An advanced broadband dielectric relaxation spectroscopy technique was developed to measure dipolar reorientation dynamics in an actively deforming amorphous polymer below the glass transition temperature. The application of a weak oscillating electric field during deformation allows for direct probing chain segment mobility. Results show that the application of a monotonically increasing strain on a glassy poly(vinyl chloride) induces a significant increase of the out-of-phase dielectric permittivity, thereby reflecting enhancement of the transition activity of chain segments. Moreover, the strain-dependent relaxation spectra suggest that the bifurcation of the relaxation processes decreases together with an overall increase in molecular mobility upon active deformation. The dielectric results also display a highly pronounced sensitivity to the apparent strain rate used for the mechanical excitation and isothermal aging prior to the deformation.

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Aspects of network formation and the physical properties of stoichiometric amine-cured epoxy resins are investigated. The state of network formation is systematically controlled by using isothermal cure conditions and a linear temperature ramp condition. Affine deformation in compression at a constant true strain rate is used to measure the true stress-strain response from small strains to large strains. Beginning with the low-strain responses and continuing through yielding, partially cured samples demonstrate characteristics of physical ageing that diminish as the networks approach full conversion. The postyield properties are characterized by a strong dependence on network connectivity with little influence of the curing method. The mechanical responses are also applied to necking and shear banding analyses. The resins are unable to achieve stable necking in tension. However, shear bands are able to be stabilized, as predicted by a model developed in this work. These results suggest that physical ageing is inherent in epoxy network formation, and the resulting strain localization significantly degrades the mechanical integrity of epoxy resins in the partially cured state, regardless of the cure conditions.

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.