Flow‐induced ordering and conformational rearrangement in hydroxypropylcellulose solutions

Abstract

A reevaluation of the previous rheo‐optical analysis of the system hydroxypropylcellulose/acetic acid (HPC/AA) (Edwards and McHugh, 1993) is made through the development of techniques to monitor the steady‐state and transient responses of the linear dichroism, linear birefringence, their orientation angles, and the circular birefringence. Shear and first normal stress measurements allow correlation of the rheological behavior with that of the optical properties. Results are reported for the quiescent circular birefringence of the HPC/AA solutions. Flow experiments indicate that the linear dichroism and birefringence, and the rheological response, are controlled by the overall orientation of the semirigid molecules; thus these optical properties have a common transient time scale. Circular birefringence is controlled by conformational rearrangements of the HPC which take place above a critical shear rate. The time scale of this intramolecular mechanism is much larger than that for the orientational ordering. Most likely, internal hydrogen bonds which enhance the optical activity of the HPC are broken under shear, causing molecular untwisting, followed by retwisting on flow cessation.