Molecular dynamics simulations modeled the aramid poly(p-phenylene terephthalamide) (PPTA) and a related aromatic-aliphatic polyamide derived from a five-carbon aliphatic diacid (PAP5) with nine different reactive and non-reactive force fields. The force fields were evaluated based on crystal structures as well as intermolecular Hbonding and π-molecular interactions. The optimum force field was then used to simulate stress-strain behavior in the chain and transverse-to-chain directions. In the chain direction, PAP5 had higher ultimate stress and failure strain than PPTA; however, the stiffness of PAP5 was lower than PPTA at low strain (0-2%) while the reverse was observed at high strain (last 5% before failure). This contrast, and differences in the transverse direction properties, were explained by the methylene segments of PAP5 that confer conformational freedom, enabling accommodation of low strain without stretching covalent bonds. The simulation approach demonstrated here for two poly-1 Macromolecules 2021 10.1021/acs.macromol.1c00974 mers with distinct chemistry but similar atomic interactions may be extended to other polyamides.

Self-assembled monolayers (SAMs) can reduce friction in boundary lubricated contacts by providing a low shear strength interface for sliding. However, the nanoscale mechanisms underlying low friction on SAMs are still not fully understood, especially in liquid environments in which hydrophobility or hydrophilicity affects friction. To understand this effect, friction of SAMs in water was measured using atomic force microscope experiments and molecular dynamics simulations, where hydrophilicity or hydrophobicity was determined by the terminal group of the alkanethiols. The friction on hydrophilic SAMs was larger than that on hydrophobic SAMs in both experiments and simulations, but this trend could not be explained by the strength of the adhesive force between the tip and the SAMs. Instead, analysis of the contributions of the water and SAMs to the total friction force revealed that the difference between the hydrophobic and hydrophilic SAMs could be explained by interactions between the tip and water during sliding. The much larger tip-water force on hydrophilic SAMs was attributed to a dense layer of water that was displaced during sliding as well as hydrogen bonds that formed between the water molecules and hydrophilic SAMs and then broken by the tip as it slid, leading to higher friction force.

Reactive molecular dynamics simulations were used to model poly(p-phenylene terephthalamide) and related aromatic-aliphatic polyamides derived from p-phenylene diamine and aliphatic diacids with different numbers of carbon atoms in the aliphatic chain (5, 6, 7, or 8). Tensile strain was applied to each polymer crystal in the chain direction and the mechanical response was characterized. All the polymers with aliphatic segments exhibited strain hardening, transitioning from an initial (low-strain) linear regime to a second (high-strain) linear regime. The modulus at high strain was similar for all polymers, but the modulus calculated at low strain decreased with increasing aliphatic chain length. The decrease in the low-strain modulus with increasing chain length was explained by the observation that polymers with longer aliphatic chains were wavier (i.e., deviated more from the fully extended conformation) in the quiescent 1 state such that they could accommodate low strain without deforming covalent bonds.Extension of wavy chains occurred through an intra-chain process for all polymers, quantified by the bond dihedral angles. In addition, for polymers with an even number of non-aromatic carbons, the strain response involved slip between chains within the hydrogen bonded sheets. The ultimate stress of the polymers exhibited an odd-even effect (even polymers were weaker) which was explained by differences in hydrogen bonding and ring-ring coplanarity prior to failure; polymers with an even number of carbon atoms had less favorable H-bonding and poorer ring alignment. The results revealed direct correlations between aliphatic chain length, intra-and inter-chain interactions, and the mechanical properties of polyamide crystals.

Atomic resolution imaging of surfaces in liquid environments using atomic force microscopy (AFM) is challenging in terms of both reproducibility and measurement interpretation. To understand the origins of these challenges, we used molecular dynamics simulations of AFM on hydrophilic self-assembled monolayers (SAMs) in water. The force on the model AFM tip was calculated as a function of lateral and vertical position relative to the SAM surface. The contributions of the water and SAMs to the overall force were analyzed, and the former was correlated to the water density distribution. Then, dynamic AFM was modeled by oscillating the tip at a driving amplitude. It was found that the contrast between amplitudes at different lateral positions on the surface was dependent on the vertical position of the tip. Lastly, amplitude maps were produced for two vertical positions at constant height, and the ability to capture atomic resolution was related to the force on the tip. These results offer an explanation for the observed instability in atomic scale imaging using AFM and more generally provide insight into the contrast mechanisms of surface images obtained in liquid environments.

Reactive molecular dynamics simulations of Ti3C2Tx with three different surface terminations were used to understand friction and failure of MXenes during sliding at normal pressures from 2–20 GPa and temperatures from 300–1100 K. The O-terminated MXene had the smallest shear stress at low pressures and temperatures, but failed at more severe conditions due to interlayer bonding and the formation of Ti–O–Ti bridges between MXene layers. Failure was not observed for the OH-terminated MXene or a heterostructure combining O- and OH-terminations. For these, at less severe operating conditions, shear stress was smaller for the OH-terminated MXene, while the opposite was observed at higher temperatures and pressures. These trends were explained in terms of adhesion and the complex effect of hydrogen atoms that can either facilitate or hinder sliding, depending on the termination and conditions. Results show that friction and failure are affected by and potentially tunable via MXene surface termination.