STABILITY OF POLY(ETHYLENE GLYCOL) GRAFT COATINGS

Electrokinetic surface characterization is sensitive to both charge gr oups (pK and surface density) and neutral polymer coatings (molecular weight, surface density, grafting chemistry). It allows nondestructive semiquantitative evaluation of coating stability, as regards alterati on of the above parameters in relation to various environments. Charac terization of surface electroosmosis versus pH (2 to 11) was used to e valuate poly(ethylene glycol) (PEG) coatings of biotechnical significa nce following a 3-week 22 degrees C exposure to 0.5 M sodium phosphate solutions of pH 4-11. The coatings involved epoxide-functionalized PE Gs of M(r)3400 (E-PEG 3400) covalently grafted to aminosilane-modified fused quartz capillaries. Coatings made from PEGs with an epoxide gro up at one end (mE-PEG 5000) were stable at pH 4-11 whereas those made from PEGs with epoxide groups at both ends (diE-PEG 3400) were only st able at pH 4-9. During storage at pH 11 the latter exhibited time-depe ndent generation of surface negative charge groups of pK approximate t o 5.3 and surface density (approximate to 0.1/nm(2)) similar to the PE G grafting density. The effect was significantly reduced by N-2 purgin g of storage solutions to remove dissolved O-2 and was mimicked by fre sh, carboxyl difunctionalized PEG 3400 (diSPA-PEG 3400) coatings of si milar surface density. The mE-, diE-, and diSPA-based coatings studied appeared to exhibit similar grafting densities (approximate to 0.1/nm (2)) and coating thicknesses (7 nm), suggesting grafting at only one e nd group. The factors responsible for limiting surface reaction by bot h end groups aided the ability of the diE-PEG coatings to secondarily tether (approximate to 0.04/nm(2)) a model amine-containing compound ( n-propylamine) at the diE-PEG-coated quartz surface. Results are discu ssed in relation to practical applications.