Structure of oriented polystyrene monofilaments and its relationship to brittle‐to‐ductile transition

Oriented atactic polystyrene monofilaments show brittle‐to‐ductile transition in the vicinity of a degree of birefringence Δn = −2 × 10−3 (at a temperature of 20°C and at the stretching rate of 100%/min) independently of the various spinning conditions. The amorphous orientation of a cylindrically symmetric system was investigated by wide‐angle x‐ray scattering experiments. The orientation of polystyrene monofilaments in real space is denoted by P2(cos α) within the precision of measurement. The coefficient D2(r) of the second term in an expansion of cylindrical distribution function in spherical harmonics has two main peaks, near r = 1.6 Å and r = 4 ∼ 5 Å. The negative peak near r = 1.6 Å indicates orientation of phenyl groups in a plane perpendicular to the fiber axis. The positive peak near r = 5 Å (brittle region) or near r = 4 Å (ductile region) indicates the piling up of phenyl groups for the direction of the fiber axis. It is most probable that the amorphous state of atactic polystyrene consists of an appropriate cohesion of planar zigzag chains of syndiotactic polystyrene. The lower spacing (r = 4 Å) of the positive second peak in ductile region suggests that there are interchain phenyl groups near r = 5 ∼ 6 Å in a plane perpendicular to the fiber axis and that the molecular chains are extended parallel to the direction of the fiber axis. This parallel packing of chain segments along the fiber axis suppresses the formation and growth of cracks of polystyrene monofilaments resulting in ductile fracture.