Structural parameters of soft PNIPAM microgel particles as a function of crosslink density

The temperature dependences of hydrodynamic radii in thermo-sensitive microgel suspensions, known as collapse curves, are commonly fitted to the benchmark Flory-Rehner theory but parameters obtained often yield little physical insights. Our study of poly(N-isopropylacrylamide) (PNIPAM) microgel suspensions in water is driven by the hypothesis that fitting to Flory-Rehner theory can yield meaningful parameters that separate into ones that are insensitive to crosslink density or deuteration and ones that are not.

Dynamic light scattering (DLS) and rheology experiments were done on 8 microgel variants, protonated and deuterated PNIPAM for four crosslink densities, synthesized under otherwise identical conditions.

Remarkably, polymer volume fractions in the microgel particle at collapse, ϕ_collapse, obtained via rheology, are independent of crosslink density. Along with collapse curves from DLS, this determines the temperature dependence of microgel water and polymer volume fractions. Fitting collapse curves to Flory-Rehner theory yields reference polymer volume fractions, ϕ₀, associated with microgel particle elasticity. ϕ₀ is much lower than ϕ_collapse, and increases with crosslink density. For all microgel sample variants, a crossover temperature, where the elastic contribution to osmotic pressure changes sign, is found to approximate the final temperature after microgel synthesis and also to the free polymer θ temperature.