Study On The Preparation And Properties Of Lignocellulosic Biomass-based Dynamic Self-adaptive Plastic Replacement Materials

With the increasing environmental pollution caused by fossil-based plastics,many countries around the world have issued strict policies and regulations to limit the production and use of fossil-based plastics in the past a few years.Development of sustainable and degradable polymers as plastic replacement materials is of great importance to protect environment and promote social sustainable development.Lignocellulosic biomass is the most abundant,renewable,and biodegradable resource on the earth and has great potential to produce plastic substitutes.However,natural lignocellulosic biomass is a complex mixture in which cellulose,hemicellulose,and lignin are crosslinked together by strong covalent bonds and hydrogen bonding networks,and the lignocellulose molecules present coexisting ordered-disordered structure and high crystalline,making it hard to be dissolved,not melt or soften and cannot be thermal-processed like conventional fossil-based plastics.In this dissertation,a variety of lignocellulosic biomass-based heat-driven self-adaptive materials are developed as plastic substitutes by integrating wood powders and cellulose papers into the networks of dynamic covalent polymers or modifying microcrystalline cellulose via dynamic covalent chemistry.The main research contents of this thesis are as follow:(1)Wood powder/dynamic imine polymer composites(WP/PI composites)were prepared through combing wood powders and polyimine powders together via a simple heat-pressing process.The interfacial compatibility of biomass phase and polymer matrix is enhanced via construction of dynamic hydrogen-bonding interfaces and dynamic imine bonding interfaces between wood powders and polyimine,thus endowing WP/PI composites with excellent mechanical strength.The Young's modulus,tensile strength,flexural modulus,and flexural strength of WP/PI composites can reach to 1.92GPa,47.1MPa,4.74GPa,and 73.2MPa,respectively.The heat-driven imine exchange reactions between polymer chains allow stress relaxation of polyimine thermoset,enabling excellent heat-driven processability including welding,reshaping,repairing,and remolding of the obtained composite materials.WP/PI composites also show excellent thermal stability,waterproof capability,and water resistance performance.The reversibility of dynamic imine linkages makes the closed-loop 100% recycling of the polymer compositions and wood powders are possible,which can be reused in the production of the next generation composites with similar mechanical properties.The method demonstrated herein paves the way for large scale industrial production of environmentally friendly and sustainable biomass-based materials to replace fossil-based plastics and composites.(2)A novel type of cellulose-based sustainable materials,paper-polyimine composites(PPCs),were developed via functionalization of cellulose papers using dynamic imine polymer by immersing cellulose papers into uncured polyimine resin.The polyimines that infiltrate into the porous structure of cellulose papers can non-covalently bonds to cellulose fiber networks via hydrogen bonding and form interpenetrating networks,enabling the high strength of PPCs.The tensile strength and Young's modulus of PPCs can reach to 71MPa and 3.2GPa,which are significantly higher than those of most commercially available plastics.The DMA,stress-relaxation,and heat-induced self-adaption testing results show PPCs have good malleability,laminating capability,re-healability,and moldability.PPCs also exhibit excellent thermal stability,waterproof capability,and good stability in water and organic solvents.Such PPCs also show superhigh oxygen and water vapor barrier capability.More importantly,the closed-loop recycling of the polymer matrix and cellulose papers has also been demonstrated by degrading PPCs using diamine solution at room temperature.The recycled materials can be 100% reused in the production of next generation composites without loss of any mechanical properties.Therefore,PPCs reported herein represent a novel class of highly green and sustainable materials that can be used as plastic replacements in many fields such as“green”packaging.(3)Cellulose-based dynamic imine polymers(Cell-PIs) were synthesized via polymerization of dialdehyde cellulose and diamine based on Schiff-base reaction.The obtained Cell-PIs exhibit excellent mechanical performance with tensile strength and Young's modulus of 46.6MPa and 2.87GPa,respectively.The temperature-dependent FT-IR test and molecular dynamics simulation characterization of Cell-PIs indicate crosslinking of dialdehyde cellulose by diamine can significantly weaken the intermolecular hydrogen bonds of cellulose and introduce dynamic imine linkages between cellulose chains,thus facilitating the heat-driven malleability of Cell-PIs.DMA testing results show the glass transition temperature(T_g)of Cell-PIs can reach to 153?185?,which are higher than those of most commercially available fossil-based plastics and bioplastics.The heat-triggered bond exchange feature of imine linkages enables Cell-PIs with stress relaxation behaviors,thus endowing them with re-healability and re-moldability.Cell-PIs also exhibit high degradation temperature(>300?),superlow coefficient of thermal expansion(CTE,0.1ppmK^-1),excellent waterproof ability,low water absorpation ratio,and high water/organic solvent resistances.Moreover,a novel type of wood-plastic composites can be fabricated by integrating wood powders into the networks of Cell-PIs.The obtained composites show high strength,high modulus,low water absorpation ratio,high water resistance,and excellent degradability.Therefore,Cell-PIs represent a new class of biomass-based plastics that can be used as the substitutes for fossil-based plastics.In addition,Cell-PIs can also be used to design and produce novel sustainable,degradable,and green composite materials.(4)A novel kind of fully sustainable dynamic imine polymers(Bio-PIs)were prepared via crosslinking dialdehyde cellulose by a plant oil-derived long-chain diamine.The obtained Bio-PIs show excellent tunability in rigidity and flexibility by regulating the oxidization degree of dialdehyde cellulose and the crosslinking degree of polymer networks.The Young's modulus,tensile strength,and enlongation at break of Bio-PIs can be controled in the range of 4.7?1320MPa,3.1?33.5MPa,and 2.5%?108%,respectively.The cycle tensile test,temperature-dependent tensile test,temperature-dependent FT-IR test,and molecular dynamics simulation results reveal there are dynamic covalent networks and multiple dynamic hydrogen-bonding networks in Bio-PIs simultaneously,and they can form synergistic effects to determine the mechanical properties of Bio-PIs.The presence of dynamic imine linkages allowing the stress relaxation of polymer netwoks also enable Bio-PIs with excellent malleability and re-healability.The T_gof Bio-PIs is in the range of 25?65?,and the recovery rate for the mechanical properties of Bio-PIs is close to 90%.Bio-PIs own superlow CTE of<0.2ppmK^-1,which is much less than those of most commercial plastics,engineering plastics,super-engineering plastics,and thermosets.Bio-PIs also exhibit ultrahigh degradation temperature of 473?,excellent waterproof capability,low water absorpation ratio,high water/organic solvent resistance,and good resistance to acid/alkali corrosion.Moreover,Bio-PIs are fully degradable in dimaine solution.In addition,Bio-PIs can be used as the polymer matrix for developing a new generation of fully sustainable wood-plastic composites.Therefore,Bio-PIs have great application potentials in both bioplastics and sustainable composite materials.