Preparation And Characterization Of Polyimide Films For Flexible Displays

Flexible display is one of the hottest research fields in technology industry.Flexible substrates play an important role in the performance and lifetime of flexible displays,thus have received extensive attention.At present,polymer film are the most common flexible substrate materials.Thought polymer films has good transparency and flexibility,its low heat resistance and dimensional stability hinders its application.Polyimide（PI）has one of the highest heat resistance in all the polymers,but its visible light transmittance is low because of the electron transfer complex（CTC）exist in PI molecules,improving the transmission rate and size stability of PI is the key to manufactor flexible display.Two fluorinated monomers 4,4’-（six fluoro isopropyl）two phthalic anhydride（6FDA）and 4,4’-two amino-2,2’-bis three trifluoromethyl biphenyl（TFMB）were used as raw materials in order to improve PI’s transparency.First,through the solution polycondensation,polyamic acid（PAA）was synthesized to explore the influence of reaction temperature and reaction time on the viscosity of PAA;then two polyimide films were prepared by thermal imidization and chemical imidization method,separately.The experimental results showed that the best reaction temperature and reaction time of two kinds of fluorinated monomer is 60 ℃ and 6 h,relatively.,for thermal imidization,the process temperature must be more than 300 ℃,while chemical imidization temperature must be more than 200 ℃.The results show that the heat resistance and mechanical properties of the fluorinated films are slightly lower than those of the common aromatic polyimide,but their light transmission at 450 nm was increased by 65% as well as the solubility and dielectric properties were greatly improved.The thermal properties and dielectric properties of films prepared by chemical imidization is lower than those prepared by thermal imidization,but ithas batter transparency,solubility and mechanical properties.The heat shrinkage of fluorine containing film reached 0.012 /℃ which is much larger than that of aromatic PI fim.In order to reduce the heat shrinkable of the films,with nano silica powder as filler and pyromellitic dianhydride（PMDA）as the third monomer,PI/nano-SiO₂ composite film and PMDA/6FDA/TFMB type copolymer films were prepared by doping and copolymerization,respectively,while their properties were tested.The test results show that the polyimide / nano SiO₂ composite film has terrible visible light transmittance.With the increase of PMDA molar ratio in the copolymer film,the thermal properties of the film increased,the dielectric constant enlarged,the mechanical properties first increased and then decreased,the thermal shrinkage reduced to 73 × 10-6 / ℃,while the transmittance decreased by 9%.To reduce the thermal contraction of the film without diminishing the transmittance of PI film,two kinds of biphenyl monomer 3,3’,4,4’-two benzophenone four acid two anhydride（BTDA）and 3,3’,4,4’-biphenyl four carboxylic acid two anhydride（BPDA）instead of PMDA were used to produce the copolymer film with two fluorinated monomers respectively.The test results show that the color of BTDA copolymer film is too heavy to be used as,flexible substrate.With increasing BPDA molar ratio in the copolymer film,the thermal properties of the films increases,dielectric constant enlarged,mechanical properties first increased and then weakened,thermal shrinkage decreases and the transmittance decreased;all the performance of BPDA type co-PI film is lower than that of PMDA type copolymer films the same mole ratio except its heat shrinkable.According to the experimental results,the introduction of fluorinecontaining group in the aromatic PI repeating unit can greatly increase the transmittance of the film,but it will result in a sharp increase of the heat shrinkage of thin films.The synthesis performance of the film can be improved effectively by introducing the rigid monomer,and the heat shrinkage will be reduced.By changing the type and dosage of the copolymer monomer,the PI film which is comprehensive enough to meet the requirement of flexible substrate would be obtained.