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Effect of the layer-by-layer (LbL) deposition method on the surface morphology and wetting behavior of hydrophobically modified PEO and PAA LbL films.

We demonstrate that the surface morphology and surface-wetting behavior of layer-by-layer (LbL) films can be controlled using different deposition methods. Multilayer films based upon hydrogen-bonding interactions between hydrophobically modified poly(ethylene oxide) (HM-PEO) and poly(acrylic acid) (PAA) have been prepared using the dip- and spin-assisted LbL methods. A three-dimensional surface structure in the dip-assisted multilayer films appeared above a critical number of layer pairs owing to the formation of micelles of HM-PEO in its aqueous dipping solution. In the case of spin-assisted HM-PEO/PAA multilayer films, no such surface morphology development was observed, regardless of the layer pair number, owing to the limited rearrangement and aggregation of HM-PEO micelles during spin deposition. The contrasting surface morphologies of the dip- and spin-assisted LbL films have a remarkable effect on the wetting behavior of water droplets. The water contact angle of the dip-assisted HM-PEO/PAA LbL films reaches a maximum at an intermediate layer pair number, coinciding with the critical number of layer pairs for surface morphology development, and then decreases rapidly as the surface structure is evolved and amplified. In contrast, spin-assisted HM-PEO/PAA LbL films yield a nearly constant water contact angle due to the surface chemical composition and roughness that is uniform independent of layer pair number. We also demonstrate that the multilayer samples prepared using both the dip- and spin-assisted LbL methods were easily peeled away from any type of substrate to yield free-standing films; spin-assisted LbL films appeared transparent, while dip-assisted LbL films were translucent.