Preparation of cellulose nanosheets/poly (acrylic acid) hydrogel and its application in sensor field

Cellulose is a kind of environmentally friendly natural polymer in the world, which has attracted much attention due to its biodegradability and biocompatibility, but its insoluble and refractory characteristics greatly limit its development and application. With the rise of nanoscience and nanotechnology, the magical properties of nanoscale materials constantly refresh people's understanding in the micron/nanometer field. The concepts of micron size and nanometer size have appeared in various fields of natural science, including natural polymers. The preparation and modification of nanocellulose have become a research hotspot in recent 20 years. Nanocellulose, one of the most promising new materials in the future, can be obtained by mechanical, chemical or other methods of reducing the size of any dimension of cellulose to 100 nm. It has low density, high strength, biocompatibility and low thermal expansion coefficient, which can be applied to many fields, such as material reinforcement, preparation of transparent paper, hydrogel, aerogel and other functional products.In this work, the effect of oxidation process on weak bond interaction between cellulose molecular chains and structure of cellulose by mechanical milling in Fenton reagent with different concentration and different ball milling time were studied. Additionally, the cellulose nanosheets/poly (acrylic acid) hydrogel was prepared, and the electrical conductivity, strain sensing ability and its application in human motion are explored.1. The effect of oxidation process on weak bond interaction between cellulose molecular chains and structure of cellulose by mechanical millingThe morphology and size of nanocellulose were regulated by controlling the concentration of Fenton reagent and mechanical milling time. The mechanism of cellulose peeling into nanosheets under the synergistic effect of oxidation process and mechanical force is proposed: The hydroxyl radicals produced by Fenton reagent attacked and captured H linked to methylene and methyne on cellulose, and then the hydroxyl radicals transferred to form C free radicals on cellulose chains. The weak bond interaction (Van der Waals forces) between cellulose chains was weakened, and the cellulose peeled into nanosheets under mechanical milling. With the increase of ball milling time, the thickness of cellulose nanosheets did not change significantly, but the size decreased from 2 mm to 20 nm. And its crystalline form was still cellulose I, crystallinity index decreased, while the chemical structure of cellulose did not change. The contact angle of cellulose milled in 0.25 M Fenton reagent with 10 h milling time was 96°, indicating that the hydrophobic surface of cellulose was exposed during milling in Fenton reagent. However, cellulose fibers were obtained without Fenton reagent under the action of mechanical force, cellulose nanofibers were still not completely obtained when the milling time prolonged to 18 h.2. Study on preparation and properties of cellulose nanosheets/poly (acrylic acid) hydrogelsCellulose nanosheets/poly (acrylic acid) hydrogels with excellent mechanical properties and viscous properties were prepared by in-situ free radical polymerization. The effects of the content of cellulose nanosheets and the concentration of Fe3+ on the mechanical properties of the hydrogels were studied. Compared with pure poly (acrylic acid) hydrogels, the mechanical properties greatly improved with the addition of cellulose nanosheets and Fe3+. Best mechanical properties of cellulose nanosheets/poly (acrylic acid) hydrogels with 14% cellulose nanosheets and 2.50 mol‰ Fe3+ content were achieved, its elongation at break and the fracture energy were 1800% and 15500 J/m2, respectively. However, the mechanical properties of hydrogels will be decreased to some extent with the further increase of cellulose nanosheets and Fe3+ content.3. Study and application of cellulose nanosheets/poly (acrylic acid) hydrogels as strain sensorThe strain sensors with excellent conductivity and self-adherence were assembled using Cel14/PAA-Fe3+ 2.50 hydrogel as conductor, its relative resistance changes increased with the increase of tensile strain, the relative resistance changes of Cel14/PAA-Fe3+ 2.50 hydrogel was found to be 6287% at 1100% strain. Moreover, the relative resistance changes and gauge factor of the sensor decreased with the increase of its width, so the sensitivity of the sensor can be regulated by its width. In addition, the sensitivity of the strain sensor has not decreased significantly after 500 cycles under small strain (20%), medium strain (100%) and large strain (400%), and it also can be monitor human motion in real time. Not only provided research ideas for wearable strain sensor with large strain range and high sensitivity, but showed its application prospects in the field of human motion monitoring