Dynamic Mechanical Relaxations in Poly(acrylonitrile) with Different Stereoregularities

Abstract

The dynamic mechanical properties of poly(acrylonitrile)s (PANs) with NMR triad isotacticities ranging from 0.25 to 0.68 and drawn to a draw ratio (DR) of 1−60 were measured at 0.5−110 Hz in the temperature range of −150 to 200 °C to study the effects of stereoregularity and DR on the relaxation behavior in PAN. Four kinds of relaxations were observed: α and αc relaxations at around 150 °C, a βc relaxation at around 100 °C, and a γ one at around 25 °C were observed as tan δ peaks measured at 3.5 Hz depending on the stereoregularity and DR. The magnitudes of the loss modulus E‘ ‘ and tan δ peaks were sensitive to the isotacticity, DR, and thermal history of the samples. These results confirm the previous assignments of the α and βc relaxations in at-PAN to the molecular motions in amorphous and paracrystalline phases, respectively, and they are also applicable to both iso- and at-PANs. Furthermore, the observed effects of the stereoregularity and DR on the relaxation behavior, crystallinity, and chain conformations studied by wide-angle X-ray diffraction suggested more details about the origins of these relaxations. The βc and γ relaxations are predominantly ascribed to the segmental motions of helical sequences and planar zigzag sequences, respectively, both in paracrystalline phases. The lower tail of the loss peak due to the γ relaxation is likely ascribed to the local mode motions of conformationally disordered sequences. It is noted, however, that an ultradrawn at-PAN fiber with a DR of 60 exhibited a sharp tan δ peak at around 150 °C (αc relaxation) that was associated with a sudden decrease in the storage modulus E‘ and a DSC endothermic peak. Therefore, both the molecular motion in amorphous regions of PANs (α relaxation) and that associated with the first-order thermal transition in paracrystalline phases of an ultradrawn at-PAN fiber (αc relaxation) contribute to the relaxation at around 150 °C.