Sustainable Self-Healing Elastomers: Chitosan-Tuned Ionic Dynamics in Natural Rubber Networks

Current self-healing elastomers face a fundamental challenge in reconciling robust mechanical properties with efficient autonomous repair due to the incompatibility between permanent covalent networks and reversible dynamic bonds. While chitosan (CTS) has shown potential in rubber modification, its role in establishing stimuli-responsive ionic networks within nonfunctionalized natural rubber (NR) to achieve intrinsic self-healing remains underexplored. Following a systematic screening of CTS molecular weight (MW) and degree of deacetylation (DDA), we engineered sustainable self-healing NR composites using CTS as a cross-linker. CTS was mechanically blended with unmodified NR, followed by thermal activation to form reversible [COO^-]-[NH₃ ⁺] ionic bonds between CTS amines and native phospholipid-derived pyrophosphate termini on NR chains. Self-healing efficiency, tensile recovery, and cyclic hysteresis were quantitatively evaluated. Critical evidence from FTIR revealed ionic cross-linking via the amide band at 1741 cm^-1 with characteristic bathochromic shifts; XRD confirmed disruption of CTS crystallinity; cross-link density analysis (T₂ relaxation) revealed chain confinement modulated by CTS DDA and MW; and SEM demonstrated homogeneous CTS dispersion below 10 phr compared to agglomeration above 30 phr. We found that 10 phr CTS enables near-complete self-healing after 20 min of ambient contact, attributed to rapid ionic bond reorganization evidenced by high cyclic residual strain. However, this occurred with and approximately 22% decline in tensile strength versus neat NR. Excess CTS (>10 phr) reduced self-healing efficiency due to restricted chain mobility from ionic clusters and steric hindrance, coupled with a 22% tensile strength decline. Low-MW CTS facilitated superior healing by minimizing entanglement constraints. This study resolves the healing-mechanics trade-off by leveraging native NR components as dynamic anchors, eliminating the need for synthetic functionalization. By interfacing renewable polysaccharides with nonreactive polymers, our work advances circular design principles for next-generation sustainable materials.