Preparation Of Polyimide Films Containing Bisbenzoazole Units In Main Chain And Study On Their Heat Resistant Dimensional Stability

As a special engineering plastic with outstanding heat resistance and mechanical properties,polyimide(PI)has been widely used in aerospace,electrical insulation,separation and purification,and microelectronics.In recent years,with the increasing development of flexible display technology,light-weight,high-strength,flexible and rollable PI films are expected to replace traditional hard glass substrates.As far as practical applications are concerned,flexible display substrate films put forward more stringent requirements on the physical properties such as heat resistance and dimensional stability of traditional PIs.For example,the glass transition temperature(T_g)of PI needs to be higher than 350°C,and the higher the better.More importantly,the coefficient of thermal expansion(CTE)of such PI films needs to be much lower than conventional PI films(CTE>40 ppm/°C),and it is hoped to match the traditional inorganic/metal substrates(CTE<20 ppm/°C).Based on the above facts,developing PIs with higher T_g and lower CTE is urgently needed.The benzoazole heterocycles,including benzimidazole,benzoxazole and benzothiazole,are a class of structural units with excellent rigidity and linearity,and have been developed into a kind of polybenzoazole branches with comprehensive properties comparable to PI.Extensive work has confirmed that the combination of benzoazole heterocycles and imide rings can achieve breakthroughs in the properties of derived polymers,especially significantly enhanced heat resistance and reduced CTE.For example,the unique intermolecular hydrogen bonds of benzimidazole derived PI can promote the close packing and in-plane orientation of molecules,thereby improving the thermal properties and reducing the CTE.However,such intermolecular hydrogen bonds also cause the hydrophilicity and high water absorption,which will damage the electrical insulation and accelerate the degradation of PIs.Compared with benzimidazole,benzoxazole units have similar molecular structure,and the optimal packing of molecular chains and low water absorption of polymers are realized due to the removal of intermolecular hydrogen bonds.Based on the above application requirements and theoretical basis,this thesis focused on the designability of molecular structure,designed and synthesized a series of bisbenzoazole derived diamines and PIs.The relationship between molecular structure and polymer properties was discussed in detail.The details were as followed:1.The benzimidazole derived PIs exhibited enhanced thermal properties due to the enhanced interchain interaction through intermolecular hydrogen bonds.Different from the widely reported benzimidazole derived diamine(PABZ),a novel bisbenzimidazole derived diamine(DPABZ)was synthesized.DPABZ showed a more rigid and linear backbone structure and stronger amine nucleophilicity than PABZ by molecular structure optimization simulation.The properties of the resulting films were compared by adjusting the molar composition of the PABZ/DPABZ in the copolyimides.In the parallel feed molar ratio,DPABZ derived PIs showed higher heat resistance and mechanical strength than PABZ derived PIs.Different from the high water absorption of PABZ derived PIs,DPABZ derived PIs all showed low water absorption(2.1%～2.5%),which may due to the closer chain packing of the latter and the restriction of intramolecular hydrogen bonds on intermolecular hydrogen bonds.Finally,all DPABZ derived PI films behaved excellent heat resistant dimensional stability,especially DPABZ:BPDA exhibited an ultra-low CTE value of 5.4ppm/°C,which was expected to be applied to flexible substrates.2.Benzoxazole showed similar molecular structure to benzimidazole without introducing intermolecular hydrogen bonds,so benzoxazole derived PIs exhibited low CTE,improved hydrophobicity but significantly sacrificed thermal properties.On the basis of traditional benzoxazole derived diamine(BOA),a novel diamine(DBOA)containing bisbenzoxazole unit and its derived PIs were synthesized.The steady-state simulations of the molecular structures revealed that the rigid bisbenzoxazole unit of DBOA was highly coplanar,and DBOA exhibited more prominent linearity and higher nucleophilicity than BOA.As a result,these DBOA based PIs exhibited more outstanding heat resistant dimensional stability than traditional BOA based PIs.More importantly,compared with PABZ based PIs,even without intermolecular hydrogen bonds,DBOA based PIs also exhibited close T_g and CTE values,and significantly reduced water absorption.Among the reported bisbenzoxazole derived diamines,DBOA derived PIs exhibited relatively lower CTE.3.Benzimidazole can strengthen the interchain interactions,and benzoxazole can optimize molecular packing.Therefore,PIs containing both benzimidazole and benzoxazole in the main chain are a kind of potential substrate films.In this chapter,the benzene ring of PABZ was chemically modified to benzoxazole,resulting in a pair of novel isomeric diamines containing bisbenzoazole(benzimidazole and benzoxazole)units.Owing to the reinforced backbone rigidity and linearity,optimized molecular packing and enhanced interchain interactions,these isomeric diamines derived PIs exhibited more outstanding heat resistant dimensional stability,mechanical properties and lower water absorption than traditional PABZ derived PIs,which were a promising class of flexible polymer base films.The wedge-shaped isomeric diamine derived PIs showed relatively higher T_g due to higher nucleophilicity and rotational barrier,while the linear isomeric diamine derived PIs tended to show lower CTE.The effects of molecular structure and interchain interactions on the molecular packing and properties of polyimide were discussed in detail.4.N-methyl substituted benzimidazole derived PIs achieved a reduction in the water absorption due to the removal of intermolecular hydrogen bonds,but also resulted in the sacrifice of heat resistant dimensional stability.In order to alleviate the above contradictions,in this chapter,isomeric diamines containing semi-N-methyl substituted bisbenzimidazole and their derived PIs were synthesized.Novel semi-N-methyl substituted bisbenzimidazole diamines inherited the rigidity,linearity and high nucleophilicity of aromatic heterocycles.Meanwhile,the steric hindrance of N-methyl groups weakened intermolecular hydrogen bonds,restricted the free rotation of rigid planes and intensified the entanglement of rigid chains.The final derived PIs exhibited comparable T_g,significantly reduced CTE and water absorption to the PABZ derived PIs.In addition,the conformational differences of isomeric diamine molecules and their effects on chain packing,intermolecular interactions and properties of derived PIs were analyzed.The above-mentioned PI films containing bisbenzoazole units in the main chain all have achieved improvements in heat resistant dimensional stability,including high T_g and low CTE,which were expected to become candidates for flexible substrate films and be applied to flexible copper clad laminates,flexible printed circuit,flexible display substrates and other electronic fields.