Atomistic modeling of the hygromechanical properties of amorphous Polyamide 6,6

Abstract

Polyamide 6,6 (PA66) is a key engineering polymer, whose unique mechanical properties arise from strong interchain hydrogen bonding. However, its hygroscopic nature makes it highly sensitive to water uptake, which markedly alters its thermomechanical behavior. Contrary to traditional experimental approaches, this study uses atomistic molecular dynamics (MD) simulations to investigate the role of water in modifying the glass transition temperature (Tg) and the viscoelastic response of amorphous PA66. Simulations capture a nonmonotonic dependence of Tg on water content. At low water concentrations, isolated water molecules bind to amide groups and restrict chain mobility, while beyond ~2.5 wt %, water clustering disrupts the hydrogen bond network and causes a pronounced Tg depression. Analysis of amide group fluctuations reveals a master correlation between local segmental dynamics and bulk density, verifying the known temperature humidity equivalence in terms of density variation. The computed Young's modulus exhibits systematic softening with increasing temperature and water content, consistent with experimental trends, albeit a more pronounced impact of water at low temperatures. Time temperature superposition behavior is observed for both dry and hydrated systems. This work provides molecular scale information on the hygromechanical coupling in PA66 and demonstrates the ability of MD simulations to predict water induced transitions that govern the macroscopic behavior of polyamides.

Figures (4)

• Figure 1: MD analysis of the glass transition behavior of amorphous PA66 as a function of water content. (a) Density $\rho$ as function of temperature $T$ for the dry system ($C_w = 0$ wt.%) and a hydrated system ($C_w = 8.7$ wt.%), illustrating the thermal expansion and the density reduction associated with water-induced swelling. Dashed lines are fits obtained using Eq. \ref{['eq:hyperbola']} (b) Variation of $T_g$ with $C_w$. An increase in $T_g$ is observed at low water contents ($C_w\leq 2.5$ wt.%), suggesting a binding effect of isolated water molecules (H$_2$O MD snapshot at $C_w = 1.1$ wt.%) that locally stabilizes the amide hydrogen-bond network. Beyond this concentration, $T_g$ decreases continuously as water clustering (H$_2$O MD snapshot at $C_w = 8.7$ wt.%) disrupts inter-chain hydrogen bonds and promotes segmental mobility, leading to an effective softening of the amorphous matrix. (c) Radial distribution function $g(r)$ of water oxygen atoms at various water contents. A peak at 2.8 Å corresponds to the nearest-neighbor O-O distance, while the emergence of a secondary peak near 4 Å (fully developed at $C_w = 3.5$ wt.%) and a third peak around 6.5 Å indicates progressive clustering of water molecules. These features mark the transition from well-dispersed water at low concentrations to the formation of bulk-like water clusters at higher $C_w$, consistent with the observed $T_g$ dependency on water content.
• Figure 2: Amide-group fluctuations in amorphous PA66 at different temperatures and water contents. (a) Distribution of RMSF of amide oxygen atoms (O$_{amide}$) at $C_w= 6.7$ wt.% obtained from MD simulations at various temperatures (Inset for H$_{amide}$). The MD data (symbols) are accurately described by log-normal fits (dashed lines) for all temperatures, reflecting the multiplicative nature of local molecular fluctuations. Increasing temperature shifts the RMSF distribution peak toward larger values and broadens its width, indicating enhanced segmental mobility and intensified uncorrelated motion of individual amide groups. (b) Correlation between the peak of the RMSF (O$_{amide}$) distribution and $\rho(T)$ for all simulated temperatures and water contents. The data collapse onto a single master curve, revealing a universal coupling between local segmental dynamics and bulk packing density. A clear transition is observed at $\rho(T) \sim$ 1.07 g/cm$^3$, separating the high-density, glassy regime from the low-density, rubbery regime associated with thermally or moisture-induced softening of the amorphous matrix.
• Figure 3: Mechanical behavior of amorphous PA66 (dry and hydrated conditions) under uniaxial monotone loading. (a) Engineering stress vs. strain curves from uniaxial deformation tests (300 K, $10^9$ 1/s) in tension and compression mode. The introduction of water results in a reduction of both stiffness and yield strength, consistent with the plasticizing effect of absorbed moisture within the amorphous PA66 matrix. (b) Young’s modulus (E) extracted from uniaxial tests at various temperatures, strain rates, and water contents. The modulus exhibits strong sensitivity to both temperature and deformation rate: an increase in temperature from 300 K to 380 K leads to a $\sim$35% reduction in E for the dry polymer, while lowering the strain rate produces an additional decrease in stiffness, highlighting the viscoelastic character of amorphous PA66. Increasing water content induces an overall softening, particularly accentuated at low temperature (300 K).
• Figure 4: Viscoelastic response of amorphous PA66 under dry and hydrated conditions analyzed using time-temperature superposition (TTS). (a) Master curves of Young’s modulus (E) as a function of strain rate for $C_w = 0$ wt.% and $C_w = 8.7$ wt.% using $300$ K as reference temperature. Data extracted at different absolute temperatures for a dry system are shown in the inset. The hydrated polymer curve is shifted to lower stiffness, indicating a reduced modulus at a given strain rate compared to the dry system (lines added for clarity). (b) Strain-rate dependence of the hydration effect on E. At high strain rates, only a reduction in modulus of $\sim 10\%$ is observed upon hydration, whereas at low strain rates the same water uptake produces a reduction in modulus of almost 50%. This behavior emphasizes the timescale-dependent nature of the plasticization process. (c) Temperature dependence of the TTS shift factor $a_T$ for different reference temperatures. The data follow a classical Arrhenius-like trend and are well described by the Eyring equation (Eq. \ref{['eq:TTS']}) in dashed lines (only shown for the dry polymer). Hydrated PA66 (filled symbols) shows little deviation from the dry system (open symbols), indicating comparable activation energies.