The liquid crystalline state of polyimide precursors

The polyimide precursor poly(4,4′‐methylenediphenylene pyromellitamic acid) (PMDPAA) in N‐methyl pyrrolidone (NMP) was a lyotropic liquid crystal as evidenced by a DSC thermogram and x‐ray spectrum on its gel, and optical properties by the existence of the critical concentration on the solution viscosity. The configuration of PMDPAA as shown by wide angle x‐ray diffraction pattern was composed of regular meta pyromellitamic acid units. These units together with the rigid character of CH2 linkage between two phenyl rings, and the hydrogen bonding of the intermesogenic units were surmised to be the driving force for liquid crystal formation. The liquid crystalline characters were dependent on molecular weight, solid content, temperature, and the extent of mechanical shearing flow. Data from rheometrics mechanical spectrometer (RMS) indicated that the elastic character of the melt of PMDPAA gel at room temperature shifted to lower frequency on standing and the complex dynamic viscosity increased with time.