Bio-based And Degradable Acetal-containing Polyimides From Natural Lignin-derived Biomass Small Molecules

With the rapid development of electronic information technology,polyimide has attracted great attention as a polymer material with excellent mechanical properties,thermal stability,low dielectric properties,and resistance.Traditional aromatic polyimides have high crystallinity due to the conjugated large ? bonds formed between imide rings and benzene rings,resulting in the rigidity of PI molecular chains and the tight accumulation between molecular chains,which makes the traditional aromatic polyimides difficult to melt,dissolve and degrade.Furthermore,we know that the raw materials for synthesizing polyimide are basically derived from fossil resources such as petroleum.In response to the above urgent problems,this thesis develops a class of polyimide based on biomass structure,and introduces acetal structure in the main chain structure to improve the degradation performance of the material.In this thesis,we focused on exploring vanillin,erythritol and syringaldehyde derived from natural lignin as raw materials.Three kinds of raw materials were subjected to acetal reaction,nucleophilic extraction reaction and reduction reaction to obtain three bisacetal diamine monomers with different structures.Diamines and commercial dianhydrides were then polymerized to obtain multiple series of high-performance polyimide films.The inquiry process is as follows:In the first part,two novel diacetal pyridine ring structure diamine,(1R,6S)-3,8-bis{3-methoxy-4-[(5-aminopyridin-2-yl)oxy]phenyl}-2,4,7,9-tetraoxabicyclo[4.4.0]decane(VEPA)and 3,9-bis {3-methoxy-4-[(5-aminopyridin-2-yl)oxy]phenyl}-2,4,8,10-tetraoxaspiro[5.5]undecane(VPPA)were successfully synthesized starting from natural lignin-derived vanillin,lignin-derived erythritol and readily available pentaerythritol through three reaction steps of acetal reaction to offer diphenol compounds,nucleophilic substitution to give dinitro compounds and then reduction reaction to give diamine compounds.Then using the two diamines and various commercially available aromatic dianhydrides 6FDA,ODPA,BTDA and BPDA as raw materials through conventional two-step polycondensation reaction,two series of tough and transparent polyimides film EPIs and PPIs were obtained.The characteristic peaks of the two series of films in the Fourier transform infrared(FI-IR)spectrum indicate complete imidization.And the two series of films maintain similar thermal properties.The glass transition temperatures(Tg)of EPIs and PPIs ranging in 189?214? and 195?223?,respectively.Compared with erythritol-based bridged biacetal structure film EPIs,pentaerythritol-based spiro biacetal structure film PPIs have relatively superior optical and mechanical properties.For EPIs,mechanical properties with tensile strengths ranging in 55.8?68.7 MPa,tensile modulus ranging in 1.47?2.02 GPa,and elongation at break ranging in 4.74?6.82%,which are slightly inferior to the PPIs with tensile strengths ranging in 77.8?85.7 MPa,tensile modulus ranging in 2.03?2.16 GPa,and elongation at break is 5.37?8.93%.Additionally,PPIs series films have higher optical transparence with 67.1?80.3%than EPIs with 63.9?76.4%at 500nm.And the two series of PI films obtained have certain solubility in a variety of organic polar solvents.The most noteworthy is that both series of PI films can be acidic degraded at room temperature.By 1H NMR monitoring the degradation process,we have proposed an acidic degradation mechanism.This thesis presents a simple and effective method for monitoring the degradation of polyimides.In the second part,a novel diamine with a fluorinated spiro diacetal structure 5,5'-(oxybis(methylene))bis(2-(3,5-dimethoxy-4-(4-amino-2-(trifluoromethyl)phenoxy)phenyl)-5-ethyl-1,3-dioxane)(SDBFA)were successfully synthesized starting from natural syringaldehyde,and easily available formaldehyde,trimethylolpropane(TMP),and n-butyraldehyde through four reaction steps of aldol condensation reaction to synthesize a tetraol compound,acetal reaction to offer a diphenol compound,nucleophilic substitution to give a dinitro compound and then reduction reaction to give a diamine compound.Then using diamine SDBFA or commercial 1,3-bis(4'-aminophenoxy)benzene(TPE-R)and various commercially available aromatic dianhydrides 6FDA,ODPA,BTDA and BPDA as raw materials through conventional two-step polycondensation reaction,two series of tough and transparent polyimides film SPIs and TPIs were obtained.The characteristic peaks of the two series of films in the Fourier transform infrared(FI-IR)spectrum indicate complete imidization.What is certain is that the overall SPIs series films still maintain relatively good thermodynamic properties.The glass transition temperatures(Tg)of SPIs and TPIs ranging in 238?265? and 256?303?,respectively.Compared with TPIs based on TPE-R with ordinary oxyaromatic structure,the SPIs series with fluorinated spiro ring bisacetal structures have relatively superior optical and mechanical properties.Among them,the tensile strength of SPIs series films is 81.3?115.9 MPa,the elastic modulus is 2.27?2.74 GPa,and the elongation at break is 6.1?10.8%,which is slightly better than that of TPIs series films;the tensile strength of TPIs series films is 77.8?87.6 MPa,elastic modulus is 2.03?2.54 GPa,and elongation at break is 4.8?7.2%.In addition,in the 500 nm UV-visible spectrum measurement,the transmittance of SPIs series is 63.8-89.3%higher than that of TPIs series 58.3-80.2%.This shows that the introduction of the fluorinated spiro ring bisacetal structures into the polyimide not only maintains the excellent thermodynamic properties and good mechanical properties,but also increases the solubility and certain hydrophobic properties of the polyimide film.In summary,this two work indicates that erythritol,vanillin and syringaldehyde are renewable aromatics origin capable of being transformed into high performance polyimide with balanced thermal,optical and processing properties by derivating bisacetal structures into polymer backbone.More elaborate degradation studies are required to better understand the degradation mechanism,products,kinetics,and feasibility under realistic environmental conditions.