Study On Preparation,Structure,and Properties Of Cellulose-Based Smart Gels

Cellulose is the most abundant biomass material.Due to its renewability,biocompatibility,nontoxicity,and degradability,cellulose and its derivatives are not only widely used in conventional industrial fields,but also have many applications in various areas such as tissue engineering scaffold,drug delivery systems and wearable sensors.Driven by engineering application requirements,smart gel materials based on cellulose and its derivatives have become a hot research topic at present.This thesis studied on preparation,structure and properties of cellulose based smart gels.First,hydrophobic cellulose aerogel（HCA）was fabricated and the ability of its bubble adsorption underwater was researched.Second,cellulose nanofibers were prepared and used to enhance mechanical properties of polyacrylamide（PAM）/Gelatin hydrogel.Third,a novel multivalent cations-triggered shape memory hydrogel was synthesized in a one-pot method.At last,sodium carboxymethyl cellulose（CMC）/polyacrylic acid（PAA）self-healing hydrogel was synthesized and soaked in saturated sodium chloride solution to further improve its mechanical properties.A cellulose aerogel（CA）with porous structure and large surface area was fabricated via combining a dry pulp disintegration process with freeze-drying.The CA became superhydrophobic after it was modified with methyltrimethoxysilane（MTMS）.FT-IR,XRD,EDX,and SEM were used to characterize the cellulose aerogel before and after modification.The silylation hydrophobic modification of cellulose aerogel was confirmed.The surface wettability of the MTMS-modified aerogels was evaluated by contact angle value.Both water droplet and oil droplet could penetrate into unmodified CA once they were applied onto the cellulose aerogel.After treatment with MTMS,HCA was only wetted by the oil droplet and repelled by the water droplet.These phenomena indicate the hydrophobic and oleophilic properties of HCA.The HCA was able to effectively collect and store transport methane bubbles under water.It exhibited exceptional performance for collecting methane bubbles under both static and dynamic absorption-release conditions.We found the cellulose concentration and submersion depth of the HCA impacted on the collection efficiency of methane bubble.With a decrease of cellulose concentration or an increase of submersion depth,the capacity of HCA to trap methane volume increased linearly.Cellulose nanofibers were isolated through TEMPO-oxidation pre-treatment followed by mechanical disintegration.And then a series kinds of shape memory TOCN/PAM/Gelatin hydrogels with varying TOCNs and gelatin contents were prepared through a facile in-situ free-radical polymerization method.After the polymerization,TOCNs,gelatin,and PAM were distributed homogeneously in the hydrogels.Importantly,there was no new covalent bond formed between gelatin,PAM,and TOCNs.As to the shape memory properties of TOCN/PAM/Gelatin composite hydrogels,the reversible nature of the gelatin network imparts the TOCN/PAM/Gelatin compositehydrogels with fast cooling-fixed（ice water for 30s）and thermo-induced shape recovery（in 90℃water for 5 s）capability.Good mechanical properties（strength>200 KPa,strain>650%）were achieved.As TOCNs content increased,the maximum tensile stress（s_b）,and elastic modulus（E）of the DN gels consistently increased.It can reach 91 KPa and 20.8 KPa,respectively.The introduction of gelatin contributed to increases inthe elastic modulus and tensile strength,further revealing the enhanced compatibility of the gelatin and the TOCN/PAM system.Such a composite hydrogel with good shape memory behavior and enhanced mechanical strengths would be an attractive candidate for a wide variety of applications.A novel multivalent cations-triggered shape memory hydrogel was synthesized in a one-pot method,and interpenetrating double network was formed by chemically cross-linked PAM network and physically cross-linked sodium carboxymethyl cellulose network.The temporary shape was fixed by complexation between a native biopolymer,CMC,and transition metal ions,specifically Fe³⁺,Ag⁺,Al³⁺,Cu²⁺,Ni²⁺,and Mg²⁺.In particular,CMC-Fe³⁺hydrogel exhibits excellent shape fixity ratio（95%）.The chelation of CMC and Fe³⁺could be applied as temporary crosslinks to stabilize the deformed shape of the hydrogel and contribute to the excellent shape memory effect in memorizing a macro-scale shape and a QR pattern.Therefore,we chose PAM/CMC_1.0-Fe³⁺hydrogel as the model material and further investigated its shape recovery process.It was found that a wide range of molecules and anions could be applied to break off the temporary cross-links between CMC and Fe³⁺.The PAM/CMC composite hydrogels also exhibited excellent tunable mechanical properties.The mechanical properties of the composite hydrogel can be adjusted by changing the cross-linking densities.The presented strategy could enrich the construction as well as application of biopolymers-based shape memory hydrogels.Finally,CMC/PAA hydrogel with excellent mechanical and self-healing properties was synthesized.The carboxyl group in the CMC molecule can complexation with metal ions,and play the role of energy dissipation when hydrogel deforms under the action of external forces,which helps to improve the strength of hydrogel.By simply soaking the CMC/PAA hydrogel in sodium chloride（NaCl）solution,the chain entanglement between CMC and PAA can be promoted,and the tensile stress can reach 1.94 MPa,which is 12.1 times of single PAA hydrogel.The self-healing property of CMC/PAA hydrogel was investigated,and we found that CMC_1.0/PAA hydrogel could lift a weight of 100 g without breaking,and its fracture stress could be restored to 1.83 MPa.CMC/PAA hydrogels with different CMC contents showed good self-healing performance,and the self-healing efficiency was more than 80%.In addition,due to the ionic property,CMC_1.0/PAA hydrogel has good conductivity,which is expected to be applied in electronic skin or wearable devices.