Aqueous Solution Characterization and Metal Nanoparticle Formation in pH-responsive PHEGMA-block-PDEAEMA Diblock Copolymers
Katsamanis, Vasilis H
The micellization behavior of pH-sensitive diblock copolymer is investigated. The pH-responsive tertiary amine block is hydrophobic at higher pH forming the core of the micelles. Metal nanoparticles can be formed within these cores, which can be used as catalysts. A series of pH-sensitive, double-hydrophilic diblock copolymers comprising a neutral, hydrophilic poly(hexa(ethylene glycol) methacrylate) (PHEGMA) block and an ionizable pH-tunable poly(2-diethylamino)ethyl methacrylate) (PDEAEMA) block were synthesized by group transfer polymerization. Molecular weights of the copolymer are between 16000 and 27000. These copolymers can self-assemble into micellar structures in water by adjusting the solution pH. At pH 2 the copolymer is in its unimer state due to the hydrophilicity of the protonated DEAEMA units, while an increase of the solution pH>7 results in the deprotonation of the amine residues, which become hydrophobic, and lead to the formation of micelles. Three different experimental techniques, dynamic light scattering (DLS), pH-metry and 1H NMR spectroscopy were used in order to study the behavior of the copolymers and the respective homopolymers in aqueous solution and characterize the polymer structures formed. Potensiometric titration curves show a plateau between pH 6 and 8 were deprotonation of the amine groups occurs. The extent of plateau was found to increase with the DEAEMA content of the copolymer. The addition of base (NaOH 0.1M) in a low pH polymer solution (forward titration curve) and the addition of acid (HCl 0.1M) in a high pH polymer solution (back titration curve) follow a similar pattern and suggest a fully reversible protonation process. DLS has been used for the investigation of the aqueous solution properties and the micellar behavior of the PHEGMA-b-PDEAEMA diblock copolymers and PHEGMA and PDEAEMA homopolymers. Single chains and aggregates are observed at pH ~2 with hydrodynamic radii of around 4nm and 80nm respectively. At pH ~10 micelles with a hydrodynamic radius of around 13.5nm and micellar aggregates with a hydrodynamic radius of around 100nm are formed. The effect of the chain length of the hydrophobic DEAEMA block and the polymer concentration on the micelle characteristics were also investigated. 1H NMR spectra recorded as a function of pH verify the formation of micelles at pH >7 consisting a PDEAEMA core and a PHEGMA corona, whereas they produce evidence of the partial deprotonation of the PDEAEMA units in the range 7-7.75. The PDEAEMA core can dissolve metal compounds due to coordination with the N atom of the amine. In this nanoreactor, metal nanoparticles can nucleate and grow upon reduction with NaBH4. Metal nanoparticle size is in the range of a few nanometers as TEM pictures show. The formation of metal nanoparticles within the micelles cores was investigated, using three different preparation procedures. The micellization process and the polymer behavior upon metal incorporation and metal reduction, while varying the solution conditions, were studied by DLS. PtCl62- was found to induce copolymer micellization by complexation even at low pH values. At high pH micelles containing metal nanoparticles in their cores with micelles of around 14nm, that depend on the copolymer molecular weight, have been found. It has been shown that these nanoparticles exhibit significantly enhanced catalytic properties for hydrogenation and oxidation reactions.