We apply a recently developed coarse-graining method to build models for polyester polyols, versatile polymers with applications in coatings, by combining models for the component monomers. This strategy employs the corresponding states correlation to the group-contribution SAFT-γ Mie equation of state [Mejia, A.; et al. Ind. Eng. Chem. Res. 2014, 53, 4131–4141] to obtain force-field parameters for the constituent monomer species. Results from simulations agree favorably with experimental values of mass density, glass transition temperature (T g), and specific heat capacity change at T g. Further simulations over a range of Mie parameters and polymer chemical compositions yield a correlation that relates the parameters directly to T g. This correlation is validated by experimental data and can be used as a predictive tool within the tested parameter space to expedite the design of these coating materials.

The sol–gel transition of a series of polyester polyol resins possessing varied secondary hydroxyl content and reacted with a polymerized aliphatic isocyanate cross-linking agent is studied to elucidate the effect of molecular architecture on cure behavior. Dynamic rheology is utilized in conjunction with time-resolved variable-temperature Fourier-transform infrared spectroscopy to examine the relationship between chemical conversion and microstructural evolution as functions of both time and temperature. The onset of a percolated microstructure is identified for all resins, and apparent activation energies extracted from Arrhenius analyses of gelation and average reaction kinetics are found to depend on the secondary hydroxyl content in the polyester polyols. The similarity between these two activation energies is explored. Gel point suppression is observed in all the resin systems examined, resulting in significant deviations from the classical gelation theory of Flory and Stockmayer. The magnitude of these deviations depends on secondary hydroxyl content, and a qualitative model is proposed to explain the observed phenomena, which are consistent with results previously reported in the literature.

No abstract

Thermoset polymers are examples of chemically cured, network-forming materials whose bulk properties depend sensitively on formulation chemistry and reaction conditions. In this work, we employ molecular dynamics simulations to model polyester-based urethane thermosets that are specifically targeted for coating applications. Parameterizing force field interactions with a statistical associating fluid theory (SAFT)-γ Mie approach in conjunction with corresponding state correlations [MejíaA. Mejía, A. Ind. Eng. Chem. Res.20145341314141; MüllerE. A. Müller, E. A. Langmuir2017331151811529] permits the facile development of effective models for our thermosetting system. We have devised a theoretical model to fit experimental kinetic data and implement a crosslinking algorithm that replicates the theoretical kinetics. Our molecular simulations capture the cure kinetics regarding the reactions of the isocyanate group with the primary and secondary hydroxyl termini. Analysis of molecular-level connections that arise during crosslinking affords new information about network structure development. Predicted glass transition temperatures and thermomechanical properties agree well with experimental data.

Because of their utility in diverse coatings applications, a model series of polyester–polyol films possessing different diol formulations and cross-linked with an aliphatic isocyanate cross-linker under two different conditions is systematically investigated to elucidate the effects of backbone chemistry and cure temperature on the ultimate thermomechanical and free-volume properties. The diol of particular interest in this study is 2,2,4,4-tetramethyl-1,3-cyclo­butanediol (TMCD). The glass transition temperature of the films, as measured by thermal calorimetry and dynamic mechanical analysis, is observed to correlate strongly with TMCD content. In the limit of 100 wt % TMCD diol, the operable temperature range for these films increases by as much as ∼60 °C. Surface mechanical properties, interrogated by nanoindentation, also depend sensitively on both TMCD content and cure conditions, and a previously reported phenomenon known as “pileup” is considered to explain some of our observations. Results from positron annihilation lifetime spectroscopy indicate that the nanoscopic free volume of films containing varying levels of TMCD is strongly dependent on chemical makeup but not on cure conditions.