Alignment of Self-Assembled Structures in Block Copolymer Films by Solvent Vapor Permeation
Edited by Qichao Hu
October 11th, 2010
Mesoscale self-assembly in block copolymer has excited interest for many applications. However, the practical realization is limited because of the inability to control the self-assembly. Thus we need to develop a facile method to precisely control the orientation and long range order of the nanoscale structure. The focus is on vertical alignment of self-assembled lamellar and cylindrical microdomains in block copolymer films. This has application in nanoporous membranes used in chemoselective transport and size-selective separations. Microdomains are known to assemble spontaneously in vertical arrangement under the influence of surface forces in block copolymer films thinner than 1 um.
The work introduces a new technique involving solvent vapor permeation, and is able to align self-assembled structures in block copolymer for much thicker films. The underlining principle is based on pressure-driven transport of solvent in the vapor phase through a polymer film, which results to long-range order and alignment of the block copolymer parallel to the vapor flux. This technique can be applied to films with thickness on the order of 1mm, and on a relatively short time scale.
The experimental setup is shown below
The block copolymer film with thickness between 0.5 to 0.7 mm, is placed on the inner surface of a vial cap, which has a small opening. A drop of solvent is used to hold the film to the cap. The vial contains about 4 ml of solvent, and the cap is sealed tightly. The vial is placed in a temperature-controlled environment, and the solvent is allowed to evaporate. The solvent vapor permeate through the polymer film.
The solvent evaporation was tested under different temperatures. At 65C, the solvent evaporation took 8 hours, and at 140C, the solvent evaporation took only 1 hour. The evaporation was also tested under reduced pressure outside of the vial.
Small angle x-ray scattering, figure shown below
shows that compared to the original polymer film, there is little improvement in order at 65C, and significant improvement at 140C. This is indicated by the presence of several new peaks.
The effect of the x-ray beam alignment relative the solvent vapor flux is studied in the figure below.
At 65C, no significant difference is observed between the two SAXS scattering patterns. But at 140C, the perpendicular direction shows that there is a well aligned morphology in which the long axes of the cylindrical microdomains are parallel to the solvent vapor flux. But in the parallel direction, the scattering is isotropic because of the uniaxial flow field of the solvent vapor.
The alignment of the self-assembled nanostructures in the polymer is due to the pressure driven flow of the solvent vapor through the polymer film.
The use of solvent vapor to order or align block copolymer films can also be used to swell the polymer to both order the microdomains and improve the mobility. This has the same effect as thermal annealing but less aggressive. Granted, directional evaporation of the solvent from the swollen polymer film is similar to vapor annealing. However, both of these are typically limited to near the surface of the polymer film, and difficult to penetrate deeper.
However this technique aligns the block copolymer through deliberate flux of solvent through the polymer film and does not involve a change of morphology or thermal anneal. The polymer microsctructures are aligned by the relative motion of the solvent molecules passing the polymer chains. It is a fast process, similar to shear of extensional flow, but much more precise.
In addition, an environment where temperature and pressure can be controlled gives the solvent evaporation process more degrees of freedom.