Polymer and colloid physics
How does a swollen microgel collapse into a dense particle?
Open access · cc by · source: Europe PMC
Soft polymer microgels shrink in two stages: first a very fast collapse into a hollow shell, then a slower rearrangement into a solid, uniform sphere.
Study at a glance
- Design
- Other — Stopped-flow solvent exchange into 20 mol% methanol with time-resolved small-angle X-ray scattering and form-factor fitting, combined with multiparticle collision dynamics plus molecular dynamics simulations of cross-linked networks.
- N
- No participant count; dispersions of monodisperse PNIPAM microgels measured over time, plus simulated microgels with varied chain lengths and quench depths.
- Population
- Poly(N-isopropylacrylamide) microgels in water-methanol mixtures (cononsolvency)
- Outcome
- Microgel radius and radial density profile versus time; simulated radius of gyration and collapse kinetics
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Key findings
The microgel radius dropped from 954 nm to about 381 nm within 5 ms, too fast to resolve, then slowly approached about 331 nm over the next 760 ms; fits needed two time constants, about 1.3 ms and 227 ms. Scattering required a core-shell model with small collapsed regions of 20 to 30 nm, meaning a dense outer shell around a looser core. Simulations reproduced the two stages: clusters form at cross-links and merge from the periphery into a hollow core-shell, driven by hydrodynamic interactions, followed by slow chain rearrangement into a compact globule.
Methodology
The authors rapidly mixed PNIPAM microgels dispersed in pure methanol or pure water with the other solvent so the mixture reached 20 mol% methanol, a composition that makes the gels collapse. They followed size and internal structure over time with time-resolved small-angle X-ray scattering, fitting detailed density-profile models. In parallel they simulated cross-linked polymer networks in an explicit fluid that includes hydrodynamic interactions, switching the monomers from good to poor solvent conditions.
Limitations
The very first fast step was faster than the experiment's time resolution, so its structure is inferred from simulations rather than measured. The scattering model has many parameters, several of which were fixed to plausible values, and the cluster term is empirical. Simulations model the solvent exchange only implicitly as a change in monomer attraction, so they do not capture specific cononsolvency chemistry, and the claim that the two-step process is generic across stimuli rests on comparison with other literature rather than direct tests.
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