Poly(N-isopropylacrylamide) (PNIPAM) microgel is a smart polymer that shows a volume phase transition temperature (VPTT) at around 32 °C in aqueous solutions, above which it collapses. In this work, combining experiments and molecular simulations, it is shown that PNIPAM microgels do not always exhibit a collapsed structure above the VPTT. Instead, PNIPAM in aqueous alcohol mixtures shows a two-step conformational transition, i.e., a collapse at low temperatures (T < 32 °C) and a reswelling when T > 50 °C. The present analysis indicates that delicate microscopic interaction details, together with the bulk solution properties, play a key role in dictating the reswelling behavior. Even when PNIPAM microgels swell with increasing T, this is not a standard upper critical solution behavior.

The present paper addresses the loading of thermoresponsive poly-N-isopropylacrylamide (PNIPAM) based microgel particles with magnetic nanoparticles (MNP: CoFe2O4@PAA (PAA = poly(acrylic acid))) and their response to an external magnetic field. The MNP uptake is analyzed by transmission electron microscopy (TEM). Obviously, the charge combination of MNP and microgels plays an important role for the MNP uptake, but it does not explain the whole uptake process. The MNP uptake results in changes of size and electrophoretic mobility, which is investigated by dynamic light scattering (DLS) and a Zetasizer. The microgels loaded with MNP preserve their thermosensitivity, and they show magnetic separability and are considered as magnetic microgels. After adsorption at a surface the magnetic microgels are studied with a scanning force microscope and indentation experiments. The magnetic microgels show an elongation along the magnetic field parallel to the surface while the height of the microgels (perpendicular to the surface and to the magnetic field) is compressed. This result is in good agreement with simulations of volume change of ferrogels in a magnetic field.

Poly(N-isopropylacrylamide) microgel particles were prepared via a ''classical'' surfactant-free precipitation polymerization and a continuous monomer feeding approach. It is anticipated that this yields microgel particles with different internal structures, namely a dense core with a fluffy shell for the classical approach and a more even crosslink distribution in the case of the continuous monomer feeding approach. A thorough structural investigation of the resulting microgels with dynamic light scattering, atomic force microscopy and small angle neutron scattering was conducted and related to neutron spin echo spectroscopy data. In this way a link between structural and dynamic features of the internal polymer network was made.Germany.

The aim of this study is to tailor the inner structure of positively charged poly-(N-isopropylacrylamid-co-allylamine) (P(NIPAM-co-AA)) microgels for a better control of the distribution of negatively charged magnetic cobaltferrite (CoFe 2 O 4 @CA) nanoparticles (MNPs) within the microgels. Therefore, two different strategies are followed for the microgel synthesis: the (one pot) batch method which leads to a higher cross-linker density in the microgel core and the feeding method which compensates different reaction kinetics of the cross-linker and the monomers. The latter one is expected to result in a homogeneous cross-linker distribution. Information about the cross-linker distribution is indirectly gained by measuring the elastic modulus via indentation experiments with an atomic force microscope. While the batch method results in a higher elastic modulus in the center of the microgel indicating a core/shell structure, the feeding method leads to a constant elastic modulus over the whole microgel. The loading with MNPs and their distribution are studied with transmission electron microscopy (TEM). The TEM images show a large difference in the MNP distribution which is correlated to the cross-linker distribution of both types of microgels. The batch method microgel has a low MNP concentration in the core. The feeding method microgel shows a much more homogeneous distribution of MNPs across the microgel. The latter one also shows a stronger charge reversal which is a hint for a higher loading of the feeding method microgel. Dynamic light scattering and electrophoretic mobility measurements demonstrate that for both types of microgels, the temperature sensitivity is preserved after loading with MNPs.