Stability of Poly(ethylene glycol) Graft Coatings

Abstract

Electrokinetic surface characterization is sensitive to both charge groups (pK and surface density) and neutral polymer coatings (molecular weight, surface density, grafting chemistry). It allows nondestructive, semiquantitative evaluation of coating stability, as regards alteration of the above parameters in relation to various environments. Characterization of surface electroosmosis versus pH (2 to 11) was used to evaluate poly(ethylene glycol) (PEG) coatings of biotechnical significance following a 3-week 22 °C exposure to 0.5 M sodium phosphate solutions of pH 4−11. The coatings involved epoxide-functionalized PEGs of Mr 3400 (E-PEG 3400) covalently grafted to aminosilane-modified fused quartz capillaries. Coatings made from PEGs with an epoxide group 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 stable at pH 4−9. During storage at pH 11 the latter exhibited time-dependent generation of surface negative charge groups of pK ≈ 5.3 and surface density (≈0.1/nm2) similar to the PEG grafting density. The effect was significantly reduced by N2 purging of storage solutions to remove dissolved O2 and was mimicked by fresh, carboxyl difunctionalized PEG 3400 (diSPA-PEG 3400) coatings of similar surface density. The mE-, diE-, and diSPA-based coatings studied appeared to exhibit similar grafting densities (≈0.1/nm2) and coating thicknesses (7 nm), suggesting grafting at only one end group. The factors responsible for limiting surface reaction by both end groups aided the ability of the diE-PEG coatings to secondarily tether (≈0.04/nm2) a model amine-containing compound (n-propylamine) at the diE-PEG-coated quartz surface. Results are discussed in relation to practical applications.