Open research questions in Polymer composites and self-healing

What the literature leaves open

• This fragmented regulatory landscape may slow the market introduction of emerging technologies such as bio-based adhesives, reversible bonding systems, or smart adhesive formulations, particularly where clear evaluation procedures for novel chemistries or deg- radation products are lacking [93] (Figure 20).

  Adhesives and Sealants in Packaging: Functional Roles and System-Level Classification (Part I) · 2026 · DOI

• The migration potential, non-intentionally added substances (NIAS), and long-term exposure effects remain insufficiently characterized under conditions relevant to food packaging.

  Natural Deep Eutectic Solvents as Green Processing and Functional Media in Food Packaging · 2026 · DOI

• The transition toward sustainable, resilient, and intelligent materials has positioned cellulose at the forefront of polymer science. However, in the self-healing subdomain, a critical evaluation of its macromolecular architecture reveals a fundamental paradox: native cellulose is intrinsically non-ideal for autonomous self-healing. The very characteristics that provide cellulose with its renowned structural integrity—its highly ordered crystalline domains and dense, rigid network of intra- and intermolecular hydrogen bonds—severely restrict polymer chain movement. Because self-healing relies fundamentally on chain mobility (reptation) and the dynamic reshuffling of bonds across a damaged interface, unmodified cellulose cannot heal autonomously [203]. Nevertheless, the true value of cellulose lies in its chemical versatility and structural hierarchy when utilized in combinations and hybrid systems. By serving as a functional co-component, cellulose resolves the classic polymer physics dilemma between mechanical robustness and healing efficiency. Through physical nanoscale extraction (yielding CNCs and CNFs) or chemical functionalization (producing derivatives like CMC, HEC, and dialdehyde cellulose), it is transformed from a rigid barrier into a dynamic building block. In these hybrid systems, cellulose acts as a reinforcing filler, a multi-site crosslinking node, or a stabilizing scaffold for dynamic covalent and supramolecular networks. Because of this combinatory approach, cellulose is successfully utilized in a diverse array of material formats, each tailored to specific operational environments. In highly hydrated or solvated states, it forms advanced hydrogels, organohydrogels, and eutectogels that are the backbone of wearable flexible electronics, e-skin, and injectable biomedical matrices. In solid-state applications, it is engineered into elastomers, transparent films, and robust industrial coatings. Furthermore, exploiting its hydrophobic derivatives (like ethyl cellulose) and macro-fibrous forms, it is formulated into microcapsules and microbial https://doi.org/10.3390/polym18111296 Polymers 2026, 18, 1296 31 of 41 carriers for extrinsic self-healing in heavy-duty structural composites, such as asphalt and concrete. A comparative analysis between current laboratory research and the global patent landscape reveals a distinct translational gap. In academic laboratories, research is heavily focused on highly sophisticated intrinsic self-healing mechanisms. Scientists are engi- neering complex, multi-dynamic networks—such as simultaneously coupling Schiff-base reactions with metal–ligand coordination and host–guest interactions—to achieve ultra-fast healing kinetics, extreme stretchability (exceeding 4000%), and multi-stimuli responsive- ness (e.g., photo-thermal, magnetic, and pH triggers). In the patented industrial sector, however, the focus shifts toward scalability, cost-effectiveness, and reliability. The patent landscape is

  Self-Healing in Cellulose-Based Materials: From Fundamentals to Future Perspectives · 2026 · DOI

• TGA performed for formulation 1 showed thermally induced mass-loss events under nitrogen; however, because the small analytical portion may not be representative of the bulk composite, the measured residue was not used to determine retained organic content or post-curing composition and was not generalised to the other formulations.

  Turmeric-Containing Polymer–Mineral Composites with a Waste Cooking Oil-Derived Binder: Physicochemical Characterisation and Exploratory Antimicrobial Screening · 2026 · DOI

• Self-healing polymer nanocomposites represent a decisive step beyond conventional self-healing polymers by enabling simultaneous durability recovery and functional rein- forcement within a single material platform. Rather than acting only as passive strength- ening agents, nanofillers actively regulate healing by modifying crack evolution, stress redistribution, interfacial dynamics, and stimulus transduction. Across intrinsic and ex- trinsic architectures, this review establishes that the most consistent pathway toward high-performance systems is not maximizing either healing efficiency or stiffness inde- https://doi.org/10.3390/polym18020276 Polymers 2026, 18, 276 27 of 36 pendently, but engineering structure–property–healing coupling so that reinforcement does not eliminate the molecular mobility required for repair. In this context, the central scientific contribution of this review is the consolidation of evidence showing that nanocom- posite performance is governed by a constrained design window defined by competing parameters—filler loading, aspect ratio, dispersion quality, interphase confinement, and activation conditions—rather than by isolated chemical mechanisms. A key conclusion is that intrinsic healing systems (dynamic covalent networks, supramolecular bonding, and chain diffusion) provide the best foundation for repeated healing, but only when nanofiller–polymer interphases are tuned to remain dynamic and not permanently immobilizing. Conversely, extrinsic systems (microcapsules and vascular architectures) can enable autonomous single-event repair with high local effectiveness, but are intrinsically limited by agent depletion, crack path dependence, and processing com- plexity. Stimuli-responsive nanocomposite systems (Joule heating, photothermal, magnetic, and humidity/solvent-triggered healing) further expand functionality by enabling spatially localized, on-demand healing; however, their long-term effectiveness depends strongly on percolation stability, interfacial fatigue resistance, and the ability to maintain conductive networks under cyclic damage. Importantly, engineering translation requires a shift in eval- uation metrics. Much of the literature still reports healing as single-cycle tensile recovery, which can overestimate true durability. For structural or fatigue-loaded applications, the more relevant targets include fracture toughness recovery, fatigue crack growth resistance, cyclic healing retention, and environmental aging stability, since service failure is typically governed by crack propagation under complex loading rather than monotonic failure. Therefore, future progress will depend as much on standardization of testing protocols as on advances in chemistry and nanofiller design. Standardized reporting of damage geometry, healing activation conditions, number of cycles, and recovery metrics will allow for cross-study comparison and will accelerate technology readiness assessment. Design principles distil

  Self-Healing Polymer Nanocomposites: Mechanisms, Structure–Property Relationships, and Emerging Applications · 2026 · DOI

• However, in experimental CANs or vitrimers, there are other factors that have not yet been explored and taken into consideration in molecular simulations that can influence dynamic crosslinking.

  Molecular Simulation of Covalent Adaptable Networks and Vitrimers: A Review · 2024 · DOI

• However, compared with traditional permeable pavement materials, the preparation process and material composition of polyurethane mixtures are not yet well understood, and their water stability has not been systematically analyzed.

  Preparation and Water Stability of Permeable Polyurethane Mixtures · 2026 · DOI

• They also highlight the need for further investigation into the effects of initiator chemistry on polymer performance and strategies to reduce residual deformation under low-rate loading.

  Self-Healing Polysulfide–Epoxy Sealants for Concrete Pavement Joints · 2026 · DOI

• While these polymorphs exhibit distinct mechanical properties, a detailed understanding of their formation conditions is lacking.

  Cooling rate-dependent polymorphism in thermoplastic polyurethanes: effect of hard segments content · 2025 · DOI

• Self-repairable materials strive to emulate curable and resilient biological tissue; however, their performance is currently insufficient for commercialization purposes because mending and toughening are mutually exclusive.

  Mechano-responsive hydrogen-bonding array of thermoplastic polyurethane elastomer captures both strength and self-healing · 2021 · DOI

• Current recycling methods for polyurethane waste have limitations, including high reagent consumption and low economic value. There is a need for a novel approach to upcycle polyurethane waste into high-value products.

  Atom-economy upcycling of commodity thermoset polyurethane into photocuring 3D printing resins based on selective cleavage—crosslink strategy · 2026 · DOI

• The paper demonstrates upcycling of one type of PUF waste formulation, but applicability to PUF from different sources with variable compositions and cross-link densities remains to be demonstrated.

  Atom-economy upcycling of commodity thermoset polyurethane into photocuring 3D printing resins based on selective cleavage—crosslink strategy · 2026 · DOI

• Investigating the long-term stability and durability of the vitrimers. Evaluating the potential of the vitrimers for various applications. Developing new curing agent chemistries to further improve vitrimer properties.

  Molecularly Structured Castor Oil-Derived Epoxy Vitrimers Crosslinked with Acidic Disulfide-Bearing Curing Agents: Efficient Dynamic Response, Degradability, And Recyclability · 2026 · DOI

• The lack of recyclability and degradability in conventional thermosets. The need for sustainable and safer alternatives to conventional epoxy resins. The limited understanding of the influence of curing agent chemistry on vitrimer properties.

  Molecularly Structured Castor Oil-Derived Epoxy Vitrimers Crosslinked with Acidic Disulfide-Bearing Curing Agents: Efficient Dynamic Response, Degradability, And Recyclability · 2026 · DOI

• The development of sustainable shape memory polymers is crucial for smart structure applications. There is a need for novel smart light-responsive shape memory polymers with improved thermal and tensile properties.

  Sustainable end-chain polybenzoxazine functionalized polyethylene glycol for smart light-responsive shape-transformable structures · 2026 · DOI

• There is a need for materials that can be used for soft actuation in functional and biomedical devices. The emergence of biomedical-grade TPUs and their compatibility with additive manufacturing provides new opportunities for fabricating customized components.

  Shape Memory Behavior of 3D-Printed Biomedical Polyurethane with Room Temperature Transition · 2026 · DOI

• The vast majority of acrylic polymers remain intractable thermosets with limited end-of-life options. The development of multifunctional acrylic vitrimers that can combine the strength and stability of thermosets with the flexibility of thermoplastics is an urgent sustainability challenge.

  Multifunctional acrylic vitrimers: advances in dynamic covalent chemistry and sustainable synthesis · 2026 · DOI

• Limited mechanical strength and solvent resistance of hot-melt adhesives due to their mostly thermoplastic nature. Need for covalent cross-links to improve performance.

  Acrylic Hot-Melt Adhesives Containing Dynamic Covalent Cross-Links · 2026 · DOI

• Investigating the biological properties of the scaffolds, - Examining the effects of different terpene systems on the properties of the scaffolds, - Developing new applications for the highly aligned and interconnected porous AESO scaffolds

  Highly Aligned, Interconnected Porous Scaffolds via Photopolymerization of Acrylated Epoxidized Soybean Oil Containing Thermoreversible Terpenes as Porogens · 2026 · DOI

• Conventional porogens have limitations in creating highly aligned and interconnected pore architectures. Existing strategies for fabricating porous AESO scaffolds have difficulty achieving high pore interconnectivity.

  Highly Aligned, Interconnected Porous Scaffolds via Photopolymerization of Acrylated Epoxidized Soybean Oil Containing Thermoreversible Terpenes as Porogens · 2026 · DOI

• Full utilization of wood resources, including poplar and other underutilized materials such as small-diameter timber, irregularly shaped wood, and processing residues

  Wood/Dynamic Covalent Polymer Network Composites · 2026 · DOI

• Conventional thermosetting polymers cannot be reshaped or reprocessed. Thermoplastic polyolefins often exhibit poor compatibility with wood powder. Limited research on dynamic covalent polymer networks for wood-based composites.

  Wood/Dynamic Covalent Polymer Network Composites · 2026 · DOI

• Further investigation of the degradation of the catalysts is needed. Immobilization and stabilization of the catalysts should be tested. The evolution of the mechanical properties during aging should be studied.

  Influence of hydrothermal exposure on the network stability and performance of an epoxy-anhydride vitrimer for aircraft cabin composites · 2026 · DOI

• The difficulty in quantifying the acid-base properties of the functional groups within the synthesized resins due to the insoluble, cross-linked polymer matrix. The challenge of identifying changes associated with metal ion complexation in the IR spectra due to the presence of broad peaks. The need to optimize the experimental protocol to ensure complete crosslinking and provide a consistent basis for comparing resins used in sorption tests.

  Advanced bis-thiourea polymer resins for efficient gold (III) recovery · 2026 · DOI

• The impact of combining different thiourea moieties within a single polymer network on the recovery of gold has not been sufficiently explored. The paper identifies a need for a deeper understanding of the effect of this dual-functional approach on the chelating properties of the resin towards Au(III) species.

  Advanced bis-thiourea polymer resins for efficient gold (III) recovery · 2026 · DOI

Most-cited papers in Polymer composites and self-healing

• Silica-Like Malleable Materials from Permanent Organic Networks · Science · 2011 · 3,497 citations
• Self-healing and thermoreversible rubber from supramolecular assembly · Nature · 2008 · 2,871 citations
• A Thermally Re-mendable Cross-Linked Polymeric Material · Science · 2002 · 2,510 citations
• Nanoparticle Polymer Composites: Where Two Small Worlds Meet · Science · 2006 · 2,446 citations
• Recent progress in self-healing polymers and hydrogels based on reversible dynamic B–O bonds: boronic/boronate esters, borax, and benzoxaborole · Journal of Materials Chemistry A · 2021 · 482 citations
• Vitrimers: Current research trends and their emerging applications · Materials Today · 2021 · 473 citations
• Disulfide-Mediated Reversible Polymerization toward Intrinsically Dynamic Smart Materials · Journal of the American Chemical Society · 2022 · 454 citations
• Functional epoxy vitrimers and composites · Progress in Materials Science · 2021 · 446 citations
• Ultrarobust, tough and highly stretchable self-healing materials based on cartilage-inspired noncovalent assembly nanostructure · Nature Communications · 2021 · 426 citations
• Mechano-responsive hydrogen-bonding array of thermoplastic polyurethane elastomer captures both strength and self-healing · Nature Communications · 2021 · 403 citations

Most recent work

• UV-Curable, Self-Healing and Rigid Polyurethane-Urea Acrylate Elastomers Reinforced with Zirconium Phosphate Nanosheets · ACS Applied Polymer Materials · 2026
• Self-Healing Polymer Nanocomposites: Mechanisms, Structure–Property Relationships, and Emerging Applications · Polymers · 2026
• Dynamic Tetraoxadiborolane as a Boron–Boron Linkage for an Impact-Resistant, Self-Healing, and Recyclable Elastomer · Macromolecules · 2026
• Dynamic Covalent Polymer Networks Based on Renewable Feedstocks: A Perspective on the Role of Vitrimers in a Closed-Loop Economy · Biomacromolecules · 2026
• Advances in polyurethane (PU)-based microfiber synthetic leather: From manufacturing to applications · Composites Part B Engineering · 2026
• Smart self-healing polymers: innovations in material design and applications for electronic skin and energy devices · Journal of Materials Chemistry A · 2026
• Diverse polymers with chemical recyclability via regioirregular polymerization of a single monomer · Nature Materials · 2026
• A multi-crosslinked bio-based waterborne polyurethane for sustainable waterproof and antifouling coatings · Progress in Organic Coatings · 2026
• Strong, Water-Resistant, Self-Healing Waterborne Polyurethane Achieved by Rigid-Flexible Design and Dynamic Networks · ACS Applied Polymer Materials · 2026
• Cross-Linker Selection Controls Glass Transition Elevation or Reduction in Dynamic Covalently Bonded Polymer Networks · Macromolecules · 2026

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