The volume transition in thermosensitive core-shell latex particles containing charged groups

An investigation of the volume transition in thermosensitive core-shell par ticles by dynamic light scattering (DLS) is presented. The core of the part icles consists of polystyrene (diameter 118 nm), whereas the thermosensitiv e shell is composed of a network of poly (N-isopropylacrylamide) containing 2 mol% acrylic acid counits. The hydrodynamic radius of these particles as determined by DLS decreases in a continuous manner when raising the temper ature. It is shown that the volume transition in the core-shell microgels r emains continuous for a wide range of ionic strengths and pH values. This b ehavior is opposite to that of macrogels of the same chemical composition, which undergo a discontinuous volume transition. The present investigation therefore demonstrates that affixing the network to solid colloidal particl es profoundly alters the volume transition of thermosensitive networks. The reason is that shrinking can take place only along the radial direction of the particles. The solid core thus exerts a strong spatial constraint onto the network, which leads to the observed behavior.