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.

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 Dynamic Impact Responses of Layered Polymer-Graphene Nanocomposite Films Using Coarse-Grained Molecular Dynamics Simulations.

Polymer nanocomposite films have recently shown superior energy dissipation capability through the micro-projectile impact testing method. However, how stress waves interact with nanointerfaces and the underlying deformation mechanisms have remained largely elusive. This paper investigates the detailed stress wave propagation process and dynamic failure mechanisms of layered poly(methyl methacrylate) (PMMA) - graphene nanocomposite films during piston impact through coarse-grained molecular dynamics simulations. The spatiotemporal contours of stress and local density clearly demonstrate shock front, reflected wave, and release wave. By plotting shock front velocity (U s ) against piston velocity (U p ), we find that the linear Hugoniot U s - U p relationship generally observed for bulk polymer systems also applies to the layered nanocomposite system. When the piston reaches a critical velocity, PMMA crazing can emerge at the location where the major reflected wave and release wave meet. We show that the activation of PMMA crazing significantly enhances the energy dissipation ratio of the nanocomposite films, defined as the ratio between the dissipated energy through irreversible deformation and the input kinetic energy. The ratio maximizes at the critical U p when the PMMA crazing starts to develop and then decreases as U p further increases. We also find that a critical PMMA-graphene interfacial strength is required to activate PMMA crazing instead of interfacial separation. Additionally, layer thickness affects the amount of input kinetic energy and dissipated energy of nanocomposite films under impact. This study provides important insights into the detailed dynamic deformation mechanisms and how nanointerfaces/nanostructures affect the energy dissipation capability of layered nanocomposite films.

Identifying Structure-Property Relationships of Micro-Architectured Porous Scaffolds through 3D Printing and Finite Element Analysis.

This study integrates 3D printing and finite element analysis (FEA) to investigate the effect of micro-architectural characteristics on the mechanical properties of porous scaffolds. The studied characteristics include the thickness of the scaffold walls and the number of domains at the cross-section. We use 3D printing to fabricate scaffolds of deliberately designed microstructures to enable strict control of the structures. The longitudinal compressive properties of different scaffolds are first analyzed through experimental testing. Then, FEA is conducted to investigate the mechanical properties and the deformation mechanisms of the scaffolds. We find that decreasing wall thickness leads to failure mechanism transition from wall compression failure to buckling instability. For scaffolds with different wall thicknesses, the failure mechanisms and the critical loads are evaluated using the theory of thin plate buckling. For the characteristic of the number of domains, both experimental and FEA results indicate increasing effective stiffness with increasing domains. Interestingly, we find that with the material properties extracted from a single wall scaffold, the computational models tend to overestimate the effective compression modulus of scaffolds with larger numbers of walls or domains than the experimental data. This observation indicates possible size-dependent material properties in 3D printed structs. Our study demonstrates that integrating experiments and computational modeling can provide fundamental insights into the mechanical properties and deformation mechanisms of micro-architectured scaffolds and unveil unique small-scale material behaviors.