| With the rapid development of world economy,depletion of petroleum-based resources and the environmental problem,searching for environmental bio-based fuel and bio-based materials has drawn extensive attention.Hemicelluloses is one of the most important components in lignocellulosic biomass which have the advantages of low price,abundance,renewable and non-toxic.However,the low degree of polymerization（DP）,complicated structure and poor solubility limited the utilization of hemicelluloses and its derivatives.In this work,a serious of studies had been done on crosslinking,esterification,oxidation and reductive amination of xylan-type hemicelluloses in order to improve their amphiphilic properties.The potential fine chemicals applications of hemicelluloses and its derivatives as adhesives and emulsifiers were also investigated.Such investigations will provide the theoretical basis and technical support for the high value utilization of hemicelluloses,and point to a new trend for better and more effective utilization of lignocellulosic biomass.（1）The modification of xylan by cross-linking with glutaraldehyde（GA）was performed at room temperature.As GA was a dialdehyde crosslinking agent,the side reaction may occur during the process of modification.Using degree of substitution（DS）to present the degree of cross-linking was found to be problematic.To better evaluate the degree of cross-linking,the CI（cross-linking index）was used to reflect the degree of cross-linking in this study.The effects of GA contents and xylan concentrations on CI were investigated.With the molar ration of GA/xylose units（XU）was 0.3 and the concentration of xylan was 0.25%,GAX with a CI of0.98（GAX-0.98）were obtained.The effects of CI on the rheological properties of GAX were investigated.The viscosity of GAX solutions increased when CI increased.GAX-0.98 solutions showed similar viscosity compared to that of the xylan solutions.With the increasing of CI,the mechanical properties of GAX films decreased.GAX with high CI showed poor film properties and coating properties compared to that of GAX with low CI.The tensile strength of low CI GAX films increased by approximately 60%compared to that of higher CI GAX films,the tensile strength of the low CI GAX-coated paper increased by approximately 15%compared to that of higher CI GAX-coated paper.When used as coating adhesives,GAX-0.98 showed comparable coating properties to that of polyvinyl alcohol（PVA）,which demonstrated its potential to substitute petroleum-based paper coating adhesives.（2）The esterification of xylan with long-chain succinic anhydride was carried out in water.The effects of dodecenyl succinic anhydride（DDSA）dosage on the DS of DDSA modified xylan were investigated.When the DDSA/xylose units（XU）molar ratio increased from 0.05to 3,the DS of DDSA modified xylan increased from 0.014 to 0.168.The effects of DS,alkenyl chain length and pH on emulsifying properties of long-chain succinic anhydride modified xylan were investigated.When DS increased from 0.014 to 0.09,the emulsifying properties of DDSA modified xylan were improved,however,when DS increased beyond 0.09,the effects of DS on emulsifying properties of DDSA modified xylan were no more significant.DDSA modified xylan showed poor emulsifying properties at pH=4.5,while showing better emulsifying properties at pH=6.5 and pH=8.5.At the same DS（0.09）,the effects of alkenyl chain length on emulsifying properties of DDSA modified xylan were evaluated.When the alkenyl chain length increased from C8 to C16,the droplet size decreased from 2.15μm to 0.75μm,the zeta potential decreased from-25.6 mV to-32.2 mV,the emulsifying activity（EA）increased from 0.443 to0.914,the emulsion stability was also improved.Furthermore,when the alkenyl chain length increased beyond C12,increasing the alkenyl chain length had not significantly impact on the emulsifying properties.The long-chain succinic anhydride modified xylan had the potential to substitute nature emulsifiers and petroleum-based emulsifiers in many applications,while the DS,alkenyl chain and pH should be kept at a certain value.（3）Different molecular weight xylan samples（noted as:A,B,C,D）were obtained by xylanases enzyme hydrolysis.The esterification of xylan（A,B,C,D）with DDSA were carried out at room temperature.The effects of DS on the emulsifying properties of DDSA modified xylan were investigated.When DS increased,the emulsifying properties initially became better,and then flatted off.At the same DS,the effects of molecular weight on emulsifying properties of DDSA modified xylan were investigated.With the increasing of molecular weight,the droplet size and zeta potential of the prepared emulsion with DDSA-modified xylan decreased,the EA increased and the emulsion stability were improved.The stability mechanisms of DDSA modified xylan emulsions were investigated.When the molecular weight of DDSA modified xylan was similar,the differences caused by steric hindrance were not significant.The stability of emulsion was mainly maintained by electrostatic repulsion.Furthermore,when the molecular weight increased,the steric hindrance of the particles increased.The stability of emulsion was maintained by both the steric hindrance and electrostatic repulsion.（4）DDSA-modified xylan with high DS（0.0440.328）were synthesized by a homogeneous esterification with DDSA in dimethyl sulfoxide（DMSO）.The effects of DS on emulsifying properties of DDSA modified xylan were investigated.The emulsifying properties of DDSA modified xylan were improved when the DS increased.The effects of pH on emulsifying properties of DDSA modified xylan were also investigated.At pH=4.5,DDSA modified xylan showed poor emulsifying properties,while at pH=6.5 and pH=8.5,DDSA modified xylan showed similar emulsifying properties.Furthermore,when DS increased beyond 0.224,the effects of pH on the emulsifying properties of high DS DDSA modified xylan were much smaller compared to that of low DS DDSA modified xylan..The effects of solvents systems on emulsifying properties of DDSA modified xylan at the same DS（0.044±0.001）were also investigated.The DS of DDSA modified xylan prepared in water systemn was smaller than that of DDSA modified xylan prepared in DMSO system.The emulsions prepared by DDSA modified xylan which were synthesized in water had a droplet size of 1.3μm,a zeta potential of-25.9 mV and a EA of 0.684,while the emulsions prepared by DDSA modified xylan which were synthesized in DMSO system only had a droplet size of 2.65μm,a zeta potential of-15.6mV and a EA of 0.635.The emulsion papered by DDSA modified xylan which was synthesized in water system showed better emulsion stability.These results suggested that DDSA modified xylan prepared in water system had better emulsifying properties.（5）The dialdehyde xylan（DAX）with various aldehyde groups contents（noted as DAX-1,DAX-2,DAX-3 and DAX-4）were prepared by periodate oxidation.The reductive amination of DAX was accomplished by reductive amination with long-chain alkylamine（octylamine,dodecylamine,hexadecylamine）at room temperature.The borane-2-picoline complex（pic-BH3）were selected as reducing agent.The emulsifying properties of reductive aminated DAX were evaluated.When the aldehyde contents increased,the contents of alkyl groups increased,which contributed to better emulsifying properties.The droplet size and zeta potential of emulsion prepared by dodecylamine modified DAX-4 decreased by 80.21%and 98.35%,while the EA increased by 275.86%compared to that of dodecylamine modified DAX-1 emulsions.When the alkyl chain increased from C8 to C16,the emulsifying properties of reductive aminated DAX were improved.Furthermore,when the alkyl chain increased beyond C12,the effects of alkyl chain length on emulsifying properties were not evident.The reductive aminated DAX showed potential to partly substitute petroleum-based emulsifiers such as Tween-20,Tween-60and Tween-80 in many applications. |
