| The development of human society constantly puts forward new requirements for sustainable development means such as green production,energy-saving and environmental protection.A trend to replace fossil resources with renewable resources has gradually been formed.Lignin is a cross-linked natural polymer containing various functional groups,such as hydroxyl,methoxyl,and carbonyl groups.As a sustainable,biodegradable,and biocompatible polymer,lignin is considered an alternative resource to produce alkanes,aldehydes,phenolics,polyesters,epoxies,and polyurethanes(PUs).Lignin,when added into polymer materials,can significantly improve the mechanical strength,the flame resistance,and the ultraviolet resistance.For instance,the thermal stability and biodegradability of PU could be greatly improved when lignin is introduced as one of the raw materials.However,due to its complex molecular structure and large steric hindrance,lignin is not so reactive as petro-based polyols.Furthermore,lignin exhibits poor compatibility with polyurethane matrix is poor and thus could not be well dispersed in it.To overcome these problems,I tried to modulate the molecular structure and relative molecular mass of lignin by chemical modification,and to investigate the mechanism of the role of modified lignin in the polymer synthesis process in this study.Based on those issues,a series of studies were carried out and the results are as follows:Highly reactive lignin-based polyols were prepared by the combination of mild hydrothermal degradation and hydroxylation of acetic acid lignin(AAL).The lignin-based polyol was chemically reactive and compatible with diisocyanate,with a reduction in number average molecular weight(M_n)from 2923 to 684?g/mol and an increase in hydroxyl content from 5.21 to 9.13?mmol/g.The results of non-isothermal DSC analysis showed that the lignin-based polyol with lower molecular weight and higher hydroxyl content exhibited lower activation energy when reacting with isocyanate.The polytetramethylethylene ether glycol(PTEMG)could partially replace lignin-based polyol in the reaction with isocyanate to synthesize LPUEs.The mechanical performance and thermal properties of the LPUEs strongly depended on the molecular weight and hydroxyl content of the lignin-based polyols.As the OH groups in lignin serve as proton donators for hydrogen bonding,intermolecular interactions can be intensified in the LPUE using lignin-based polyols with high hydroxyl content and low molecular weight.In this case,the mechanical strength and thermal properties of the LPUE could be dramatically improved.In addition,the amount of substitution of lignin-based polyol had a significant effect on the mechanical properties of LPUEs.When the lignin addition was15%,the LPUE exhibited excellent mechanical properties with tensile strength of 55 MPa and elongation at break of 1501%.A novel nano functionalized lignin nanoparticle(Nano-FL)containing nitrogen(N)and phosphorus(P)moieties was developed via mild grafting reactions combined with the ultrasound method.The Nano-FL incorporated in PUE acted as both crosslinking agents and flame retardants.The novel Nano-FL showed good compatibility and dispersibility in the PUE matrix,thereby overcoming the weakening effect of adding traditional lignin flame retardants on the mechanical properties of the PUE materials.PUE/Nano-FL exhibited strong tensile properties.The PUE containing 10%(by mass)Nano-FL attained a limiting oxygen index as high as 29.8%and it also passed the UL-94 V-0 rating.Furthermore,Cone Calorimetry Test(CCT)showed that the addition of Nano-FL not only reduced the heat release rate and the total heat release but also decreased the total smoke production rate during combustion.The char residues in PUEs with Nano-FL showed dense and continuous morphologies and improved oxidation resistance.The combined barrier and quenching effects of the char layer provided excellent flame retardancy.The amidated modified lignin(NHL)was prepared by introducing reactive amino and amide groups on a part of hydroxyls and carboxyls located on the side chain of AAL.The self-healable,reusable and strong LPUE were successfully synthesized via partially substituting polyether polyol with NHL and introducing a chain extender containing dynamic disulfide bonds.NHL endowed the polyurethane elastomer with significantly enhanced mechanical strength by introducing intensive hydrogen bonding interactions.The thermostability of LPUE was also improved by NHL.In addition,the dynamic hydrogen and disulfide bonds played a key role in improving self-healing and reusable performance of LPUEs.After the LPUE was completely broken by external forces,it still maintained more than 80%of tensile strength and elongation at break after hot pressing treatment.An azo-functionalized lignin(Azo-NL)containing reactive amino groups,amide groups and azo-phenyl groups was prepared by esterification reaction.A reduced phenolic hydroxyl content and an increased carboxyl group content were observed after the esterification reaction.The reduction of phenolic hydroxyl content can effectively weaken the UV absorption ability of lignin and promote the response of azo phenyl groups to light stimulation.The self-healable,reusable and strong LPUE were successfully synthesized via partially substituting polyether polyol with NHL and introducing a chain extender containing dynamic disulfide bonds.The amino and amide groups in the Azo-NL structure promote the formation of more dynamic hydrogen bonds,facilitating the microphase separation of the"hard"and"soft"polyurethane chain segments in the LPUE matrix.The LPUE exhibits good shape-memory performances with a shape fixing rate of greater than 99%and a shape recovery rate of 96%.In addition,the macroscopic deformation of the elastomer was triggered by the photoisomerization of azo-benzene to achieve a remotely photocontrolled driving polymer.The elastomer exhibits a dual photothermal response of deformation under UV light,fixation under visible light,and recovery under heat source. |
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