| Abstract |
The aim of the present study is to investigate the solution properties of pH-responsive polymeric materials in water. The formation of colloidal metal nanoparticles within such polymeric matrices is also studied and in particular the polymer behavior upon metallation and the metal nanoparticle characteristics are examined. Double-hydrophilic block copolymers of poly(hexa(ethylene glycol) methacrylate) (PHEGMA) and poly(2-diethylamino)ethyl methacrylate) (PDEAEMA) alter their characteristics in aqueous media as a function of the solution pH. At low pH the copolymer is in its unimer state due to the hydrophilicity of the protonated DEAEMA units, while an increase of the solution pH above 7 results in the deprotonation of the amine groups, which become hydrophobic and lead to the formation of micelles. Dynamic light scattering and 1H NMR spectroscopy were used to study the micellization process in aqueous solutions of the above copolymers as a function of the degree of protonation (α) of the DEAEMA units. It was found that for 0.2 ≤ α ≤ 1 the polymer is in its unimer state while micelles are formed only for α < 0.2. The gradual deprotonation of the DEAEMA units was also observed in the 1H NMR spectra of the copolymers as a progressive shift of the DEAEMA peaks to higher fields due to the more effective shielding of the protons from the electron pair of the deprotonated nitrogen. Finally, the formation of micelles leads to the disappearance of the peaks due to the DEAEMA block which becomes non-hydrated and forms the core of the micelles. The core of these micelles is able to dissolve metal compounds due to coordination with the amine units. Such a micellar core can be considered as a nanoreactor, where nucleation and growth of metal nanoparticles upon metal reduction with NaBH4 are restricted to mesoscale level. The polymer behavior after metal incorporation and reduction was studied by dynamic light scattering and 1H NMR spectroscopy. Upon addition of H2PtCl6 or K2PtCl6 metal-induced micellization of the PHEGMA-b-PDEAEMA diblock copolymers at pH values below 7 was found due to the electrostatic interactions between the PtCl62- divalent anions and the positively charged amine groups. At high pH metal-induced micellization was not observed. The metallation process was also followed by UV/Vis spectroscopy while TEM was used to study the metallated micelles and the characteristics of the nanoparticles formed after metal reduction. Finally, the crystalline structure of the platinum nanoparticles was confirmed by X- Ray diffraction which also allowed to estimate the size of the nanoparticles around 3.5 nm. Another class of smart materials which have also attracted great attention in recent years is that of stimuli- responsive microgel particles. In the present work acid-swellable microgels synthesized by emulsion copolymerization of the pH-sensitive 2-(diethylamino)ethyl methacrylate monomer with a bifunctional PPO-based cross-linker have been studied. These microgels exhibit reversible swelling properties in water by adjusting the solution pH. Thus, at low pH the microgel particles are swollen due to the hydrophilicity of the protonated amine units, while an increase of the solution pH leads to the formation of hydrophobic latex particles comprising hydrophobic DEAEMA units. The aqueous solution behaviour of the microgel particles was investigated as a function of solution pH by dynamic light scattering. At pH above 7 a radius of 111 nm was obtained for the latex particles while upon decreasing the solution pH below 7 the hydrodynamic size of the microgels increases to ~180 nm. Platinum nanoparticles were formed within these nano-objects upon addition of H2PtCl6 followed by metal reduction with NaBH4. Similar to the metallation of the diblock copolymers, the microgel properties upon incorporation of the metal precursor (H2PtCl6) and the subsequent metal reduction was studied by dynamic light scattering, while UV/Vis spectroscopy was used to investigate the polymer-metal interactions. The metallated microgel particles and the metal nanoparticle characteristics were studied by TEM. X- Ray diffraction was also used to verify the crystalline structure of the nanoparticles from which the size of the nanoparticles was estimated to be 4.5 nm.
|
| Subject |
αμινοομάδες, μικύλλια, pH, βαθμός πρωτονίωσης, H2PtCl6, K2PtCl6, ηλεκτροστατικές αλληλεπιδράσεις, NaBH4, αναγωγή, νανοσωματίδια πλατίνας, αποκρίσιμα μικροπηκτώματα, amine groups, micelles, pH, degree of protonation, H2PtCl6, K2PtCl6, metal-induced micellization, NaBH4, reduction, platinum nanoparticles, responsive microgel particles, latex particles |
Πολυμερικά Υλικά που Αποκρίνονται στο pH: Επίδραση του Βαθμού Ιοντισμού και Σύνθεση Νανοκρυστάλλων Πλατίνας
