Examination of the flow behaviour of HEC and hmHEC solutions using structure–property relationships and rheo-optical methods

The effect of solely intermolecular interactions due to hydrophobic alkyl substituents on the flow behaviour of hmHEC solutions was determined via comparison of the structure–property relationships of hmHECs and HECs based on the overlap parameter c[η]. For this purpose the η₀–[η]–c relationship for HEC was determined to be η₀=8.91·10^–4+8.91·10^–4·c[η]+1.07·10^–3(c[η])²+1.83·10^–7(c[η])^5.56. In addition the structure–property relationship for the longest relaxation time via the λ–[η]–c relationship λ·c^1+1/a=2.65·10^–8(c[η])²+4.25·10^–8(c[η])³+5.44·10^–12(c[η])^5.27 has been determined. Although the hmHECs had a higher zero shear viscosity than HECs of comparable overlap parameters at a range of 1<c[η]<13, the flow curves could be described via the same λ–[η]–c relationship in that range, indicating a timescale of the intermolecular interactions below the longest relaxation time.

The behaviour of the supramolecular structures in solution with an applied shear field was characterized by rheo-optical analysis of the shear thickening behaviour which occurs with addition of surfactant. Contrary to expectations, a slope >1 of the flow birefringence Δn′ as a function of shear rate could be observed in double logarithmic plotting. The degree of orientation of the flow birefringence φ primarily decreases with increasing shear rate, but increases later on at a characteristic shear rate. These two exceptional phenomena can be explained by a pronounced anisotropy of the polymer coils caused by the dilatant flow.

This assumption is backed up by the occurrence of a maximum in the dichroism curves which is caused by a finite stability of the aggregated structures in solution. On a molecular basis, these observations agree with the theoretically predicted (Witten and Cohen) transition from intra- to intermolecular polymer micelles. The detected aggregates correspond with the polymer chains that are aligned in one micelle.