| Polyimides（PIs）,which possess excellent mechanical,thermal and chemical resistant properties,are extensively used in electronic and microelectronic industries.The intrinsic dielectric constant（ε）of traditional PIs lies at approximately 3.5.To minimize the power loss and signal delay during signal transmission of interlayer dielectrics for ultra-large-scale integration（ULSI）,lowerεvalue（ε<2.5）is required for dielectric materials.The common methods for decliningεof PI include reducing the polarizability of PI,decreasing the stacking density of PI chains,and introducing nonpolar nanomaterials.Crown ethers can be used as host molecules,and form supramolecular host-guest inclusion complexes with guest molecules containing amino or ammonium salts.Crown ethers,which possess cavity structures and low polarizability,are well dispersible in PI matrix because of the interaction between ether and imide bond.The introduction of crown ethers to PI matrix will be beneficial to achieve a good balance between excellent mechanical properties and lowεof PIs.Whereas it might cause the dramatical decrease in thermal properties of the composites,owning to the low thermal decomposition temperature of crown ethers.Further modification of PI/crown ether composites is desiderated,so as to achieve a lowerε,meanwhile effectively maintain or even improve the thermal,mechanical and other properties of PIs,and meet the application in ULSI.In order to improve the heat resistance of PI/crown ether composites,organosilicon was introduced into polyamic acid（PAA）/crown ether complexes to construct PI/crown ether/organosilicon composites with cross-linked network structure.Ladder-like polypropylsilsesquioxane（LPSQ）was introduced into PAA/18-crown-6（18C6）complexes by in-situ polymerization to prepare PI/18C6/LPSQ nanocomposite films.Here3,3’,4,4’-pyromellitic dianhydride（PMDA）was used as dianhydride monomer,and4,4’-diaminodiphenyl ether（ODA）was used as diamine monomer.Theεof PI/18C6/LPSQ decreased to 2.74 at the loading of 4.5 wt%Si-O-Si.Young’s modulus,tensile strength and fracture energy was 5.08 GPa,135.0 MPa and 27.5 MJ/m3,respectively.The temperature at which the material has 5%weight loss(T5%)and glass transition temperature（Tg）of PI/18C6/LPSQ increased from 412.2°C and 306.4°C of PI/18C6 to 456.0°C and 342.5°C,respectively.Furthermore,the hydrosilylation between tetramethylsiloxane（TDSS）and PAA/γ-allyloxy-18-crown-6（A18C6）complexes was carried out during thermal imidization,to prepare PI/silane cross-linked crown ether（PI/SiCL-A18C6）composite films.The modification effect was similar to the above results.Theεof PI/SiCL-A18C6 decreased to2.61.T5%and Tg of PI/SiCL-A18C6 increased to 453.1°C and 341.9°C,respectively.In PAA/crown ether composites,only a part of crown ethers can be threaded by PAA chains.The preparation process of PI/crown ether composite films was optimized by removing the free crown ether from the corresponding PAA,and the thermal properties of PI composites were improved.Three new crown ethers with different ring-cavity structures,dicyclohexane-18-crown-6（DCH18C6）,dibenzo-18-crown-6（DB18C6）and dibenzo-24-crown-8（DB24C8）,were selected to investigate the effect of the outer structure and the size of crown ether ring on properties of PI films.The introduction of cyclohexyl and phenyl groups outside the crown ether ring and the increase in the molecular size of crown ether contributed to the decline inε,as well as to the improvement in the heat resistance of PI/crown ether composites.Theεof PI/DB24C8 composite film at the loading of 3.6 wt%DB24C8 decreased from 2.99 of 18C6 to 2.84,meanwhile its T5%and Tg were increased by126.3°C and 31.2°C to 538.5°C and 337.6°C,respectively.Young’s modulus,tensile strength and fracture energy of PI/DB24C8 were 5.13 GPa,141.2 MPa and 55.0 MJ/m3,respectively,which were increased by 83%,18%and 163%,compared with pristine PI.In order to effectively decease the intrinsicεof PI,three new diamines（XDA）containing large side groups（biphenyl,naphthalene and pyrene）were synthesized.4,4’-（Hexafluoroisopropylene）diphthalic anhydride（6FDA）was selected as dianhydride monomer to prepare 6FDA-XDA polyimide films.In addition,2-（4-aminophenyl）-5-aminobenzimidazole（DAPBI）was introduced as the third monomer to prepare 6FDA-DAPBI/XDA copolyimides（co-PIs）,and it was expected to further improve the thermal resistance of PIs by strengthening intermolecular hydrogen bonding.The existence of large side groups helped to increase the molecular chain spacing and free volume of PI,and endowed PI films with excellent solubility and transparency.Theεof6FDA-DAPBI/XDA co-PI films decreased to 3.01.T5%s of the films were in the range of518°C-526°C,and Tgs exceeded 375°C.For the purpose of optimizing the mechanical properties of co-PIs,DB24C8 was introduced into 6FDA-DAPBI/XDA co-PIs to prepare co-PI/DB24C8 composite films.Theεof PI further declined to 2.73.Noteworthy,the rigidity and toughness of co-PIs were improved effectively.Young’s modulus,tensile strength and fracture energy of co-PI/DB24C8 were increased by 8%,25%and 88%,to 5.01 GPa,153.7MPa and 12.0 MJ/m3,respectively,compared to 6FDA-DAPBI/XDA co-PIs with the similar monomer molar ratio.Increasing the molecular size of the monomers is an effective way to prepare PI with aεvalue lower than 2.5.Two new tetramines（XTA）with large molecular size were synthesized.4,4’-Oxodiphthalic anhydride（ODPA）was chosen as dianhydride monomer,and4,4’-diamino-2,2’-bistrifluoromethyl biphenyl（TFDB）was used as diamine monomer.ODPA-TFDB/XTA co-PIs were synthesized through chemical imide reaction.The co-PI films could be completely dissolved in polar solvents at room temperature.Its transmittance at 450nm and 550 nm was as high as 83.5%and 88.5%,respectively,and the cut-off wavelength was as low as 363 nm,meanwhile theεdecreased to 2.55.DCH18C6 was added into co-PIs to prepare ODPA-TFDB/XTA co-PI/DCH18C6 composite films.Theεof PI further dropped to 2.41,at the same time the transparent performance was further improved,and the good thermal properties of co-PIs was maintained.Co-PI/DCH18C6 exhibited high Young’s modulus,tensile strength and fracture energy of 5.12 GPa,136.8 MPa and 15.1 MJ/m3,respectively,which were increased by 6%,18%and 101%,respectively,compared with the corresponding co-PI film.PI/crown ether composite films prepared in this paper displayed the great application potential in electronics and microelectronics industry. |
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