The electrostatic self-assembly of microgels on polymer brushes and its effects on interfacial friction

In this article, a series of monodisperse poly(N-isopropylacrylamide-co-acrylic acid) [P(NIPAm-AA)] microgels were prepared with different content of acrylic acid (AA) by surfactant-free emulsion polymerization, and their electrostatic self-assemble and tribological behavior on polymer brushes were investigated. The ζ-potential of microgels became more negative with the increase content of AA, which means a stronger hydration capability. For cationic poly[2-(methacryloyloxy)ethyltrimethylammonium chloride] (PMETAC) brushes, negative P(NIPAm-AA) microgels adsorbed on the surfaces of brushes as a result of the electrostatic interaction, and more AA content means stronger absorption ability. However, compared to the polymer brushes, P(NIPAm-AA)_2:1 and P(NIPAm-AA)_5:1 microgels possessed the weaker hydration capability, which led to a concomitant increase in friction of interface. In terms of P(NIPAm-AA)_10:1 microgels, due to the weak adsorption, they could be sheared off easily, leading to the PMETAC brushes swell again, and thus, a lower friction of interface was obtained. Moreover, the tribological behavior of microgels was significantly affected by the pH, especially the P(NIPAm-AA)_2:1 microgels exhibited good lubrication property in high pH solution due to high hydration of deprotonated carboxylic acid groups.