Graft Copolymerization Of Organosolv Lignin And Its Effect On The Properties And Structure Of Gel-Spun Polyvinyl Alcohol Fibers

Polyvinyl alcohol(PVA)is a hydrophilic polymer,which has good mechanical properties.It is often used as a raw material for the preparation of films and fibers.Next to cellulose,lignin is one of the world's most abundant natural polymers.It is often used as an inexpensive bio-based filler for synthetic fibers.However,the amorphous chemical structure and natural hydrophobicity of lignin make it poorly compatible with certain hydrophilic polymers(e.g.,PVA),which results in the mechanical properties of lignin-based reinforced fibers not being enhanced.Although lignin/PVA fibers have been successfully prepared by gel spinning technique,the inhomogeneity of the spinning solution observed under optical microscope indicates that the affinity problem of lignin and PVA still exists.In this work,organosolv lignin(OL)was chemically modified by using graft copolymerization to graft a large amount of acrylic monomer to the lignin macromolecules,thus improving its hydrophilicity and enabling better integration with PVA to further improve the mechanical properties of lignin/PVA fiber.In this paper,the effects of graft-modified lignin on the properties and structure of PVA fibers are investigated.The main aim is to investigate the structure and reinforcing mechanism of gel-spun modified lignin/PVA fibers.To achieve this target,this study is divided into two sections.First,the optimal chemical modification conditions for OL graft copolymerization and the effect of the prepared modified lignin on the structure and mechanical properties of gel-spun PVA fibers were explored.The results of the chemical modification experiments showed that the modification time of 6 h was too short and therefore the reaction was not successful.The reaction of 12 h showed that the yield of modified lignin was 75.0%and the molecular weight increased from 485 g/mol to 1333 g/mol.The reaction of 24 h showed that the yield of modified lignin was 90.4%and the molecular weight further increased to 2054 g/mol.Thus,the effective time of chemical modification was determined to be 12 and 24 h.Neat PVA,5%OL/PVA,5%OL₁₂/PVA and 5%OL₂₄/PVA fibers were prepared by gel spinning technique.5%was the concentration of fillers(filler weight by total weight).In terms of mechanical properties,the tensile strength of neat PVA fiber was 714.94±44.43 MPa,Young's modulus was 8.97±0.30GPa,and toughness was 18.09±2.00 J/g.When adding 5%OL₂₄,the overall mechanical properties of the 5%OL₂₄/PVA fiber were enhanced over the neat PVA fiber as well,with an increase in tensile strength to 1.15±0.05 GPa(61.4%)and Young's modulus to 13.09±1.36GPa(45.9%),and toughness increased to 26.72±3.65 J/g(47.7%).This indicates that the affinity between OL₂₄ and PVA is better and stronger intermolecular forces are formed,and the formation of a rubber-like network structure inside the 5%OL₂₄/PVA composite fiber has a good impact on the improvement of the mechanical properties of the fiber,which also identifies24 h as the optimal grafting reaction time.In the second stage,PVA composite fibers with different OL₂₄ concentrations(5%?10%?20%?30%)were prepared to investigate the effect of OL₂₄ content on the structure and properties of fiber.It was shown that fibers had good mechanical properties when the OL₂₄ was added at 5%and 10%(lignin by weight of PVA),with 5%OL₂₄/PVA having a tensile strength of 1.01±0.09 GPa,Young's modulus of 18.63±2.24 GPa,and toughness of 19.64±3.50 J/g,and 10%OL₂₄/PVA fiber had a tensile strength of 1.02±0.12 GPa,Young's modulus of 18.62±1.47 GPa,and toughness of 20.90±3.02 J/g.This agrees with the conclusion reached in the first stage of the experiments that a interconnected structure was formed between the PVA macromolecular chains and that the stronger intermolecular forces result in the enhancement of the mechanical properties of the fibers.However,after the percentage of OL₂₄ addition increased to 20%and 30%(lignin by weight of PVA),the mechanical properties of fibers decreased significantly,with the tensile strength of 20%OL₂₄/PVA and 30%OL₂₄/PVA fiber being 604.45±54.24,643.89±38.28 MPa,respectively,and Young's modulus being 9.94±1.06,11.56±1.28 GPa,respectively,and toughness being 12.49±0.77,13.88±1.03 J/g,respectively.From the fiber microstructure analysis,swelling behavior,and water dissolution results,it is likely that aggregation of the high content fillers occurred,which eventually leads to the deterioration of fiber properties and structure.