Anomalous Transition in Aqueous Solutions of a Thermoresponsive Amphiphilic Diblock Copolymer

Abstract

The aggregation and disaggregation caused by temperature and shear rate in aqueous solutions of the thermoresponsive methoxy-poly(ethylene glycol)-block-poly(N-isopropylacrylamide) (MPEG53-b-PNIPAAM113) copolymer was studied with turbidity, dynamic light scattering, steady shear, capillary viscometer and rheo-small angle light scattering (rheo-SALS) methods. The results from the turbidity measurements exhibited a transition peak in the turbidity curve at an intermediate temperature followed by a visible drop and then an upturn with further temperature increase. This effect is caused by the combination of temperature induced aggregation and species shrinkage. Dynamic light scattering provides a more local study on the polymer dynamics. For the dilute samples used in this study, it gives the temperature dependency of the mean relaxation time, the hydrodynamic radius and the mutual diffusion coefficient. The steady shear measurement results showed a peak in the relative viscosity curve at the same intermediate temperature as the turbidity. At a high shear rate, the peak amplitude was reduced, due to the shear flow induced breakup of aggregations. Shear rate enhances the interpolymer aggregation before large clusters are formed; shear rate breaks up the large clusters, and high shear rates have more effect on the breakup process. The capillary viscometer measurements shows that shear flow induced disaggregation dominate the concentration induced aggregation until the temperature is close to the lower critical solution temperature (LCST); the interpolymer aggregation is more pronounced at LCST. Aggregation at low temperatures and disaggregation at high temperature due to shear stresses on the solutions are studied by rheo-SALS experiments. Thermoresponsive aggregation is also discussed. A phenomenon of an increase in the cloud point temperature as the shear rate increases is pointed out, which is explained by the disruption of aggregation complexes under shear stresses.