The diverse uses of papers have expanded from simple material for recording information to lightweight and flexible materials in portable electronics. Paper is made from cellulose, which is the most available natural polymer. The incorporation of electrically conducting fillers has made paper exhibit sensitive resistive change to a number of stimuli, including mechanical deformations, temperature shifts, and the presence of gases and liquids. These smart materials are very attractive in the fields of packaging, healthcare, military, and sportswear. Carbon nanotubes are filler materials that have drawn significant attention due to their high electrical conductivity, thermos stability, and mechanical modulus.
The process of preparing smart papers takes two different procedures. For the first one, a conductive carbon nanotube layer is coated on the external surface of a paper sample. In the second method, carbon nanotubes are mixed with cellulose fibers in the papermaking process to produce a percolated carbon nanotube network with electric paths running throughout the entire bulk of the paper. Both processes are simple to implement and cost effective, but the latter has higher potential for sensing applications owing to better interactions between the carbon nanotubes and the cellulose fibers.
Fiber inter-cohesion of cellulose is composed of hydrogen bonding. Therefore, the introduction of carbon nanotubes is faced with the challenge that addition of carbon without hydroxyl groups to the pulp limits the interaction between cellulose fibers. This greatly affects the mechanical integrity.
Researchers led by Professor Anthony Dichiara at the University of Washington, Seattle, United States in collaboration with Professor Bai at University Paris-Saclay in France demonstrated a considerable improvement in the paper’s strength characteristics despite a high carbon nanotube loading, about 10 wt%. Their research work is now published in Journal of Material Chemistry A.
The authors made paper hand sheets with unbleached Kraft softwood pulp, which was modified by a layer-by-layer nano assembly method, and comprised a subsequent coating of cationic polyacrylamide and hydroxyl-functionalized carbon nanotube films on the cellulose microfibers. By pre-adsorbing the carbon nanotubes with alkali lignin, the second most abundant natural polymer after cellulose, they were able to achieve up to 10 wt% CNT loading without obvious aggregation, which is remarkable for such a hydrophobic filler in a water-based process. They studied the mechanical features of the as-produced papers together with their performance as liquid water detector and as strain-sensors.
The as-produced papers were flexible, light weight, and indicated enhanced wet and dry strength features that could be referenced to the improved interfacial interactions between the cellulose and functionalized carbon nanotubes, which originated from hydrogen bonding. The embedded carbon nanotubes formed an electrically conductive network, which made the smart papers sensitive to a number of external stimuli.
The as-produced papers were implemented as highly sensitive reproducible detectors for the presence of water in various systems. “Water sensing is very challenging to do due to the polar nature of water, and current systems are either expensive or not practical to implement,” said lead author Anthony Dichiara, a UW assistant professor of bioresource science and engineering in the School of Environment and Forest Sciences. These materials are so sensitive that they can also detect trace amounts of water in mixtures of various liquids, which is molecules is particularly valuable for the fuel industry, where water is commonly regarded as impurity. In addition, their high tensile strain sensitivity as opposed to the typical foil gauges makes the as-produced papers attractive for a number of applications ranging from remote detection of water leakages to actuators and water content determination.
The findings of the study will be significant in the fabrication of other renewable materials for multifunctional applications.
A. B. Dichiara, A. Song, S. M. Goodman, D. He and J. Bai. Smart papers comprising carbon nanotubes and cellulose microfibers for multifunctional sensing applications. Journal of Material Chemistry A, volume 5 (2017), 20161.
Go To Journal of Materials Chemistry A