Esterification Of Lignin And Its Effect On The Structure And Properties Of High-Performance Gel-Spun Poly （Vinyl Alcohol） Fibers

With the continuous exploitation of petroleum resources and the growing awareness of environmental protection,biomass resources have received widespread attention due to their environmental friendliness and renewable and biodegradable nature.Lignin is a biomass resource with great potential for application because of its low cost,high aromatic ring content,abundant reserves in nature,excellent UV resistance and antioxidant activity.However,the low content of polar groups in the structure of lignin and its poor interfacial bonding with water-soluble polymers affect its application in polar polymer enhancement.To improve the reinforcement effect of lignin fillers on polyvinyl alcohol（PVA）fibers,this paper adopts direct esterification to modify lignin hydrophilically and prepares a series of lignin/PVA fibers by gel spinning method and investigates the effect of esterification of lignin on the structure and properties of composite fibers.The main contents of this paper are as follows.1.The structure of lignin was modified by direct esterification using the aqueous gluconic acid solution as raw material to investigate the structure and properties of modified lignin with different reaction time.Infrared spectroscopy（FTIR）and nuclear magnetic carbon spectroscopy(¹³C NMR)indicated that the reaction between gluconic acid and lignin was dominated by esterification,with possible limited etherification and depolymerization side reactions as the modification time was extended.Gel permeation chromatography（GPC）tests showed that the relative molecular mass（M_w）of modified lignin increased and the polydispersity index（PDI）improved with increasing reaction time.Differential scanning calorimetry（DSC）tests revealed that the glass transition temperature（T_g）of modified lignin increased and combined with FTIR results indicated that the hydrogen bonding of modified lignin was enhanced.The solubility test results showed that the water solubility of modified lignin gradually improved with the increase of reaction time,but the solubility of modified lignin in dimethyl sulfoxide gradually decreased.2.Three low content modified lignin/PVA composite fibers(5%ML₁₂/PVA,5%ML₂₄/PVA and 5%ML₄₈/PVA fibers)were prepared by gel spinning method using the above-mentioned modified lignin as reinforcing filler to study the effects of various modified lignins on the structure and properties of the composite fibers.It was shown that all the modified lignin with low content could effectively increase the draft multiplier and improve the strength and toughness of the composite fibers.5%ML₄₈/PVA fibers had the most excellent mechanical properties with a breaking strength of 1.27 GPa,Young’s modulus of 20.7 GPa,and toughness of 31.5 J·g^–1,which were 82%and 26%higher than 5%unmodified lignin（OL）/PVA fibers,respectively.SEM observations showed that the modified lignin/PVA fibers were compatible and had a dense and fibrillar morphology at the fracture tip.FTIR and X-ray diffraction patterns indicated that the modified lignin reduced the intermolecular forces of PVA and promoted the crystallization of PVA.The swelling test results showed that the modified lignin increased the water resistance of the composite fiber at high temperature,and the 5%ML₄₈/PVA fiber still maintained its structural integrity at 100℃.3.By utilizing the above-mentioned modified lignin(ML₄₈)with the best enhancement effect to blend with unmodified lignin in different mass ratios,five kinds of high-content lignin/PVA composite fibers（30%OL/PVA,30%OL₂-ML₁/PVA,30%OL₁-ML₁/PVA,30%OL₁-ML₂/PVA and 30%ML/PVA fibers）were prepared through gel spinning method.The effect of blending of modified lignin on the mechanical properties of 30%lignin/PVA fibers was investigated by tensile testing.The results showed that the mechanical properties of the composite fibers deteriorated first and then improved compared to those of 30%OL/PVA fibers due to the gradual increase in the proportion of modified lignin at high lignin content.The deterioration of the compatibility of the composite fibers containing modified lignin was observed by SEM,which may be the main reason for the deterioration of the mechanical properties.30%OL₁-ML₁/PVA fibers had the best mechanical properties with a breaking strength of 1.1 GPa and Young’s modulus of 28 GPa,an increase of up to 50%on its tensile strength and 60%on its Young’s modulus.FTIR and X-ray diffraction patterns showed that the improved mechanical properties of the composite fibers containing modified lignin were mainly related to the enhanced interaction between lignin and PVA and the increased crystallinity.The swelling test showed that the modified lignin could decrease the moisture absorption and swelling of the composite fibers at room temperature but had little effect on the water resistance of the composite fibers at high temperatures.