Dianhydride architectural effects on the relaxation behaviors and thermal and optical properties of organo-soluble aromatic polyimide films

摘要

Dianhydrides of specific molecular architecture was designed and synthesized based on 2,2'-disubstituted 4,4',5,5'-biphenyltetracarboxylic dianhydrides (2,2'-disubstituted BPDAs). Eight dianhydrides were polymerized with two 4,4'-diamino-2,2'-disubstituted biphenyl diamines (trifluoromethyl disubstituted groups, or PFMB, and methyl disubstituted groups, or DMB) to obtain two series of PFMB- and DMB-based aromatic polyimides. As the backbone structures of these two series of polyimides are unchanged throughout each series, the effects of the 2,2'-disubstituted groups of both the dianhydride and diamine constituents on the solubility and thermal and optical properties as well as the relaxation behavior of these polyimides can be identified. It was found that the PFMB-based polyimides with 2,2'-disubstituted BPDAs show excellent solubility while the DMB-based polyimides with the same dianhydrides are less soluble. The same trends can be found for both thermal and thermo-oxidative stability and optical transparency in the ultraviolet and visible light regions. These two series of polyimides exhibit glass transition temperatures (T_g) which show a competition between chain rigidity and linearity with regards to molecular packing. When the size of the 2,2'-disubstituted groups is small and their shape is close to spherical, the T_g initially increases with the size of these 2,2'-disubstituted groups. This is because of the fact that the steric hindrance of these groups prevents the appearance of a cis-conformation of BPDA. However, once these groups possess large size and exhibit anisotropic shapes, their effect on the molecular packing becomes dominant and the T_g starts to decrease. Further, this is the first time that three relaxation processes (the #beta#_1, #beta#_2, and #alpha# processes) were observed above room temperature in these aromatic polyimides. We have identified that the #beta#_1 process is attributed to the local motion of the diamine constituents while the #beta#_2 process is caused by the local motion of the dianhydride constituents. The #alpha# process is associated with the glass transition. The cooperativity of the molecular motion associated with the #beta#_1 and #beta#_2 processes are also discussed.