Dynamic mechanical relaxations in poly(acrylonitrile) with different stereoregularities

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 degreesC to study the effects of stereoregularity and DR on the relaxation behavior in PAN. Four kinds of relaxations were observed: alpha and alpha(c) relaxations at around 150 degreesC, a beta(c) relaxation at around 100 degreesC, and a gamma one at around 25 degreesC were observed as tan delta peaks measured at 3.5 Hz depending on the stereoregularity and DR. The magnitudes of the loss modulus E and tan delta peaks were sensitive to the isotacticity, DR, and thermal history of the samples. These results confirm the previous assignments of the a and 0, 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 beta(c) and gamma 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 gamma 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 d peak at around 150 C (ac. 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 (alpha relaxation) and that associated with the first-order thermal transition in paracrystalline phases of an ultradrawn at-PAN fiber (alpha(c) relaxation) contribute to the relaxation at around 150 degreesC.