Frontier Applications | Exploring the Optimal Processing Temperature for Sustainable Composite Manufacturing: Low-Field NMR Technology Challenges Conventional Wisdom

Background

In the field of sustainable manufacturing, vitrimer materials containing dynamic covalent bonds (DCBs) have become a research hotspot, as they combine the stability of thermosets (below the topology freezing temperature, Tv) with the processability of thermoplastics (above Tv). However, the conventional method of determining Tv via rheological techniques (e.g., stress relaxation experiments) has limitations. These measurements are influenced by both chemical bond exchange and frictional dynamics, making it difficult to pinpoint the true onset temperature of chemical exchange. This issue becomes even more complex in fibre-reinforced composites (FRCs), where fibre-matrix interactions further obscure accurate Tv evaluation.

To overcome this challenge, the research team employed multiple characterisation techniques. Among them, low-field nuclear magnetic resonance (NMR) relaxation measurement emerged as a key method due to its high sensitivity to molecular dynamics. Unlike traditional rheological tests, which are typically destructive, low-field NMR enables non-destructive insights into the chemical exchange processes within vitrimer systems.

Application of Low-Field NMR Technology

1. Experimental Design

Materials studied: Epoxy–anhydride-based polyester vitrimer (both neat matrix and cellulose fibre-reinforced composites; functional group ratio of epoxy to anhydride: R = 1:1).

Key parameters: Molecular mobility across temperatures was monitored by measuring transverse relaxation time (T₂) and changes in rigid/flexible phase components.

Technical advantages: Non-invasive measurements reflect segmental mobility at the molecular level without interference from the material’s bulk mechanical behaviour—allowing direct correlation with dynamic covalent bond exchange processes.

2. Key Findings

Figure A: Pure R=1:1 Vitrimer

A (i): T₂ relaxation time increases with temperature. Since T₂ is positively correlated with molecular motion (higher mobility yields longer T₂), both rigid and flexible phases show gradual increases beyond 75 °C (near Tg). A sharp increase is observed above 150 °C, indicating the activation of bond exchange (i.e., ester exchange), which reduces dipole-dipole coupling.

A (ii): Phase fraction changes. Below 100 °C, the material is predominantly rigid (stable phase fraction). Above 100 °C, the rigid fraction decreases while the flexible fraction increases, with the most notable transition between 125–160 °C. This confirms that bond exchange becomes active in this range and begins at 150 °C—significantly lower than the rheologically measured Tv of 200.7 °C.

Figure B: VC (1:1) Composite

B (i): The composite shows overall longer T₂ relaxation times than the neat vitrimer, with a more pronounced increase as temperature rises—indicating enhanced molecular mobility due to fibre–matrix interactions.

B (ii): Phase fraction analysis shows the rigid-to-flexible transition occurs at lower temperatures in the composite, eventually converging into a single phase. This reflects cooperative molecular motion facilitated by dynamic bonding between fibre-surface hydroxyls and matrix ester groups (via ester exchange).

Technical Value & Application Significance

1. Overcoming Limitations of Traditional Methods

Conventional rheological Tv reflects bulk flow behaviour. In contrast, low-field NMR uncovers the onset of chemical exchange at a much lower temperature, suggesting that Tv is a combined result of both bond exchange and frictional dynamics—rather than a singular “phase transition” point.

This finding is critical for optimising vitrimer composite reprocessing: materials can be reshaped below the rheological Tv, reducing high-temperature damage to fibres (e.g., cellulose).

2. Advancing Sustainable Composite Development

Low-field NMR revealed the dynamic covalent mechanism between cellulose fibres and the vitrimer matrix (hydroxyl-to-ester ester exchange), explaining how the composites retain over 90% of their mechanical strength (e.g., ≈70 MPa shear strength, >10% strain rate) after multiple reprocessing cycles.

This technology provides a molecular-level design pathway for creating “self-healing” and “recyclable” composites—for example, by tailoring hydroxyl group density on fibre surfaces to optimise dynamic interfacial bonding efficiency.

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References:

Rohewal, S. S., Damron, J. T., Seo, J., Kanbargi, N., Gupta, S., Humphrey, H. E., Kearney, L. T., Chang, J., Tetard, L., & Naskar, A. K. Hierarchically Structured Vitrimer Biocomposites for Sustainable Manufacturing [J]. Small, 2025, 25 (00721): 1 – 14.