Preparation And Characterization Of Polyimide Membrane Covalently Hybrid With POSS

Polyimide?PI?,as high-performance material,has been widely used in insulation,new energy,aviation,aerospace,laser,and microelectronics due to its excellent mechanical properties and thermal stability as well as good photoelectric properties.However,the special structure of aromatic polyimide easily forms charge transfer complexes and dense packing of chains,resulting in poor processability,low transparency and high dielectric constant,which greatly limits their applications in advanced microelectronics and optoelectronics.Therefore,designing and developing polyimide membrane with high transparency,easy dissolution and excellent comprehensive properties is of great significance for the wide application of polyimide.Polyhedral polysilsesquioxane?POSS?has a unique cage-like molecular structure that combines the superior properties of inorganic and organic materials.In this paper,POSS is selected as nano-hybrid material,and functionalized POSS has been introduced into the polyimide chain through chemical bonding in order to achieve"molecular-level hybridization"between POSS and polyimide.The properties of polyimide membranes covalently hybrid with POSS were investigated.The details are as follows:?1?Cardo-type polyimide containing hydroxyl was synthesized,and POSS was connected to the side chain of the polyimide through a covalent grafting method to prepare polyimide membranes containing POSS of the side chain.POSS-polyimide composite films not only had high mechanical properties and thermal stability as well as excellent dissolution processing performance,but also had low and stable dielectric constant and dielectric loss.At 20 ^oC and 10⁶ Hz,the dielectric constant of pure PI is3.92,and the dielectric constants of PI-POSS-10 and PI-POSS-30 were reduced to 3.70and 3.78,respectively.Compared with the ungrafted polyimide film,the dielectric constant and dielectric loss of the hybrid film had better temperature stability.The tensile strength of the hybrid membranes decreased slightly and the elongation at break increased with the increase of POSS content.It was notable that PI-POSS-30 and PI-POSS-50 showed good solubility even in common low-boiling solvents such as chloroform,tetrahydrofuran,and 1,4-dioxane.The introduction of POSS has achieved a good balance between the mechanical properties,dissolution processing properties,and dielectric properties of polyimide,making the overall performance of polyimide more excellent.?2?A polyimide hybrid material with a cross-linked network structure was prepared by using epoxy POSS as a cross-linking agent and reacting with a polyimide containing hydroxyl groups and fluorene structure.The tensile strength and Young's modulus of the crosslinked membrane were significantly increased compared with the pure PI membrane.The tensile strength of the hybrid film with a degree of crosslinking of 87.5%was 141.00 MPa,and the Young's modulus was 4.23 GPa,while the tensile strength and Young's modulus of the pure PI film were 72.58 MPa and 2.65 GPa,respectively,the tensile strength of the former was about twice of the latter.At the same time,the epoxy-based POSS formed SiO₂ passivation layer on the surface of the material under the bombardment of atomic oxygen,making the hybrid material have excellent atomic-oxygen-durable properties.Therefore,the introduction of epoxy-based POSS plays a role in enhancing the mechanical properties and improving the membrane antigenic atomic-oxygen-durable performance of the hybrid membrane.In summary,this paper used I-POSS and epoxy-based POSS to modify polyimide,preparing grafted and cross-linked hybrid polyimide membrane covalently hybrid with POSS,respectively.The covalent introduction of POSS significantly improved the physical and chemical stability and mechanical properties of the material on the basis of ensuring the original performance of the polyimide.