Thermal shaping of shell-crosslinked (SCK) nanoparticles, facilitated by nanoconfinement of fluid-like cores

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Abstract

The controlled manipulation of the shape of shell-crosslinked (SCK) nanoparticles is described as a novel methodology for the construction of complex nanostructured materials. The presence of a hydrophobic, fluid-like (low Tg) core domain in the SCK allows for substrate-induced shaping of the nanoparticle. The SCK nanoparticles are prepared by the self-assembly of poly(acrylic acid)-block-polyisoprene (PAA-b-PI) diblock copolymers into polymer micelles, followed by crosslinking of the hydrophilic shell layer via amidation reactions between the acrylic acid functionalities and 2,2′-(ethylenedioxy)bis(ethylamine). The synthesis of two amphiphilic diblock copolymers, one consisting of an unmodified PI block (PAA-b-PI, PI having 86% 4,1-microstructure, Tg(PI) = −75 °C) and the other composed of a hydrochlorinated PI block (PAA-b-PI(HCl), Tg(PI(HCl)) = 27 °C), the assembly of the copolymers into polymer micelles and the subsequent shell-crosslinking of the micelles into SCKs is described. The synthesis of a poly(tert-butyl acrylate)-block-polyisoprene (PtBA-b-PI) precursor via nitroxide-mediated radical polymerization (NMRP) and the subsequent methanesulfonic acid catalyzed cleavage of the precursor's tert-butyl ester groups is discussed. Treatment of poly(tert-butyl acrylate)-block-polyisoprene (PAA-b-PI) with HCl(g) is also described as a method for hydrolysis of the tert-butyl esters and for hydrochlorination of the unsaturated groups of PI to yield PAA-b-PI(HCl) in one step. The results of the analyses of solution properties are presented and these findings, in conjunction with transmission electron microscopy (TEM) and atomic force microscopy (AFM) size measurements, indicate spherical shapes for the SCKs in aqueous solution. Using a combination of TEM and AFM dimension determinations, an SCK containing a PI core (predominantly cis-4,1-repeat units) of Tg = −63 °C is shown to deform, at room temperature, from the solution-state spherical shape upon deposition onto a mica substrate. By contrast, deposition of an SCK containing a PI(HCl) core (Tg = 33 °C) shows little deformation at room temperature from spherical shape as determined by its particle aspect ratio of 1.4 (sphere = 1.0). Thermally-driven particle shape manipulation, behavior analogous to the processing of nanoscopic thermoplastic materials, is demonstrated with the hydrochlorinated SCK (PAA-b-PI(HCl)). A large decrease in the average AFM particle height of this SCK is found upon heating above the Tg of the hydrochlorinated PI core (33 °C). This finding demonstrates the major role played by the core (glassy vs. fluid-like) in defining the equilibrium shape of the SCK. Crosslinking within the core domain (in addition to the initial shell crosslinking) is also shown to dramatically increase the rigidity of the nanoparticles and prevent nanoparticle shape deformation when in contact with substrates.

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