The physical properties of polyisocyanates in solution

Abstract

Electric birefringence measurements have been made on four samples of poly-n-butyl isocyanate in three solvents. The value of M W/ M N for three of the samples was 1·2. A description of the apparatus and of its use with both alternating and pulsed direct current electric fields is given. The method has been used to obtain molecular relaxation times and dipole moments. These data have been analysed, together with the results of dielectric experiments reported in the literature, in terms of both rigid helices and weakly bending helices. The results indicate that flexibility is encountered for molecular weights in excess of 5 × 10 4. This is somewhat lower than had previously been reported from other relaxation experiments. Below this molecular weight, the observed molecular relaxation times, which were associated with an end-over-end rotation due to the helices having a permanent dipole moment along their helical axis, were related to a number average molecular weight. Data were consistent with a helix monomer projection length of 1·3 Å and a diameter of 11 Å. In the range 5 × 10 4 < M N < 10 5 , the data have been interpreted in terms of Hearst's recent theory for weakly bending rods. These results were consistent with the worm-like chain model, used to interpret the data for molecular weights in excess of 100,000. It was shown that the assignment of a specific persistence length was unreliable even for molecular weights as high as 5 × 10 5. The study demonstrates the merits and sensitivity of the electric birefringence method. It is the first time that the “double frequency” component of the birefringence, which is generated when alternating fields are applied to the solutions, has been detected and used to determine molecular parameters for a synthetic polymer.