Modeling surface segregation of smart PU coatings at hydrophilic and hydrophobic interfaces via coarse-grained molecular dynamics and mesoscopic simulations

Developing adaptive coatings having desired functionalities at targeted interfaces is one of the major efforts in the coatings science area. The adaptation of the surface functionality to the changing surface conditions can be maintained by introducing dangling chains with different properties to the cross-linked polymer coatings. In this work, we strive to investigate the change in interfacial morphology of PU coatings as exposed to hydrophilic (HPI) and hydrophobic (HPB) interfaces by employing molecular simulations at the coarse-grained and mesoscopic levels. The molecular structure and surface segregation dynamics are studied for PU coatings having pure HPI, mixture of HPI and HPB, and amphiphilic dangling chains. The dual-scale simulations, Dissipative Particle Dynamics (DPD) and MARTINI model, yield results about the dangling chain structures at the interface in terms of their end-to-end distances, where HPI chains adopt a more extended conformation in water in comparison to oil interfaces. The reverse is observed to be valid for the HPB chains. Regarding the dangling chain dynamics, a swift migration towards the interfaces is noticed at about 10 ns for both of the simulation methods. The structures of the dangling chains and their interaction with the interfaces are also characterized by computing the radial distribution function (RDF) profiles. Preferential interactions between the HPI/water and HPB/oil are clearly noted. The switchability of the surfaces is also studied by simulating the system in cycles, such that the interface is changed from water to oil and back to water. The migration of HPI groups in the dangling chains towards water and vice versa in each cycle is clearly shown by the simulations. In all, the inherent structure and dynamics of the dangling chains is obtained at the molecular level by the dual-scale molecular simulations. Our findings reveal a significant level of understanding about interfacial morphology of thermoset coatings modified by dangling chains, where the results can be extended to find applications in guiding the experimental studies

Supramolecular Engineering of Alkylated, Fluorinated, and Mixed Amphiphiles

The rational design of perfluorinated amphiphiles to control the supramolecular aggregation in an aqueous medium is still a key challenge for the engineering of supramolecular architectures. Here, the synthesis and physical properties of six novel non-ionic amphiphiles are presented. The effect of mixed alkylated and perfluorinated segments in a single amphiphile is also studied and compared with only alkylated and perfluorinated units. To explore their morphological behavior in an aqueous medium, dynamic light scattering (DLS) and cryogenic transmission electron microscopy/electron microscopy (cryo-TEM/EM) measurements are used. The assembly mechanisms with theoretical investigations are further confirmed, using the Martini model to perform large-scale coarse-grained molecular dynamics simulations. These novel synthesized amphiphiles offer a greater and more systematic understanding of how perfluorinated systems assemble in an aqueous medium and suggest new directions for rational designing of new amphiphilic systems and interpreting their assembly process

fluorination, density, roughness, and Lennard-Jones cutoffs

The interplay of fluorination and structure of alkane self-assembled monolayers and how these affect hydrophobicity are explored via molecular dynamics simulations, contact angle goniometry, and surface-enhanced infrared absorption spectroscopy. Wetting coefficients are found to grow linearly in the monolayer density for both alkane and perfluoroalkane monolayers. The larger contact angles of monolayers of perfluorinated alkanes are shown to be primarily caused by their larger molecular volume, which leads to a larger nearest-neighbor grafting distance and smaller tilt angle. Increasing the Lennard-Jones force cutoff in simulations is found to increase hydrophilicity. Specifically, wetting coefficients scale like the inverse square of the cutoff, and when extrapolated to the infinite cutoff limit, they yield contact angles that compare favorably to experimental values. Nanoscale roughness is also found to reliably increase monolayer hydrophobicity, mostly via the reduction of the entropic part of the work of adhesion. Analysis of depletion lengths shows that droplets on nanorough surfaces partially penetrate the surface, intermediate between Wenzel and Cassie–Baxter states

Morphology of conducting polymer blends at the interface of conducting and insulating phases: insight from PEDOT:PSS atomistic simulations

Having phase-separated conductive and less-conductive domains is a common morphology in semiconducting polymer blends as it exists in the case of PEDOT:PSS, which is a representative example with a wide range of applications. In this paper, we constructed atomistic models for the interface between the PEDOT-rich (conductive) grains and the PSS-rich (less-conductive) phase through molecular dynamics simulations. Our models are obtained from experimentally relevant compositions, based on precise force field parameters, and through a robust relaxation procedure. We show that both PEDOT-rich and PSS-rich phases consist of PEDOT lamellae embedded in PSS chains. The size of these lamellae depends on the PEDOT concentration in each phase and our model predictions are in quantitative agreement with the experimental data. Furthermore, our models suggest that neither the phases nor the interfaces are entirely connected by π-π stacking. Thus, inter-lamellae tunnelling is essential for both intra- and inter-grain charge transport. We also show that a small increase (≈8 wt%) in the PEDOT concentration results in rather larger lamellae sizes, considerably more oriented lamellae, and slightly better inter-lamellae connectivity, which result in enhanced intra-grain conductivity. Moreover, we show how enhancing phase separation between PEDOT-rich and PSS-rich domains (similar to the effect of polar co-solvents), i.e., pulling out PEDOT from the PSS-rich phase and adding it in the PEDOT-rich phase, highly enhances the intra-grain connectivity but decreases the inter-grain conduction paths through the interface. Our results explain how the marginal extra degree of phase separation (based on experimentally obtained values) could result in a great enhancement in the overall film conductivity

Microstructural Model of Indacenodithiophene-<i>co</i>-benzothiadiazole Polymer: π-Crossing Interactions and Their Potential Impact on Charge Transport.

Morphological and electronic properties of indacenodithiophene-co-benzothiadiazole (IDTBT) copolymer with varying molecular weights are calculated through combined molecular dynamics (MD) and quantum chemical (QC) methods. Our study focuses on the polymer chain arrangements, interchain connectivity pathways, and interplay between morphological and electronic structure properties of IDTBT. Our models, which are verified against GIWAXS measurements, show a considerable number of BT-BT π-π interactions with a (preferential) perpendicular local orientation of polymer chains due to the steric hindrance of bulky side chains around IDT. Although our models predict a noncrystalline structure for IDTBT, the BT-BT (interchain) crossing points show a considerable degree of short-range order in spatial arrangement which most likely result in a mesh-like structure for the polymer and provide efficient pathways for interchain charge transport

Microstructural Model of Indacenodithiophene-<i>co</i>-benzothiadiazole Polymer: π‑Crossing Interactions and Their Potential Impact on Charge Transport

Morphological and electronic properties of indacenodithiophene-co-benzothiadiazole (IDTBT) copolymer with varying molecular weights are calculated through combined molecular dynamics (MD) and quantum chemical (QC) methods. Our study focuses on the polymer chain arrangements, interchain connectivity pathways, and interplay between morphological and electronic structure properties of IDTBT. Our models, which are verified against GIWAXS measurements, show a considerable number of BT-BT π–π interactions with a (preferential) perpendicular local orientation of polymer chains due to the steric hindrance of bulky side chains around IDT. Although our models predict a noncrystalline structure for IDTBT, the BT-BT (interchain) crossing points show a considerable degree of short-range order in spatial arrangement which most likely result in a mesh-like structure for the polymer and provide efficient pathways for interchain charge transport

MARTINI-based simulation method for step-growth polymerization and its analysis by size exclusion characterization: a case study of cross-linked polyurethane

Simulation studies of step-growth polymerization, e.g., polymerization of polyurethane systems, hold great promise due to having complete control over the reaction conditions and being able to perform an in-depth analysis of network structures. In this work, we developed a (completely automated) simulation method based on a coarse-grained (CG) methodology, i.e., the MARTINI model, to study the cross-linking reaction of a diol, a tri-isocyanate molecule and one-hydroxyl functional molecule to form a polyurethane network without and with dangling chains. This method is capable of simulating the cross-linking reactions not only up to very high conversions, but also under rather complicated reaction conditions, i.e., a non-stoichiometric ratio of the reactants, solvent evaporation and multi-step addition of the reactants. We introduced a novel network analysis, similar to size-exclusion chromatography based on graph theory, to study the growth of the network during the polymerization process. By combining the reaction simulations with these analysis methods, a set of correlations between the reaction conditions, reaction mechanisms and final network structure and properties is revealed. For instance, a two-step addition of materials for the reaction, i.e., first the dangling chain to the tri-isocyanate and then the diol, leads to the highest integrated network structure. We observed that different reaction conditions lead to different glass transition temperatures (Tg) of the network due to the distinct differences in the final network structures obtained. For example, by addition of dangling chains to the network, the Tg decreases as compared to the network without dangling chains, as also is commonly observed experimentally