Anna C. Balazs,
University of Pittsburgh
Research Objectives
The aim of our research is to isolate conditions for creating patterned polymer films. Tethering polymers onto a substrate and immersing the system into a poor solvent provides a unique opportunity for creating such patterned layers. For example, tethered homopolymers form arrays of "pinned micelles" on the surface. We extended this concept by investigating the behavior of tethered copolymers in poor solvents. By introducing greater chemical complexity within the chains, we could drive the system to form more complicated surface patterns.
Computational Approach
In carrying out this research, we used numerical self-consistent field calculations, Monte Carlo simulations, and scaling theory.
Accomplishments
We determined the behavior of tethered polyelectrolytes confined between two walls and the properties of thin films of ABC triblocks confined between smooth plates. In the case of polyelectrolytes, we demonstrated that surfaces covered with like-charged polymers and immersed in a poor solvent show an attraction as the layers are compressed. Furthermore, at high degrees of ionization, compressing the layers results in a novel first-order phase transition: the stretched, charged chains spontaneously associate into aggregates on the surfaces. At both low and high degrees of ionization, the free energy versus distance profiles reveal distinct minima, which indicate an optimal separation between the surfaces. The results provide guidelines for driving highly concentrated solutions of mesoscopic particles to self-assemble into ordered arrays, or colloidal crystals. Such colloidal crystals can be used as Bragg defraction devices or optical switches.
In the case of confined ABC triblocks, the B segment is chosen to be the central block, and all the blocks are incompatible. The chains microphase segregate into a lamellar phase, with the stripes either perpendicular or parallel to the walls. When all the monomer-surface interactions are identical, the perpendicular orientation has the lowest free energy. When a repulsion is introduced between the surface and the A and C monomers, the surface interactions further stabilize the perpendicular orientation. At strong surface interactions, the morphology of the perpendicular structure is controlled by the overall thickness of the molten layer.
Significance
In comparing diblocks to triblocks as candidates for forming laterally patterned films, our work indicates that triblocks possess distinct advantages over diblocks. Therefore, triblocks can be used to fabricate patterned polymer surfaces whose features are in the tens of nanometers scale and, thus, an order of magnitude smaller than typically achieved through photo-lithography. These surfaces can be used for novel optical or electronic applications. Both of the above studies illustrate the dramatic effect that confinement has on the phase behavior of polymeric systems.
Publications
Singh, C., E. B. Zhulina, and A. C. Balazs. 1997. Attraction and novel phase behavior between like-charged polymer layers. Macromolecules 30:7004.
Singh, C., G. Pickett, E. B. Zhulina, and A. C. Balazs. 1997. Controlling the interactions between polymer-coated surfaces. J. Phys. Chem. B 101:10614.
Balazs, A. C., C. Singh, E. B. Zhulina, G. Pickett, S.-S. Chern, and Y. Lyatskaya. 1997. Theory of chains tethered at interfaces. Prog. in Surf. Sci. 55:181.
Density profiles for two surfaces coated with end-grafted polyelectrolyte chains. The plots
show the effect of decreasing the separation between the surfaces and reveal that the
stretched, charged chains spontaneously associate into aggregates as the surfaces are
brought closer together. The parameter f denotes the polymer density. Here the shape of the
profiles for the counterions are qualitatively the same as those for the polymers. (a) At
surface separation h = 80, the grafted chains form homogeneous layers. (b) At h = 40,
micelles form on each of the surfaces. (c) At h = 16, compression causes the micelles from
the surfaces to merge.