X-ray studies on the structure of polyacrylonitrile fibers

Abstract

Using an X-ray fiber diffractometer, 2D intensity distributions of homopolymer polyacrylonitrile (PAN) fibers are measured in reciprocal space in the plane containing the fiber axis. The equatorial intensity distributions show four relatively sharp reflections and three diffuse maxima. The sharp reflections correspond to a pseudohexagonal 2D lattice of intermolecular packing. The diffuse maxima are due to the Laue scattering of a rotational disorder in this lattice. The off-equatorial intensity distribution is mainly determined by intramolecular scattering. A comparison with calculated 2D intensity distributions for various conformations of short PAN chain segments reveals that the predominant conformation is planar zigzag. 2D correlation functions obtained by Fourier transformation of the intensity distribution corroborate this interpretation.To reconcile the results of the WAXS studies with tacticity measurements using 13C NMR, the continuity of the zigzag conformation is considered to be interrupted by defects (kinks) related to sterically unfavorable sequences of CN groups. No evidence is found for the existence of a two-phase structure with ordered and disordered domains.

Why ferroelectric polyvinylidene fluoride is special

“…It is reasonable that application of a large electric field aids in the alignment of monomer dipoles, which suffer from the disorder caused by the random kinking down the polymer chain, and thus helping to stabilize a polar phase. Liu and Roland [81] found no evidence for the coexistence of multiple phases, as is the case in untreated cast PVDF films, in which polymorphous films possess regions of both all-trans and trans-gauche conformation [81,84]. Further, piezoelectric studies have shown that stretching PAN fibers results increases the piezoelectric response, which, again, is consistent with the "unkinking" of the polymer chains (removing gauche bonds) due to strain.…”

“…Another system that has proven fruitful in the study of ferroelectricity and electroactivity in cyanopolymers is that of polyacrylonitrile, PAN [79][80][81]. Although similar to PVDCN, PAN has only one cyano group per monomer, with the missing -C≡N group replaced by a single hydrogen atom, as shown in Figure 7a.…”

“…One advantage that PAN has over PVDCN is that the PAN homopolymer is quite stable in ambient conditions [79,80], therefore permitting detailed study of the homopolymer's electroactive properties. Studies of PAN include piezoelectric and pyroelectric response [80,82], dielectric constant [83], and crystal structure [81]. Structural studies performed with x-ray diffraction and NMR [81] indicate that the PAN homopolymer packs in a pseudohexagonal manner with the predominantly planar zigzag chain conformation, analogous to the conformation in the all-trans ferroelectric β-phase found in PVDF.…”

“…Studies of PAN include piezoelectric and pyroelectric response [80,82], dielectric constant [83], and crystal structure [81]. Structural studies performed with x-ray diffraction and NMR [81] indicate that the PAN homopolymer packs in a pseudohexagonal manner with the predominantly planar zigzag chain conformation, analogous to the conformation in the all-trans ferroelectric β-phase found in PVDF. In addition, it has been shown that along the PAN chain exist randomly spaced kinks, resulting in a relatively large degree of disorder, perhaps similar to a disordered version of the γ phase found in PVDF, where there are about 25% gauche bonds (compared to 0% in the ferroelectric phase and 50% in the paraelectric phase).…”

“…In addition, it has been shown that along the PAN chain exist randomly spaced kinks, resulting in a relatively large degree of disorder, perhaps similar to a disordered version of the γ phase found in PVDF, where there are about 25% gauche bonds (compared to 0% in the ferroelectric phase and 50% in the paraelectric phase). The proposed packing model is that of a hexagonal arrangement of rigid rods [81]. Additionally, XRD measurements of cast samples show a strong dependence of the intensity of the x-ray reflection on the strength of the poling electric field used to prepare the system [79].…”