Amidoamines are commercially used in epoxy coatings in part to enhance water barrier properties for corrosion prevention. However, a systematic understanding of the links between monomer molecular structure and material properties of such systems remains elusive, which limits our ability to design newer and more environmentally friendly coating systems. The effects of alkyl chain length in these crosslinkers on the properties of the polymer and on corrosion performance is not yet known and has hitherto not been investigated. The purpose of this article is to report results of a combined experimental and molecular-simulations based approach to understanding the role of pendant alkyl chain length on thermomechanical properties of thermoset polymers comprising conventional epoxy resins and model amidoamine crosslinkers. The aim here is to systematically investigate this single molecular-level feature using newly synthesized pure amidoamine (AMAM) crosslinkers of varying alkyl chain lengths. These crosslinkers are cured with the diepoxy diglycidyl ether of bisphenol A (DGEBA) and the effect of alkyl chain length on the properties of the resulting thermosets are investigated. The main properties that are addressed here are density, glass transition temperature, and coefficient of volume thermal expansion. Both experiments and simulations show that density decreases and volume-expansion coefficients increase with increasing pendant chain length. Furthermore, it is shown that the trends in density and coefficient of volume thermal expansion with chain length obtained from simulations are consistent with an ideal mixing approximation. Interestingly, however, the glass-transition temperature is found to be insensitive to chain length. Molecular simulations reveal that increasing the alkyl chain length from four to ten carbons does not introduce new flexibility mechanisms to the dense thermosets, which explains the insensitivity in glass transition temperature. This work demonstrates a new way to significantly decrease the density of a thermoset polymer without compromising desirable properties such as high glass transition temperatuer and low coefficient of thermal expansion, and therefore may provide sounder rationale for molecular-based design of epoxy-based coatings.

Effect of fatty acid chain length in amidoamine crosslinkers on material properties of epoxy systems

253rd National Meeting (2017)

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