Thermodynamic and dynamic of concentrated PNIPAM microgels

Poly(N-isopropylacrylamide) (PNIPAM) microgels are nanometer-sized hydrogel particles able to change their behaviour between hard- and ultra-soft colloids, undergoing to a rapid but reversible volume phase transition (VPT), at critical values, from a swollen hydrophilic state to a collapsed hydrophobic state in response to rapid change of temperature. This behavior is a very interesting model to study the dynamic of the arrested state.1 PNIPAM microgels are one of the most popular thermo-responsive systems studied, since they exhibit the VPT in diluted solutions in water at about 32°C, which is an attractive range for biomedical applications, particularly drug delivery. On the other hand, it has been reported a shifts of the VPT temperature toward higher values at very high PMIPAM concentrated.2,3 The aim of this work is to study the thermodynamic and the dynamic properties of PNIPAM around the volume phase transition at high concentration values, that is for PNIPAM concentration larger than 10%. Microgels made of poly(N-isopropylacrilamide) was prepared in water by precipitation polymerization with a good control over their size,4 about 100 nm at 20°C. Mixtures of PNIPAM in D2O at concentration in the range 10 - 95 % w/w were prepared by two cycles of liophylization- solubilization in D2O and final dry in desiccator at room temperature up to the target concentration. The particle size and the VPT temperature in dilute solution of D2O were analyzed by dynamic light scattering (DLS), while the VPT at concentration in the 10 - 95 % w/w range was investigated by differential scanning calorimetry. Finally. the dynamic of the system was investigated by means of neutron scattering, studying the quasi-elastic response of the different concentrated samples in the 280 - 320 K temperature range at the IN13 and IN5 facilities of ILL in Grenoble.