Rodlike Behavior of Poly(n-butyl)isocyanate from Dielectric Measurements

Dielectric measurements have been made on dilute benzene solutions of poly(n‐butyl)isocyanate $${\left(\begin{array}{c}\mathrm{\hspace{0.17em}\hspace{0.17em}}\mathrm{O}\\ \mathrm{\hspace{0.17em}\hspace{0.17em}=}\\ –\mathrm{C}–\mathrm{N}–\\ \mathrm{\hspace{0.17em}\hspace{0.17em}\; \hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}–}\\ \mathrm{\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}\hspace{0.17em}}\mathrm{R}\end{array}\right)}_n,$$ where R = n‐C₄H₉, at 22.5°C over a frequency range 10⁻¹ to 2×10⁶ Hz. Measurements of relaxation time τ on six samples which have molecular weights in the range 1.4×10⁵ to 2.3×10⁶ showed that τ depends on molecular weight to the 2.7 power. In addition, only one dispersion region was observed for each sample and the corresponding dielectric increments were very large. We conclude from these observations that (1) the poly(n‐butyl)isocyanate molecule is a rigid rod whose relaxation time in dilute solution is associated with the end‐over‐end rotation of the entire rodlike molecule around its minor axis, and (2) the net dipole moments of the molecule have a direction parallel to the major axis of the molecule. Calculation of the length of the molecule from relaxation data indicates that the conformation of the polymeric chain appears to be nonplanar and possibly helical. The dipole moments calculated from the static dielectric constant were not directly proportional to the molecular weight as expected. To explain this a model of the polymerization growth step, in which dipole reversal takes place, was proposed. In this model chain element dipolar senses are not uniform throughout the chain but undergo reversal after a finite sequence length which is independent of the over‐all length.