Surface hydroxylation of polyethylene by plasma polymerization of allyl alcohol and subsequent silylation

Silylation of silica and glass is commonly used to prepare model surfaces for studies of the interactions of biological fluids and tissues with materials. In this work, the silylation of modified polyethylene surfaces has been investigated in the same context. Microwave frequency plasma polymerization of allyl alcohol was used to hydroxylate polyethylene (PE) surfaces followed by treatment with NaBH4 and then with chlorosulphonylphenylethyl trichlorosilane (CSPES) or glycidoxypropyl trimethoxysilane (GPS). Surface characterization revealed changes in composition and structure at each step. Water contact angles indicated that the plasma modified surfaces were less hydrophobic than the untreated polyethylene. X-ray photoelectron spectroscopy (XPS) showed a large increase in oxygen content of the NaBH4 treated plasma polymerized allyl alcohol modified surface (PE–Ar–AA–OH) compared to PE. Reaction with trifluoroacetic anhydride and determination of fluorine by XPS was used as a means of determining the surface hydroxyl content of PE–Ar–AA–OH. The data showed a high concentration of hydroxyl groups to be present. High resolution analysis of the C1s spectra showed the presence of peaks at higher binding energy following successive treatments, consistent with the formation of COH and other groups. Atomic force microscopy images of the plasma modified surfaces revealed changes in both surface morphology and roughness of the PE modified surfaces. Following silylation of PE–Ar–AA–OH with CSPES and GPS, further decreases in advancing and receding water contact angles were observed. The CSPES surface showed the lowest contact angles. XPS data for CSPES showed increases (relative to the PE–Ar–AA–OH) in sulphur, silicon and chlorine content; for GPS an increase in silicon content was noted. High resolution analysis of the C1s peak showed increases in the 285 eV (alkyl and aromatic groups) contribution for CPSES and in the 286.5 eV (ether and hydroxyl groups) contribution for GPS. AFM images showed that the two silylated surfaces have morphologies similar to each other, but different from that of the underlying substrate. The silylated materials may be used for subsequent attachment of bioactive amino acids, peptides or proteins in the development of biocompatible polymers.