Temperature-sensitive composite hydrogels: Coupling between gel matrix and embedded nano- and microgels

Abstract

Temperature sensitive composite hydrogels have been prepared by incorporating poly (N-isopropylacrylamide) (PNiPAM) nano- and microgels into a polyacrylamide hydrogel matrix. Composite gels with different compositions, i.e. contents of microgel, polyacrylamide and cross-linker have been prepared with different initiation reactions and at different preparation temperatures (above and below the volume phase transition temperature of PNiPAM). The size of the embedded nano- and microgels was determined by means of small angle neutron scattering and confocal laser scanning microscopy. The results show that the temperature sensitivity of the embedded PNiPAM particles can be retained depending on the size of the embedded microgels and the composition of the matrix. A corset-like functionality of the hydrogel matrix can be achieved when the matrix is highly cross-linked or an interpenetrating network (IPN) is formed. The internal structure of the composite gel depends on the preparation conditions as well as on the size of the PNiPAM particles. IPNs can be formed with microgel beads, but hardly with nanogels. Furthermore the mobility of dissolved fluorescently labeled dextran tracers was probed spatially resolved inside the composite gels by means of 2-focus fluorescence correlation spectroscopy. Tracer mobility depends on the local environment and depends on the coupling between embedded microgel and hydrogel matrix e.g. by IPN formation, and can be tailored by the composition of the system. The composite gel does not reveal a temperature dependent volume change because the matrix takes up the water released by the embedded particles. Nevertheless, the temperature dependent swelling of the PNiAM nano- and microgels affects the properties of the composite hydrogel.