Effect of cross-linker density of P(NIPAM-co-AAc) microgels at solid surfaces on the swelling/shrinking behaviour and the Young's modulus

The effect of the amount of cross-linker in poly(N-isopropylacrylamide-co-acrylic acid) microgel particles on the swelling behaviour and their elasticity is studied. The distribution of the stiffness through the particle is also investigated. Therefore, the swelling ratio obtained from dynamic light scattering measurements in aqueous solutions is compared with the one after adsorption at polycation-coated silicon wafers. The studies of the swelling behaviour at the surface are carried out with scanning force microscopy (SFM) against liquid. The Young's modulus is determined by indentation experiments with an SFM. With increasing amount of cross-linker, the ability to shrink as well as the shift in the lower critical solution temperature and in particle size (hysteresis) during the heating and cooling processes decreases. In addition, the particles at the surface preserve their height/width ratio at high amount of cross-linker, while at low amounts the shrinking and swelling mainly takes place with respect to changes in height. The particles show their highest Young's modulus in the centre of the particles and become stiffer with increasing the amount of cross-linker and the temperature.