Disentangling Heat and Moisture Effects on Biopolymer Mechanics

Abstract

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/ ⟨u 2 ⟩, temperature divided by the amplitude of segmental motion ⟨u 2 ⟩), 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.

Scaling relations in rheology of concentrated starches and maltodextrins

“…The found scaling of horizontal shift factors a T with T g /T is also in line with various studies on time-temperature-moisture superposition (Fabre et al, 2018;Ishisaka et al, 2004;Krauklis et al, 2019;Patankar et al, 2008;Zhan et al, 2019;Zhang et al, 2020). This superposition is found for materials, where hydrogen bonding is important, such as wood, polyelectrolytes, polyamides, and epoxies.…”

Predicting Solute Diffusivity in Polymers Using Time–Temperature Superposition

“…TTS has also proven useful outside of rheology. For example, it has been employed to account for the combined effects of heat and moisture on biopolymers, as well as to analyze temperature-accelerated studies of nickel leaching rates and bioresorbable polymer degradation in biomedical applications. , TTS has even been used with the mean-squared displacement, based on the similar shapes of ⟨Δ r 2 ⟩ curves at varying T (Figure a,b). For example, Khare and Phelan used TTS to build master curves of the mean-squared displacement from atomistic simulations of a cross-linked polymer across the glass transition, while Sprakel and co-workers took the same approach with simulated colloidal suspensions .…”

“…90 TTS has also proven useful outside of rheology. For example, it has been employed to account for the combined effects of heat and moisture on biopolymers, 91 as well as to analyze temperature-accelerated studies of nickel leaching rates and bioresorbable polymer degradation in biomedical applications. 92,93 TTS has even been used with the meansquared displacement, based on the similar shapes of ⟨Δr 2 ⟩ curves at varying T (Figure 1a,b).…”

Quantifying Chemical Composition and Cross-link Effects on EPDM Elastomer Viscoelasticity with Molecular Dynamics

“…This process is commonly used to equilibrate a polymer system. , The annealing process aims to eliminate any large internal stresses that may form in these systems and allows for chains to relax rapidly at elevated temperatures. Three criteria were applied to check the equilibration of the system: (1) the density of the system remains at approximately the same value, (2) the system energy remains approximately constant, (3) the Hermans orientation parameter is less than 0.05, , to ensure that the system is amorphous. From our simulation results, the density and energy fluctuations remain within 0.5% for the final 10 ns equilibration at 300 K. The absolute value of the Hermans orientation parameter was smaller than 0.05, indicating that the EPDM model is indeed amorphous.…”