Surface and Interface Structure of Diblock Copolymer Brushes

Bulent Akgun University of Akron, Akron Ohio

Diblock copolymer brushes (BCBs) have garnered enormous interest in recent years due to their stimuli-responsive behavior. Their surface character can be substantially altered by treatment with a solvent selective for one or the other of their blocks. Although polymer brush chemistry has widely studied, the internal brush structure and surface dynamics are still not clear. We have resolved the internal structure of ultrathin BCBs using neutron reflectivity and grazing incidence small-angle X-ray scattering. BCBs of polystyrene-b-poly(methyl acrylate) and polystyrene-b-poly(n-butyl acrylate) were synthesized using atom transfer radical polymerization. Internal brush structure depends strongly on the block sequence and the value of ÇN. For the thinnest films a model of two layers with smooth interfacial gradient provides a good description of the data. For films of thicker and sufficiently asymmetric composition of dPS-b-PMA samples a third layer must be included. This is consistent with the presence of a lateral ordering of some type in the center of the brush, as evidenced by GISAXS data. The interface width is found to be smaller for PMA-b-dPS than for dPS-b-PMA brushes. After a brush is treated with a selective solvent which is a good or theta solvent for the bottom block and poor solvent for the top block, Bragg rods appear in GISAXS pattern. The lateral spacing corresponding to the Bragg rods is on the order of the total thicknesses of the brushes. This lateral correlation is also detected by the power spectral density analysis of the AFM measurements using tapping mode imaging. The Bragg rods disappear upon heating to 80 °C and this behavior does not depend on which polyacrylate block was used. We have also investigated the surface fluctuations of molten homopolymer brushes using X-ray photon correlation spectroscopy (XPCS). The surface fluctuations were highly suppressed within the experimental window of the XPCS instrument, even 130 °C above the bulk glass transition temperature of the polymers.

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