Multiscale Effect of Carboxyl Modification on the Mechanical Properties of Palm Oil-Based Elastomers Studied by...

Plant oil-based elastomer has advantages such as sustainability, environmental friendliness, good biocompatibility, and high stretchability. However, the multiscale mechanical mechanisms of these materials remain unclear, limiting further optimization and application. This study focuses on palm oil-based elastomers, which feature hydrogen bond-toughened branched polymers. The effect of carboxyl modification of carbon-carbon double bonds in fatty acid chains on the mechanical behaviors of molecular segments (1-10 Å), single polymer molecules (1-10 nm), and polymer aggreagates (10-100 nm) was investigated using molecular dynamics simulations. Carboxyl modification significantly enhances the flexibility of molecular segments. As the number of carboxyl groups on a linoleic acid chain increases from 0 to 2, the average end-to-end distance reduces by 29% and the standard deviation increases from 0.23 to 6.6 Å. At the scale of single polymer molecules, carboxyl modification leads to more compact conformations and reduced conformational variability. With the number of carboxyl groups increasing from 0 to 6, the fluctuation of the radius of gyration reduces by 42.9%. At the scale of polymer aggregates, as the carboxyl modification rate increases from 0 to 100%, hydrogen bond density rises from 1.8 to 2.8 nm^-3, free volume fraction decreases from 16.8 to 14.5%, and diffusion coefficient drops from 6.7 × 10^-4 to 3.3 × 10^-4 Ų/ps. Simultaneously, the strengthened intermolecular interactions induce stretching and expansion of the polymer chains. Hydrogen bonds are mainly formed between the hydrogen and oxygen atoms on the polymer backbone. The maximum tensile stress increases from 0.23 to 0.26 GPa and the stress at a strain of 120% increases by 58%. The greater changes in relative positioning of polymer mass centers indicate a significant role of viscous deformation after carboxyl modification. Correlation analysis reveals that increased carboxyl modification enhances chain flexibility at the molecular segment scale, improves conformational compactness and stability at the single polymer-molecule scale, and strengthens the mechanical performance at the polymer aggregate scale. This study provides insights into the multiscale mechanisms of hydrogen bond-toughened palm oil-based elastomer and offers the basis for the design and optimization of plant oil-based materials.