Cononsolvency Effects on the Structure and Dynamics of Microgels

Abstract

Sensitive microgels are submicrometer sized, cross-linked polymer particles with a unique swelling behavior changing in response to surrounding conditions like temperature, pH and ionic strength. In this study we influence the swelling capability of thermosensitive microgels microgel by the composition of the solvent (cononsolvency). In particular, we investigate the effects on the structure and dynamics of poly(Nisopropylacrylamide) (PNIPAM) and poly(N,N-diethylacrylamide) (PDEAAM) microgels with different degree of swelling in MeOD/D 2 O solvent mixture at 10 °C using a combination of small angle neutron scattering (SANS) and neutron spin echo (NSE) spectroscopy at nanoseconds scales in the range of several nanometers. The structural characterization including size and density profiles was determined by fitting SANS data. The dynamical behavior of partially collapsed and swollen microgels is comprehensively described within the theory of semidilute polymers in solutions where hydrodynamic interactions are dominant. The partially collapsed PNIPAM microgel particles are not solid diffusing objects but they have relevant contributions from internal motions. Thus, Zimm segmental dynamics can be detected with elevated apparent viscosity. The swollen PDEAAM microgel particles have a faster internal dynamics compared to the partially collapsed PNIPAM. It can also be explained by Zimm-like relaxations with relatively high apparent viscosity and an additional diffusive contribution coming from the cross-linkers.

Bulk Phase and Surface Dynamics of PEG Microgel Particles

“…This observation corresponds to results of NSE measurements on microgels given in the literature. 25,26 Here, a similar trend of the Zimm dynamics was observed indicating that the polymer segments experience a higher effective viscosity due to the The inset in Figure 5 represents the contribution of cooperative diffusion in case of sample p-ME 3 O 5 . According to eq 6 the diffusion coefficient of the cooperative mode is D c = (1.3 ± 0.1) Å 2 /ns and the dynamic correlation length ξ of 12 nm was obtained using the pure solvent viscosity.…”

“…In particular, at low and intermediate Q values, several contributions to the dynamics of microgels may be observed. 25,26 In our experiments, a superposition of collective and single chain dynamics described by With increasing Q-value, the curves decay faster. All curves are assumed to decay down to zero at Fourier times τ NSE > 40 ns.…”

Inner structure and dynamics of microgels with low and medium crosslinker content prepared via surfactant-free precipitation polymerization and continuous monomer feeding approach

“…This tendency is in agreement with previous NSE investigations of microgels. 29,42,56,68 At the highest q (0.17-0.19 Å À1 ), deviations from the Zimm model are obtained, whose origin is not yet clear, but seems to be rather diffusive-like. Here, we should note that due to the complex dynamics behaviour, further investigations in a higher q-range are necessary.…”

“…Earlier reported structural investigations of PNIPAM-based microgels prepared via classical precipitation polymerization in different solvents studied by means of atomic force microscopy (AFM), dynamic light scattering (DLS) and small-angle X-ray or neutron scattering (SAXS/SANS) 9,[27][28][29][30][31] give averaged information about the polymer density distribution inside the polymer microgels. For the structural characterization of the PNIPAM microgels, a core-shell or a core-fuzzy shell model is commonly used.…”

“…For the structural characterization of the PNIPAM microgels, a core-shell or a core-fuzzy shell model is commonly used. 6,17,29,[32][33][34] In such models, the microgel has a homogeneous core with an uniform crosslinker distribution and a less dense shell with a polymer density gradient from the core to the outer surface.…”