Adsorption of poly(ethylene oxide)-poly(lactide) copolymers. Effects of composition and degradation

The effect of chemical degradation of two diblock copolymers of poly(ethyle ne oxide) (E) and poly(lactide) (L), E39L5 and E39L20, on their adsorption at silica and methylated silica was investigated with in situ ellipsometry. Steric stablization of polystyrene dispersions was investigated in relatio n to degradation. Hydrolysis of the poly(lactide) block of the copolymers w as followed at different temperatures and pH by using HPLC to measure the o ccurrence of lactic acid in solution. The block copolymers were quite stabl e in pH-unadjusted solution at low temperature, whereas degradation was fac ilitated by increasing temperature or lowering of the pH. Lower degradation rates of E39L20 where observed at low temperature in comparison with those of E39L5, whereas the degradation rates of the copolymers were quantitativ ely similar at high temperature. The adsorption of the copolymers at methyl ated silica substrates decreased with increasing degree of degradation due to the reduction in the ability of hydrophobic block to anchor the copolyme r layer at the surface. At silica the adsorption initially increased with i ncreasing degradation, particularly for E39L20 due to deposition of aggrega tes onto the surface. After extensive degradation the adsorption of the cop olymers at both silica and methylated silica resembled that of the correspo nding poly(ethylene oxide) homopolymer. Overall, it was found that the even tual reduction in adsorption occurred at a lower degree of degradation for E39L5 than for E39L20 Mean-field calculations showed a reduced anchoring fo r the block copolymers with decreasing poly(lactide) block length at hydrop hobic surfaces. In accordance with this finding, it was observed that polys tyrene dispersions were stabilized by E39L20 or E39L5 in a way that depende d on both the lactide block length and the degree of degradation. Upon degr adation of the hydrophobic block, stabilization of the polystyrene dispersi ons was maintained initially, but eventually degradation resulted in destab ilization. The average residual copolymer concentration required for stabil ization of the polystyrene dispersions was much higher than the correspondi ng concentration of intact copolymer required for stabilization. (C) zool A cademic Press.