Block copolymers and nanotechnology

Abstract

Recent years have witnessed a large number of fundamental studies aimed at understanding the structure–property relationships of polymers with nanoscale features. Support for this work is largely based on the potential applications of polymers in nanotechnology. Emerging applications have demonstrated the utility of block copolymers in the creation of functional nanostructures. Building on the pioneering work of Russell et al.,1 Register et al.,2 and Kramer et al.,3 the IBM group4 has created nanostructured thin films for flash-memory applications. A patterned block copolymer film is deposited on a silica substrate, and the pattern is transferred into the substrate by reactive ion etching. The resulting ceramic layer contains 20-nm-diameter holes in which doped silicon crystals are subsequently grown. The presence of this composite thin film between the control and floating gates improves the robustness of flash-memory devices. This development, which may signal a profound shift in computer data storage away from disk drives, does not require registry between the nanoscale features (the holes in which the crystals were grown). If registry between features can be enforced, then ultrahigh-density magnetic storage devices may be created with this approach.5 Although the advances described in the preceding paragraph are impressive, it is our view that we are at the very early stages of our search for applications based on polymer nanoscience. Like all disciplines in their infancy, our understanding of the factors that control the structure and functionality of nanostructured polymers is limited. Thus, the number of fundamental studies that can be undertaken at this point is virtually limitless. A simple example is the morphology of ABC triblock copolymers. From the pioneering experimental work of Stadler et al.,6 it is clear that a wide variety of morphologies can be obtained with these systems. It is likely that the number of phases that will ultimately be found if these systems are fully explored (including different values and signs of the Flory–Huggins interaction parameter, molecular weights of the blocks, and statistical segment lengths) will exceed 10,000. Does it make sense to embark on a study to enumerate all of these phases? Although the answer to this question is obvious, deciding which ABC triblock copolymers to study is not. Similar questions can be raised about other branches of polymer nanoscience, such as nanoparticle/polymer mixtures, polymer thin films, and multicomponent polymer blends. To answer such questions, it is instructive to examine the paths adopted by some of the highly successful fields based on physical sciences in the 20th century. One example of such a field is conventional polymer science. In this case, compelling technological applications such as vulcanized rubber were in the market many decades before the concept of chain molecules took root.7 Similarly, in the case of solid-state physics, devices such as the transistor radio and telephones were available to the public despite a limited understanding of semiconductor physics. It is our view that the fundamental challenge facing polymer nanoscience is the identification of truly compelling applications. Although making more effective computers with the help of block copolymers is certainly a very important beginning, it will probably not change the world in the manner that polyethylene and silicon did. Big problems such as incurable diseases, harnessing energy not based on fossil fuels, and alternate sources8 for carbon-based synthetic materials (the fact that the world is addicted to petroleum-based synthetic materials that are not recycled seems to be lost in the energy debate) are easy to spot, but identifying the connection between polymer nanoscience and the solutions to these problems is not. An important goal for the near future must be to identify such connections and demonstrate solutions, despite our limited fundamental understanding of polymer nanoscience. Just as solid-state physicists had no difficulty deciding on the composition of the crystals on which they would focus their attention, it will then be straightforward for polymer nanoscientists to identify important directions of research and convince society to support these directions. The authors gratefully acknowledge the National Science Foundation for its financial support [through the Division of Materials Research (grant 0514422) and the Division of Chemical and Transport Systems (grant 0305711)] and the U.S. Department of Energy [through the Office of FreedomCAR and Vehicle Technologies and the Office of Basic Energy Sciences] for its support of their research efforts in the area of block copolymer nanostructures.