Rodlike and Random-Coil Behavior of Poly(n-butyl Isocyanate) in Dilute Solution

Abstract

The polymer chain configuration, molecular relaxation, and the dipole moment of poly(n‐butyl isocyanate), (–CO–NR–) n , where R = C4H9, have been studied by dielectricmeasurements of the polymer in dilute solution. The complex dielectric constant, ε* = ε′ − jε″ , was measured at room temperature over a frequency range 10−1–1 × 106 Hz for fractionated samples whose molecular weights cover a very broad range, 20 × 103–10 × 106. From observation of the dielectric relaxation time as a function of molecular weight we deduce that the low‐molecular‐weight molecule, M w < 80 × 10 3 , is rodlike and helical. Upon increasing the molecular weight, this rodlike conformation can not be sustained and chain flexibility accumulates with the addition of each monomer unit to the chain. Finally at high molecular weight, M w > 10 6 , the polymer chain configuration can be described as random coil. The relaxation time and dipole‐moment measurements indicate that there is a one‐to‐one correspondence between the end‐to‐end distance and the dipole moment of the poly(n‐butyl isocyanate) (PBIC) molecule. Using the dipole‐moment data alone, we are able to show that the PBIC chain configuration as a function of molecular weight can be described by the Kratky–Porod wormlike model. The relationship between PBIC and biological molecules, especially the polypeptides, is discussed.