Self-assembly of poly(ethylene glycol)-block-polypyridine copolymer into micelles and at silica surface: effect of molecular architecture on silica dispersion

Abstract

We have newly synthesized amphiphilic block copolymers composed of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic pyridine segments (PEG-b-Py). Chain transfer agent-terminated PEG was subsequently chain-extended with 3-(4-pyridyl)-propyl acrylate to obtain PEG-b-Py by reversible additional-fragmentation chain transfer polymerization. Particularly, the effect of varying molecular weight (Mn) of PEG (Mn = 2,000 and 5,000) and Py in the block copolymers was investigated in terms of critical micelle concentration, pyrene solubilization, micelle size distribution, and association number per micelle. Based on the amphiphilic balance, PEG-b-Pys formed core-shell type polymer micelle. The association number of PEG2k-b-Py was higher than that of PEG5k-b-Py, suggesting the degree of phase separation strongly depended on PEG Mn. Furthermore, the adsorption of PEG-b-Py copolymer onto silica nanoparticles as dispersant was studied to estimate the effect of PEG Mn in the copolymers and their solubility in the medium on the adsorption. Adsorbed density of PEG2k-b-Py copolymer onto silica nanoparticle was higher than that of PEG5k-b-Py, which was significantly correlated with the degree of phase separation. Furthermore, the adsorbed amount of copolymer increased with the increase in ionic strength due to the reduced solubility of PEG in the buffer solution. The resultant dispersion stability was highly correlated with the graft density of copolymer onto silica surface. However, the stability of PEG2k-b-Py coated particles was lower than that with PEG5k-b-Py, this is attributed to the relatively thin layer of PEG at the silica surface, which cannot provide the system with sufficient steric stabilization as the salt concentration increases. These fundamental investigations for the surface modification of the nanoparticle provide the insight into the highly stable colloidal dispersion, particularly in the physiological condition with high ionic strength.