Silk Fibroin and Cellulose Interactions at Different Length-scales

Recent developments in biocomposite research are leading towards a more sustainable future with higher performance materials. Silk fibroin (SF) and cellulose are two abundant
biopolymers capable of forming composites with biocompatibility, high mechanical performance, and biodegradability. Optimising biodegradable materials offers a replacement to traditional plastics that reduces reliance on recycling for sustainability and minimizes the impact of unavoidable leaching into the environment. This thesis investigates the dissolution, molecular interactions, and macroscopic material properties of SF–cellulose hybrid systems with the ultimate aim of producing self-reinforcing biocomposites from waste textile streams.
Using a solvent system of the ionic liquid 1-ethyl-3-methylimidazoliumacetate (EmimAc) and the cosolvent dimethyl sulfoxide (DMSO), optimal conditions for dissolving cellulose and SF were established. Interestingly, these were found to differ. Cellulose was most quickly and efficiently dissolved in a 2:8 EmimAc:DMSO solvent mixture, and SF fibres were most efficiently dissolved in an 8:2 EmimAc:DMSO solvent mixture. The macroscopic and microscopic behaviours were seen to differ with the bulk saturation being below the molecular saturation concentration investigated with nuclear magnetic resonance, as observed by polarized optical microscopy.
Understanding the dissolution of both biopolymers enabled the preparation of homogeneous hybrid solutions and fabrication of coagulated SF and cellulose films with varied compositions. Macroscopic mechanical testing showed that hybrid films exhibited enhanced modulus and strength (2.2 ± 0.1 GPa and 28 ± 1 MPa), with properties maximised at SF contents of 5–15 wt %. Creep and stress relaxation measurements identified strain-dependent viscoelastic relaxations in hybrid samples. This was rationalised with double network theory and comparisons with relevant biological materials which also exhibit similar relaxation behaviours. Structural analyses by thermogravimetric analysis confirmed the presence of mixed phases and β-sheet crystallinity, underpinning the observed mechanical strength and increased relaxation behaviours.
Building on these findings, isotropic short-fibre reinforced composites were prepared using optimised hybrid matrices reinforced with short-fibre cotton flock. The short fibres used mimicked potential textile waste for future manufacture of these materials. Sample morphology was investigated with X-ray diffraction to characterise the reinforcing fibre volume percentage. Compared with all-cellulose composites, samples with hybrid matrices achieved elevated Young’s moduli, strains at failure, and tensile strengths (3.4 ± 0.2 GPa, 1.3 ± 0.2 %, and 72 ± 2 MPa) at 42 - 55 vol % fibre reinforcement. This behaviour was rationalised with theoretical modelling and imaging with optical microscopy and scanning electron microscopy. Optimal composites showed reduced density and increased material performance while retaining full biodegradability and recyclability. Additional acoustic characterisation highlighted the potential for application in insulation materials as hybrid samples achieved an average acoustic transmission loss of 47 ± 7 dB which exceeds the typical standard for internal interior soundproofing between rooms (45 dB).
Overall, this work demonstrates how controlled dissolution and blending of SF and cellulose can yield a new class of tunable, self-reinforcing hybrid biocomposites. By combining molecular-level miscibility with waste-fibre reinforcement, this thesis establishes fundamental understanding and practical strategies for advancing sustainable, circular biological material technologies.