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.
Highlights
To characterize the microgel structure, small-angle scattering on particles suspended in solvents is appropriate to characterize spatial extent and internal density, as well as local chain structure
We investigated the deswelling and swelling behaviour of this system with different core cross-linker contents (CCC) by Photon correlation spectroscopy (PCS) in detail
No hysteresis effects are observed in cooling/heating cycles.)[25], and we have shown that the magnitude of the slope of the linear region of the swelling curves increases with a decrease in CCC
Summary
To characterize the microgel structure, small-angle scattering on particles suspended in solvents is appropriate to characterize spatial extent and internal density, as well as local chain structure. We have recently adapted a general form-free small-angle scattering data analysis procedure based on a (reverse) Monte-Carlo (RMC) optimization, describing a priori arbitrary monomer density profiles[35] Such form-free descriptions have the advantage that no parametrisation is necessary. Berndt et al identified a core-shell structure for a microgel containing a pNIPMAM-core and a pNIPAM-shell[28] This microgel system showed a linear change in size between the two VPTTs of the polymers, similar to the microgel system studied in this work. The smaller size allows capturing the complete form factor in a standard SANS experiment, and allows a more detailed data analysis as outlined below, providing a structural understanding of the nature of the corset-effect This has been motivated by recently published evidence from infrared spectroscopy indicating interpenetration of the core and shell polymers[16]. We have investigated a series with different core cross-linker contents (CCC), for a temperature series (15, 30, 35, 40, and 55 °C) encompassing the transition temperatures of both polymers
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