| Lignocellulose,the world's most abundant biopolymer,is widely present in woody and herbaceous plants.Chitin is the second most abundant polysaccharide after cellulose.However,the chitinous biomass,mainly shrimp and crab shells,is usually discarded as industrial and domestic waste without effective utilization.The complex structure of lignocellulosic and chitinous biomass,along with the inaccessibility of the semi-crystalline cellulose and chitin polymer chains,results in their high recalcitrance to chemical processing.Thus,it will be of positive significance to separate the main biopolymers from the biomass and transform the biopolymers into high-performance materials for the purpose of relieving the resource starvation crisis.The present dissertation focused on the preparation of functional hydrogel and aerogel products from biomass(poplar wood and shrimp shells)and biopolymers(lignocellulose and chitin)using different solvent systems,crosslinking methods and formation patterns based on the biomass separation technologies with ionic liquid.The potential applications of the products in medical industry,food packaging and environment remediation were explored.We first studied the effect of various ionic liquid dissolution methods on the yield and purity of the extracted biopolymers-lignocellulose and chitin from lignocellulosic and chitinous biomass(poplar wood and shrimp shells),respectively.Then,through "dissolution-coagulation" or"dissolution-curing" process,the cellulose or chitin solutions in ionic liquid or NaOH/urea solvents were reconstituted into hydrogels.The adsorption capacity of the functionalized lignocellulosic hydrogel beads twords heavy metal ions were determined and compared with the pure cellulosic beads.Using another biopolymer produced during ionic liquid processing of wood-lignin,we fabricated a novel,high-performance lignin-based hydrogel by crosslinking lignin polymers with an unique crosslinker.The main contents and results can be summarized as follows:(1)The imidazolium acetate ionic liquid effectively extracted lignocellulose and lignin from poplar wood.The optimum dissolution methods include polyoxometalate catalytic oxidation and high-temperature dissolution,which guarantee the purity and the yield of the extracted biopolymers,respectively.Chitin could be extracted from shrimp shells within 5 min under microwave irradiation.The ionic liquid was well recycled,and maintained good dissolution ability during the second processing.(2)Bulk hydrogels and membrane alcogels were prepared from biomass-and biopolymer-ionic liquid solutions by coagulation in the mold with water or on a glass plate with ethanol,respectively.Gel forming ability of the biopolymers were determined by their molecular weight(MW)and purity.The higher the MW and purity,the higher the formability.After supercritical CO2 dyring,bulk and membrane aerogels were obtained.Membranes were also air-dried to obtain dense films.Cellulosic aerogel membranes were firm and foldable,whereas chitinous aerogel membranes were too fragile and brittle.Aerogels were able to rehydrate,but the masses and volumes couldn't recover back to the original state.The rehydrated hydrogels had higher mechanical strength and good elastoplasticity,which are thus promising materials as carriers for active substances or bio-adsorbents for pollutants.The air-dried films had high tensile strength,low water vapor transmission(WVT),and thus were potential food packaging materials.Cellulosic aerogel membranes possessed lower tensile strength,higher WVT and antioxidant activity than the air-dried films,being promising wound dressings for dry to lightly exudating wounds.(3)Physically and chemically crosslinked hydrogels and aerogels were prepared from biopolymer/NaOH/urea solutions by thermal curing.Cellulose and chitin of high MWs could form both physical and chemical hydrogels,while cellulose and chitin of lower MWs only formed chemical hydrogels.Cellulosic chemical hydrogels were transparent or semi-transparent,and the transparency increased with purity of the biopolymer and swelling of the hydrogel.The aerogels were highly rehydrated,especially for the chemical ones with low biopolymer MW and concentration.Active species(dye molecules)were loaded in the aerogels and released within 3-24 h.The higher the biopolymer concentration,the higher the loading amount and the slower the release rate.In addition,there were two rules about the volume shrinking during supercritical CO2 drying and the density and porosity of the resulting aerogels:a)all physical hydrogels based on biopolymers of high purity shrink slightly during supercritical drying,and the shrinkage increases with the increase of impurities;highly swollen chemical hydrogels shrink severer than physical ones.b)the density and porosity of the aerogels were related to the biopolymer concentration and the shrinkage during drying.The lower the concentration and shrinkage,the higher the porosity.(4)Since the high-temperature dissolution method produced high MW cellulose-rich material(CRM),we used this CRM to prepare functionalized(magnetized and carboxymethylated)lignocellulosic hydrogel beads for the removal of heavy metal ions(Cu2+and Pb2+)from water resources.Compared to the Kraft pulp(KP)and microcrystallime cellulose(MCC)beads,and to the beads made directly from wood-IL solution,CRM beads had higher specific surface area,higher number of hydroxyl groups,lower density,higher porosity,more ordered pore structure,and higher Fe3O4 content and degree of substitution after magnetization and carboxymethylation,respectively.Combining both functionalization(magnetization followed by carboxymethylation),the resulting carboxymethylated magnetic CRM beads had excellent adsorption ability towards Cu2+ and Pb2+ ions-percentage metal removal increased from 5.5%to 90%with the increase in the adsorbent dose(0.004-0.09 g dry weight/100mL solution)in the 0.1 mM solution,and the maximum Langmuir adsorption capacity for copper and lead was 0.144 mmol g-1(9.15 mg g-1)and 0.161 mmol g-1(33.36 mg g-1),respectively.(5)Ionic-liquid-isolated(IL-)lignin and Kraft(KF-)lignin were crosslinked with epoxide-terminated polyethylene glycol(ETPEG),forming lignin/ETPEG hydrogels in both bulk and membrane forms.These lignin/ETPEG hydrogels were air-dried,and showed excellent properties including good water absorbency,antioxidant efficiency,antimicrobial activity and reusability,suggesting the potential in applications including active species delivery,food packaging,and wound dressing,etc.The shrinkage of the lignin hydrogels during air-drying was related to the thickness of the hydrogels-the higher the thickness,the higher the shrinkage,and thus the lower the porosity and WVT of the dried gels,but the water uptake ability increased with thickness.Compared to the KF-lignin,IL-lignin hydrogels had three advantages:a)the concentration of IL-lignin solution required for hydrogel formation was lower,i.e.,gel forming ability of IL-lignin was better;b)IL-lignin/ETPEG hydrogels were more stable and strong,less brittle,and able to repeat the "drying-rehydration" cycles,thus facilitating preparation and usage;c)IL-lignin/ETPEG hydrogels had lower thickness,and thus higher WVT and antioxidant efficiency. |
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