Article pubs.acs.org/JPCB
Thermally Reversible Physically Cross-Linked Hybrid Network Hydrogels Formed by Thermosensitive Hairy Nanoparticles Roger A. E. Wright, Daniel M. Henn, and Bin Zhao* Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37996, United States S Supporting Information *
ABSTRACT: This Article reports on thermally induced reversible formation of physically cross-linked, three-dimensional network hydrogels from aqueous dispersions of thermosensitive diblock copolymer brush-grafted silica nanoparticles (hairy NPs). The hairy NPs consisted of a silica core, a water-soluble polyelectrolyte inner block of poly(2(methacryloyloxy)ethyltrimethylammonium iodide), and a thermosensitive poly(methoxydi(ethylene glycol) methacrylate) (PDEGMMA) outer block synthesized by sequential surface-initiated atom transfer radical polymerizations and postpolymerization quaternization of tertiary amine moieties. Moderately concentrated dispersions of these hairy nanoparticles in water underwent thermally induced reversible transitions between flowing liquids to self-supporting gels upon heating. The gelation was driven by the lower critical solution temperature (LCST) transition of the PDEGMMA outer block, which upon heating self-associated into hydrophobic domains acting as physical cross-linking points for the gel network. Rheological studies showed that the sol−gel transition temperature decreased with increasing hairy NP concentration, and the gelation was achieved at concentrations as low as 3 wt %.
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INTRODUCTION Polymer brush-grafted nanoparticles, i.e., hairy NPs, are unique hybrid materials consisting of polymer chains end-tethered to the surface of core NPs.1−3 They have shown great promise in a wide range of applications such as advanced polymer nanocomposites, chemical sensing, catalysis, water remediation, and lubrication.1−8 The NP cores are typically inorganic6−10 or metallic4,11−14 in nature with a variety of possible shapes and functionalities.11−15 While hairy NPs can be made by grafting end-functionalized polymers to the surface of core NPs (“grafting to”),5 those with high grafting densities are often synthesized by growing the polymer brushes from the NP surface, i.e., “grafting from”.1−3 By coupling this approach with “living”/controlled radical polymerization techniques such as atom transfer radical polymerization (ATRP), nitroxidemediated radical polymerization, or reversible addition− fragmentation chain transfer polymerization, well-defined polymer brushes with controlled molecular weights and narrow polydispersities have been synthesized from a variety of NPs.6−16 A unique feature of hairy particles is that they allow for a combination of desired properties from both the polymer, such as stimuli-responsiveness5,17 or environmental compatibility,2−4 and the NPs, including optical,12−14,18 magnetic,7,19 or other physical properties.20 We have been particularly interested in environmentally responsive polymer brush-grafted NPs, e.g., thermosensitive hairy NPs, and their behavior upon application of external stimuli.17,21,22 Thermoresponsive hairy NPs are typically made with water-soluble polymers displaying a lower critical solution temperature (LCST) transition in water.5,21−23 These hybrid © XXXX American Chemical Society
NPs exhibit a decrease in hydrodynamic volume upon heating to above the LCST, and the LCST transition of polymer brushes is usually broader and begins at a lower temperature compared with the corresponding free polymer in water. The grafted stimuli-responsive polymers on the core NPs often determine the interactions between hairy NPs and their environments. For example, thermosensitive hairy NPs can undergo spontaneous, quantitative, and reversible transfer between water and an immiscible liquid phase, either an organic solvent or a hydrophobic ionic liquid in response to temperature changes,21,22 similar to the thermoresponsive diblock copolymer micelle shuttles reported by Lodge et al.24,25 Thermoresponsive hairy NPs are structurally similar to block copolymer micelles in some sense. Thermosensitive block copolymers have received considerable attention, especially for their utility in the formation of physical hydrogels, which exhibit reversible, in situ sol−gel transitions in response to temperature variations.26−29 These hydrogels have advantages over their chemically cross-linked analogues for certain applications, e.g., site-specific drug delivery where their unique sol−gel transition behavior can allow them to be injected as a free-flowing solution which turns into a gel immediately after injection due to the temperature change.27−30 There are generally two types of injectable block copolymer hydrogels: gels based on the packing of discrete micelles26,31−33 and gels based on the bridging of micellar cores by a water-soluble Received: June 14, 2016 Revised: July 20, 2016
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DOI: 10.1021/acs.jpcb.6b06009 J. Phys. Chem. B XXXX, XXX, XXX−XXX
Article
The Journal of Physical Chemistry B
PDMAEMA were converted to charged trimethylammonium iodide moieties by alkylation with methyl iodide,42 yielding polymer brushes with a hydrophilic inner block and a thermosensitive outer block. At sufficiently high concentrations and above the LCST of the outer block, the PDEGMMA blocks self-assembled into hydrophobic domains (i.e., hydrophobic micellar cores), which were linked to core silica NPs by the bridging polyelectrolyte chains in a manner similar to thermosensitive ABA or ABC micellar hydrogels (Scheme 1). The hairy NPs were characterized by thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and 1H NMR spectroscopy; the brush architecture was confirmed by cleavage with hydrofluoric acid and subsequent size exclusion chromatography (SEC) analysis. The hydrogel properties were studied by rheological measurements.
block, usually the center block of ABA or ABC triblock copolymers (i.e., physically cross-linked three-dimensional network gels).26,34−36 The latter usually requires a significantly lower polymer concentration for gelation, typically,
