SPONTANEOUS MOLECULAR-ORIENTATION OF POLYIMIDES INDUCED BY THERMAL IMIDIZATION .3. COMPONENT CHAIN ORIENTATION IN BINARY POLYIMIDE BLENDS

The degrees of uniaxial and in-plane molecular orientation for homopol yimides (PI) and PI/PI binary blends were estimated from the absorptio n dichroism of perylenetetracarboxydiimide (PEDI) bound to the main ch ains. Thermal cure of a slightly drawn film of poly(amic acid) (PAA) d erived from 3,4,3,'4'-biphenyltetracarboxylic dianhydride (BPDA) and p -phenylenediamine (PDA), BPDA/PDA, showed marked spontaneous orientati on toward the stretching direction. The dependence on the chain struct ure revealed that in addition to the chain stiffness (linearity) the m olecular packing is an important factor for the spontaneous orientatio n behavior. In a miscible binary blend system composed of semirigid BP DA/ PDA labeled with PEDI and flexible BPDA/ODA (ODA: 4,4'-oxydianilin e), the extent of spontaneous orientation for the labeled BPDA/PDA cha ins decreased with increasing BPDA/ODA content in the blends. From the result, the marked spontaneous orientation behavior of homo-BPDA/PDA is interpreted as a kind of ''cooperative phenomenon'' where the neigh boring chains promote the molecular orientation of each other in the c ure process. Effects of film thickness, amount of residual solvent, cu re method, and heating rate on the extent of spontaneous orientation w ere also discussed for the homo-BPDA/PDA system.