Spatial distribution of core monomers in acrylamide-based core-shell microgels with linear swelling behaviour

Abstract

The peculiar linear temperature-dependent swelling of core-shell microgels has been conjectured to be linked to the core-shell architecture combining materials of different transition temperatures. Here the structure of pNIPMAM-core and pNNPAM-shell microgels in water is studied as a function of temperature using small-angle neutron scattering with selective deuteration. Photon correlation spectroscopy is used to scrutinize the swelling behaviour of the colloidal particles and reveals linear swelling. Moreover, these experiments are also employed to check the influence of deuteration on swelling. Using a form-free multi-shell reverse Monte Carlo approach, the small-angle scattering data are converted into radial monomer density profiles. The comparison of ‘core-only’ particles consisting of identical cores to fully hydrogenated core-shell microgels, and finally to H-core/D-shell architectures unambiguously shows that core and shell monomers display gradient profiles with strong interpenetration, leading to cores embedded in shells which are bigger than their isolated ‘core-only’ precursor particles. This surprising result is further generalized to different core cross-linker contents, for temperature ranges encompassing both transitions. Our analysis demonstrates that the internal structure of pNIPMAM-core and pNNPAM-shell microgels is heterogeneous and strongly interpenetrated, presumably allowing only progressive core swelling at temperatures intermediate to both transition temperatures, thus promoting linear swelling behaviour.

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

“…As illustrated in Fig. 5a, the unexpected result of our analysis is that the cores embedded in core-shell microgels occupy a larger space than the original "core-only" microgels [61]. Due to conservation of monomer numbers-this is exactly the advantage of using the same cores-the cores possess the same mass and thus the same low-q intensity in absence of interaction.…”

“…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 some cases, interactions between particles generate a very weak structure factor appearing in the low-q domain, in particular below the VPTT where microgels are swollen. For this reason, usually very low volume fractions are measured; moreover, it has been checked that the results are virtually identical when cutting off the low-q oscillations [61].…”

“…In a following paper, we have reported on the synthesis and scattering analysis of particles made of a p(NIPMAM) core and a p(NNPAM) shell [61]. These experiments have been set up to verify the spatial structure of the core embedded in a shell.…”

“…In the same paper, we have estimated the shell profiles without having the corresponding measurements with corematching [61]. This was achieved by comparing the H-H with the H-D monomer density profiles, which by subtraction provide the D-profile.…”