Responsive P(NIPAM-co-NtBAM) microgels: Flory-Rehner description of the swelling behaviour

In the present work, a systematic series of poly(N-isopropylacrylamide)(NIPAM) -poly(N-tert-butylacrylamide) (NtBAM) copolymer microgels is prepared by surfactant-free emulsion polymerisation using N,N′-methylenebis(acrylamide) (BIS) as cross-linker. The thermoresponsive behaviour of these colloids was studied in detail applying different scattering techniques. The swelling curves obtained on the basis of photon correlation spectroscopy (PCS) are analysed using the theoretical model of Flory and Rehner. The PCS measurements reveal the narrow particle size distribution of the poly(NIPAM-co-NtBAM) microgels and a decreasing hydrodynamic radius as well as a decreasing volume phase transition temperature (VPTT) with increasing comonomer content. The description of the swelling ratio α as a function of temperature by the Flory-Rehner theory for uncharged homopolymer gels was only partially satisfying and fails for higher comonomer contents. In addition, small angle neutron scattering (SANS) is used to study the internal network structure of these microgels, and the polymer network is characterised in terms of the correlation length ξ. Above the transition temperature, only interfacial scattering from the totally collapsed particles in water can be observed. Due to a certain surface roughness of the copolymer microgels, the SANS profiles beyond the VPTT were fitted using a modified Porod law.