Synthesis And Properties Of Polyimides Containing Piperidine Rings And Asymmetric Structures

Polyimide（PI）is at the apex of high temperature resistant polymer materials due to its excellent thermal and chemical stability.From the earliest aerospace to the current microelectronics and flexible displays,polyimide has been practically applied in wide range of fields.Traditional PI has gradually become inadequate to meet the needs of emerging fields,which put forward higher performance requirements for various products,such as high optical transmittance,combined high glass transition temperature with low dielectric constant,as such,a great deal of researches have been carried out on structural modification and performance optimization of PIs.Traditional aromatic PIs contain a large number of aromatic benzene rings,which leads to strong conjugation,and intra-and inter-chain charge transfer complex effect（CTC）lead to dark coloring,poor in solubility and difficult to achieve processing.The introduction of an alicyclic ring into the PI backbone is one of the most effective methods to weaken the above two effects.It can effectively break the conjugation,reduce the charge transfer,and increase the non-coplanarity of the molecule,thereby improving the processing performance of the material.The introduction of pendent substituents and asymmetric linkage can increase the rotational freedom of polymer mainchain,reduce the degree of chain packing,and improve the optical transmittance of the PIs.The introduction of pyridine heterocyclic structure,in which the molar refractive index of C=N is higher than C=C double bond can increase the molecular polarity,meanwhile,the lone pair of electrons on the pyridine N atom can increase the electronic polarization,thus offering better solubility.Herein,this thesis focuses on the structural design and performance optimization of two new type of polyimides.In the first part of this work,nucleophilic addition of phenol and piperidone was carried out to give 4,4’-（1-methylpiperidine-4,4-diyl）diphenol（MPDH）,which is then reacted with 1-chloro-4-nitrobenzene to give 1-methyl-4,4-bis[4-（4-nitrophenoxy）phenyl]piperidine（MBPD）,followed by reduction to generate the diaminemonomer4,4’-{[（1-methylpiperidine-4,4-diyl）bis（4,1-phenylene）]bis（oxy）}dianiline（MBDBPD）.The obtained MBDBPD and ODA in different molar ratio were polymerized with BTDA by two-step solution polycondensation method to obtain PIs films,and the imidization degree at different temperatures was analyzed by IR to find that the imidization reaction was complete at300℃.It is shown that,compared with commercial PI,the solubility of MBDBPD modified PIs was improved while maintaining the optical properties,and the thermal stability was further improved,with T_g ranging in 290-311°C,T_d5 480-515°C,T_d10519-568°C,and residue rate at 800°C（in N₂）higher than 45%.The tensile tests showed that the tensile strength of the modified PI films is in the range of 73.27-132.53MPa,elongation in the rage of 4.22-12.31%,and the tensile modulus in the range of 2.0-3.5GPa.The general properties of the material can be adjusted by the ratio of the two diamines.Due to the tertiary amine structure in the piperidine ring,the modified PI copolymers become more hydrophilic,which can be used as potential bio-medical material with anti-bacterial performance in the case of further quaternization reaction with 1,3-propanesultone.In the second part of this work,4-（2-methyl-4-nitrophenoxy）phenol（MNP）was obtained by controlling the reaction conditions with hydroquinone and 2-fluoro-5-nitrotoluene as raw materials,and then reacted with 2-Methoxy-4-fluoronitrobenzene substitution to give 2-methoxy-4-[4-（2-methyl-4-nitrophenoxy）phenoxy]-1-nitrobenzene（MNB）,and finally reduced to obtain the diamine 4-[4-（4-amino-2-methoxyphenoxy）phenoxy]-2-methylaniline（MNH）.One the other hand,hydroquinone and 1-fluoro-4-nitrobenzene were react to obtain 4-（4-nitrophenoxy）phenol（NPP）,which is then reacted with 2-chloro-5-nitropyridine to obtain 5-nitro-2-[4-（4-nitrophenoxy）phenoxy]pyridine（NPY）,followed by reduction to give the diamine 6-（4-（4-aminophenoxy）phenoxy）pyridin-3-amine（APA）.The two newly synthesized diamine monomers,MNH and APA,were homopolymerized with BPADA,BTDA and ODPA,respectively,to obtain two series of films of JPIs and BPIs.It is shown that both series of materials exhibit good solubility while maintaining good thermodynamic stability,especially in low boiling point solvents,which is confirmed by XRD and the average interchain distance d value.The optical transmittance of the films has been greatly improved,with T₅₀₀of JPIs ranging in 30.9-68.3%,and the T₅₀₀of BPIs 53.9-75.1%,which can meet the requirements of most flexible display.Microporous membranes with different pore sizes were obtained by adding BTDA and pore-forming agent DBP to diamine monomers APA and ODA in different proportions.