Hydrophilization of silicone-urea copolymer surfaces by UV/ozone: influence of PDMS molecular weight on surface oxidation and hydrophobic recovery

Abstract

Hydrophilization of polydimethylsiloxane urea copolymer (PSU) surfaces and the extent of hydrophobic recovery were investigated as a function of; (i) UV/ozone (UV/O) exposure time, (ii) the aging period after UV/O exposure, (iii) sample preparation method, and (iv) polydimethylsiloxane (PDMS) soft segment molecular weight of the copolymer (1500, 3000 and 11,000 g/mol). All copolymers had a constant urea hard segment content of 15% by weight. Samples were prepared by three different methods, which were; solution casting, spin-coating and electrospinning. XPS spectra clearly showed transformation of PDMS into SiO2 and sub-oxides, which increased gradually with increasing UV/O exposure time. XPS and ATR-FTIR measurements also revealed that the copolymer based on PDMS-11000 displayed the highest amount of SiO2 formation and overall surface modification. Static water contact angle values of the spincoated silicone urea copolymer films decreased significantly from 1100 to 430 after 3 h of UV/O exposure. Interestingly, the hydrophobicity of the electrospun fibers was retained under similar UV/O exposure conditions, most probably due to the preserved surface roughness. Hydrophobic recovery was evaluated after 2 months of storage at ambient conditions. The slowest recovery was observed for spincoated copolymer film based on PDMS-11000, due to higher amount of surface oxidation and formation of a thicker SiO2 barrier layer. (C) 2013 Elsevier Ltd. All rights reserved.