Disentangling Heat and Moisture Effects on Biopolymer Mechanics

Heat and moisture are known to have important mechanical effects on polymers such as hygric swelling, thermal expansion, and mechanical weakening. A common approach when investigating such effects is to assume the effects of heat and moisture to be similar - the so-called time-temperature-moisture superposition. Through molecular dynamics simulation, this study evaluates the extent of the similarity of the effects of moisture and heat on the hygric swelling, thermal expansion, and mechanical weakening of a biopolymer: an uncondensed type of lignin, one of the most abundant polymers in the plant regime. We introduce as a microscopic metric the local stiffness (T/ u²⟩, temperature divided by the amplitude of segmental motion u²⟩), to analyze the mechanisms of mechanical effects of heat and moisture. The local stiffness of polymer skeleton and the overall stiffness of the composite material are shown to be strongly correlated, with a Pearson correlation coefficient of 0.96. Under the assumptions of harmonic vibration and isotropy, an explicit equation relating bulk moduli and the local stiffness can be derived, and the theoretically predicted moduli are in good agreement with measurement. The thermal expansion and weakening are shown to be related to each other and both dependent on the local stiffness. The analysis of the potential energy further points out that heating weakens both primary and secondary bonds of the polymer skeleton, while hydration only affects the secondary bonds. This major difference is thought to be the origin of the different impacts of heat and moisture on biopolymer mechanics, offering a different view of the time-temperature-moisture superposition principle.