The surfaces of nano-particles have been functionalized to provide a method for dispersion of the insoluble materials in liquid media. Cadmium sulfide (CdS) quantum dot (QD)-poly(acrylate) nano-composites were prepared using AGET ATRP in a miniemulsion.(1) The QD surface was initially functionalized with a tris(alkyl)phosphine, previously modified with an ATRP chlorine initiator, and a controlled polymerization was subsequently carried out from the functionalized surface of the nano-particles. The final material provided a homogeneous dispersion of the QDs in a suspension medium. The optical-absorption edge in the visible spectra of the nano-composites confirmed the presence of CdS QDs in the suspension. Quantum confinement effects were assigned, though a blue shift in relation to the optical spectrum of the initial QDs.
Indeed this example is quite exemplary of the advantages of conducting synthesis of composite particles by conducting a grafting from reaction in AGET ATRP miniemulsion systems. Indeed an efficient synthesis of hybrid organic/inorganic nanoparticles was also conducted using silica particles functionalized with surface tethered initiators in an AGET ATRP miniemulsion process. In comparison to the bulk polymerization, using the same stoichiometry, the use of a miniemulsion polymerization process allowed the preparation of hybrid materials with a higher yield, i.e., higher monomer conversion, and a higher polymerization rate without macroscopic gelation. Direct visualization by AFM provided additional evidence for the formation of well-controlled hybrids.(2) This approach could be applied to the synthesis of various well-defined polymers with complex architectures based on multifunctional initiators.
Another advance on the novel robust technique developed for the synthesis of cadmium sulfide quantum dot-poly(acrylate) nano-composites was the recent development of a one-pot synthesis of thermally stable core/shell gold nanoparticles (Au-NPs) via surface-initiated ATRcoP of butyl acrylate and a dimethacrylate-based cross-linker. The higher reactivity of the methacrylate cross-linker enabled the formation of a thin cross-linked polymer shell around the surface of the Au-NP before the growth of linear polymer chains from the shell.(3) The procedure could be conducted in a one-pot synthesis simplifying the purification step. There was no inter-particle coupling and no coupling between linear polymer chains.
The cross-linked polymer shell served as a robust protective layer, prevented the dissociation of linear polymer brushes from the surfaces of Au-NPs, and provided the Au-NPs excellent thermal stability at elevated temperatures (e.g., 110 DegC for 24 h). This synthetic method could be easily expanded for preparation of other types of inorganic/polymer nano-composites with significantly improved stability.
Remaining on the subject of continued research efforts, several hybrid nanocomposites consisting of a magnesium dihydroxide (MDH) core and tethered poly(meth)acrylate chains were synthesized via ATRP to exemplify a fire retardant that can be tailored to the targeted polymer matrix, in this case for meth(acrylate) matrices.(4) MDH is a representative of flame retardants which release water when heated. Released water can block the flame and exclude oxygen by diluting the presence of flammable gases in the contacting atmosphere. In addition, char formed on the surface of the polymer works as a heat insulating barrier so it interrupts the flow of flammable decomposition products. The hydroxyl groups inherent on the surface of the MDH particles were modified by reaction with 2-bromopropionyl or 2-bromoisobutyryl bromide to attach ATRP initiator moieties to the particle. n-Butyl acrylate, methyl methacrylate, dodecyl methacrylate and octadecyl methacrylate were polymerized from the functionalized MDH particles using the "grafting-from" technique.
The polymer chains attached to the MDH particles provided the composites with enhanced compatibility in blends with common polymers. The efficiency of attachment, and the molecular weight and polydispersity of the polymers attached to the nanoparticles were investigated by GPC and TGA after post-polymerization cleavage from the particles. AFM was used to analyze morphologies and structure of the composites. The resulting hybrid nanocomposites formed stable suspensions and showed good dispersability in solvents.
The dispersability of hybrid particles with matrix selected graft copolymers was examined by studies on the effect of dispersing different concentrations of a SiO2-poly(butyl acrylate) hybrid nanoparticles in a poly(butyl acrylate matrix.
The modulus of the poly(butyl acrylate)/hybrid composite structure progressively increased with increasing concentration of the hybrid particle uniformly dispersed in the blended poly(butyl acrylate matrix). One potential drawback from reinforcing a polymer by addition of a hybrid composite particle is a loss of transparency due to scattering from the embedded particles - a consequence of the significantly different refractive index n of most inorganic materials and the organic embedding medium. A method to suppress the scattering of inorganic nano-particle inclusions within an organic embedding media was recently presented.(5) Suppression of scattering takes place by means of appropriate surface modification of the particle using ATRP such as to match the effective refractive index of the resulting core-shell particle to the refractive index of the embedding medium. The key to the approach is the observation that for core-shell particles with a size less than the wavelength of light the optical properties are approximately equal to those of a homogeneous particle with an effective dielectric constant that depends on the optical properties and volume fractions of the respective constituents. The reference provides a selection of common particle filler materials with appropriate composition of surface-grafted polymer to achieve index-matching and compatibilization, the selected polymer pairs were chosen because of their respective negative Flory-Huggins interaction parameter, for a variety of matrix polymers.
A potential application for homo-arm stars was illustrated by preparation of two well defined star macromolecules with two oppositely charged arm structures; a poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) star and a poly(acrylic acid) (PAA) star with cross-linked cores.(6) It is possible to exploit the electrostatic interactions between the polyelectrolyte arms of PDMAEMA star and PAA star polymers to generate all-star polyelectrolyte LbL films with alternating multilayer thin films using layer-by-layer (LbL) assembly.
The star/star multilayer films possess non-uniform and nanoporous structures, which result from the characteristic architecture of star polymers. The thickness, porosity, and refractive index of star/star multilayer films are precisely tunable by assembly pH conditions. Furthermore, as-assembled star/star multilayer films exhibit distinct morphological changes by undergoing extensive structural reorganization upon post-treatment under different pH conditions that do not lead to any changes with their linear compositional counterparts; it is hypothesized that these differences are due to the star polyelectrolyte's compact structure and decreased extent of entanglement and interpenetration, which lead to a low degree of ionic crosslinking compared to their linear counterparts. We have observed an enhanced ionic (proton) condution of star/star multilayers following the pH-induced structural reorganization.
(1) Esteves, A. C. C.; Bombalski, L.; Trindade, T.; Matyjaszewski, K.; Barros-Timmons, A. Small 2007, 3, 1230-1236.
(2) Bombalski, L.; Min, K.; Dong, H.; Tang, C.; Matyjaszewski, K. Macromolecules 2007, 40, 7429-7432.
(3) Dong, H.; Zhu, M.; Yoon, J. A.; Gao, H.; Jin, R.; Matyjaszewski, K. J. Am. Chem. Soc. 2008, 130, 12852-12853.
(4) Ok, J.; Matyjaszewski, K. Journal of Inorganic and Organometallic Polymers and Materials 2006, 16, 129-137.
(5) Bombalski, L.; Dong, H.; Listak, J.; Matyjaszewski, K.; Bockstaller, M. R. Advanced Materials 2007, 19, 4486-4490.
(6) Kim, B.-S.; Gao, H.; Argun, A. A.; Matyjaszewski, K.; Hammond, P. T. Macromolecules 2009, 42, 368-375.