X‐ray investigations on annealed fibers of poly(<i>p</i>‐phenylene‐1,3,4‐oxadiazole)

Abstract

Abstract The influence of annealing on the supermolecular structure of commercial, thermostable fibers, spun from solutions of poly( p ‐phenylene‐1,3,4‐oxadiazole) (POD) in H 2 SO 4 , is examined. The crystalline α‐modification of thermally treated POD fibers has an orthorhombic unit–cell probably of space group P2 1 2 1 2 1 . The symmetry of the single POD chain in these crystallites is 2 1 . The unit–cell dimensions are a = 1.235 nm, b = 0.655 nm, c = 1.40···1.47 nm, where c depends on the annealing temperature T a . The unit cell contains 4 chains of two monomers each. Annealing up to T a of about 755 K causes increases in crystallite size, crystalline orientation, and linear degree of order, combined with an improved axial Young's modulus E . Thermal degradation at higher temperatures leads to the breaking of tie molecules in general, while UV‐radiation selectively damages tie molecules that are not taut.