Observed frustration in confined block copolymers

Abstract

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...

Monte Carlo simulations of diblock copolymer thin films confined between two homogeneous surfaces

“…The confined lamellae are compressed and stretched in a cyclic manner as a function of D. These are in agreement with experimental evidence. [1][2][3] …”

“…[1][2][3] When the same blocks are preferred by both surfaces, the number of bilayers within the parallel lamellae ͑denoted by n) is an integer between D/L 0 Ϫ 1-3 A firstorder transition appears to occur between the compressed and the stretched states of the copolymer chains. 1,2 Further experiments also show that, as surface-block interactions become less specific, an additional increase in the frustration between D and L 0 can lead to formation of perpendicular lamellae with LϷL 0 near at least one of the surfaces. 3,4 TEM images show that the perpendicular orientation is short-ranged.…”

Orientational Wetting in Hybrid Liquid Crystalline Block Copolymers

“…If the thickness d of the film is incommensurate with these values, say d n , d , d n11 , then the system will separate into coexisting domains of thicknesses d n and d n11 [6], in a process which has been compared to 2D topological coarsening [7]. Experimental and theoretical results on isotropic diblocks confined between parallel hard walls have illustrated the effects of frustration between the lamellae period and an externally imposed length scale [8][9][10][11][12]. Side-chain liquid crystal polymers have recently been incorporated into diblock copolymers [13][14], where the smectic ordering of side groups can lead to the competition between two inherently different length scales, and can frustrate generic diblock ordering.…”

Parallel and Perpendicular Lamellae on Corrugated Surfaces

“…As an example, we mention that lamellar block copolymers (BCP) confined between two flat and parallel surfaces have been shown to orient parallel or perpendicular to the surfaces, depending on the surface separation [1,2]. Furthermore, chemically patterned surface can induce perpendicular or even tilted lamellae, if the chemical interaction is strong enough [3,4].…”