Abstract:

Due to the excellent thermal stability, good mechanical properties and low dielectric constant, polyimide  is  a  high-performance  polymer  materials,  which  mainly  used  in  the  defense  industry, aerospace  and  microelectronics  industries.  In  the  field  of  optoelectronics,  transparent  polyimide films  can  be  used  as  solar  cell  substrate  materials,  flexible  transparent  conductive  film  substrate materials, etc. However, due to the presence of a highly conjugated structure in the molecular chain of  the  polyimide,  a  charge  transfer  complex  having  strong  absorption  of  light  in  the  visible  light region  can  be  easily  formed  between  the  inter-  and  intra-molecule,  leading  to  the  polyimide  film exhibit yellow and limit its application in the optical field. In this paper, to improve the transparency of  polyimide,  the  cardo  structure  is  combined  with  the  alicyclic  structure,  and  the  effect  of  the structure of the polymer on its transparency, thermal properties, and solubility is also studied. Works have been mainly conducted as follows:

(1) Starting from phenolphthalein, the ester bond in phenolphthalein was replaced with amide bond to synthesize bisphenol monomer, while trans-1,4-cyclohexanediamine was reacted with 4-  fluorophthalic anhydride  to  synthesize  1,4-bis(4-fluorophthalimide)cyclohexane.  The  bisphenol monomer  and  the  1,4-bis(4-fluorophthalimide)cyclohexane  are  polymerized  into  a  series  of polyimides by nucleophilic substitution reaction. The series of polyimides can be dissolved in most common organic solvents at room temperature, and can be partially dissolved in tetrahydrofuran, which is a low-boiling solvent, their glass transition temperature exceeds 270℃and can reach up to 316℃, and the temperature at which decomposition occurs at 5% is above 400℃(in nitrogen atmosphere), and the transmittance of the films are almost all above 80% (at 450 nm), which have good transparency.

(2) A series of polyimides were synthesized from the cardo fluorene-substituted  bisphenol containing  pendant  methyl  groups  as  raw  materials  and  the  1,4-bis(4-fluorophthalimide)cyclohexane monomers with 1,4-cyclohexane by nucleophilic substitution. The influence of fluorene structure and the different trans-/cis- ratios of 1,4-cyclohexane were studied. The series of polymers have good solubility at room temperature, the glass transition temperature is higher than 400℃and can even reach up to 352 400℃, and the temperature when decomposed at 5% is higher than 400℃ (in nitrogen atmosphere). The transmittances of the polyimide films at 400 nm are almost above 80%, and can reach more than 85% at 450 nm, which have excellent transparency.

Key Words: polyimide, nucleophilic substitution reaction, cardo, 1,4-cyclohexanediamine