantly with the presence of a strong electric field without immediately affecting the chain conformations. The increment of dipole moment is due to the aligning of side atoms within the chains and the increment becomes more significant with elevated temperature. In contrast, chain conformations change significantly under mechanical stretching. Specifically, before yielding, the total dipole moments are still governed by local orientations of atoms. Later, the chain segments begin to straighten in the large deformation stage, which leads to the increment of the total dipole moment. Our results also show that there exists an optimal temperature window for maximum ⟨G⟩ to ⟨T⟩ transformation rate. Moreover, we look into the synergistic effect of electric poling and mechanical stretching and explain molecular-level mechanisms for this effect. This study contributes to the fundamental understanding of the underlying molecular mechanisms for the piezoelectric PVDF system under different processing conditions.

Mechanical and Viscoelastic Properties of Wrinkled Graphene Reinforced Polymer Nanocomposites - Effect of Interlayer Sliding within Graphene Sheets

Multilayer graphene sheets (MLGSs) are promising nano-reinforcements that can effectively enhance the properties of polymer matrices. Despite many studies on MLGSs-reinforced polymer nanocomposites, the effect of wrinkles formed in MLGSs on the reinforcement effect and the viscoelastic properties of polymer nanocomposites has remained unknown. In this study, building upon previously developed coarse-grained models of MLGSs and poly(methyl methacrylate) coupled with molecular dynamics simulations, we have systematically investigated nanocomposites with different numbers of graphene layers and various wrinkle configurations. We find that with decreasing degree of waviness and increasing numbers of layers, the elastic modulus of the nanocomposites increases. Interestingly, we observe a sudden stress drop during shear deformation of certain wrinkled MLGSs-reinforced nanocomposites. We further conduct small amplitude oscillatory shear simulations on these nanocomposites and find that the nanocomposites with these specific wrinkle configurations also show peculiarly large loss tangents, indicating an increasing capability of energy dissipation. These behaviors are attributed to the activation of the interlayer sliding among these wrinkled MLGSs, as their interlayer shear strengths are indeed lower than flat MLGSs measured by steered molecular dynamics technique. Our study demonstrates that the viscoelastic properties and deformation mechanisms of polymer nanocomposites can be tuned through MLGS wrinkle engineering.