Molecular mechanisms of thermo-oxidative stabilization in antioxidant-modified polyisoprene: bridging DFT calculations with macroscopic performance

Abstract

Context

Thermo-oxidative and ozone-induced degradation of natural rubber (NR) in modified asphalt necessitates the use of robust antidegradants, yet the precise quantum–mechanical prediction mechanisms underlying their action remain largely unexplored. This study integrates macroscopic experimental characterizations with density functional theory (DFT) simulations to evaluate the competitive efficacies and thermodynamic profiles of 6PPD, TMQ, and BHT. Theoretical evaluation reveals that hydrogen atom transfer (HAT) exclusively dominates radical scavenging, where the endergonic nature of TMQ (ΔG_rxn = + 1.4 kcal/mol) establishes it as a superior high-temperature thermodynamic reservoir, experimentally yielding a 99.2% tensile retention post-aging. Conversely, BHT suffers from rapid depletion due to its highly spontaneous dissociation (BDE = 74.0 kcal/mol). Transition state analyses provide a robust theoretical kinetic rationale for the antiozonant superiority of 6PPD; its simultaneous hydrogen abstraction of ozone requires a remarkably low activation barrier (ΔG^‡ = + 3.9 kcal/mol), successfully outcompeting the concerted cycloaddition required to cleave the polyisoprene backbone (ΔG^‡ = + 10.2 kcal/mol). The irreversible oxidative transformation of 6PPD into a quinone derivative was theoretically mapped, yielding a computed asymmetric C = O stretching fingerprint at 1700.6 cm⁻¹, which provides a precise spectroscopic target for its trace identification. Additionally, the drastic curing acceleration induced by 6PPD (t_s2 = 1:44 min) is fundamentally rationalized by its exceptional binding affinity toward the Zn²⁺ activator (ΔG_bind = − 275.2 kcal/mol). This synergistic approach robustly bridges coordination thermodynamics and transition-state kinetics with empirical macroscopic stability.

Methods

All quantum chemical calculations were performed using density functional theory (DFT) at the B3LYP-D3(BJ)/def2-TZVP level of theory. The ORCA 6.1.1 software package was employed for all computational modeling, conducted in the gas phase under standard conditions (298.15 K, 1 atm) with unscaled zero-point energy (ZPE) corrections applied. Intrinsic molecular reactivities were extracted via conceptual DFT (CDFT) using frontier molecular orbital (FMO) energies. Transition-state (TS) geometries were approximated using relaxed surface scans (RSS) and subsequently subjected to rigorous saddle-point optimizations (OptTS), with authenticity verified by the presence of a single imaginary frequency in harmonic vibrational analysis. Theoretical infrared (IR) spectra were simulated via analytical frequency calculations. Experimentally, formulated NR compounds (F1–F4) were prepared via a two-roll open mill and vulcanized at 150 °C. Accelerated thermal aging was conducted in an air-circulating oven at 100 °C for 72 h. Macroscopic performance was evaluated using a moving die rheometer, universal testing machine (ISO 37:2017), durometer (ASTM D2240-15), SEM–EDX, and FTIR spectroscopy to validate the theoretical paradigms.