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

\u3cp\u3eSimulation 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.\u3c/p\u3

The influence of the exposure conditions on the chemical and physical changes of polyester-urethane coatings during photodegradation

\u3cp\u3eThe 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 <295 nm) in the SPD largely influences the depth-inhomogeneity of degradation with respect to the optical properties and the chemical composition of the coating. The availability of oxygen in the exposure atmosphere determines the degradation pathway that is followed, such as to what extent the photo-oxidative breakage of urethane bonds or the formation of yellow chromophores due to aromatic crosslinking reactions occurs. Indentations at the surfaces of virgin and degraded coatings showed an increase in Young's modulus and hardness when degraded under aerobic conditions, while degradation under anaerobic conditions did not lead to significant changes. The presence of water is responsible for increasing the surface roughness of the coating during degradation, which directly influences the coating's gloss retention. Several time-independent correlations between the changes in chemical, optical and mechanical properties of coatings resulting from different exposure experiments have been established.\u3c/p\u3

A simulation approach to study photo-degradation processes of polymeric coatings

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

Degradation of a polyester-urethane coating:physical properties

\u3cp\u3eIn 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.\u3c/p\u3