Wetting behaviour and direct observation of thermally responsive polystyrene-block -poly(N -isopropylacrylamide)-block -polystyrene electrospun fibres in aqueous environment

Abstract

Electrospun fibres of thermally responsive triblock copolymer polystyrene-block-poly(N-isopropylacrylamide)-block-polystyrene were prepared. Fibre morphology and swelling were studied below and above the lower critical solution temperature of poly(N-isopropylacrylamide) (PNIPAM) using cryo-electron microscopy. Cryo-transmission electron microscopy showed that the fibre diameter increased up to 150% after immersion in water at 20 °C. In contrast, at 45 °C the fibre diameter increased considerably less. The sessile drop technique was used to characterize temperature-dependent wetting of fibre mats. Contact angle (θCA) measurements revealed that a block copolymer fibre mat changed from hydrophobic (θCA > 90°) to hydrophilic (θCA < 90°) state within seconds after applying a water droplet on it at 20 °C. At 40 °C the initial contact angle was measured to be higher (135°) and it decreased much less than at 20 °C during the first minute of measurement. We observed using scanning electron microscopy that the electrospun fibres of the block copolymer having 77 wt% of PNIPAM lost their cylindrical shape and changed from fibres to thin sheets at both 20 and 40 °C within seconds after applying water on the fibres. Fibres having 55 wt% of PNIPAM were observed to be stable in water at both 20 and 40 °C, which resulted, surprisingly, in fibre mats with the strongest effects on thermally sensitive wetting. We discuss the surprising results and the implications that the evolution of fibre surface roughness has on the long-term wetting behaviour, demonstrating a self-adaptable hydrophilicity/hydrophobicity nature of the fibre mats. © 2013 Society of Chemical Industry