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Symmetric, diblock copolymers confined between two solid surfaces were studied by neutron reflectivity. A multilayered morphology with an integral number of layers oriented parallel to the solid interfaces was found in all cases. The period of the confined multilayers deviated from the bulk period in a cyclic manner as a function of the confined film thickness. A first-order transition occurred between the expanded and contracted states of the copolymer chains. The data suggest that the deviation of the period from the bulk value decreases with increasing separation distance.PACS numbers: 61.41.+e, 61.12.Ex, 68.55.3k Symmetric, diblock copolymers are polymer chains comprised of two chemically distinct polymer chains covalently bonded together at one end where the volume fraction of each constituent is 0.5. In the bulk, copolymers above a critical molecular weight microphase separate [I] into lamellar microdomains having a period commensurate with the size of the copolymer chain, i.e. , on the size scale of hundreds of angstroms. However, in the vicinity of a surface, the specific interactions of the segments of each block with the surface force an orientation of the lamellar microdomains parallel to the surface with a period equal to the bulk value, Lrt [2-6]. The formation of such oriented multilayers, coupled with the high resolution of neutron reflectivity, has provided a route for quantitatively characterizing the morphology of ordered copolymer systems [5]. More generally, these self-assembled multilayers have proven to be ideal models for the investigation of such phenomena as two-dimensional coarsening processes [7][8][9] and finite size scaling effects on phase transitions [10,11]. Typically, copolymer films are prepared on a solid substrate leaving the polymer surface unconstrained. At equilibrium, the total film thickness at any point has been shown to be given by nLO, if the segments of one block segregate preferentially to both the air and substrate interfaces [3,6, 12], or by (n+ 2 )La, if one block segregates to the substrate interface and the other to the free surface [2]. Here, n is an integer. If the initial film thickness t does not conform to this condition, then the constraint is met by the formation of steps on the free surface with a height of Lo and a surface coverage commensurate with the relation of t to nLO or (n+ -, ' )Lo, respectively. In this work the first experimental studies on thin films of diblock copolymers confined between two flat rigid surfaces are reported. Replacing the free surface with another solid surface suppresses the surface topography and elirninates this route for meeting the thickness constraint.Consequently, the copolymer is forced from its bulk equilibrium morphology into a frustrated state. Theoretically this problem has been studied by several groups resulting in conIIicting predictions. Shull [13],from self-consistent field calculations, predicts that the copolymer period will either increase or decrease to accommodate the thickness constraint. Simulations...

The dewetting behavior of a liquid film from a liquid substrate has been studied as a function of the substrate viscosity, using two highly viscous polymers as a model system. The dewetting velocity exhibits a minimum as a function of substrate viscosity. This behavior results from the competition of (1) the mass transport in the substrate, and (2) the retarded deformation of the liquid-liquid interface. Atomic force microscopy is used to image both the liquid-air interfaces and the buried liquid-liquid interface. The shape of the latter changes significantly with increasing substrate viscosity.The dewetting of thin liquid films from a flat surface is a common phenomenon with crucial impact on various technological processes. In recent years, the stability of thin liquid films has received considerable scientific interest as well, and a basic understanding of spreading and dewetting has evolved both for simple liquids and complex fluids [1][2][3][4][5][6][7][8][9][10][11][12]. Thin polymer films with high molecular weights have proven to be ideal model systems in these studies owing to their low vapor pressure and due to the fact that their high viscosity facilitates the study of the dynamic behavior. While most studies so far have dealt with thin liquid films on a solid substrate, the more complex situation of a liquid dewetting from a liquid substrate has received much less attention [12][13][14][15][16]. Brochard-Wyart, Martin, and Redon [12] recently presented a detailed theoretical study suggesting that liquid-liquid dewetting should exhibit a variety of different regimes depending mainly on the relative viscosities of the two liquids, the thicknesses of the respective liquid layers, and the surface and interfacial tensions involved. Martin, Buguin, and Brochard-Wyart [13] showed that the dewetting velocity depends on the viscosity of the substrate and decreases with increasing substrate viscosity. This apparently intuitive result points to the importance of the particular shape of the liquid-liquid interface around the growing hole and suggests that, in contrast to a solid substrate, the rim extends into the underlying liquid. However, so far direct verification of this model has not been possible, as the necessary three-dimensional imaging of the liquid-liquid interface during the dewetting process is a tedious experimental task. As will be shown below, the situation can be even more involved as the amount to which the rim penetrates into the substrate is in itself determined by the relative mobilities in the two liquids.In this Letter, we present a systematic study of the dewetting process at a polymer/polymer interface for varying molecular weight of the polymeric substrate. It is found that the dewetting velocity initially decreases with increasing substrate molecular weight; however, at large substrate viscosities, the opposite trend is observed. We present a novel technique to image the shapes of both the growing hole and the interface between the two polymers at any stage of the dewetting process. Th...

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