STRUCTURE OF MINIMUM THICKNESS AND TERRACED FREESTANDING FILMS OF BLOCK-COPOLYMERS

The morphologies of thin, substrate-free block copolymer films have be en examined by cross-sectional TEM. Two poly(styrene-b-butadiene) dibl ock copolymers were studied: one that forms PS cylinders and the other that forms PB cylinders in the bulk. Films were annealed while suppor ted by metal TEM grids, embedded, and ultramicrotomed in cross-section . We find that at the metal support the film forms a meniscus-like reg ion, or Plateau border, which exhibits the bulk morphology. Away from the border, the film thickness decreases and regions of terraced in-pl ane cylinder domains occur until a minimum thickness is reached. The m inimum thickness region of the PB majority copolymer in cross-section shows a PS interlayer penetrated by a hexagonal array of circular PB c hannels that connect upper and lower PB surface layers, and a total th ickness of 25-27 nm. The minimum thickness region of the PS majority c opolymer in plan view shows no image contrast, but in cross-section re veals a continuous PS interlayer covered by layers of PB, and a total thickness of 20 nm. Comparisons with the chain dimensions suggest a bi layer arrangement for both morphologies with strongly perturbed chain conformations in the surface layers. (C) 1996 John Wiley & Sons, Inc.