Self-healing Polymers

Living organisms can repair damaged tissues and recover their functions, providing an inspiring motif for the design of self-healing materials. Conventional polymer coatings and networks, however, often accumulate scratches and cracks that compromise their appearance, mechanical integrity, and service life. Self-healing polymers address this limitation by incorporating molecular mechanisms that enable damaged regions to reconnect. In intrinsically healable systems, reversible interactions and dynamic covalent bonds allow polymer networks to reorganize under suitable conditions, providing a route to material repair and, in some systems, reprocessing.

Our laboratory explores how dynamic bonding, polymer-chain mobility, and structural organization can work together to create durable yet repairable materials. A central design challenge is to balance mechanical strength with the molecular mobility required for healing: crosslinking stabilizes a material, but can also restrict the movement of chains across a damaged interface. We therefore investigate dynamic polymer networks, including hindered urea-based systems, and examine how microphase-separated structures influence scratch healing and thermomechanical properties. A further research direction involves integrating renewable cellulose nanocrystals into self-healing polymer networks. Here, nanocellulose serves as both a reinforcing component and a functional interface that participates in network interactions. Our recent work combines carboxylated cellulose nanocrystals with a bismuth catalyst to develop organotin-free, healable, and reprocessable urethane-crosslinked copolymers through dynamic bond exchange, hydrogen bonding, and coordination interactions. Through these complementary approaches, we aim to establish molecular and structural design principles for polymer coatings and composites that combine damage recovery, useful mechanical properties, and extended material lifetimes.

Self-healable CNC/Polymer Composites

We develop self-healable cellulose nanocrystal (CNC)/polymer composites that combine mechanical reinforcement with reversible network reconstruction. By tailoring CNC surface chemistry and its interactions with dynamic polymer networks, we use CNCs as both reinforcing nanofillers and active components in the healing process. Hydrogen bonding and metal–ligand coordination enable efficient damage repair while maintaining mechanical strength. Our CNC-based composites have demonstrated 99% healing efficiency within 1 h at 5 °C, while composites incorporating solvent-free, silane-modified CNCs achieved ≥99% healing efficiency in less than 20 min at 5 and 25 °C. Our recent work extends this approach to organotin-free, self-healable and reprocessable polymer networks containing TEMPO-oxidized CNCs and bismuth neodecanoate. These materials combine bismuth-catalyzed transesterification with hydrogen bonding and bismuth–carboxylate coordination to facilitate network reconstruction. The resulting transparent composites exhibit enhanced strength, efficient healing, and retention of more than 90% of their tensile strength after repeated reprocessing. This research provides a foundation for durable protective coatings and repairable materials for flexible and wearable devices.

Selected Publications

Saddique, A.; Lee, H. M.; Kim, J. C.; Bae, J.; Cheong, I. W. “Cellulose nanocrystal nanocomposites capable of low-temperature and fast self-healing performance.” Carbohydrate Polymers 296 (2022), 119973. DOI

Saddique, A.; Kim, J. C.; Bae, J.; Cheong, I. W. “Low-temperature, ultra-fast, and recyclable self-healing nanocomposites reinforced with non-solvent silylated modified cellulose nanocrystals.” International Journal of Biological Macromolecules 254 (2024), 127984. DOI

Saddique, A.; Hussain, M.; Cheong, I. W. “Carboxylated cellulose nanocrystals enable tin-free, reprocessable, self-healing urethane-crosslinked copolymer networks via bismuth-mediated dynamic bonds.” Chemical Engineering Journal 533 (2026), 174603. DOI

Stimuli-responsive Polymers

Inspired by these capabilities, we developed random and block copolymers incorporating thermoresponsive and photoresponsive moieties. These copolymers can form droplets, hollow particles, and polymersomes that exhibit stimuli-induced colour changes, phase separation, and volume changes. To impart photoresponsiveness, we incorporated spironaphthoxazine (SPO) or spiropyran derivatives into the copolymers. Upon UV irradiation, SPO undergoes cleavage of the spiro C–O bond and ring opening to form a deep-blue merocyanine species with a broad absorption band centred at approximately 610 nm. This transformation is reversible: the merocyanine form returns to the closed-ring spiro form under visible-light irradiation or through thermal relaxation, including in the dark. In our recent work, irradiation at 365 nm induced directional aggregation and internal phase separation of SPO-containing random copolymers confined within water-in-oil (W/O) droplets. This response enabled the transformation of initially symmetric droplets into asymmetric Janus particles. By inducing phase separation within preformed droplets, this approach provides a route to Janus particle formation that addresses mixing difficulties associated with coflow-based fabrication. These findings demonstrate how photoresponsive polymer architectures can translate a molecular-scale transformation into changes in both colour and particle morphology.

Graphene/CNT Dispersions

Graphene, a two-dimensional material composed of carbon atoms arranged in a hexagonal structure with sp² bonding, exhibits exceptional properties such as optical transparency, mechanical strength, thermal conductivity, and high electron mobility. However, its poor processability limits its widespread availability.

Recently, various methods for graphene preparation have been developed, including chemical vapor deposition, mechanical exfoliation, molecular assembly, epitaxial growth on SiC, and liquid-phase exfoliation. Among these, liquid-phase exfoliation is considered the most efficient approach for industrial applications due to its low cost and scalability.

In liquid-phase exfoliation, both covalent and non-covalent functionalizations of graphene have been extensively studied. Covalent functionalization typically involves oxidation of graphite, which introduces defects such as carboxylic acid, hydroxyl, and carbonyl groups or even structural holes. In contrast, non-covalent functionalization can yield pristine graphene, but it requires the use of dispersants or stabilizers to prevent aggregation after exfoliation.

To achieve stable graphene dispersions with minimal aggregation, various types of solvents (e.g., inorganic, organic, and fluorinated oils) and surfactants (ionic, non-ionic, short-chain, and polymeric) have been employed. Among them, block copolymer dispersants offer several advantages over short-chain surfactants, including improved colloidal stability, reduced need for additional polymeric binders, slower migration, and tunable compatibility with different systems.