Green Extraction Of Hemicellulose And Its Application In Conductive Gels

Hemicellulose,as one of the main components of lignocellulosic raw materials,has the characteristics of abundant reserves,biodegradable,renewable,and excellent biocompatibility.However,the utilization rate of hemicellulose is obviously lower than that of cellulose and lignin for a long time.On the one hand,it is difficult to separate and purify hemicellulose,and the current separation methods have some problems such as environmentally unfriendly,expensive,and poor economic applicability.On the other hand,hemicellulose possesses complex molecular structure and poor water solubility,and hard to form a membrane structure,which greatly restrict their development and utilization.In order to solve the above problems,this article had conducted exploratory researches on the efficient extraction of hemicellulose,hemicellulose structure,hemicellulose dissolution and its high value-added application.To solve the aforementioned problems,the systematic research about the hemicellulose including efficient extraction method,structure,dissolution behavior and high value-added application were carried out in this paperFirstly,freeze-thaw cycle pretreatment combined with hot water hydrolysis was proposed to extract the hemicellulose from wood chips,which was simple,efficient,low energy consumption,and chemical free.The results showed that after four freeze-thaw cycles at-20℃followed by hot water extraction at 170℃for 1 h,the extraction rate of hemicellulose reached the maximum of 85.87 mg·g^-1,which was 32.25%higher than that of direct hot water extraction;In addition,the yield of monosaccharide gradually increased with the increase of freeze-thaw times,the yield of monosaccharide in hydrolysis liquor was 27.40%after freeze-thaw cycle treatment at-20℃for 6 times（based on the extracted hemicellulose）,which was 23.1%higher than that of direct hot water extraction..Also,SEM results confirmed that the frost heaving effect caused by freeze-thaw cycle pretreatment endows the poplar chips and cell wall with cracks and irregular micro morphology,and also provided more small fragments for the wood chips and fiber cell wall.The composition and structure of hemicelluloses extracted from polar chips by freeze-thaw cycle combined with hot water treatment were studied and compared with other four hemicelluloses,and their water solubility and film-forming properties were also evaluated.The results showed that extraction method had a great influence on the components of hemicellulose,the content of glucose in hemicellulose extracted by hydrothermal treatment was relatively high,about 7.34%（relative to the extracted hemicellulose）,and the average molecular weight of the obtained hemicellulose was low,the number average molecular weight（Mn）and xylan content were 2031 g/mol and 68.79%,respectively The molecular branched degree of hemicellulose extracted by sodium hydroxide extraction was 1.58%higher than that of hot water extraction,and the number average molecular weight was 27500 g/mol.Meanwhile,the content of xylan reach 78.96%（relative to the extracted hemicellulose）.The content of xylan in three commercial hemicelluloses was significantly higher than that of the poplar hemicellulose,Among them,the content of beech wood hemicellulose xylan was highest of 91.04%,and the content of arabinose and uronic acid were 1.91%and 3.09%,respectively,indicating that the branches chain of xylan in beech wood hemicellulose was mainly existing in 4-o-methylglucuronic acid;The content of arabinose and glycolic acid in corn cob were 5.95%and1.75%,respectively,indicating that arabinose was the main branch chain of hemicellulose xylan in corncob.The molecular weight and branched degree were the important factors that affect the water solubility and film formation of hemicellulose,and the higher the branched degree,the better the water solubility and the easier the film formation.The feasibility of eutectic solvent as a green solvent for hemicelluloses was explored to overcome the water-insoluble problem of hemicellulose.The results indicated that the solubility of xylan in choline chloride:ethanolamine（Chcl:E）,Choline chloride:urea（Chcl:U）,band choline chloride:glycerol（Chcl:G）was as follows Chcl:E>Chcl:U>Chcl:G at the same temperature.The solubility of xylan in Chcl:E achieved the maximum at 90°C,307.81 mg·g^-1.In Chcl:U and Chcl:G,the solubility of xylan increased with the increase of the temperature,296.24 and 52.47 mg·g^-1 DES at 110°C,respectively.¹H-NMR and ¹³C-NMR showed that the hydroxyl hydrogen of xylan can form hydrogen bonds with N6,N8,O9,O10 and Cl^-in DES,producing a stable hydrogen bond donor receptor complex system.Meanwhile,xylan,acted as hydrogen bond acceptor,its hydroxyl oxygen atoms can combine with N6-H,N8-H,O9-H,and O10-H of DES and form hydrogen bonds,and finally another form of hydrogen bond acceptor donor stable complex system was formed.Additionally,the presence of choline chloride can promote the hydrogen bond formation of DES.Xylan/Polyacrylamide（PAM）/DES ionic gel was prepared by free radical polymerization via introducing xylan to DES（Chcl:U G=1:2:0.15）solvent due to the excellent performance of DES,.When the addition amount of xylan was 4%,the tensile strength of xylan/PAM/DES ionic gel was 16.5 k Pa,which was 82.12%higher than that of pure PAM/DES ion gels.The xylan/PAM/DES composite ionic gel had good conductivity,when the addition amount of xylan was3%（relative to acrylamide）.the initial ionic conductivity was 0.87 m S·cm^-1,and the conductivity decreases rapidly with the increase of tensile strain of ionic gel.In addition,the composite ion gel had excellent transparency,stability,and adhesion ability.Meanwhile,it can respond quickly to strain and has excellent sensitivity and stability when it was used as a strain sensor.Using the metal coordination effect of Al³⁺,polyacrylic acid（PAA）and DES were introduced into the nanoscale cellulose（CNF）and xylan quaternary ammonium（QAX）crosslinking system to prepare transparent composite hydrogels with strong mechanical properties,excellent electrochemical properties,self-healing and good adhesion.The mechanical strength of the composite hydrogel increased with the increase of Al³⁺dosage,while the elongation at break decreased.The maximum tensile strength was 11.53 k Pa and the maximum elongation was 708%in the experimental range.At the same time,the conductivity decreased with the increase of Al³⁺dosage.The ionic conductivity of composite hydrogel was fast and sensitive to strain.When the elongation was 10%,the ionic conductivity of the composite hydrogel was 1.07 m S·cm^-1,which was 25.55%lower than that of the original conductivity 1.174 m S·cm^-1.Therefore,the prepared composite hydrogels could be used to make wearable and flexible strain sensors for wearable and flexible devices,and has application prospects.