Effect of Polymer/Solid and Polymer/Vapor Instantaneous Interfaces on the Interfacial Structure and Dynamics of Polymer Melt Systems

Polymers are used in a wide range of applications that involve chemical and physical processes taking place at surfaces or interfaces which influence the interaction between the polymer material and the substance that comes into contact with it. Polymer surfaces are usually modified either chemically or physically for specific applications such as facilitating wetting, reducing friction, and enhancing adhesion. The variety and complexity of surface and interfacial processes requires a molecular-level understanding of the structural and dynamical properties of the surface/interface layer to help in the design of materials with desired functional properties. Using molecular dynamics (MD) simulations, we investigate the structure and dynamics at the surface of polymer films. We find that the density profiles of the films as a function of distance relative to an instantaneous surface have a structure indicative of a layering at the polymer/vapor interface similar to the typical layered structure observed at the polymer/substrate interface. However, the interfacial molecules at the polymer/vapor interface have a higher mobility compared to that in the bulk while the mobility of the molecules is lower at the polymer/substrate interface. Time correlation of the instantaneous polymer/vapor interface shows that surface fluctuations are strongly temperature dependent and are directly related to the mobility of polymer chains near the interface

Characterizing the Hydrophobicity of Surfaces Using the Dynamics of Interfacial Water Molecules

As most interfacial processes of practical interest occur in aqueous media where the presence of water may have an impact on desired functional properties, it is important to understand the structural and dynamical properties of interfacial water. Using molecular dynamics simulations, we investigated the properties of interfacial water molecules in contact with model atactic polystyrene surfaces of varying polarity. We find that interfacial water molecules, which do not make hydrogen bonds with the substrate, have a faster dynamics and appear to have a universal water–water hydrogen bond relaxation time of about 5 ps. The diffusion coefficients and the relaxation times of the water molecules involved in hydrogen bonding with the surface, on the other hand, have strong dependence on surface polarity and reveal a hydrophobic to hydrophilic transition regime with contact angle in the range of 40–50°. The results presented will be of broad interest to researchers working in the area of surface science, biotechnology, nanotechnology, and in all forms of coating applications

Theta Temperature Depression of Mechanically Interlocked Polymers: [2]catenane as a Model Polymer

Polycatenanes have recently attracted considerable attention due to their potential for many applications and as model systems for understanding the role of mechanical interlocking in the physical properties of mechanically interlocked polymers. We used molecular dynamics simulations to investigate the conformational properties of [2]­catenane polymers in solution as a function of the solvent quality and molecular weight. We found the θ-temperature of [2]­catenane polymers to be depressed compared to their linear and ring counterparts and follow the relationship θ[2]catenane ring linear. The conformation of the two rings in [2]­catenane is found to be strongly dependent on the solvent quality. In a good solvent, their conformation is similar to that of an analogous free ring polymer, while, in a poor solvent, their conformation significantly deviates from an analogous ring polymer. Furthermore, the thermal blob size (Nblob) follows the theoretical prediction of the linear relation between Nblob and 1/v2, where v is the excluded volume, and is found to be strongly dependent on polymer topology in a poor solvent condition than in a good solvent condition

Nanostructures and Electronic Properties of a High-Efficiency Electron-Donating Polymer

The development of organic photovoltaic (OPV) solar cells has seeded a bright hope of achieving low-cost solar energy harvesting. Practical realization and successful commercialization require enhancing the efficiency of solar energy harvesting, which, in turn, relies on the core understanding of structure–property relationships in OPV materials. Here, we report the first large-scale density functional calculations of the nanoconformational and electronic properties of the thieno­[3,4-b]­thiophene-<i>alt</i>-benzodithiophene copolymer (PTB7), a high-efficiency OPV material. These first-principles results include the chain length dependence of the torsional potential, the nearest-neighbor torsional coupling, the band gap, and the electronic conjugation length. Importantly, PTB7 was found to have a torsional potential almost independent of chain length, very weak nearest-neighbor torsional coupling, a low band gap (∼1.8 eV), and a very long conjugation length (∼147 Å) compared to the other conjugated polymers like polythiophene and poly­(3-alkylthiophene). These results suggest that PTB7 can be an efficient electron donor for OPV devices

Local Structure Contributions to Surface Tension of a Stereoregular Polymer

We have used all-atom molecular dynamics (MD) simulations to calculate the surface tension of melt poly­(methyl methacrylate) (PMMA) as a function of tacticity. Computation of surface tension using the Kirkwood-Buff approach required hundreds of nanoseconds of equilibration. The computed slopes of surface tension versus temperature are in very good agreement with reported experimental values. Using a rigorous treatment of the true interface, which takes into account the molecular roughness, we find that isotactic PMMA, in comparison to syndiotactic and atactic PMMA, shows a larger surface concentration of polar ester-methyl and carbonyl groups on the surface versus nonpolar α-methyl groups. A mechanistic hypothesis based on the helical nature of the isotactic PMMA chains, their relative flexibility, and their reported conformational energies is proposed to explain the trends in composition near the surface. We highlight here how surface composition and surface tension are controlled by both polarity and steric constraints imposed by tacticity

Constraints on Knot Insertion, Not Internal Jamming, Control Polycatenane Translocation Dynamics through Crystalline Pores

The translocation of polymers through pores and channels is an archetypal process in biology and is widely studied and exploited for applications in bio- and nanotechnology. In recent times, the translocation of polymers of various different topologies has been studied both experimentally and by computer simulation. However, in some cases, a clear understanding of the precise mechanisms that drive their translocation dynamics can be challenging to derive. Experimental methods are able to provide statistical details of polymer translocation, but computer simulations are uniquely placed to uncover a finer level of mechanistic understanding. In this work, we use high-throughput molecular simulations to reveal the importance that knot insertion rates play in controlling translocation dynamics in the small pore limit, where unexpected nonpower law behavior emerges. This work both provides new predictive understanding of polycatenane translocation and shows the importance of carefully considering the role of the definition of translocation itself

Adsorptive Structure and Mobility on Carbon Nanotube Exteriors Using Benzoic Acid as a Molecular Probe of Amphiphilic Water Contaminants

Benzoic acid is the simplest aromatic carboxylic acid that is also a common water contaminant. Its structural and amphiphilic properties are shared by many other contaminants of concern. Based on a molecular dynamics study, this work reports the competitive adsorption of benzoic acid with water on the curved exteriors of carbon nanotubes of varying oxygen content. With the help of cylindrically approximated pair correlation functions, carboxyl–carboxyl associations were found to serve as an additional mechanism providing stability to the adsorbed benzoic acid on tube exteriors. These associations are secondary to the main aromatic–aromatic interactions during the adsorption process and therefore were not sufficient to establish the energy hierarchy at the adsorbed state with increase in surface oxygen content. The same mechanism was previously ascribed to the adsorption of the structurally similar but bulkier tannic acid. Both water and benzoic acid were organized into numerous mobility groups and a correspondence was established between species residence time and the average translation time taken to escape the tube vicinity. Vigorous exchange of water molecules among the first adsorption shell, the second adsorption shell, and the immediate vicinity radially outside was estimated to take place within a short time of about 10 ps

Template-Induced Enhanced Ordering under Confinement

We report a surprisingly strong ordering of Si−(CH3)2 groups upon confinement between two surfaces, an oxidized poly(dimethyl siloxane) (PDMSox) elastomer and a methyl-terminated self-assembled monolayer (octadecyltrichlorosilane (OTS)) on sapphire substrates. This enhanced ordering is induced by the template of ordered methyl groups of OTS and is not observed for other surfaces (fluorinated monolayers and sapphire substrates). This strong ordering is reminiscent of layering observed for confined symmetric molecules between two mica surfaces but was expected to vanish between rough macroscopic surfaces. These results provide new insights on confined structure at the interface between two solids and are important in the understanding of surface-controlled processes of practical importance

Molecular Structure of Poly(methyl methacrylate) Surface II: Effect of Stereoregularity Examined through All-Atom Molecular Dynamics

Utilizing all-atom molecular dynamics (MD), we have analyzed the effect of tacticity and temperature on the surface structure of poly­(methyl methacrylate) (PMMA) at the polymer–vacuum interface. We quantify these effects primarily through orientation, measured as the tilt with respect to the surface normal, and the surface number densities of the α-methyl, ester-methyl, carbonyl, and backbone methylene groups. Molecular structure on the surface is a complex interplay between orientation and number densities and is challenging to capture through sum frequency generation (SFG) spectroscopy alone. Independent quantification of the number density and orientation of chemical groups through all-atom MD presents a comprehensive model of stereoregular PMMA on the surface. SFG analysis presented in part I of this joint publication measures the orientation of molecules that are in agreement with MD results. We observe the ester-methyl groups as preferentially oriented, irrespective of tacticity, followed by the α-methyl and carbonyl groups. SFG spectroscopy also points to ester-methyl being dominant on the surface. The backbone methylene groups show a very broad angular distribution, centered along the surface plane. The surface number density ratios of ester-methyl to α-methyl groups show syndiotactic PMMA having the lowest value. Isotactic PMMA has the highest ratios of ester- to α-methyl. These subtle trends in the relative angular orientation and number densities that influence the variation of surface structure with tacticity are highlighted in this article. A more planar conformation of the syndiotactic PMMA along the surface (<i>x</i>–<i>y</i> plane) can be visualized through the trajectories from all-atom MD. Results from conformation tensor calculations for chains with any of their segments contributing to the surface validate the visual observation

Tension Amplification in Tethered Layers of Bottle-Brush Polymers

Molecular dynamics simulations of a coarse-grained bead–spring model have been used to study the effects of molecular crowding on the accumulation of tension in the backbone of bottle-brush polymers tethered to a flat substrate. The number of bottle-brushes per unit surface area, Σ, as well as the lengths of the bottle-brush backbones <i>N</i>_bb (50 ≤ <i>N</i>_bb ≤ 200) and side chains <i>N</i>_sc (50 ≤ <i>N</i>_sc ≤ 200) were varied to determine how the dimensions and degree of crowding of bottle-brushes give rise to bond tension amplification along the backbone, especially near the substrate. From these simulations, we have identified three separate regimes of tension. For low Σ, the tension is due solely to intramolecular interactions and is dominated by the side chain repulsion that governs the lateral brush dimensions. With increasing Σ, the interactions between bottle-brush polymers induce compression of the side chains, transmitting increasing tension to the backbone. For large Σ, intermolecular side chain repulsion increases, forcing side chain extension and reorientation in the direction normal to the surface and transmitting considerable tension to the backbone