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In recent years, ionic microgels have garnered much attention due to their unique properties, especially their stimulus-sensitive swelling behavior. The tunable response of these soft, permeable, compressible, charged colloidal particles is increasingly attractive for applications in medicine and biotechnologies, such as controlled drug delivery, tissue engineering, and biosensing. The ability to model and predict variation of the osmotic pressure of a single microgel with respect to changes in particle properties and environmental conditions proves vital to such applications. In this work, we apply both nonlinear Poisson–Boltzmann theory and molecular dynamics simulation to ionic microgels (macroions) in the cell model to compute density profiles of microions (counterions, coions), single-microgel osmotic pressure, and equilibrium swelling ratios of spherical microgels whose fixed charge is confined to the macroion surface. The basis of our approach is an exact theorem that relates the electrostatic component of the osmotic pressure to the microion density profiles. Close agreement between theory and simulation serves as a consistency check to validate our approach. We predict that surface-charged microgels progressively deswell with increasing microgel concentration, starting well below close packing, and with increasing salt concentration, in qualitative agreement with experiments. Comparison with previous results for microgels with fixed charge uniformly distributed over their volume demonstrates that surface-charged microgels deswell more rapidly than volume-charged microgels. We conclude that swelling behavior of ionic microgels in solution is sensitive to the distribution of fixed charge within the polymer-network gel and strongly depends on bulk concentrations of both microgels and salt ions.
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STUDYING THE EFFECT OF CROSSLINKER ON POLYMERIC MICROGELS
Microgels are spherical particles suspended in solution, comprised of crosslinked polymer chains. Due to the amphiphilic property of the parent polymer, microgels display a temperature dependent volume phase transition (de-swelling), and thus have the potential to be used for drug delivery. Previous studies suggest that increasing the concentrations of the chemical cross-linker reduces the hydrodynamic radius (Rh) and the de-swelling ability, thus primary experiments focused on the variation of cross-linker to polymer ratios. Microgels were synthesized using the polysaccharide polymer hydroxypropyl cellulose (HPC) and chemical cross-linker divinyl sulfone (DVS), in a surfactant solution. Synthesized particles were characterized using dynamic light scattering (DLS) for temperature and angular dependence to study their shape and determine the apparent Rh of the swollen and de-swollen states. Initial microgel synthesis revealed a dependence of Rh on microgel concentration in samples, requiring a correction for infinite sample dilution during analysis. Increasing DVS:HPC ratio from 1 to 30 causes Rh to decrease from 150 to 190 nm at 25 °C, and from 65 to 95 nm at 50 °C. Ratios from 40 to 50 resulted in swelling from 70 nm at 25 °C to 165 nm at 50 °C. At a ratio of 60, an apparent bulk gelation occurred. The increase in DVS:HPC ratio allowed for the controlled synthesis of more compact microgels that display reversible temperature controlled deswelling. However, at ratios above 30, particles were found to grow in size above the transition temperature.
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- Award ID(s):
- 1659541
- PAR ID:
- 10130602
- Date Published:
- Journal Name:
- The Ohio journal of science
- Volume:
- 118
- Issue:
- 1
- ISSN:
- 2471-9390
- Page Range / eLocation ID:
- A49
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.18061/ojs.v118i1
