The development and application of combinatorial methods and techniques to pharmaceutical drug discovery has revolutionized the process of bringing new drugs to market. Similarly, application of combinatorial principles to materials science is expected to greatly influence the future direction of research, as is already being witnessed for catalyst studies. One area of materials research where these principles are just now beginning to be applied is the characterization of polymer physical properties. Towards this goal we have developed a technique to create polymer thin films with a controlled, continuous gradient in film thickness. As a demonstration of the unity of this technique, we have revisted the well-studied phenomena of the morphology of symmetric diblock copolymer thin films.Symmetric diblock copolymers are defined as block copolymers where the two polymers are nearly equal in molecular weight. Previous studies have shown that in annealed thin films of these materials surface energetics induces formation of lamellae parallel to the substrate surface having a thickness L11 equal to the bulk equilibrium lamellae spacing. When one block segregates to both the substrate and air interface the film thickness is given by nL, while (n + 1/2)L describes the film thickness when one block segregates to the substrate and the other to the air. In the case where the film thickness deviates from these integer thicknesses, the excess material forms incomplete layers with islands or holes of height L. The percentage coverage of the incomplete layer depends on the fraction t/L where t is the film thicknwss above the last integer layer height. This dependence is utilized to demonstrate the efficacy of combinatorial methods applied to polymer characterization by creating thin films of symmetric polystyrene-b-poly(methyl methacrylate) (PS-b-PMMA)copolymer having continous gradients in thickness. The morphology and incomplete layer formation is characterized by atomic force microscopy.
Proceedings Title: 2000 American Chemical Society National Meeting (ACS PMSE)
Conference Dates: August 20-24, 2000
Conference Title: American Chemical Society
Pub Type: Conferences
combinatorial, diblock copolymers, polymer thin films