Recent experimental studies have demonstrated that the introduction of oxygen-containing functional groups in graphene sheets can greatly enhance the mechanical properties of their nanocomposites with polar polymers even at extremely low loadings. Motivated by these reports, we determine here the elastic constants of syndiotactic poly(methyl methacrylate) (sPMMA) at small wt % loadings of graphene sheets through atomistic modeling. To carry out a comparative study of the effect of graphene functionalization on the degree of mechanical reinforcement, we address both pure (i.e., unfunctionalized) and functionalized graphene sheets bearing epoxy and hydroxyl groups randomly bound on both sides of their surface in the host sPMMA matrix. The calculation of elastic constants (which involves no adjustable parameters) follows the methodology originally proposed by Theodorou and Suter [Macromolecules198619139], and has been based on the use of the Dreiding all-atom force-field. Our predictions for the elastic constants (which for the pure sPMMA matrix are within the error bars of experimentally computed values) suggest a substantial increase in the elastic constants, especially in the case of functionalized graphene sheets. For example, at just 5.67 wt % loading of the host matrix in functionalized graphene sheets, they indicate an improvement in Young’s modulus E by ∼74%, in the bulk modulus B by ∼19%, and in the shear modulus G by 83%. Our results fully corroborate recent experimental measurements about the unique opportunities that functionalized graphene sheets offer for the design of new, very strong multifunctional materials at low nanofiller content.

Piezoelectric fluoropolymers convert mechanical energy to electricity and are ideal for sustainably providing power to electronic devices. To convert mechanical energy, a net polarization must be induced in the fluoropolymer, which is currently achieved via an energy-intensive electrical poling process. Eliminating this process will enable the low-energy production of efficient energy harvesters. Here, by combining molecular dynamics simulations, piezoresponse force microscopy, and electrodynamic measurements, we reveal a hitherto unseen polarization locking phenomena of poly(vinylidene fluoride–co–trifluoroethylene) (PVDF-TrFE) perpendicular to the basal plane of two-dimensional (2D) Ti3C2Tx MXene nanosheets. This polarization locking, driven by strong electrostatic interactions enabled exceptional energy harvesting performance, with a measured piezoelectric charge coefficient, d33, of −52.0 picocoulombs per newton, significantly higher than electrically poled PVDF-TrFE (approximately −38 picocoulombs per newton). This study provides a new fundamental and low-energy input mechanism of poling fluoropolymers, which enables new levels of performance in electromechanical technologies.

High-performance, unpoled and recyclable piezoelectric generators are produced by combining dipole templating via single-walled carbon nanotubes with shear-induced polarisation via 3D printing of fluoropolymers.

Molecular dynamics simulations and Rouse theory suitably adapted for polymer chains adsorbed by one or both of their ends are combined to offer a quantitative description of the local structure and microscopic dynamics in attractive polymer nanocomposite melts using a poly­(ethylene glycol) (PEG)/silica nanocomposite as a model system. Our work reveals that the adsorbed layer around the silica nanoparticle is far from being characterized as “glassy” or “immobilized” since adsorbed polymer segments in the form of tails and loops on silica exhibit appreciable mobility locally, which helps adsorbed chains to relax at short length scales, albeit rather slowly. The simulations also reveal significant differences in the structural and dynamic properties of the PEG/silica nanocomposite melts studied for different terminal groups (hydroxyl versus methoxy) of the PEG chains, originating from the different ways that polymer chains adsorb on the silica surface: hydroxyl-terminated PEG chains are adsorbed by their ends giving rise to a brush-like structure, whereas methoxy-terminated ones are adsorbed equally probably along their entire contour, thus resulting in better packing of adsorbed segments. Due to the dense interfacial layer that develops in both cases, the diffusive behavior of free chains is also affected (it slows down compared to that in the corresponding pure PEG melt), especially in the nanocomposite where PEG chains are terminated with hydroxyl groups. Direct comparison of simulation and theoretical predictions with previously reported experimental data in the literature for the dynamic structure factor [Glomann et al., Phys. Rev. Lett. 2013, 110, 178001] for the same systems under the same temperature and pressure conditions reveals excellent agreement.

Molecular dynamics (MD) is used to simulate a model atactic poly(methyl methacrylate) (PMMA) system in which carbon nanotubes (CNTs) have been randomly dispersed. Our purpose is to elucidate the equilibrium structure and dynamic behavior of PMMA chains at the interface with a CNT. CNTs with different diameters and at different concentrations in the host PMMA matrix are studied, and their effect on the equilibrium squared radius-of-gyration and squared end-to-end distance of PMMA chains is examined. We have analyzed PMMA density, structure, and conformation both axially and normal to the CNT surface. Our MD simulations indicate that the presence of CNTs causes a small decrease in the size of the polymer chains, which becomes more pronounced as the concentration (volume fraction) and diameter of CNTs in the nanocomposite increases. We also provide a detailed analysis of adsorbed PMMA chain conformations in terms of trains, loops, and tails, and their statistical properties. An important finding of our work is that PMMA chains tend to penetrate significantly into the CNTs through their faces; as a result of CNT filling by PMMA chains, the area near the CNT mouths is characterized by significantly higher polymer mass density (almost by 45%) than the bulk of the nanocomposite. Additional simulation results for local and terminal relaxation in the PMMA-CNT nanocomposites reveal that due to strong PMMA-CNT attractive forces, all relaxation times in the interfacial region are significantly prolonged in comparison to the bulk, and the same happens with the diffusive (translational) motion of the chains. The density profile that develops (both axially and radially) in the vicinity of CNTs appears to significantly delay PMMA dynamics at all length scales. How this affects the glass-transition temperature of the nanocomposite is also analyzed.

Stable graphene dispersions in chloroform, using very low pyrene-functionalized polymer/graphite mass ratio.

Pyrene-functional PMMAs were prepared via ATRP-controlled polymerization and click reaction, as efficient dispersing agents for the exfoliation of few-layered graphene sheets (GS) in easily processable low boiling point chloroform. In parallel, detailed atomistic simulations showed fine dispersion of the GS/polymer hybrids in good agreement with the experiment. Moreover, the molecular dynamics simulations revealed interesting conformations (bridges, loops, dangling ends, free chains) of GS/polymer hybrids and allowed us to monitor their time evolution both in solution and in the polymer nanocomposite where the solvent molecules were replaced with PMMA chains. Microscopic information about these structures is very important for optimizing mechanical performance. It seems that the combination of atomistic simulation with advanced chemistry constitutes a powerful tool for the design of effective graphene dispersing agents that could be used for the production of graphene-based nanocomposites with tailor-made mechanical properties.

Femtosecond electron diffraction is established as goniometer of ultrafast nanocrystal rotations.