Dynamic birefringence and conformation of poly- p-phenylene terephthalamide molecules in solution

Abstract

Flow birefringence (FB) for samples of poly- p-phenylene terephthalamide (PPPhTPhA) dissolved in sulphuric acid has been investigated; intrinsic viscosities [η] of the solutions have been measured. Characteristic values of FB [ n] and orientation angles [ χ/ g] of the solutions were determined. Coefficients of rotatory diffusion D r and molecular weights M of all the samples of PPPhTPhA were calculated using the values of [ χ/ g] and [η]. Dependences of [η] and D r on M show that the conformation of the PPPhTPhA molecules in solution resembles a slightly bending rod. Quantitative data on the equilibrium rigidity of the PPPhTPhA molecules were obtained from comparison of characteristic values of [ n] and [ χ/ g] at different M and from the dependence of [ n]/[η] on M, using the theory of optical anisotropy of persistent chains and also from the dependence of [η] on M. It was shown that the number of monomer units in a statistical segment of the PPPhTPhA chain is S = 200 ± 20; this corresponds to a segment length of (1300 ± 130) Å. The equilibrium rigidity of the PPPhTPhA chain is slightly lower than that of the PPBA molecules. Possible reasons for this difference are discussed.