Block copolymer synthesis and self-assembled nanostructures
In TPCI we are interested in the synthesis of well-defined block copolymers, i.e. block copolymers whose molecular characteristics, like overall molecular weight and molecular weight of each block, chemical composition and architecture can be controlled at will. To this end, we employ well-established living/controlled polymerization methodologies, primarily anionic polymerization high vacuum techniques and controlled free radical polymerization schemes, as well as post polymerization functionalization strategies. By judicious choice of the monomers, the desired chemical structures and physicochemical properties can be built into the macromolecules, leading to the formation of functional block copolymers. Research is focused on the development of novel amphiphilic and double hydrophilic block copolymers with polyelectrolyte blocks. These well-defined block copolymers can be utilized for basic structure/properties relationship determinations but also for functional nanomaterials designs and nanotechnological applications, since they can self-assemble in a variety of nanostructures in solution, in bulk or on surfaces, they can form hybrid nanostructures with other building blocks or they can act as nanoscale templates or nanoreactors in materials synthesis or as nanocarriers in drug and gene delivery systems.
Poly(methacrylic acid-b-p-hydroxystyrene) (PMAA-PHOS) double hydrophilic block copolymers were synthesized from poly(p-tert-butoxystyrene-b-tert-butyl methacrylate) precursors, prepared via anionic polymerization high vacuum techniques, via a one step post polymerization hydrolysis reaction of both precursor blocks. The PMAA-PHOS copolymers showed pH-responsive self-assembly behavior in aqueous solutions as light scattering and AFM investigations indicated. The copolymers are in the form of unimers or loose aggregates at pH>9, where phenolate and carboxylate groups are dissociated. Micellar like structures, having compact PHS cores and PMAA coronas, and micellar clusters are present at 9>pH>4, while the copolymers precipitate at pH<4 (Macromolecules 39, 4767 (2006)). Multicompartment micelles from triblock terpolymers
A new amphiphilic triblock copolymer poly((sulfamate-carboxylate)isoprene)-b-polystyrene-b-poly(ethylene oxide), PISC230-PS52-PEO151, was synthesized via the post polymerization reaction of the anionically prepared precursor copolymer polyisoprene-b-polystyrene-b-poly(ethylene oxide) with chlorosulfonyl isocyanate. The self-assembly behavior of the polyelectrolyte triblock copolymer in dilute aqueous solutions was studied by static and dynamic light scattering, atomic force and cryogenic transmission electron microscopy, fluorescence and 1H NMR spectroscopy. In acidic solutions, the copolymer self-assembles into kinetically trapped multicompartment micelles with the core consisting of discrete PS and PISC domains and with a PEO shell. If the solution pH is adjusted to the alkaline region, the multicompartment micelles undergo irreversible transition to regular micelles with the PS core and a hybrid shell formed by PEO and PISC blocks (Macromolecules 2009, 42, 5605).
Complexes of polyelectrolyte-neutral block copolymers with surfactants and polyelectrolytes
Double Hydrophilic Block Copolymer Complexes
Complexes between anionic-neutral sodium (sulfamate-carboxylate)isoprene/ethylene oxide double hydrophilic diblock copolymers (SCIEO) and the cationic polyelectrolyte quaternized poly(2-vinylpyridine) (QP2VP), as well as the cationic surfactant dodecyltrimethylammonium bromide (DTMAB) were studied in aqueous solutions, at pH 7. The mass and size of the complexes depend on the mixing ratio between the components. A transition from intrachain to an interchain association was observed for block copolymer/surfactant complexes. SCIEO/QP2VP complexes were found to respond to increasing concentrations of added salt. Spherical and ellipsoid shaped complexes with a core-shell micellar like structure were formed in the systems studied, as evidenced by light scattering and AFM experiments (J. Phys. Chem. B 111, 8351 (2007)).
Complexes ofpolyelectrolyte-neutral block copolymers with proteins
Complexes between sodium (sulfamate-carboxylate)isoprene/ethylene oxide double hydrophilic block copolymers (SCIEO) and lysozyme, a globular protein, were formed in aqueous solutions, at pH 7, due to electrostatic interactions between the anionic groups of the polyelectrolyte block of the copolymers and the cationic groups of lysozyme. The structure of the complexes was investigated as a function of the anionic/cationic charge ratio of the two components in solution and ionic strength by static, dynamic and electrophoretic light scattering, atomic force microscopy and fluorescence spectroscopy. The mass and size of the micellar like complexes depend on the mixing ratio and the molecular characteristics (molecular weight, composition and architecture) of the copolymer used. Complexation persists at 0.15 M NaCl, the value for physiological saline, as a result of additional hydrophobic interactions between the copolymers and the enzyme. Fluorescence spectroscopy measurements indicate that the secondary structure of lysozyme does not change substantially after complex formation. This is a model system that can be useful to the design of nanocarrier and delivery systems for protein drugs (J. Polym. Sci. Part A: Polym. Chem. 45, 509 (2007), Macromol. Biosci. 2010, 10, 139).
Au nanoparticles were formed through polymer induced reduction of gold ions within the corona of poly[tert-butylstyrene-b-sodium (sulfamate-carboxylate-isoprene)] (BS-SCI) micelles in water. Temperature accelerates the formation of Au nanoparticles, however once the Au reduction is complete no effect on the stability of the BS-SCI/Au colloidal material was noticed. The complex BS-SCI/AuNPs hybrid colloid displayed reversible pH sensitivity, due to the chemical/polyelectrolyte structure of the SCI corona block, taking a more compact conformation under acidic conditions, and a more loose one under alkaline conditions. These structural changes resulted in changes in the optical properties, observed via shifts in the SPR band of the system, due to changes in the AuNPs interparticle distances. An increase in the ionic strength of the solution promoted agglomeration of AuNPs within the corona followed by changes in the UV-Vis absorption spectra. Finally, lysozyme could be complexed with the BS-SCI/Au colloid resulting in a multifunctional protein carrier system that also includes a metal nanoparticle based marker that may also be utilized in protein diagnostics (Polymer 50, 2743 (2009)).