Bio-Based Thermoplastic Phosphorescent Materials with Closed-Loop Recyclability at Room TemperatureThe growing demand for sustainable materials has catalyzed a new wave of innovation at the intersection of polymer science and environmental engineering. Traditional petrochemical-derived phosphorescent materials, while functionally powerful, are burdened with environmental liabilities including toxicity, energy-intensive processing, and poor recyclability. Recent breakthroughs in bio-based phosphorescent polymers—especially those with room-temperature phosphorescence (RTP)—mark a new era of environmentally responsible luminescent technologies.
Among these, the development of Poly(TA)/Cell, a bio-based, thermoplastic RTP material synthesized from thioctic acid (TA) and carboxylated cellulose nanofibers (CNF), offers a compelling solution. This innovation integrates high-performance photophysics with closed-loop recyclability and excellent mechanical processing flexibility.
Fig 1. Schematic illustration for the processing and recycling of Poly(TA)/Cell with RTP. (Qian Y., et al., 2025)
Thioctic Acid (TA): A Disulfide-Based Dynamic Monomer
Thioctic acid, also known as α-lipoic acid, is a naturally occurring organosulfur compound that plays a role in biological redox systems. It contains a disulfide bond (-S-S-), which is both thermally polymerizable and alkali-cleavable, making it an ideal building block for dynamic and recyclable polymers. Upon heating, TA undergoes ring-opening polymerization, forming Poly(TA), a flexible polymer with embedded disulfide bonds that endow it with self-healing and degradable properties.
Carboxylated Cellulose Nanofibers (CNF): Structural Reinforcement and Optical Enhancer
Cellulose, the most abundant biopolymer on Earth, brings rigid crystalline domains, abundant hydroxyl and carboxyl groups, and excellent hydrogen bonding capability. When carboxylated, CNFs provide improved interaction sites for Poly(TA), forming molecular clusters that serve as phosphorescent emission centers. The CNF scaffold stabilizes triplet excitons, essential for persistent RTP, and creates a semi-crystalline matrix conducive to photonic activity.
Green RTP Emission with Environmental Sensitivity
Poly(TA)/Cell exhibits green phosphorescence (~500 nm) under UV excitation, with an afterglow lifetime reaching up to 600 milliseconds. This persistent luminescence results from suppressed non-radiative decay pathways, facilitated by:
Excitation and Humidity Responsiveness
The material demonstrates excitation wavelength-dependent behavior, with RTP lifetime peaking under 300 nm irradiation. More notably, the RTP is highly sensitive to humidity:
These properties render Poly(TA)/Cell highly suitable for humidity sensors and dynamic displays.
Förster Resonance Energy Transfer (FRET) with RhB
The emission spectrum of Poly(TA)/Cell overlaps with the absorption band of Rhodamine B (RhB), a red fluorescent dye. This spectral alignment enables triplet-to-singlet energy transfer, producing red afterglow (~610 nm) through TS-FRET.RhB Loading Effects
Varying the RhB content in the composite modulates the optical output:
These tunable emissions broaden Poly(TA)/Cell's application range to anti-counterfeiting, optical encryption, and smart labeling systems.
FTIR and XPS Confirmation of Hydrogen Bond Networks
Fourier-transform infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) confirm redshifts in C=O and O–H signals, indicative of strong hydrogen bonds between Poly(TA) and CNF. These interactions underpin the RTP behavior by:
Computational Simulation of Binding Energies
Theoretical models identify three interaction modes:
Among these, the mixed scenario exhibits the strongest binding energy (~−28.3 kcal/mol), consistent with enhanced exciton trapping and stability.
Comparison with Analogous Composites
Control experiments with Poly(AA)/Cell and Poly(TA)/PVA reveal markedly reduced RTP lifetimes (~24–30 ms), attributed to:
These findings validate the synergistic role of sulfur chemistry and cellulose crystallinity in producing efficient RTP.
Thermal Moldability and Form Stability
Poly(TA)/Cell demonstrates true thermoplastic behavior, with a glass transition temperature (Tg) of ~59.9 °C. Upon heating above this threshold:
No degradation of phosphorescence was observed after:
Self-Healing via Disulfide Bond Reformation
The dynamic disulfide bonds enable self-repair under mild heating. When fractured, the polymer chains realign and reconnect across the interface, restoring mechanical and optical integrity. Tensile stress-strain curves confirm retention of original strength post-healing, enhancing lifecycle utility.
Depolymerization and Separation Process
Poly(TA)/Cell can be degraded in alkaline (NaOH) solution, cleaving disulfide bonds and depolymerizing Poly(TA) into its monomeric form. The recycling process involves:
Yield and Reusability
This approach supports true molecular-level circularity, reducing material waste and environmental impact while preserving resource value.
Dual-State Information Encoding
A phosphorescent label was created with numeric patterns that shift visibility under different lighting conditions:
This dual-mode visibility is ideal for product authentication, tamper-proof packaging, and covert security tagging.
2D Luminous Puzzle Structures
Poly(TA)/Cell and Poly(TA)/Cell/RhB components were thermally bonded to create bicolor phosphorescent puzzle pieces. These structures demonstrate material modularity and potential in interactive displays, branding, and educational tools.
| Property | Poly(TA)/Cell | Poly(AA)/Cell | Poly(TA)/PVA |
|---|---|---|---|
| RTP Lifetime (ms) | ~600 | ~66 | ~30 |
| Self-Healing | Yes | No | No |
| Thermoplastic | Yes | No | Yes |
| Recyclability | Closed-loop (92% TA) | Not demonstrated | Not demonstrated |
| Color Tunability | Green to Red (with RhB) | Monochrome | Monochrome |
| Humidity Sensitivity | High, reversible | Low | Low |
| Metal Ion Response | Yes | Not evaluated | Not evaluated |
Poly(TA)/Cell exemplifies how material sustainability, photonic performance, and process engineering can coalesce into a single advanced platform. With its:
…it provides a blueprint for the future of eco-functional optoelectronics. As industries seek scalable, non-toxic, and reprocessable alternatives to conventional photonic materials, Poly(TA)/Cell stands out as a luminous innovation in both function and philosophy.
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Reference
This article is for research use only and cannot be used for any clinical purposes.
