A. J. Guenthner
Tideway Arts and Sciences, LLC,
United States
Keywords: cyanate ester, water uptake, composite resins
Summary:
Even though the differences in bond length and bond strength between carbon-carbon and carbon-silicon bonds are relatively modest, these differences have been successfully exploited in surprising ways to improve many disparate aspects of performance in polymer composite resins, including lower melting points for monomers and lower moisture uptake in cured networks. Because these benefits are recognized in some but not all networks containing silicon-carbon bond, a detailed understanding of the mechanisms by which the properties of polymer networks emerge from molecular-level characteristics is required in order to realize the benefits. Historically, developing such an understanding, particularly in a quantitative form that is well-suited to modern, computationally-driven methods of materials design, has been challenging due to the often ill-defined chemical structure present in thermoset network polymers as well as the difficulty in conductive quantitative analysis of insoluble network macromolecules. This presentation describes how many of these challenges have been overcome recently, resulting in the successful development of high-performance thermosetting polymer networks for aerospace and defense applications, due to careful selection of materials and experiments, as well as more powerful computational methods of structure analysis. Polymer networks derived from cyanate ester monomers (also known as polycyanurate networks) offer the advantages of structural homogeneity down to the molecular level (as confirmed by AFM and TEM experiments) as well as a well-defined and relatively simple chemical structure for cured networks (as confirmed by FT-IR and occasional solid-state NMR experiments). These features have enabled not only precision and reliability in physical property measurements such as density, glass transition temperature, and water uptake, but also a relatively high level of fidelity in classical atomistic simulations. This presentation will demonstrate how computational capabilities currently available for both confirmational studies of repeat units as well as the physics of cross-linked networks provide a means of tracing the influence of changes in atom and bond-level properties through their effects on network microstructure to the resultant changes in physical properties, all in a highly quantified manner. As a result, experimental structure-property relationship studies have been utilized to quantitatively validate the mechanisms, rather than simply empirically describe the observed effects. An important consequence of obtaining the detailed mechanisms underlying observed structure-property relationships in silicon-containing polycyanurate networks is that the mechanisms often apply to, and have been successfully exploited in, other types of high-performance polymer networks where more complex chemical structures and cure mechanisms make it difficult to discover these mechanisms directly. This presentation will illustrate how trends in properties such as the water uptake for many thermosetting polymers may now be predicted using both very rapid informatics-driven screening approaches, or more computationally-demanding but more reliable physical simulations of cross-linked networks. These approaches are many times faster and less costly than empirically-driven experimentation in cases where custom chemical synthesis of materials is required.