Preparation And Application Of Lignin Containing Nanocellulose Fibril Based On γ-Valerolactone/Water System

Nanocellulose is a nanoscale polymer derived from natural biomass,which not only retains the characteristics of natural cellulose,but also has many advantages such as large specific surface area,excellent mechanical strength,good hydrophilicity and biocompatibility.However,most of the current research on nanocellulose uses bleached chemical pulp as raw material,which not only needs to consume a large amount of chemical raw materials,but also prevents the biomass resources from being fully utilized.In this thesis,the mechanical pulp was pretreated by a ternary GVL/water/acid system,and then nanocellulose fibrils（LCNF）with different lignin contents were prepared by ball milling method.Meanwhile,the effect of different lignin contents on the characteristics of LCNF were studied.Afterwards,LCNF was applied to polyacrylic acid（PAA）hydrogel to enhance its mechanical properties,and the PAA-LCNF composite hydrogels and sodium alginate/Ca²⁺/PAA/LCNF double network structure hydrogel with super-stretchable properties were fabricated respectively,which laid the foundation for the high-value application of biomass and the research of ultra-stretched hydrogels.The main research contents are as follows:（1）Preparation of LCNFs with different lignin contents.Firstly,the mechanical pulp was pretreated by using a ternary GVL/water/acid system,and the lignin removal rate was controlled by changing acid concentration and treatment time to acquire the pulp with different lignin contents.Subsequently,LCNFs with different lignin contents were produced by the mechanical grinding with ball mill,and the properties of the pretreated pulp and LCNF with different lignin contents were characterized by various testing means.The results showed that the lignin content of the pretreated pulp was 23.11%,18.18%,12.98%,7.44%and 2.25%,respectively,which indicated that the lignin removal effect became more obvious with increasing acid concentration and treatment time.The obtained LCNFs were named as LCNF-23%,LCNF-18%,LCNF-12%,LCNF-7%and LCNF-2%according to the difference of lignin content.SEM analysis revealed that the more drastic pretreatment conditions,the greater the degree of pulp fiber surface damage,the larger the surface area,and the smaller the diameter of the prepared LCNFs.XRD and FTIR results demonstrated that the functional groups and cellulose crystalline structure did not change during the preparation of LCNF,and it still retained the cellulose I structure.Also,the lower the lignin content in LCNF,the higher its crystallinity.When the lignin content was 18.18%,LCNF displayed the best hydrophobic and UV-shielding properties.（2）Preparation of LCNF reinforced polyacrylic acid composite hydrogel with super-stretchable performance.Briefly,LCNF with different lignin content was mixed with acrylic acid monomer,and then tetramethylethylenediamine（TEMED）and ammonium persulfate were used as cross-linker and initiator to prepare PAA-LCNF composite hydrogels by thermal initiation of free radical polymerization of acrylic acid in lignin-containing nanocellulose fibril suspension.The influences of LCNF with different lignin contents on various properties of hydrogels,including tensile and compressive properties,morphological structure,swelling behavior,UV-shielding ability and thermal stability,were investigated by diverse testing methods.The results showed that the addition of LCNF significantly improved the mechanical properties of PAA hydrogels.When the lignin content in LCNF was 18.18%,the composite hydrogels had the best tensile properties,with 3466%super-tensile strain and457 k Pa stress,which increased 124%tensile strain and 207 k Pa tensile stress compared with the pure PAA hydrogel.Moreover,the compressive stress of PAA-LCNF18%reached the maximum value of 679 k Pa,which was 479 k Pa higher than that of the pure PAA hydrogel.SEM images displayed LCNF could make the network structure of PAA hydrogel in the shape of regular honeycomb coal with uniform pore distribution,which was due to the cross-linking between PAA molecular chains and nanocellulose groups to generate a large number of hydrogen bonds after the introduction of LCNF.At the same time,the lignin in LCNF was dispersed in the PAA matrix,which played a certain spacing role.In addition,the addition of LCNF endowed PAA hydrogel with excellent UV-blocking capability,and the UV-shielding performance of the composite hydrogel was optimal when the lignin content was about 18%.（3）Preparation of sodium alginate reinforced PAA-LCNF hydrogels with double network（DN）and super-stretched performance.Based on the synthesis of PAA-LCNF hydrogel,various loadings of sodium alginate were introduced into the PAA-LCNF 18%hydrogel system,and then physically cross-linked with Ca²⁺to fabricate double-network hydrogel.The effects of sodium alginate in different contents on various properties of the double-network hydrogels,such as tensile,compression,SEM,FTIR,and conductivity,were investigated by various testing methods.The FTIR results revealed that the broad peak strength of DN hydrogels increased significantly at 3650～3100 cm^-1,and the contour was more obvious.This indicates the formation of double-network hydrogels,because in addition to the hydroxyl group on the surface of PAA and the-OH on the surface of LCNF to form hydrogen bonds,the crosslinking of sodium alginate and calcium ions also produces a large number of hydrogen bonds,which increases the intensity of the absorption peak.The results showed that the introduction of sodium alginate-Ca²⁺network could significantly enhance the mechanical properties of the DN hydrogels.When sodium alginate was added at 0.4 wt%,the elongation at break and tensile strength of the hydrogel were 5293%and 363 k Pa,respectively.Compared with the PAA-LCNF18%single network hydrogel,the elongation at break of the DN hydrogels increased by 1827%and the tensile strength decreased by 94 k Pa.Although the tensile strength was reduced,the DN-0.4%sodium alginate hydrogel at 300%strain of each cycle was greater than that of the PAA-LCNF 18%hydrogel.Furthermore,the maximum compressive stress of DN-0.4%sodium alginate hydrogel was 594 k Pa at 80%strain.SEM analysis indicated that the addition of sodium alginate significantly reduced the pore size of the DN hydrogel.When the sodium alginate content was less than 0.5 wt%,the pore of the DN hydrogel became smaller and the network structure became denser with the increase of sodium alginate content.But,at the sodium alginate content of 0.6 wt%,agglomeration appeared in the DN hydrogel,and the hydrogel network was not uniform and the pore size became larger.In addition,the as-prepared DN hydrogels possessed good conductivity and could sensitively sense the change of electrical signal,which had a great application potential for biosensing filed.