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In this study, by using X-ray powder diffraction profiles as blueprints, we successfully mapped the most probable molecular-level structural arrangement of the PMMA super-helix stereocomplexes through molecular dynamic simulations. Molecular-level resolution of the PMMA triple-helix supramolecule was not previously achievable by experimental methods. After constructing molecular models of stereo-regular complexes composed of linear it- and st-PMMAs, our all-atom molecular dynamics simulations identified the stereocomplex structure that best reproduces experimental diffraction profiles and thermodynamic properties as a double helix of isotactic (it-)PMMA with a helical pitch of 1.8 nm and 9 units per turn surrounded by a single helix of syndiotactic (st-)PMMA with an average helical pitch of 0.9 nm and 20 units per turn. The it-/st- complexing stoichiometry in the PMMA triple-helix is therefore 9 : 20. This presents the first all-atom model of the it-/st-PMMA triple-helix stereocomplex that accurately fits experimental X-ray diffraction profiles. In addition, the simulation results revealed the outer st-PMMA helix of the PMMA stereocomplex has a fiber diameter of at least 1.85 nm and adopts a non-ideal helical geometry. Furthermore, through dynamic simulations, surprising new sights into the effect of the structural configuration of the PMMA stereocomplex (i.e., helical pitch and direction, and tilt angle) on the physical properties of their crystal structures were obtained. Those crystal properties include X-ray diffraction profile, packing density, chain-chain spacing, chain width and cohesive energy density.

The novel duolayer system, comprising a monolayer of ethylene glycol monooctadecyl ether (C18E1) and the water-soluble polymer poly(vinylpyrrolidone) (PVP), has been shown to resist forces such as wind stress to a greater degree than the C18E1 monolayer alone. This paper reports all-atom molecular dynamics simulations comparing the monolayer (C18E1 alone) and duolayer systems under an applied force parallel to the air/water interface. The simulations show that, due to the presence of PVP at the interface, the duolayer film exhibits an increase in chain tilt, ordering, and density, as well as a lower lateral velocity compared to the monolayer. These results provide a molecular rationale for the improved performance of the duolayer system under wind conditions, as well as an atomic-level explanation for the observed efficacy of the duolayer system as an evaporation suppressant, which may serve as a useful guide for future development for thin films where resistance to external perturbation is desirable.

Understanding, and improving, the behavior of thin surface films under exposure to externally applied forces is important for applications such as mimicking biological membranes, water evaporation mitigation, and recovery of oil spills. This paper demonstrates that the incorporation of a water-soluble polymer into the surface film composition, i.e., formation of a three-duolayer system, shows improved performance under an applied dynamic stress, with an evaporation saving of 84% observed after 16 h, compared to 74% for the insoluble three-monolayer alone. Canal viscometry and spreading rate experiments, performed using the same conditions, demonstrated an increased surface viscosity and faster spreading rate for the three-duolayer system, likely contributing to the observed improvement in dynamic performance. Brewster angle microscopy and dye-tagged polymers were used to visualize the system and demonstrated that the duolayer and monolayer system both form a homogeneous film of uniform, single-molecule thickness, with the excess material compacting into small floating reservoirs on the surface. It was also observed that both components have to be applied to the water surface together in order to achieve improved performance under dynamic conditions. These findings have important implications for the use of surface films in various applications where resistance to external disturbance is required.

We have applied a theoretical modelling approach to aid in the rational design of contamination resistant coatings. Using in silico nano-indentation we have characterised the adhesion between a contaminant particle and engineered polymer surfaces that include functionalised-surface-crosslinked and functionalised non-surface-crosslinked polyesters. The roughness, density and morphology of the surfaces were dynamically monitored as the coatings responded to the approaching particle. Our results suggest that surface crosslinking provides a stable platform for incorporation of functional groups that would otherwise migrate into the bulk substrate upon aging. However surface crosslinking with rigid cyclic curing agents renders the coatings far too stiff, which can result in slip whereby surface crosslinkers shift to the side of the incoming contaminant particle, leading to both the exposure of the flexible substrate and the envelopment of the contaminant. This type of deformation negates steric repulsions of the functional groups, crucial for preventing undesirable adhesion. In contrast, functionalisation combined with flexible acyclic surface crosslinks results in superior contamination resistance.

Responsive surfaces show potential for many applications; however, the molecular mechanisms of their responsive behavior are often dependent on the nature and properties of the substrate and this dependence is not fully understood. We present a molecular dynamics study on the mechanical response of poly(ethylene glycol) (PEG) grafted on substrates of varying flexibility in "dry" conditions. Our in silico surface loading tests show that when PEG is grafted onto a hard substrate (silica), there is a significant reduction in adhesion to a solid surface, owing to augmented steric repulsions at the interface. However, when the same chains are tethered onto a soft substrate (polyester), interfacial adhesion is strengthened. We find that the deformable substrate allows significant rearrangement of the subsurface and grafted segments during loading. Asperities along the rough soft surface also provide free volume for the tethered chains to occupy, enabling them to carpet the surface and increasing the density at the interface. Our results explain the molecular basis of the mechanical response of PEG when grafted onto hard and soft substrates and provide a rationale for surface protection using PEG.

The leaf of the lotus plant provides an elegant example of how a natural surface can remain clean even in the dirtiest of environments. The leaf’s fine-scale surface structure combined with its hydrophobic chemistry ensures that water droplets bead off its surface, carrying away contaminant particles [1]. An alternative dirt-shedding surface, also found in nature, is based on hydrophilic surface chemistry. Water droplets that come into contact with hydrophilic surfaces spread, forming a thin film of water and lifting contaminant from the surface. There is significant motivation to pursue both of these types of behaviour for artificial systems because of the wide range of possible applications ranging from self-cleaning paints to dirt-resistant clothing. However, for some applications, pertinent to the system in question, the synthesis of a self-cleaning surface is indeed a difficult task to achieve, and one needs to first elucidate the precise properties required to generate a certain level of resistance to the adhering contaminant. Atomistic simulations provide a useful tool to gain such insight and add value to surfaces where stay-clean properties are highly desirable. Here, using force-field-based molecular mechanics and dynamics, we explore the properties of polymer surfaces in order to develop design aspects for self-cleaning industrial paint-coatings. A polyester surface model is constructed, on the basis of a realistic cured paint coating [2] while various carbon models are selected [3–6], to emulate commonly encountered atmospheric dirt particulates [7–10]. The purpose of this work is to gain a fundamental understanding of the nature of interactions between contaminant and polymer coating in various environments, including aqueous conditions [11], and explore nanoscale modifications of the coating that help reduce the strength of the adhering contaminant. The modifications are based on hydrophobic or alternatively hydrophilic surface treatments. Here we demo

This study compares the performance of the all-atom molecular dynamics force fields OPLS-AA and COMPASS, and the united-atom GROMOS96 ff53a6 force field, for organic monolayers at aqueous interfaces, as a function of surface density, temperature, and system size. Where possible, comparison with experimental data was undertaken and used to scrutinize the performance of each force field. We find close agreement between the all-atom force fields (OPLS and COMPASS) and experiment for the description of organic monolayers on water. However, the united-atom force field 53a6 tends to exhibit poorer agreement than the all-atom force fields

We present a simulation methodology to chemically cross-link the surface of fully atomistic models of polymers. We simulated nanoindentation of the cross-linked model surfaces whereby nanoparticles (fullerene in this case) are used to probe different regions of the polymer surface enabling us to characterize the surface hardness of the polymer film and calculate the work of adhesion between polymer and nanoparticle. The cross-linked polyester surfaces are constructed using hexamethylene and isophorone di-isocyanate cross-linkers. Our results indicate that, despite surface cross-linking with hexamethylene di-isocyanate molecules, the polymer top surface still maintains significant softness. The hexamethylene cross-links that protrude from the surface are deformable, resulting in a strong adhesion between the surface and carbon particle. In contrast, isophorone surface crosslinking results in significantly weaker adhesion with carbon, due to the less deformable rigid structure formed at the top layer. In this case, while rigidity is imparted to the polymer top surface, the flexibility of the core region of the polymer is maintained.

We present a molecular dynamics study of the interaction of fullerene (C60) with polyester surfaces in water. Three different polyester surfaces are studied, including untreated polyester (Polyester), fluorine-modified polyester (Polyester75F), and hydroxyl-modified polyester (Polyester75OH). Results indicate that in the absence of any surface modification, the fullerene particle is drawn down onto the polyester, even for a significant initial depth of water separating the particle and the surface. However, the addition of hydroxyl or fluorine groups on the surface of the polymer leads to a stronger interaction between water and surface atoms and, when combined with structural features such as rigidity and surface roughness, can reduce adhesion of the C60 particle in water.

This theoretical study examines the adhesion of amorphous carbon to functionalized polyester surfaces. Several modifications to the polyester surface have been investigated including the addition of hydroxyl, carboxyl, or fluorine substituents at varying surface concentrations. Initially, the polyesters were artificially held rigid, and the surface modification not only imparted chemical changes to the polymer surface but also accentuated its atomic scale roughness. Under this rigid regime, low concentrations of surface modifiers have been shown to lead to the greatest reduction in adhesion with amorphous carbon, and increasing the level of modification does not provide further advantages in the reduction of adhesion, due to saturation. The effects of surface reorganization were also investigated and have been shown to be strongly dependent on the structure of the carbon surface, the chemical type, and surface density of modifiers. The surface modifiers reorganize during relaxation, which leads to significant differences in their effects on adhesion.

This study uses classical molecular dynamics to examine how aging (relaxation) of functionalized polyester surfaces effects their adhesion to carbon particles. We show that relaxation leads to significantly increased adhesion compared to the rigid polymers. Functionalization of polyester surfaces followed by aging results in negligible reductions in adhesion to carbon compared to the unmodified polyester and, in some cases, leads to slight increases in interfacial adhesion. These results indicate that for surface modification to be effective, i.e., reduce interfacial adhesion with carbon, the polyester surfaces need to maintain both the rigidity and atomic roughness.

A continual testing development program for Colorbond, a prepainted steel product carefully designed multilayer system that offers corrosion protection and theoretical studies of the material's performance at the nanoscale are conducted. Results suggest that increasing the hardness and atomic roughness of the polyester surface by crosslinking and/or carefully chosen nano-functionalisation can significantly decrease its interactions with the carbonaceous solids and nanoparticles.

This study examines the adhesion of graphite to functionalized polyester surfaces using a range of qualitative and quantitative measures of theoretical adhesion. Modifications to the polyester surfaces include the addition of hydroxyl, carboxyl, or fluorine substituents with coverages of 0.4 and 0.9 groups per nm2. In each case, the introduction of substituents to the surface of the polyester was calculated to lead to reduced adhesion to graphite. Effects of surface relaxation on adhesion are studied by employing different simulation protocols. The theoretical results suggest one mechanism to reduce adhesion to carbonaceous solids is to increase atomic roughness using strongly hydrophilic or alternatively strongly hydrophobic substituents.