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Research
POLYMER BRUSHES AS TEMPLATES FOR THE FORMATION OF METALLIC NANOPARTICLES
Polymer brushes, due to their nanometer dimensions and well-defined structure, offer many advantages as templates for the preparation and application of nanoparticles when compared to other polymeric templates. These advantages include the ability to control assembly of nanoparticles over multiple length scales, superior precision over template architecture, the availability of a greater variety of functional groups, and ease of use and recycling. The goal of this project is to develop tethered functional polymer brush structures to act as templates for the preparation of both mono- and multi-component catalytic systems.
To achieve this goal, controlled/“living” free radical polymerization techniques, including atom transfer radical polymerization (ATRP) and reversible addition-fragmentation chain transfer (RAFT) polymerization, will be utilized to prepare functional block copolymers covalently attached to flat and high-surface area substrates. The selection and position of functional monomers within the block copolymers will facilitate attachment of metal cations and the subsequent formation of metal nanoparticles with well-defined spatial and compositional control. In previous work, we have demonstrated to use of block copolymer brushes, consisting of polystyrene (PS) and poly(acrylic acid) (PAA), as templates for the synthesis of both silver and palladium nanoparticles. The formation of the nanoparticles was confirmed using both X-ray photoelectron spectroscopy (XPS), attenuated total reflectance infrared spectroscopy (ATR-FTIR), and atomic force microscopy (AFM).
Other examples of monometallic nanoparticles prepared in diblock copolymer brushes of PS and PAA include cobalt, gold, platinum, and copper. Below is a series of ATR-FTIR spectra for the formation of cobalt nanoparticles within a PS-b-PAA polymer brush. Of primary interest in determining the success of this method is monitoring changes in the carbonyl peak of the PAA. In spectrum (a), the carbonyl peak of the PAA is a broad peak at approximately 1700 wavenumber. After treatment with cobalt acetate, there is a complete shift in the carbonyl peak from 1700 to approximately 1500 wavenumbers which corresponds to complete formation of the carboxylate anion in the PAA block (spectrum (b)). Following loading of the cobalt ions onto the PAA block, reduction with sodium borohydride results in a shift back from the carboxylate anion to the carbonyl peak at approximately 1700 wavenumbers (spectrum (c)). This shift corresponds to little if any change in the cobalt content of the diblock copolymer brush as measure by XPS.
In conjunction to the formation of monometallic nanoparticles, bimetallic nanoparticles can be formed by treatment of the PS-b-PAA polymer brush with a solution containing two different metal cations:
A specific example of this is treatment of a PS-b-PAA polymer brush with palladium and silver acetate. In this case XPS was used to confirm the loading of both metals onto the PAA block of the polymer brush. The XPS spectra indicates that before loading of the metals only carbon, oxygen and silicon and detected in the sample. After loading we can see the presence of both silver, 0.7 atomic%, and palladium, 2.8 atomic%. Using an approximation we can calculate the degree of loading on the PAA block which was approximately 48% palladium and 10% silver. This suggests that loading of the PAA block was incomplete, which was subsequently confirmed by ATR-FTIR.
We are currently working on methods to improve the loading efficiency in the bimetallic systems and the application of both the monometallic and bimetallic systems for catalytic reactions.
