Controlling phase separation in microgel-polymeric micelle mixtures using variable quench rates

Abstract

We investigate the temperature dependent phase behaviour of mixtures of poly(Nisopropylacrylamide) (pNIPAM) microgel colloids and a triblock copolymer (PEO-PPO-PEO) surfactant, which undergoes micellisation. Gelation in these systems results from an increase in temperature. Here we alter the heating rate and observe that the mechanism for aggregation changes from one of depletion of the microgels by the micelles at low temperatures to association of the two species at high temperatures. We use this to access multiple structures at one microgel concentration. Samples with a micelle concentration above 7 wt% were found to demix into phases rich and poor in microgel particles at temperatures below 33 • C, under conditions where the microgels particles are partially swollen. Under rapid heating full demixing is bypassed and gel networks are formed instead. The temperature history of the sample therefore allows for kinetic selection between different final structures, which may be metastable.

Real space analysis of colloidal gels: triumphs, challenges and future directions

“…The gelation is caused by the temperature responsive association (i.e. bridging) of the microgel colloids and triblockcopolymer, driven at elevated temperatures by hydrophobic interactions [35,239].…”