Preparation And Properties Of Superheat-resistant Polyimide Films Containing Benzimidazole Moieties

Polyimides(PI),especially as flexible substrates in the flexible display fields,have attracted growing attention due to high thermal stability,good chemical resistance,and excellent mechanical properties.To meet the high-temperature requirements in the low-temperature polysilicon(LTPS)process and avoid the dimensional changes due to thermal or mechanical stress,it is of great significance to develop polyimides with superheat resistance(high T_g and Td)and dimensional stability(low CTE1).Current research has proven that a structural modification in the PI chains was an effective approach to obtain high-performance intrinsic PI materials.Rigidity,conjugation,linearity,hydrogen bonding and crosslinking are considered as characteristic “strengtheners” of intermolecular interactions.Incorporating heterocyclic units that bear functional groups,e.g.benzimidazole,can offer favorable conditions to form hydrogen bondings was an effective way to enhance intermolecular interactions and improve the thermal properties.Rigid and aromatic benzimidazole moieties with a hydrogen bonding donor N-H can enhance intermolecular interaction and the linearity of PI chains favored the in-plane orientation,leading an increase of thermal resistance and dimensional stability.Polyimides derived from 5-amino-2-(4-amino-benzene)benzimidazole(PABZ)have been synthesized and corresponding T_g was around 400 oC.In order to further achieve PI films with higher heat resistance and dimensional stability,diamines containing bis-benzimidazole were proposed and corresponding PI films were prepared.The effects of molecular structure,the ratio of copolymerization,imidization process and annealing atmosphere on the properties of PI films were systematically investigated.The main contents are as follows:(1)Based on diamine PABZ,PI films with different copolymerization ratios and imidization process were prepared.The PI films obtained by different copolymerization ratios exhibited excellent thermal resistance and dimensional stability: the T_g was from 417 oC to 482 oC;5% weight loss temperature(Td 5)was from 552 to 585 oC;CTE values averaged from 50 to 200 oC was in the range of-2.3-1.7 ppm/K.All the PI films showed excellent mechanical properties with a tensile strength from 195.7 to 239.5 Mpa,a tensile modulus from 4.3 to 8.1 GPa,and an elongation at break from 2.7 to 9.3%.ATR-FTIR spectra of PI films obtained from different imidization process and atmosphere suggested crosslinking would occur when the content of BPDA was high and cured to 400 oC in air.When the annealing temperature increased,the crosslinking would gradually disappear and become the degradation of molecular chains.In addition,with a higher content of PMDA,and higher annealing temperature in air,the T_g of PI films would increase,while Td 5 and mechanical properties decreased a lot.(2)Three kinds of diamino-substituted bis-benzimidazoles(DP,BB and i-BB)that enable the formation of extensive hydrogen bonding networks were synthesized via a simple one-step method.A series of self-standing homopolyimide films were prepared from BB or i-BB polymerized with dianhydrides respectively.At the same time,a series of copolyimides from PABZ,BB and ODA with the flexible structure were also synthesized.1H NMR,13 C NMR,mass spectrometry and elemental analysis all showed the synthesized diamines were consistent with the designed structure and had a high purity.The T_g of PI films from BB or i-BB were in the range of 400-466 oC.For PI films copolymerized with ODA,the T_g increased from 337 to383 oC and 448 oC respectively when the content of PABZ and BB was 50 mol%,which indicated the introduction of bis-benzimidazole moieties was more effective in improving the heat resistance of PI films.The CTE of homopolyimides synthesized from BB was lower(6.7-15.3 ppm/K)than their counterparts from i-BB(31.1-39.5 ppm/K)due to BB has a higher level of linearity and coplanarity.In copolyimides containing ODA,the introduction of 50 mol% PABZ and BB could decrease the CTE from 49.6 ppm/K to 22.6 ppm/K and 16.1 ppm/K respectively.T_d~5 of homopolyimides from BB or i-BB was between 550-563 oC and copolyimides from ODA was between 531-554 oC.(3)A series of copolyimides from PABZ,5-amino-2-(3-aminobenzene)benzimidazole(i-PABZ),BB and i-BB with 5-amino-2-(4-aminobenzene)benzoxazole(BOA)were prepared in air or in vacuo.These resultant copolyimides exhibited excellent thermal and mechanical properties with a high T_g to 422 oC,T_d~5 above 556 oC,lower CTE to 5.1 ppm/K and tensile strength above151.1 Mpa.The T_g and Td 5 values of copolyimides cured in air were higher than those of copolyimides cured in vacuo,whereas opposite trend was seen in crystallinity and mechanical properties.The T_g of copolyimides from PABZ was lower than their distorted counterparts from i-PABZ,but exhibited higher dimensional stability(lower CTE).However,copolyimides from BB showed higher T_g and lower CTE than kinked counterparts from i-BB.In addition,a series of copolyimides from different proportions of PABZ and i-BB were prepared.The T_g of copolyimides annealing in vacuo were from 394 to 458 oC,while those annealed in vacuo and additional 1h in air was 446 to 459 oC?The higher content of PABZ,the improvement of T_g and Td 5 was more obvious in copolyimides annealed in vacuo and additional 1h in air.The trend was same for the reduction of CTE.(4)Copolyimides from DP,BB with PABZ and i-PABZ were prepared and their thermal and mechanical properties were investigated.After copolymerized with DP or BB containing bis-benzimidazole moieties,the T_g of PI films were all higher than 452 oC and Td 5 were higher than 553 oC.The tensile strength of resultant PI films was above 117.4 Mpa,even to 232.8 Mpa.All the polyimides exhibited outstanding heat resistance,thermal stability and mechanical properties.At the same time,the PI films showed excellent dimensional stability when copolymerized with PABZ(CTE <10 ppm/K).The T_g of PI films derived from i-PABZ was higher than their counterparts from PABZ.This can be ascribed to the distorted and bent conformation of i-PABZ,which lead to a more difficult chain rotation and higher molar rotational energy.The mechanical properties and thermal stability(T_d~5)of PI films showed a decrease trend with the content of DP or BB increase.Nevertheless,these PI films still retained sufficient properties for application as a flexible substrate.