Numerical insights on ionic microgels: structure and swelling behaviour

Abstract

Recent progress has been made in the numerical modelling of neutral microgel particles with a realistic, disordered structure. In this work we extend this approach to the case of co-polymerised microgels where a thermoresponsive polymer is mixed with acidic groups. We compare the cases where counterions directly interact with microgel charges or are modelled implicitly through a Debye-Hückel description. We do so by performing extensive numerical simulations of single microgels across the volume phase transition varying the temperature and the fraction of charged monomers. We find that the presence of charges considerably alters the microgel structure, quantified by the monomer density profiles and by the form factors of the microgels, particularly close to the volume phase transition (VPT). We observe significant deviations between the implicit and explicit models, with the latter comparing more favourably to available experiments. In particular, we observe a shift of the VPT temperature to larger values as the amount of charged monomers increases. We also find that below the VPT the microgel-counterion complex is almost neutral, while it develops a net charge above the VPT. Interestingly, under these conditions the collapsed microgel still retains a large amount of counterions inside its structure. Since these interesting features cannot be captured by the implicit model, our results show that it is crucial to explicitly include the counterions in order to realistically model ionic thermoresponsive microgels.

Modeling Microgels with a Controlled Structure across the Volume Phase Transition

“…However, the dependence of R H on N seems to saturate at the largest investigated sizes, so that the discrepancy between simulations and experiments seems to remain present even by extrapolating N to realistic values (∼O(10 7 )). A possible explanation of this result could be attributed to charge effects, which have been shown to become relevant above the VPT temperature 39,40 and therefore should be taken into account for a more faithful representation of the behavior of the numerical R H at high T . A second possibility would be a fine tuning of the interaction potential between the beads beyond V α in order to obtain a polymer chain elasticity in closer agreement with the experimental one.…”

Osmotic pressure of suspensions comprised of charged microgels

“…Note that this is true for ionic microgels (with charges distributed along all the polymeric networks) but also for neutral microgels since they possess charges in the particle periphery due to the polar initiator used in the precipitation polymerization synthesis [53]. This is particularly important for microgel suspensions since microgels are compressible and can shrink in response to an unbalanced osmotic pressure inside and outside the particles [29,45,59,60,78].…”

“…Finally, we mention that methods to achieve an in silico synthesis of microgels have been recently developed [76][77][78][79].…”

Contrast variation SANS measurement of shell monomer density profiles of smart core–shell microgels

“…The response of these materials has great potential in the context of different applications like smart catalyst carriers, [11][12][13] drug delivery, 14,15 sensors, [16][17][18] optical devices, [19][20][21] responsive surface coatings 22 and actuators 23 or in biomedical applications. 24 Combining two monomers of different transition temperatures in statistical copolymers can be used to tune the swelling transition temperature, [25][26][27][28][29] whereas the subsequent synthesis of core and then shell 27,30-35 may lead to a continuous, linear temperature dependent swelling. 27,36,37 The linearity of the response of these coreshell systems might be beneficial especially for sensors and actuators.…”