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Smart coatings have aroused a growing interest because of the performance of predefined surface functions upon reacting to external stimuli. Among them, responsive polymer coatings to water, which often benefit from the presence of a mobile hydrophilic material, are of great interest. Polyurethanes (PUs) are versatile materials with respect to the structure-property relationship. Therefore, the incorporation of hydrophilic segments in PUs is a rational way to produce water-sensitive smart coatings; however, having a considerable amount of hydrophilic material deteriorates the physical properties because of a large amount of water uptake. In this study, we have analyzed previously synthesized smart PUs, based on hydrophobic polycarbonate (PC) and hydrophilic polyethylene glycol (PEG) soft segments, in which only a limited amount of PEG is used. These coatings maintain, more or less, zero water contact angle, whereas the water uptake remains below 15 wt %. The combination of experimental analysis and coarse-grained molecular dynamics (CG MD) simulations reveals that PEG segments migrate to the coating/water interface and partially cover the surface, whereas the hydrophobic nature of the PC keeps the bulk of the coating intact when the coating is covered with water. Moreover, our CG MD simulations and experimental analysis suggest a reversible phase arrangement under wet/dry cycles on molecular and macroscopic scales.

Various physical and chemical surface parameters, e.g., surface roughness and surface chemistry, contribute to anti-soiling (AS) properties. Nevertheless, the effect of surface chemistry has not been distinctly elucidated yet. In this study, a set of mechanically stable and durable hydrophobic AS coatings with controlled surface chemistry were synthesized, while surface roughness was kept below 1 nm. Fluoroalkylsilane (FAS), alkylsilane (AL), and tetraethyl orthosilicate (TEOS) were employed for synthesis. Surface chemistry and surface roughness were quantified by XPS and AFM. A soiling lab simulator was designed to accelerate the soiling process. AS properties were quantified by UV–vis spectroscopy and optical microscopy. The surface free energy of coatings was estimated through (polar and apolar liquids) contact angle measurements (ranging from 16 to 30 mN/m), and a clear correlation was discovered between the AS properties and surface free energy. Moreover, mechanical properties and weathering resistance of the coatings were analyzed by nano-indentation and QUV accelerated weathering tester. While all coatings showed acceptable AS properties (transmission loss after dust deposition between 1/5 and 2/5 of uncoated glass) and excellent mechanical strength (above 27 GPa modulus and 2 GPa hardness), FAS-based coatings showed a significantly higher durability against weathering as compared to the AL-based ones. © 2022 Elsevier B.V.

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.

A kinetic Monte Carlo method to simulate photodegradation of a polymer coating is applied to the weathering process of a polyester-urethane clearcoat during artificial exposure under different conditions. Firstly, the optimised simulation parameters that yield the best match with experimentally measured results on the depth-resolved ester and urethane bond fractions are determined and compared for two different aerobic exposure experiments (one in a Weather-Ometer (WOM) and one in a Suntest equipment). Secondly, several other quantities that are obtained from the simulations, but cannot be determined experimentally, are compared, such as the fraction of newly formed crosslink bonds, absorptivity states, oxidised states, the fraction of radicals, the concentration of oxygen and the total amount of remaining material. Depth-inhomogeneity of the rate of photon absorption leads to the formation of distinct depth gradients in the WOM simulation, while a much more homogeneous evolution is obtained for the Suntest-air simulation. Photo-oxidative damage in the WOM simulation is more concentrated on the upper layer of the coating, resulting in the extensive evaporation of highly oxidised material, whereas degradation in the Suntest-air simulation is more spread out over the entire coating thickness, resulting in less material loss.

The influence of the conditions of artificial degradation experiments on the photodegradation process of polyester-urethane clearcoats has been studied by comparing three types of exposure experiments with different conditions regarding the spectral power distribution (SPD), the exposure atmosphere (aerobic and anaerobic) and the presence of water. The presence of short wavelengths (λ

In this work we simulate the full sequence of steps that are also typically performed in an experimental approach when studying photo-degradation of a polymer coating, namely, i) sample preparation, ii) photo-degradation and iii) thermo-mechanical analysis of the material during photo-degradation. In the current paper, we focus on performing several molecular dynamics simulations to study the thermo-mechanical properties of a virgin thermoset coating as well as degraded ones. Using an atomistic structure that is obtained by fine-graining the mesoscopic structure, we obtain consistent correlations between the simulated thermo-mechanical properties of the material and those measured experimentally. In addition, it is shown that by using oscillatory strain fields in MD - instead of the commonly applied linear tensile/compression strain fields – one can acquire greater knowledge on the structure-property relation of polymeric materials. Eventually, we show that our simulation approach gives rise to a remarkable insight into the mechanism of the photo-degradation process.

This paper is devoted to the surface analysis of a polyester-urethane coating during weathering under different conditions using artificial weathering machines. By means of atomic force microscopy (AFM), the evolution of the surface topology of the coatings is studied. Degradation is shown to be a laterally inhomogeneous process. The presence of water facilitates material removal and leads to an increase in the surface roughness and consequently a gloss loss. In addition, by comparing degradation under aerobic and anaerobic conditions, it is shown that oxidation reactions are the main cause of lateral inhomogeneous degradation of coatings.

Transmission FTIR analysis of polyester-urethane coatings (PUC), that were degraded under different accelerated laboratory weathering conditions, are compared. The aim of this comparison is to deepen our insight into the chemical pathways of weathering of polyester-urethane coatings. We monitored the chemical changes for different environments, such as aerobic or anaerobic conditions as well as wet or dry conditions, in order to increase our insight into the effect of each individual stress factor, i.e., photons, oxygen, temperature and water, on the chemical pathways of weathering. We showed that the degradation of urethane groups proceeds via photo-oxidative pathways and that the ester groups mainly degrade via photolytic reactions. The ester bond scission accelerates after an initially-slow-rate stage of weathering in the presence of urethane groups. This is due to an increase in the optical absorptivity of the coating as a result of degradation under anaerobic conditions, as shown before. By means of a kinetic analysis using a combination of FTIR and UV–Vis spectroscopy results (obtained before), we found that a first-order kinetic model can perfectly describe the rate of ester bond scission during the weathering and that the increase in the rate of reaction is due to the increase in the light absorptivity of the coating as a result of degradation. Finally, using the interference fringes of FTIR spectra, we showed that evaporation and water-caused removal of degraded material cause a particularly pronounced decay in the thickness of the coating. In the absence of water spray, the material loss takes place in the same period as urethane groups decompose and stops afterwards, even though the ester bond scission proceeds with higher rates. This supports the hypothesis of photo-oxidative pathways for the urethane group decomposition and photolytic mechanisms for ester bond scission. Dark experiments showed that PUC coatings are highly resistant to hydrolysis and thermal degradation.

The effect of graphene (G) and graphene oxide (GO), used as the nanofiller in polymer nanocomposites (NC), on the structural and dynamic properties of polymer chains, has been studied by means of molecular dynamics (MD) simulations. Two polymers, i.e., poly(propylene) and poly(vinyl alcohol), are employed as matrices to cover a wider range of polymer�filler interactions. The local structural properties, e.g., density profile, average Rg, and end-to-end distance as well as dynamic properties, e.g., estimated translational and orientational relaxation times, of polymer chains are studied. In addition, the interaction energies are estimated between polymers and nanofillers for different hybrid systems using MD pullout simulations. Strong heterogeneities in polymer structural and dynamic properties have been observed such that chains are more oriented and exhibit slower dynamics in the vicinity of the nanofillers (G and GO) as compared to bulk. It is also found that the orientation of polymer chains at the interface is more influenced by the nanofiller in such a way that the more oriented polymer chains are observed in G-based NC for both polymers. However, the immobilization of polymer chains at the interface proves to be very much dependent on the polymer�filler interactions. (Figure presented.). Scopus

Chemical degradation of polymer coatings via a photo-oxidative pathway, denoted as photo-degradation, results in physical changes which, in spite of the long service life of coatings, eventually lead to failure of the material. Conventional molecular simulations cannot cope with this process with its wide range of time and length scales, related to the rare occurrence of ‘degradation events’ as compared to the time scale of structural relaxation of the polymer chains. Therefore a combination of suitable simulation techniques is needed to overcome this problem. By coupling a kinetic Monte Carlo simulation to a Dissipative Particle Dynamics method, a novel simulation approach has been developed that makes it possible to take into account chemical and physical pathways of the photo-degradation process. For a model polyester-urethane coating photo-degradation under inert conditions was studied with and without taking structural relaxation into account as well as by varying the ratio of reaction rate constants. For the model coating studied, taking physical relaxation into account proved to be essential for modeling the photo-degradation process.

In this article we studied the evolution of thermomechanical properties of a polyester-urethane coating during degradation under different degradation conditions, i.e., aerobic and anaerobic conditions with and without dry/wet cycling during degradation. Dynamic mechanical and thermal analyses show that under aerobic conditions the coatings become stiffer and more brittle in the glassy state. This stiffening is probably due to the increase in the amount of hydrogen bonding and the formation of oxidized groups which increase the polarity of the material and enhance the interactions of the polymer segments. However, oxidation reactions result in a considerable decrease in cross-link density and stiffness in the rubbery state. Both changes, in the glassy and rubbery states, give rise to development of internal stresses. These stresses increase as the degradation process proceeds. Nevertheless, for samples exposed to anaerobic conditions, the stiffness remains constant in the glassy state and the cross-link density slightly increases as a result of degradation. This reconfirms the dominance of the effect of oxidation reactions on the mechanical failure of the coatings. Oxygen permeation measurements show a more-or-less time-independent diffusion coefficient and a gradual decrease in solubility of oxygen as a function of exposure time. This results in a slight decrease in oxygen permeation (mainly in the early stage of the degradation) as degradation proceeds. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 659–671

Polyester-urethane resins are important candidates for high performance, outdoor coating applications. Despite their relevance, quantitative information regarding the photodegradation of these materials is scarcely available. In the present study, a model polyester-urethane clearcoat without additives is artificially degraded and the changes in optical properties and chemical composition are studied by UV-VIS spectroscopy and FTIR-ATR microscopy, respectively. The change of the optical properties throughout the coating thickness is quantified and interpreted using a newly developed optical model. Chemical changes are quantified in a depth-resolved manner by combining FTIR-ATR microscopy with optical profilometry in order to visualise the time evolution of compositional gradients during photodegradation by accurate assignment of the correct depth position to all recorded ATR spectra. The rate of change for the concentration of several chemical entities in the model polyester-urethane was found to become constant after the initial stage of weathering. The loss of urethane crosslinks in the coating occurs faster and to a much larger extent as compared to ester bond scission. Results from the optical and the chemical characterisation are combined to propose a kinetic model for ester bond photolysis that provides an estimate of the quantum efficiency of this process.