Adsorption of diblock copolymers of poly(ethylene oxide) and poly(lactide)at hydrophilic silica from aqueous solution

The adsorption of a series of amphiphilic poly(ethylene oxide)poly(DL-lacti de) (PEO-PL) diblock copolymers at the water/silica interface was investiga ted by ellipsometry and reflectometry. For all copolymers, a much higher sa turation adsorption is found compared to that of the PEO homopolymers, indi cating the importance of the PL block for the adsorption. The copolymers di splay a saturation adsorption that increases with increasing hydrophobic co ntent of the polymer, and decreases with increasing hydrophilic content of the polymer. Despite this, however, the layer thickness observed is rather similar for all polymers, regardless of the length and composition of the c opolymers. Moreover, the layer thicknesses were significantly higher than w hat would be expected for the unperturbered copolymer dimensions. The initi al adsorption kinetics of the different copolymers are comparable, The init ial adsorption rate increases cooperatively with concentration and is slowe r than that expected for diffusion-controlled adsorption. Moreover, the ads orption increases only slightly over a concentration the range from 20 to 5 0 degrees C, Furthermore, pH titrations show that all polymers exhibit a cr itical desorption pH of 8-9, which is lower than the corresponding value of pH 10.5 observed for the PEO homopolymer. These results are discussed in t erms of the adsorption mechanism and the adsorbed layer structure and forma tion, (C) 2000 Academic Press.