Optimizing the performance of the near-infrared (NIR) photothermal conversion via modulating the domain size of the chiral nematic phase in co-assembled cellulose nanocrystal composite films

Abstract

Near-infrared (NIR) photothermal materials based on conjugated polymers have shown great potential in energy storage, therapeutics, and diagnosis. Optimizing the dispersion of conductive polymers in these composites is an effective pathway to achieve better performance for the NIR photothermal conversion. Herein, we first show that the nematic liquid crystal structure of cellulose nanocrystals via self-assembly can be used as a skeleton to optimize the distribution of conjugated polymers in NIR composites. To be specific, we demonstrate a composite composed of poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), cellulose nanocrystals (CNCs), oxidized starch (OS), and tannic acid (TA), in which the dense compaction of small domains of CNCs’ chiral nematic structure allows the formation of an efficient scaffold for the phase-segregation of PEDOT, leading to a significantly improved efficiency in photothermal conversion. The composite with a domain size of the chiral nematic structure of 2 μm (5.77 wt% of PEDOT) presents the largest surface temperature change (115 °C) under 750 nm laser irradiation (1.5 W) within 20 s and the highest temperature coefficient of resistance of −1.285% °C⁻¹ in a temperature range from 20 to 100 °C, which is superior to many other reported photothermal systems. In addition, its photothermal conversion efficiency (η_PT) reaches 77.6%, which is even higher than that of the pure PEDOT:PSS film (68.7%). The enhanced photothermal conversion of CNC-based materials can be integrated with their chiral-optical property for developing unique multifunctional sensors.