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: R and R c 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 R 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 β c 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 (R c 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 (R relaxation) and that associated with the first-order thermal transition in paracrystalline phases of an ultradrawn at-PAN fiber (Rc relaxation) contribute to the relaxation at around 150 °C.

A Novel Method for Investigation of Entropic Stress in Prestabilization of PAN‐Based Precursor Fibers

“…The lowest paracrystalline orientation results in higher temperature β relaxation. 39 Therefore, as it can be seen from microstructure characteristics in Table III, FST for P1 fibers is higher than other samples. However, in under stress treatment the imposed stress can elongate the less oriented chains in the paracrystalline phase more easily and FET for P1 is lower than other samples.…”

“…These points are β relaxation ascribed to the segmental motions of helical sequences and planar zigzag sequences in paracrystalline phases. 39 The first shrinkage temperature (FST) for P1, P2, and P3 is 70, 63, and 69°C, respectively. In the under stress treatment with an increase of temperature, every sample is initially elongated.…”

The synergistic effect of dope concentration and jetdrawing on structure development of wet-spun poly(acrylonitrile)

“…Sawai et al [18,19] also confirmed the dependence of the final properties of ultrahigh-molecular-weight PAN on draw ratio. They observed that the polymer chain orientation, density and modulus increased with increasing draw ratio and reached a plateau at high draw ratios.…”

“…Another important parameter in determining proto-fiber structure is jet stretch ratio in the coagulation bath [12,[17][18][19][20]. Mikolajczyk et al [12] reported macrovoids reduction via increasing draw ratio.…”

Dynamic dielectric properties and the γ transition of bromine doped polyacrylonitrile

“…In the semicrystalline polymers, such as polypropylene and PAN, the three transitions are designated in descending order from the melting temperature as α, β, and γ [7]. A recent paper on dynamic mechanical relaxation in PAN with different stereoregularities presents a summary of the relaxation temperatures for PAN, as reported previously in several references, and concludes by pointing out the characteristics of each transition and assigning it to corresponding molecular processes [8]. In agreement with the conventional claim that the lowest temperature transition (e.g., the γ transition) in side-group polymers (PP, PVC, PS, etc.)…”