Homogeneous Chemical Modification Of Hemicelluloses And Preparation Of Biocomposites Based On Its Derivatives

Hemicelluloses,one of the main components of lignocellulosic biomass,have the advantages of low price,easy availability,renewability,biocompatibility and biodegradability.However,with the inherent shortages of heterogeneous structure,low degree of polymerization（DP）,strong hydrophilicity and poor solubility,research in the field of hemicelluloses was neglected and limited.In this study,a series of research was conducted on the homogeneous esterification and graft copolymerization of xylan-type hemicelluloses,the regulating mechanism of the structure of hemicelluloses derivatives as well as its potential applications,which will provide the theoretical basis and technical support for the high-valued utilization of hemicelluloses.（1）The homogeneous esterification of xylan with different linear anhydrides and acyl chlorides was carried out in dimethylsulfoxide/N-methylimidazole（DMSO/NMI）at room temperature.The effects of reaction conditions on the degree of substitution（DS）and weight percent gain（WPG）of xylan esters as well as their distributions were investigated.The results indicated that NMI could act as solvent,acid binding agent and catalyst,simplifying the reaction conditions.With the molar ratio of acetic anhydride to anhydroxylose units（AXU）in xylan of 8:1,the per-O-acylation of xylan with the maximum DS of 1.98 was readily obtained.With the molar ratio of butyric anhydride to AXU of 8:1,the maximum WPG of xylan ester was 86.88%.The results of solubility test indicated that the solubilities of xylan in N,N-dimethylformamide（DMF）and chloroform（CHCl₃）improved with the increase of DS after esterification.The results of TGA/DTG indicated that the thermal stability of xylan increased after the esterification with anhydrides,while decreased with acyl chlorides,probably due to the degradation of xylan and xylan esters in the presence of hydrochloric acid by-product.（2）Homogeneous ring-opening graft polymerization（ROGP）of ε-caprolactone（ε-CL）onto xylan was investigated in dual polar aprotic systems N,N-dimethylacetamide/lithium chloride（DMAc/LiCl）,DMF/LiCl and 1-methyl-2-pyrrolidinone（NMP）/LiCl.The effects of reaction temperature,molar ratio of ε-CL to AXU and the kinds of solvents on the DS and DP of xylan-graft-poly（ε-caprolactone）（xylan-g-PCL）copolymers were investigated.The results indicated that these dual polar aprotic systems could act as solvent and catalyst.With the molar ratio of ε-CL to AXU of 20:1,the xylan-g-PCL copolymers with DS and DP of 0.55 and 1.53,respectively,could be synthesized at 100 ℃ after 24 h in NMP/LiCl.The results of ¹H-¹³ C HSQC and ¹H-¹³ C HMBC indicated that more poly（ε-caprolactone）（PCL）side chains were attached to C-3 position than to C-2 position of xylan,illustrating the higher reactivity of hydroxyl groups at C-3 position.The results of TGA/DTG and tensile test indicated that the thermal stability and film-forming property of xylan improved upon the increased DS and DP of xylan-g-PCL copolymers.The ROGP of ε-CL with xylan was conducted in ionic liquid 1-allyl-3-methylimidazolium chloride（[Amim]Cl）with 4-dimethylaminopyridine（DMAP）as a catalyst.The DS and DP of xylan-g-PCL copolymers were controlled by varying the reaction temperature,reaction time,concentration of catalyst as well as molar ratio of ε-CL to AXU.The solubility test indicated that all of xylan-g-PCL copolymers could be easily dissolved in DMSO and DMF,whereas the solubility in water depended on DS under the selected conditions to a large extent.The xylan-g-PCL copolymers could be swollen in water with DS less than 0.09,or dissolved in water with DS ranging from 0.14 to 0.56,or undissolved in water with DS over 0.65.FT-IR,NMR and TGA/DTG were applied to characterize the chemical structure and thermal stability of xylan-g-PCL copolymers.The results of NMR indicated the higher reactivity of hydroxyl groups at C-3 position than those at C-2 position in xylan.The results of TGA/DTG indicated that the thermal stability of xylan-g-PCL copolymers increased as the increase of DS.By comparison,there are some similarities and differences between the homogeneous ROGP of xylan with ε-CL in dual polar aprotic systems and [Amim]Cl.The similarities included:（a）an appropriate increase of the reaction temperature and molar ratio of ε-CL to AXU was conducive to the improvement of graft efficiency of xylan-g-PCL copolymers;（b）the solubility of xylan increased after the graft of PCL side chains.The differences included:（a）the ROGP reaction could be carried out in dual polar aprotic systems without additional catalyst;（b）the graft efficiency of xylan-g-PCL copolymers in [Amim]Cl was higher than that in dual polar aprotic systems.（3）The ROGP of L-lactide（L-LA）was successfully achieved on xylan-type hemicelluloses in [Amim]Cl using 1,8-diazabicyclo[5.4.0]undec-7-ene（DBU）as an effective organic catalyst.The effects of reaction temperature,molar ratio of L-LA to AXU as well as concentration of catalyst on the DS and DP of xylan-graft-poly（L-lactide）（xylan-g-PLA）copolymers were investigated.Xylan-g-PLA copolymer with the maximum DS of 0.58 and DP of 5.51 was synthesized at 50 ℃ for 12 h with the molar ratio of L-LA to AXU of 20:1 and the concentration of DBU of 4 wt%.The results of ¹H-¹³ C HSQC and ¹H-¹³ C HMBC indicated that more poly（L-lactide）（PLA）side chains were attached to the hydroxyl groups at C-3 position than to C-2 position in xylan.The results of TGA/DTG indicated that the thermal stability of xylan could be increased by grafting with L-LA.The results of DSC revealed that the xylan-g-PLA copolymers exhibited a single glass-transition temperature（T_g）,which decreased with the increase of molar substitution（MS）.Thus,modification resulted in the xylan-g-PLA copolymers with thermoplastic behavior and tunable T_g,broadening its application as thermoplastic materials.The results of XRD indicated the amorphous structure of xylan-g-PLA copolymers for the low content of PLA side chains.（4）The ROGP of propylene carbonate（PC）with xylan was conducted in [Amim]Cl in the presence of organocatalyst DBU.The effects of reaction temperature,reaction time and molar ratio of PC to AXU on the structure and properties of xylan-graft-poly（propylene carbonate）（xylan-g-PPC）copolymers were investigated.With the molar ratio of PC to AXU of 10:1 and concentration of DBU of 2 wt%,the xylan-g-PPC copolymer with DS and DP of 0.47 and 1.73,respectively,was obtained at 120 ℃ after 12 h.The physico-chemical properties of xylan-g-PPC copolymers and the resultant composite films were characterized by FT-IR,NMR,TGA/DTG and tensile analysis.TGA/DTG indicated that the thermal stability of xylan increased after the grafting of PPC side chains.The results of tensile analysis showed that with the increase of DS and DP of xylan-g-PPC copolymers,the tensile strength and Young’s modulus of films decreased,while the elongation at break increased.（5）Xylan-based composite films were fabricated by grafting copolymerization of xylan with p-dioxanone（PDO）,and then blending with poly（vinyl alcohol）（PVA）.The mechanism of interactions between xylan-graft-poly（p-dioxanone）（xylan-g-PPDO）copolymers and PVA was studied.The effect of PPDO content（denoted as WPG）on the mechanical properties of films was also investigated.As the increase of the molar ratio of PDO to AXU from 0.5:1 to 20:1,the WPG of xylan-g-PPDO copolymers increased from 5.30% to 86.97%.FT-IR and SEM analysis proved the existence of intermolecular hydrogen bonding interactions between the hydroxyl groups of xylan-g-PPDO copolymers and PVA.From DSC and XRD data,the addition of PVA led to increase in the T_g and the crystallinity（X_c）of xylan-g-PPDO copolymers.TGA/DTG curves indicated that the thermal stability of xylan-g-PPDO copolymers could be improved by the blending with PVA.Tensile testing showed a dramatic increase in the elongation at break of films with the increase of WPG of xylan-g-PPDO copolymers,and the maximum elongation at break of film was 133.5% with the WPG of 86.97%.In terms of its excellent flexibility and tunable T_g,these materials can be used as thermoplastic packaging films.（6）Xylan-graft-polyethylene glycol（xylan-g-PEG）copolymers with moisture-absorption/retention abilities were synthesized in [Amim]Cl with 4,4-diphenylmethane diisocyanate（MDI）as the coupling reagent.The effects of DS,molecular weitht of poly（ethylene glycol）monomethylether（mPEG,1000 and 5000 g mol-1,denoted as 1K and 5K,respectively）and the relative humidity of environment（RH,43% and 81%）on the moisture-absorption/retention ratios of xylan-g-PEG copolymers were investigated.With the increase of the molar ratio of mPEG to AXU from 1:1 to 5:1,the DS of xylan-g-PEG_1K and xylan-g-PEG_5K copolymers increased from 0.07 and 0.03 to 0.21 and 0.14,respectively.The results indicated that with the increase of DS,molecular weight of m PEG and RH,the moisture-absorption/retention ratios of xylan-g-PEG copolymers increased.After 96 h,the maximum moisture-absorption/retention ratios of xylan-g-PEG copolymers were 39% and 47%,respectively.The ratio of biochemical oxygen demand to chemical oxygen demand of xylan-g-PEG copolymer proved its good biodegradability.The MTT assay indicated that there was no obvious cytotoxicity of xylan-g-PEG copolymer,and thus has great potential as moisture-absorption/retention biomaterials.