In a recent article published in Advanced Science, researchers explored the thermomechanical properties of n-type polymer poly(benzodifurandione) (PBFDO) and assessed its suitability as a yarn-coating material. PBFDO and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) coated silk yarns were utilized to create two out-of-plane thermoelectric textile devices—a thermoelectric button and a thermopile—for comparative analysis.
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Background
Electronic textiles (e-textiles) are versatile materials that bring additional functions to fabrics, including health monitoring, diagnostics, communication, displays, motion sensing, and thermal regulation. E-textiles equipped with energy-harvesting devices offer the advantage of converting local energy sources, such as body heat, into electricity for powering integrated electronics.
Thermoelectric devices show potential for energy harvesting by converting body heat (temperature differences between skin and surroundings) directly into electrical energy through the Seebeck effect. Various integration methods exist, including coating or printing thermoelectric generators onto existing fabrics, stitching or embroidering conductive fibers, and knitting or weaving with conductive yarns.
Thermoelectric materials used in textiles should be comfortable, safe, and non-toxic. Therefore, developing truly functional polymer-based thermoelectric textile devices requires an n-type yarn with greatly improved air stability, mechanical robustness, and processability.
Methods
PBFDO ink was prepared using dimethyl sulfoxide (DMSO) solvent. Thin films were spin-coated (1500 rpm for 120 s and 3000 rpm for 10 s) onto cleaned glass slides and annealed on a hotplate at 50 °C for one hour. Additionally, free-standing films (7-18 µm thick) of PBFDO were prepared.
PBFDO-coated silk yarn was created by immersing the yarn in PBFDO ink for 10-20 minutes, followed by oven drying at 40 °C. Dip-coating in PBFDO ink was repeated twice for yarns used in characterization and thermopile sewing, and three times for prototype yarns, with a final drying step at 40 °C.
The samples underwent characterization using atomic force microscopy (AFM), scanning electron microscopy (SEM), and wide-angle X-ray scattering (WAXS). Electrical resistance was measured in a vacuum over a temperature range of 80 to 400 K using the four-point probe method. Tensile deformation tests on free-standing films and yarn sections were conducted at various temperatures, and yarn samples were machine-washed 1 to 7 times (15 minutes each at 20 °C) with 3 L of water.
A thermopile device, with one n-type and one p-type leg, was assembled by hand-stitching the conductive yarns with a sewing needle. Additionally, PBFDO and PEDOT yarns were stitched onto six layers of felted wool fabric to create an out-of-plane thermoelectric generator with eight n-type and eight p-type legs.
For validation, the textile thermopile was modeled in COMSOL Multiphysics alongside the experimental characterizations.
Results and Discussion
The prepared free-standing PBFDO films demonstrated strong mechanical robustness, maintaining integrity when handled below room temperature and even when bent in liquid nitrogen for a few minutes. However, thermal annealing at 200 °C for 5 minutes resulted in brittleness, causing fractures during bending, indicating that excessive thermal annealing reduces the mechanical robustness of PBFDO.
In tensile deformation tests at room temperature, PBFDO exhibited stiffness along with a degree of ductility. Cooling to -10 °C increased stiffness but made the polymer more brittle. SEM images of the yarn surface and freeze-fractured cross-sections showed no distinct outer shell layer, suggesting that PBFDO adhered to individual yarn filaments, with partial ingression into each.
Stress-strain curves showed that immersion in PBFDO ink decreased Young's modulus of the silk yarn while increasing strain at break. This allowed the PBFDO-coated yarn to endure cyclic stretching up to 5 % strain for 30 cycles without breaking, indicating that the yarn is well-suited for textile applications.
The electrical conductivity of the PBFDO-coated yarn remained stable over 14 months under ambient conditions, highlighting its remarkable durability. Additionally, both the thermoelectric button and generator maintained stable performance after 41 weeks and 10 days of storage, respectively, though the generator's efficiency dropped by 30 % after 90 days.
Conclusion
The researchers effectively demonstrated the processability of n-type PBFDO into free-standing films exhibiting high stiffness and ductility at room temperature. The multifilament yarn coated with this durable polymer showed stable electrical conductivity, remaining nearly unchanged over 14 months under ambient conditions. Even after seven washing cycles, conductivity only increased threefold, underscoring the yarn's exceptional long-term stability and washability.
The robustness and functionality of the n-type yarn were validated through two out-of-plane thermoelectric textile devices: a thermoelectric button and a textile thermopile. The thermopile's output power was sufficient to support sensors based on organic electrochemical transistors for monitoring electrophysiological signals. Thus, PBFDO-coated multifilament silk yarn shows considerable potential for developing reliable thermoelectric textiles.
Journal Reference
Craighero, M. (2024). Poly(benzodifurandione) Coated Silk Yarn for Thermoelectric Textiles. Advanced Science. DOI: 10.1002/advs.202406770, https://onlinelibrary.wiley.com/doi/10.1002/advs.202406770
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