Mixtures of pH-responsive microgels and temperature-responsive star-like copolymers; from heteroaggregation to gelation

Abstract

Heteroaggregation of dispersions has attracted much interest in the literature, especially when one or more components are stimulus responsive. Here, we study binary mixtures of microgels (MG) and star-like copolymers for the first time. The study investigated the use of complementary hydrogen bonding between carboxylic acid and amide groups to construct heteroaggregates and gels that contained temperature- and pH-responsive components. The pH-responsive MG contained methacrylic-acid and had an apparent pK_a of 8.2. Two new star-like copolymers were introduced which comprised a cationic backbone with poly(N-isopropylacrylamide) side-chains. They are abbreviated as M1–PNP. A combination of complementary hydrogen bonding and hydrophobic interactions was shown to cause formation of heteroaggregates for mixed MG/M1–PNP dispersions at room temperature and at pH values less than the MG pK_a. MG/M1–PNP heteroaggregate formation occurred over a wide pH-range and also in the presence of 0.2 M NaNO₃. The heteroaggregates exhibited temperature-dependent hydrodynamic diameters and zeta potentials. Concentrated MG/M1–PNP dispersions formed self-supporting hybrid gels at 45 °C and gel formation also occurred over a wide pH range. The gels contained 80% MG with respect to total polymer content and were remarkably ductile. They had yield strains greater than or equal to 290%. There was evidence that the elasticity and ductility of the hybrid gels were controlled by the MG and M1–PNP components, respectively. The new M1–PNP star-like copolymers introduced here had superior temperature-triggered gel-formation properties compared to related copolymers and should be a versatile system for conferring temperature-responsive gelation properties to polymer colloids containing carboxylic acid groups.