Determination of Internal Density Profiles of Smart Acrylamide-Based Microgels by Small-Angle Neutron Scattering: A Multishell Reverse Monte Carlo Approach

Abstract

The internal structure of nanometric microgels in water has been studied as a function of temperature, crosslinker content, and level of deuteration. Small-angle neutron scattering from poly(N-isopropylmethacrylamide) (pNIPMAM, volume phase transition ≈ 44 °C) microgel particles of radius well below 100 nm in D2O has been measured. The intensities have been analyzed with a combination of polymer chain scattering and form-free radial monomer volume fraction profiles defined over spherical shells, taking polydispersity in size of the particles determined by AFM into account. A reverse Monte Carlo optimization using a limited number of parameters was developed to obtain smoothly decaying profiles in agreement with the experimentally scattered intensities. Result are compared to the swelling curve of microgel particles in the temperature range from 15 to 55 °C obtained from photon correlation spectroscopy (PCS). In addition to hydrodynamic radii measured by PCS, our analysis provides direct information about internal water content and gradients, the strongly varying steepness of the density profile at the particle-water interface, the total spatial extension of the particles, and the visibility of chains. The model has also been applied to a variation of the crosslinker content, N,N′-methylenebisacrylamide (BIS), from 5 to 15 mol%, providing insight in the impact of chain architecture and crosslinking on water uptake and on the definition of the polymer-water interface. The model can easily be generalized to arbitrary monomer contents and types, in particular mixtures of hydrogenated and deuterated species, paving the way to detailed studies of monomer distributions inside more complex microgels, in particular core-shell particles.

Microgel PAINT – nanoscopic polarity imaging of adaptive microgels without covalent labelling

“…6b-d with DLS data, more complex models with a higher number of parameters are not needed. 28,47 The radial distribution of polymers within the microgel is compared to (i) the respective radial distributions of localizations obtained by PAINT measurements averaged over 20 microgels (black dotted line) as also outlined in the study by Siemes et al 48 and (ii) to the median solvatochromic values (red dotted line). At 21 C, the localization density distribution is similar to the polymer distribution within the core but gradually decreases towards the outer microgel regions whereas the polymer density in the shell shows only a small drop.…”

Recent advances in stimuli-responsive core-shell microgel particles: synthesis, characterisation, and applications

“…In recent contributions, we have implemented a reverse Monte Carlo (RMC) optimisation of general, form-free density profiles Φ(r) [59]. Such arbitrary profiles allow the description of SANS intensities without a priori parametrisation.…”

“…Our approach combined several measurements: at first, a pure p(NIPMAM) core ("core-only"), onto which either deuterated or hydrogenated p(NNPAM) shells have been grown. It is stressed that all cores are the same in our studies [57,59,61,89], i.e. the second synthesis step of the shell has been performed with pre-existing cores, enabling thus direct comparisons.…”

“…In the inset, the initial guess for the intensity (in red) and the final simulation result (in green) are compared to the experimental intensity (black). Reprinted with permission from reference [59]. Copyright 2018 American Chemical Society 23°C) [60] for the shell synthesis, thereby generating the linear swelling.…”

“…b Density profiles of the Hp(NIPMAM) core with a CCC of 10 mol% in D 2 O. Adapted with permission from reference[59]. Copyright 2018 American Chemical Society linear swelling and shrinking property is intimately related to the exact shape of the density gradient.…”