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

纤维素是一种全球范围内广受关注的环保天然高分子，因其可生物降解性与生物相容性而备受瞩目，但它难溶、难加工的特性极大限制了其发展与应用。随着纳米科学与纳米技术的兴起，纳米尺度材料的神奇特性不断刷新人们在微米/纳米领域的认知，微米、纳米尺度的概念已渗透到包括天然高分子在内的自然科学各领域。近二十年来，纳米纤维素的制备与改性成为研究热点。纳米纤维素作为未来最具潜力的新型材料之一，可通过机械、化学或其他方法将纤维素任意维度的尺寸降至100 nm，具备低密度、高强度、生物相容性佳、热膨胀系数低等优势，可应用于材料增强、透明纸制备、水凝胶、气凝胶等诸多功能产品领域。

本工作研究了不同浓度芬顿（Fenton reagent）试剂体系下，机械球磨氧化工艺对纤维素分子链间弱键相互作用及纤维素结构的影响。此外，制备了纤维素纳米片/聚丙烯酸（poly(acrylic acid)）水凝胶，并探究了其导电性能、应变传感能力及在人体运动监测中的应用。

1. 氧化工艺对球磨过程中纤维素分子链弱键相互作用及结构的影响
通过调控芬顿试剂浓度与机械球磨时间，实现对纳米纤维素形貌与尺寸的精准调控。本研究提出了氧化工艺与机械力协同作用下纤维素剥离为纳米片的机理：芬顿试剂产生的羟基自由基攻击并夺取纤维素上与亚甲基、次甲基相连的氢原子，随后自由基发生转移，在纤维素分子链上形成碳自由基，削弱了纤维素链间的弱键相互作用（范德华力（Van der Waals forces）），最终在机械球磨的作用下，纤维素剥离为纳米片。随着球磨时间延长，纤维素纳米片的厚度无显著变化，但尺寸从2 mm降至20 nm；其晶型仍为纤维素I（Cellulose I），结晶度指数下降，但纤维素的化学结构未发生改变。在0.25 M芬顿试剂中球磨10 h的纤维素接触角为96°，表明在芬顿试剂中球磨过程中暴露了纤维素的疏水表面。而未添加芬顿试剂仅受机械力作用时，仅能获得纤维素纤维；即便将球磨时间延长至18 h，仍无法完全得到纤维素纳米纤维。

2. 纤维素纳米片/聚丙烯酸水凝胶的制备与性能研究
通过原位自由基聚合法制备了兼具优异力学性能与黏附性能的纤维素纳米片/聚丙烯酸水凝胶。研究了纤维素纳米片含量与Fe³+浓度对水凝胶力学性能的影响。与纯聚丙烯酸水凝胶相比，添加纤维素纳米片与Fe³+后，水凝胶的力学性能得到显著提升。当纤维素纳米片含量为14%、Fe³+浓度为2.50 mol‰时，纤维素纳米片/聚丙烯酸水凝胶的力学性能最优，其断裂伸长率与断裂能分别可达1800%与15500 J/m²。但若进一步提高纤维素纳米片与Fe³+的含量，水凝胶的力学性能会出现一定程度的下降。

3. 纤维素纳米片/聚丙烯酸水凝胶作为应变传感器的研究与应用
以Cel14/PAA-Fe3+ 2.50水凝胶为导电介质，组装得到兼具优异导电性与自黏附性能的应变传感器。该传感器的相对电阻变化率随拉伸应变的增大而升高，在1100%应变下，Cel14/PAA-Fe3+ 2.50水凝胶的相对电阻变化率可达6287%。此外，传感器的相对电阻变化率与灵敏系数（Gauge Factor）随传感器宽度的增加而降低，因此可通过调整传感器宽度来调控其灵敏度。此外，该应变传感器在小应变（20%）、中应变（100%）与大应变（400%）下经过500次循环后，灵敏度未出现明显下降，且可实时监测人体运动。本研究不仅为宽应变范围、高灵敏度的可穿戴应变传感器提供了研究思路，同时在人体运动监测领域展现出良好的应用前景。