Development of biobased ionomers as self-healing materials

Steyn, Willem

Development of biobased ionomers as self-healing materials

Thesis / Dissertation

2026

Publisher

University of Cape Town

Department

Department of Mechanical Engineering

Faculty

Faculty of Engineering and the Built Environment

Abstract

Over the past few decades, concerns about the environment, climate change and our finite petroleum resources have led to research focusing on alternatives to petroleum-based polymers. It has thus become necessary to address the necessity for more sustainable polymers. This study focused on the development of bio-based ionomeric polymers as self-healing materials. These bio-based polymers were first prepared as unsaturated poly(ester amides)s from diacids, diols, anhydrides and amino acids via polycondensation polymerisation with sulfonated itaconic acid. These tailor-made monomers were synthesised via the ring opening reaction of succinic anhydride using para-aminobenzoic acid (PABA), L-phenylalanine (PHE), or aminobutyric acid (AB). The sulfonation reaction of itaconic acid was performed using sodium metabisulfite and led to the ionic moiety being incorporated into the polymer backbone. The incorporation of norbornene anhydride provided the unsaturated moiety in the polymer backbone. These bio-based unsaturated poly(ester amides)s were subsequently crosslinked under UV-light using pentaerythritol tetrakis(3-mercaptopropionate) (PETMP) as crosslinker. Structural characterisation was performed using ¹H-NMR and FTIR spectroscopy to confirm successful reaction syntheses. From the Soxhlet extraction results a gel content ranging from 76% to 79% was confirmed, thus confirming successful crosslinking of all polymer samples. The PABA-based polymers exhibited the lowest gel content, which could be attributed to its UV-absorbing properties, thus affecting crosslink efficiency. The crosslinked polymers displayed varying mechanical properties, with the ultimate tensile strength (UTS) values ranging from 0.268 MPa to 0.641 MPa, the Young's modulus values ranging from 0.156 MPa to 2.384 MPa, the elongation at break from 65.66% to 169.47% and the toughness values from 0.248 MPa to 0.369 MPa. The fractured samples were healed thermally at 130°C for 48 hours. Post-healing, the UTS increased drastically in the PABA and PHE-based samples, with PABA reaching 2.34 MPa and the Young's modulus increasing by 1243% to 32.016 MPa. The hardness values ranged from 21 to 30 Shore A in the crosslinked state, while increasing to 90 Shore A post-healing, with particular reference to the PABA-based samples. This increase corresponds with the increase in tensile strength and in the Young's modulus. Thermogravimetric analysis (TGA) demonstrated that the polymers had an onset thermal degradation temperature (Tonset) ranging from 295°C to 299°C, with a residual mass percentage of 17.6% to 22.7% at 800°C. The PABA-based polymers had the highest thermal stability which can be attributed to the aromatic groups in the backbone, while AB-based polymers had the lowest stability. The crosslinked polymers exhibited slow, yet gradual mass loss in a pH 7.4 environment at 37°C over 30 days in PBS solution. The AB-based polymers degraded the fastest, while the PABA-based samples showed the highest resistance to degradation. Further research and development of these polymers could lead to fully bio-based alternatives to self-healing materials, thus making them promising candidates for biomedical and synthetic skin applications.

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