Time-resolved surface rearrangements of poly(2-hydroxyethyl methacrylate-block-isoprene) in response to environmental changes

The hydrophilic-hydrophobic diblock copolymer (2-hydroxyethyl methacrylate, HEMA)(41)-block-(isoprene)(84) was synthesized by anionic living polymeriz ation using a protection technique for the hydroxyl group of HEMA. The surf ace structures of the block copolymer film under dry and wet conditions wer e analyzed by transmission electron microscopic observation and contact ang le and angle-dependent X-ray photoelectron spectroscopic measurements. The top surfaces of the films cast from DMF/THF and THF/methanol are covered wi th a polyisoprene microdomain. When the as-cast film is exposed to water, t he polyisoprene layer at the top surface disappears and is replaced with a PHEMA microdomain. In particular, the hydrated surface clearly emerges unde r transmission electron microscopy (TEM) by the double-staining method and reverts again to a hydrophobic polyisoprene layer by annealing the film at 52 degrees C for 1 h. Such surface rearrangement occurs repeatedly by soaki ng in water and annealing. The short-time-scale reorientation of the surfac e microdomain of the film under dry to wet conditions is successively obser ved with TEM. The difference between the surface rearrangements of the PHEM A-block-polyisoprene and PHEMA-block-polystyrene films in response to envir onmental change is also investigated.