Confined Block Copolymer Thin Films

186

318

We have used a dynamic density functional theory ͑DDFT͒ for polymeric systems, to simulate the formation of micro phases in a melt of an asymmetric block copolymer, A n B m ( f A ϭ1/3), both in the bulk and in a thin film. In the DDFT model a polymer is represented as a chain of springs and beads. A spring mimics the stretching behavior of a chain fragment and the spring constant is calculated with the Gaussian chain approximation. Simulations were always started from a homogeneous system. We have mainly investigated the final morphology, adopted by the system. First, we have studied the bulk behavior. The diblock copolymer forms a hexagonal packed array of A-rich cylinders, embedded in a B-rich matrix. Film calculations have been done by confining a polymer melt in a slit. Both the slit width and surface-polymer interactions were varied. With the outcomes a phase diagram for confined films has been constructed. Various phases are predicted: parallel cylinders (C ʈ ), perpendicular cylinders (C Ќ ), parallel lamellae (L ʈ ), and parallel perforated lamellae (CL ʈ ). When the film surfaces are preferentially wet by either the A or the B block, parallel oriented microdomains are preferred. A perpendicular cylindrical phase is stable when neither the A nor B block preferentially wets the surfaces. The predicted phase diagram is in accordance with experimental data in the literature and explains the experimentally observed differences between films of asymmetric block copolymers with only two parameters: the film thickness and the energetic preference of the surface for one of the polymer blocks. We have also observed, that confinement speeds up the process of long range ordering of the microdomains.

Section: A Block Copolymer Thin Filmsmentioning

confidence: 99%

“…6,7,18,19,20 In confined films, frustration cannot be avoided and the lamellar period deviates from the bulk value. 13,16 Random copolymers have been used to tune the interactions between the interfaces and the two different copolymer blocks. 14,15 Films with nonselective surfaces have been prepared in this way.…”

Section: A Block Copolymer Thin Filmsmentioning

confidence: 99%

Asymmetric block copolymers confined in a thin film

Huinink

Brokken-Zijp

Dijk

et al. 2000

186

318

“…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] …”

Section: A Parallel Lamellae Between Two Strongly Preferential Surfacesmentioning

confidence: 99%

“…[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.…”

Section: Introductionmentioning

confidence: 99%

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

Wang

Yan

Nealey

et al. 2000

161

219

Thin films of symmetric diblock copolymers confined between two hard, flat and homogeneous surfaces have been investigated by means of Monte Carlo simulations on a simple cubic lattice. For such simulations, the match between bulk lamellar period L 0 and the simulation box size is crucial to obtain meaningful results. The simulations have been performed in an expanded grand-canonical ensemble, where the chemical potential and the temperature of the confined films are specified and the density is allowed to fluctuate. The dependence of morphology, density, and chain conformation in the confined films on the type of surfaces, surface separation, and the strength of surface-block interactions has been studied systematically. Our results are consistent with experimental findings.

“…[1][2][3][4][5][6][7][8][9][10][11] This reduces the surface-block interfacial energy. In the case of parallel lamellae confined between two hard ͑impenetrable͒ surfaces, the confined lamellar period L may vary from the bulk value L 0 to accommodate the frustration between surface separation D and L 0 .…”

Section: Introductionmentioning

confidence: 99%

Symmetric diblock copolymer thin films confined between homogeneous and patterned surfaces: Simulations and theory

Wang

Nath

Graham

et al. 2000

We have investigated the ability of a simple phenomenological theory to describe the behavior of symmetric diblock copolymer thin films confined between two hard surfaces. Prior knowledge of the morphology in the confined films is crucial for applying this theory to predict the phase diagram of such systems. Taking advantage of our observations in Monte Carlo simulations, we use the theory to construct phase diagrams for thin films confined between patterned-homogeneous surfaces, and obtain good agreement with our results of simulations. Two conditions are essential for obtaining long-range ordered perpendicular lamellae: a lower stripe-patterned surface with the surface pattern period L s comparable to the bulk lamellar period L 0 , and an upper neutral or weakly preferential surface. We have also examined the undulation of perpendicular lamellae between two hard surfaces. For the cases of two homogeneous ͑preferential͒ surfaces and patterned-preferential surfaces, our calculations using the phenomenological theory indicate that the amplitudes of the undulation are on the same order of magnitude as observed in our Monte Carlo simulations, and are one order of magnitude larger than previously reported. The theory, however, is unable to capture the shape of the undulation. For the case of patterned-neutral surfaces, we find that an earlier analysis is unable to yield the undulations that would stabilize the perpendicular lamellar morphology. We have addressed this issue and obtained undulations that are consistent with our observations from Monte Carlo simulations.