Synthesis And Properties Of High Heat-resistant,Soluble And Transparent Polyimides With Side Groups

With the rapid development of flexible wearable devices and flexible displays,there is sharply increasing demand for substrate films with high-performances,including colorlessness,transparency,high thermal stability and strong flexibility.Traditional inorganic transparent materials especially glass are difficult to meet the requirements due to their fragility and poor flexibility.Otherwise,flexible polymers,such as polyethylene terephthalate,polycarbonate,polyethylene naphthalate and polyethersulfone,usually possess excellent transparency but are limited by the low glass transition temperatures(T_g),which are difficult to meet the increasingly harsh high-temperature processing conditions.Polyimide is one of the highest heat-resistant polymer materials.Meanwhile,polyimide also presents excellent solvent resistance,good mechanical property and low dielectric constant,showing irreplaceable advantages.However,majority of polyimides are usually insoluble and difficult for processing because of the rigid molecular chains and the strong intermolecular interactions caused by heterocycles and polar groups in the molecular structure.Apart from insolubility,polyimide also suffers from dark color and non-transparency,causing by the intra-and intermolecular charge-transfer effect between the five-membered imide rings and the diamine residues.To address these,it's necessary to reduce the rigidity of the molecular structure which can weaken the interactions among chains and reduce the formation of the charge-transfer complex(CTC).However,these strategies might also decrease other properties,especially thermal property.Therefore,it is still challenging in molecular design for the balance of the comprehensive performances.On contrary,the introduce of side groups with large steric hindrance seems to be well balance between transparency and thermal performance,which can not only effectively destroy the close packing of molecular chains and increase the torsion angles,thereby improving the solubility and transparency,but also hinder the internal rotation of molecular chains to maintain or improve the thermal property,especially T_g of polyimides.Based on the molecular structure,this dissertation designed and prepared several aromatic diamines with large steric hindrance groups and corresponding polyimides,and systematically studied the effects of molecular structure and preparation process on polyimide properties.The main contents are as follows:1.Two aromatic diamines containing large pendant biphenyl groups and methyl groups on the ortho-side of the amino groups were synthesized and used for preparation of polyimide films through one-step high-temperature method with various common dianhydrides,respectively.Accordingly,the PIs can be divided into two categories,namely fluorine-containing FPI and non-fluorine PI.All polyimides had good solubility in common high-boiling polar solvents and low-boiling solvents.The average transmittances of all PI and FPI films in the visible light region were above 80%.Especially,the films derived from 4,4-oxydiphthalic anhydride(ODPA),4,4'-(hexafluoroisopropylidene)diphthalic anhydride(6FDA) and cyclobutane-1,2,3,4-tetracarboxylic dianhydride(CBDA)were very light in color.At the same time,the films had high heat resistance(T_g>340?)and thermal stability(5 wt%weight loss temperature(T_d5)>479?).The overall performances of PI and FPI films were found to be comparably similar,except for better solubility and transparency of FPI.2.To reveal the impacts of the aromatic pendant groups and the ortho-methyl groups on the property of polyimide,various aromatic diamines containing methyl,benzene ring and biphenyl groups were synthesized.Then,two series of polyimide films were prepared accompanying with 3,3',4,4'-biphenyl tetracarboxylic diandhydride(BPDA)and 6FDA.It generally improved overall properties including solubility and T_g via using the aromatic pendant groups and methyl side groups,but losing the performance in coefficient of thermal expansion(CTE).Detailly,the introduction of methyl side groups could significantly increase the twist angle between the imide ring and the benzene ring,showing a much greater transparency.While,the aromatic pendant groups would lead to an increase in the yellowness index(YI)due to the conjugation of benzene rings.When comes to thermal performance,the introduced methyl groups still maintained the good thermal stability and the aromatic ring structures as pendant groups could improve the thermal weight loss temperature.Herein,the methyl side groups were more conducive to the improvement of the overall performance of polyimide.3.To further improve the comprehensive performances of the polyimide films,biphenyl diamine monomers with larger rigidity were equipped with various numbers of methyl substituents.Using 6FDA as dianhydride,PIs were prepared by one-step high-temperature polycondensation.Subsequently,effects of the number and location of methyl steric hindrance on the properties of polyimide films were well-investigated from both experimental and theoretical(using computer simulation)aspects.The different amounts and positions of methyl steric hindrance resulted in giant differences in electronic effect,spatial structure and chain packing,thereby showing different performances.Notably,even though the methyl groups might increase charge-transfer interaction via increasing the electron-donating ability of the diamines,the PI films with methyl groups were turned out to be transparent due to steric hindrance on the distortion of molecular conformation and the loosening of chain packing.2255TMB-PI with methyl groups both at ortho-and meta-positions of amino groups had the best transparency with a transmittance of 82%at 400 nm and YI of 2.88.Besides,it had good overall performances including good solubility,high T_g(407?),low dielectric constant(2.84)and CTE at 34.3 ppm/K.4.Taking film-processing into consideration,the polyimides with methyl groups by two-step thermal imidization,two-step chemical imidization and one-step high-temperature method were systematically compared.Furthermore,the polyimides with different substituents based on two-step thermal imidization and chemical imidization were obtained.Polyimides by chemical imidization and one-step high-temperature method had better comprehensive properties than the ones by thermal imidization.The overall performances of PI films via the chemical imidization and one-step high-temperature method were similar but the former films exhibited higher transparency and lighter color.Compared with section 3,the transparency of 2255TMB-PI prepared by chemical imidization was further improved with yellowness index at 1.7,presenting almost completely colorless.Contributing to the various electronic effects,it found that different substituents leaded to various differences in transparency between film-processing methods.The strong electron-withdrawing trifluoromethyl groups made PI films with small differences between the thermal and chemical imidization.While,PI films with electron-donor methyl groups exhibited huge disparity using different processing methods.5.As mentioned above,the comprehensive performances of PI films can be well improved via introducing methyl steric hindrance and optimizing film processing pathways.Unfortunately,although with good heat resistance,solubility and high transparency,the CTE was sacrificed.Therefore,three aromatic diamines with amide bonds,methyl,sulfone and trifluoromethyl groups were then designed to decrease the sacrificed CTE.A series of homo-and co-polyimides were prepared with 6FDA and CBDA dianhydrides by two-step chemical imidization process.It proved that the introduction of amide bonds significantly reduced CTE of the films with slightly damaged transparency and thermal stability to a certain extent.With amide bonds,the high transparent and completely colorless film was obtained by introduction of trifluoromethyl groups.While the films with methyl and sulfone still had relatively poor transparency.When copolymerized with 50%rigid CBDA dianhydride,the comprehensive properties(except thermal stability)of the films were improved to a certain extent,especially dimensional stability(CTE)and mechanical properties.The fluorinated copolymer 55FA-TFMB-PI film had T₄₀₀of 85%,YI of 2.0,low CTE to 21.1 ppm/K,at the same time,the T_g was 337? and the tensile strength,modulus and elongation at break were 179 MPa,4.2 GPa and 8.1%.