Generation of metal nanoparticles in spherical polyelectrolyte brushes and their application in heterogeneous catalysis
This thesis describes synthesis of spherical cationic polyelectrolyte brushes by “grafting from” technique. The spherical polyelectrolyte brushes have been used as “nanoreactors” for the synthesis of metal nanoparticles (Gold, Platinum and Silver). The catalytic activity of the resulting metal/polymer nanocomposites has been tested by heterogeneous hydrogenation of carbonyl groups. The synthesis of the cationic spherical polyelectrolyte brushes is achieved by a three-step procedure. Firstly, cationic polystyrene core particles are synthesized by emulsion polymerization by using a cationic surfactant and a cationic thermal initiator. In the second step, a thin layer of photoinitiator is generated around the particles by addition of the photoinitiator under “starved conditions” . The photoinitiator is a monomer, which is added when the formation of core particles is in the last stage. The photoinitiator polymerizes with the styrene to give a covalently bound photoinitiator. In the third and last Step, the brushes are grafted on the core particles by photoemulsion polymerization, where the initiation is triggered by UV/VIS radiation. The important parameters of the brushes- contour length and grafting density are determined by the cleavage of the chains from the surface by alkaline hydrolysis. The ester functionality within the photoinitiator is hydrolyzed under the harsh conditions of hydrolysis. The cleaved chains are analyzed to determine the molecular weight. The grafting of the charged polymeric chains stabilizes the colloids even under unfavorable conditions like high ionic strength and high pH. The behavior of brushes is investigated at different ionic strength and pH. At different ionic strengths brushes show three regimes- osmotic, salted and neutral regime. At increasing ionic strength, a shrinking in brush thickness is observed due to the screening of ionic charges. The cationic brushes (polyamino ethylmethacrylate hydrochloride and poly vinylbenzylamine hydrochloride) bear protonated amine functionality. The brushes are classified as annealed brushes as they are sensitive towards pH. The brushes lose the protons at high pH, to result in the uncharged brushes, causing them to shrink, which is followed by dynamic light scattering. Polyaminoethylmethyacrylate brushes are used as nanoreactors to synthesize the metal nanoparticles of gold and platinum. The water-soluble metal salts are to introduce metal ions in the brushes. The negatively charged metal ions (AuCl4- and PtCl6-2) interact with the cationic chains of the brushes. Dynamic Light Scattering is used to study the influence of the metal ions on the brushes. It is observed that metal ions induce much more pronounced shrinking as compared to the monovalent ions. The shrinking can be compared with the shrinking caused multivalent ions such as MgSO4. The metal ions once introduced are localized within the brushes due to strong correlation of counterions with the polyelectrolyte chains. The excess ions are cleaned by ultrafiltration. The trapped counterions render high contrast to the brushes and hence visualize brushes in Cryo-TEM. The metal ions can be reduced chemically by NaBH4 to generate nanoparticles. The particles are studied by high-resolution Transmission microscopy and cryogenic TEM. The metal nanoparticles formed are well spaced and crystalline in nature. The particles formed are found to be stable against aggregation. The silver nanoparticles are formed by using the anionic brushes (polyacrylic acid) and AgNO3 as the precursor salt. The catalytic activity of Platinum is tested by the heterogeneous hydrogenation of the carbonyl functionalities with dihydrogen under optimum conditions. The platinum nanoparticles give a 90% conversion of butyraldehyde to 1-butanol. The catalyst is found to be recyclable for a number of runs without losing the efficiency. The time dependent studies are carried out to gain insight in the mechanism and the kinetics of the reaction. The particles are found to be stable after the catalytic cycles. The gold nanoparticles supported on the polystyrene are found to be catalytically active for the same reaction. The gold particles are found to be recyclable for a number of runs without losing the efficiency. The bulk gold is completely inactive, hence the origin of catalytic activity is attributed to the quantum size effects. As the polystyrene particles are inert, the role of support particles in the origin of catalytic activity can be ruled out. The nanoparticles are found to become inactive for catalysis after 6 months of standing with a manifold increase in the particle size as revealed by microscopy.
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