Self-Assembly and Pattern Formation

Pattern formation on micrometer and nanometer length scales has direct technological application and is of fundamental interest. These patterns can be achieved using sophisticated lithography techniques, but soft materials such as polymers can self-assemble into regular patterns by merely changing e.g. the temperature of the system. Our work on self-assembly and pattern formation has focused on two different polymer trilayer systems. For freely-standing trilayer films, in which the central layer is much softer than the two capping layers, we have observed the self-assembly of a novel, periodic, in-plane morphology for trilayer films consisting of a wide variety of materials combinations [1]. We have developed a simple theory, based on linear stability analysis, which correctly predicts the dependence of the wavelength of the morphology on the individual film thicknesses, as determined by the interplay between the energy decrease associated with the attractive dispersion force acting across the trilayer film and the energy increase associated with the bending of the solid capping layers. For an all-polymer system, PS/PI/PS, we have shown that the morphology can be removed by altering the dispersion interaction [1], and can be reformed with a larger periodicity merely by decreasing the temperature [2]. Above the Tg value of the PS capping layers, holes form and grow in the central PI layer, forming a distinct rim around the edge of the hole [2]. Line defects, e.g. cracks, in the films can lead to the self-assembly of isolated PI “dots”, which consist of only ~10 6 PI chains, creating microencapsulated PI on micrometer (in-plane) and nanometer (out of plane) length scales [2]. We have also studied supported trilayer films, consisting of alternating layers of two polymers with similar mechanical properties (PS/PMMA/PS) [3]. At elevated temperatures, the trilayer films undergo an instability that is qualitatively different from that observed for the PS/PI/PS freely-standing trilayer films. By combining optical and scanning probe microscopy together with the use of an exclusive solvent for PS, we have identified a two-stage dewetting process in which the top layer dewets first, exposing the central polymer layer that subsequently dewets. This process can be understood in terms of the similarity of the mechanical properties and the surface and interfacial tensions of the two polymers. The resulting morphologies are very complex, with very different morphologies obtained as the thickness and Mw values of the different layers are varied. We have developed an image analysis method based on the use of dark field optical microscopy and Voronoi tessellation to quantify a transition in the morphology as the capping layer thickness is increased. We have also studied the self-assembly of polystyrene microspheres on both hydrophilic glass substrates and hydrophobic polymer substrates and developed an image analysis procedure for characterizing the changes to the morphology produced by annealing below and above the glass transition temperature of the PS microspheres [4].

[1] K. Dalnoki-Veress, B.G. Nickel and J.R. Dutcher, Phys. Rev. Lett. 82, 1486 (1999). [2] C.A. Murray, J. Thomas and J.R. Dutcher, submitted for publication. [3] C. Schultz-Nielsen and J.R. Dutcher, in preparation. [4] C. Gigault, K. Dalnoki-Veress and J.R. Dutcher, J. Colloid Interf. Sci. 243, 143 (2001).