Polymer Alignment

Spontaneous organization of nanoscale structures

Microphase separation occurs in diblock copolymer thin films in directions both perpendicular and parallel to the underlying support substrate due to the immiscibility and differential wetting properties associated with the two components of these fascinating materials. Microphase separation creates islands and holes via film thickness quantization in the direction perpendicular to the substrate while, under carefully balanced thermodynamic conditions, microphase separation can also expose both polymer components to the air/polymer interface in the direction parallel to the substrate. This latter phenomenon can generate spacially periodic microdomains consisting of the different chemical constituents of the diblock, whose scale and geometry reflect the chemical and physical properties of the polymer. The repeat spacing of such microdomains can be precisely controlled on the nanometer scale by changing, for example, the molecular weight of the diblock copolymer.

One of the strategic goals for these self-organizing materials is to experimentally control the perfection of the resulting nanostructures, leading to their utilization in a variety of fundamental and technological applications including electron transport in confined and periodic geometries. For example, the controllable architectures of these materials may lead to their use in electronic and magnetic nanostructures via selective decoration of individual diblock components with either conductive or magnetic nanoparticles. Another potential application of these periodic diblock microdomains is in nanolithography. These efforts are currently hindered by the lack of long-range order in these soft materials, induced by the presence of topological defects such as dislocations and disclinations; that is, the persistence lengths of the spontaneously formed microdomains are limited by the presence of structural defects. Various methods have been developed to circumvent this problem, including the use of either externally applied electric or shear fields, or controlled solvent evaporation to induce alignment of the microdomains.

Figure 2. 35 µm × 35 µm AFM image of a 21 µm diameter annulus after thermal treatment at 513 K for 24 h. The image appears rectangular, as a tilted view was used to accentuate the annulus. The irregular topology present in the core region before annealing (Figure 1) is absent after the annealing treatment, leaving only islands and holes. Annealing leads to rim width expansion and rim height contraction, in this instance to 5.5 µm and 130 nm, respectively. This annealing procedure allows the polymer chains to achieve equilibrium heights for each polymer layer. The cross-sectional height map of the ring topology (measured along the 35 µm profile line) clearly shows quantized height gradations. Island and hole height profiles in the core region are not shown. Height quantization occurs for both odd and even multiples of L/2 where L is 43 nm. The inset shows a 7 µm × 7 µm AFM image to demonstrate that height quantization has occurred in the rim region after annealing.

We introduce a new and simple spin-casting method which produces annular structures from cylinder-forming polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA) which exhibit a high degree of long-range, that is, micron-length, microdomain spatial coherence with radial alignment. Using this procedure, microdomain alignment is achieved without the application of any externally applied guiding fields. The system chosen for these experiments was PS-b-PMMA supported on a silicon oxide substrate. This combination is known to produce thin films suitable for nanotemplating applications; that is, both diblock components are exposed to the air/polymer interface, thereby providing sites for subsequent selective chemical reaction on the nanometer scale. We address the microphase separation charateristics of these annuli in directions perpendicular and parallel to the substrate; these latter findings are of particular interest, as these structures exhibit a remarkable degree of microdomain alignment clearly visible in the cylinders which form when using these new procedures.

Figure 3. (a and b) Comparative 1 µm × 1 µm AFM images which demonstrate the difference in the degree of cylinder alignment in the L-thick region when using the same annealing conditions (513 K for 24 h) but with or without the addition of the minor polar solvent. (a) Cylinders pack with no preferential alignment when the sample is prepared without the minor solvent. (b) Highly aligned microdomains form with orientation perpendicular to step boundaries when the substrate is precoated using the minor solvent film. (c and d) Two rim images taken at different locations of the same ring which demonstrate radial alignment. Cylinders on nL thick regions align perpendicularly to step boundaries (marked by dashed lines) with extent up to 2 µm (limited by the step width). Radial alignment persists around the entire rim.

by Seth B. Darling
created 05/01

Reference:

  1. "Cylinder alignment in annular structures of microphase separated polystyrene-b-polymethylmethacrylate" J. Hahm and S.J. Sibener, Langmuir 16 4766-4769 (2000)