Properties of lignin and hydrogel for optimal network compatibility.
| Property | Role in network compatibility | Characteristic of lignin | Characteristics of hydrogel | Impact on composite performance | References |
|---|---|---|---|---|---|
| Hydroxyl group | Determines the density of potential covalent crosslinking sites (e.g., with crosslinkers like epichlorohydrin, glutaraldehyde) or strong hydrogen bonding. | 450–900 mg KOH/g (for Kraft softwood lignin, representing 5–10 mmol/g of OH groups). Contains aliphatic and phenolic OH. | Varies widely. polyvinyl alcohol (PVA): ~18 mmol/g OH groups. Polyacrylamide: requires functionalization; OH content is low natively. | High OH content increases crosslinking density, enhancing tensile strength up to 400% and reducing swelling ratio. | [29–32] |
| Solubility & hydrophilicity (log P) | Governs miscibility and dispersion of lignin within the hydrophilic hydrogel matrix. Prevents macroscopic phase separation. | Log P ≈ 2.5–4.0 (hydrophobic). Poor water solubility; often requires chemical modification (e.g., sulfonation) for dispersion. | Log P < 0 (hydrophilic). Designed to have high affinity for water, with equilibrium water content often greater than 90%. | Good compatibility requires modifying lignin to improve hydrophilicity. Poor dispersion leads to brittle composites. | [33–36] |
| Glass transition temperature Tg | Indicates polymer chain mobility. A matched Tg between components suggests better molecular-level mixing and integration. | Tg between ~90–180°C (for Kraft lignin). High Tg due to aromatic, rigid structure and hydrogen bonding. | Tg between –20 to 120°C. Highly tunable. PVA: ~85°C. PAAm: ~165°C (theoretical). Often well below 0°C when swollen. | A large mismatch can lead to phase separation. Lignin can act as a rigid filler, increasing the composite’s Tg and modulus. | [37–40] |
| Surface energy & interfacial tension | Low interfacial tension promotes spontaneous wetting and adhesion between lignin particles and the hydrogel polymer chains. | ~40–55 mJ/m2 (Dispersive component ~40 mJ/m2, Polar component variable). | ~30–45 mJ/m2 (e.g., PVA ~42 mJ/m2). High polar component due to hydrophilicity. | Lower interfacial tension minimizes aggregation, leading to a more homogeneous composite and efficient stress transfer. | [41–43] |
| Particle size & morphology | Determines the available surface area for interaction and defines the composite’s microstructure. Nanoscale size is critical. | Nanoparticles (LNPs): 50–300 nm (via precipitation/ultrasonication). Micro-particles: 1–100 µm. Spherical or irregular. | N/A (continuous polymer network). Pore size typically 1–100 nm in hydrogels. | Lignin nanoparticles enable uniform distribution, act as multifunctional crosslinkers, and enhance mechanical properties. | [23, 44, 45] |
| Rheological properties (G’, G”) | Reflects the viscoelastic behavior and structural integrity of the pre-gel solution, indicating how well components mix. | Aqueous dispersions can show G’ > G” at high concentrations (> 10 wt%), indicating gel-like behavior. | Pre-gel solutions are typically viscous liquids with G” > G’. Crosslinking inverts this relationship. | Incorporating lignin can increase the complex viscosity of the pre-gel solution, promoting better suspension and processability. | [37, 46] |
AB: Conceptualization, Data curation, Investigation, Visualization, Writing—original draft. YA: Resources, Methodology, Software, Validation, Writing—review & editing. TKB: Conceptualization, Software, Supervision, Writing—original draft, Writing—review & editing. MTI: Conceptualization, Supervision, Project administration, Writing—review & editing. All authors read and approved the submitted version.
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