F abricating N ano -S cale D evices : B lock C opolymers and their A pplications B S J Aditya Limaye In the earliest days of synthetic chemistry, scientists in the field attempted to control the ordering of atoms on a molecular scale in order to create useful compounds for various applications, such as synthetic medicines, dyes, and catalysts. Molecular-scale synthetic chemistry continues to thrive today, and has led to the discovery of synthesis schemes for millions of organic molecules, ranging from simple hydrocarbons such as methane to complex biological molecules such as Vitamin B12. However, as molecular-scale chemistry has flourished, new techniques in macromolecular chemistry, dealing with compounds of a thousand atoms or more, have emerged rapidly, enabling the control of atomic placement in macromolecules, which are found widely in commercial products and in biological systems. One of the major focuses of macromolecular chemistry today is the production and processing of polymers, which are macromolecules made from repeating sub-units known as monomers. Molecules such as deoxyribonucleic acid and cellulose, polymers of ribose and glucose, respectively, play crucial roles in biological monomers, creating a large macromolecule with varying chemical composition at different points along the chain (Matsen and Bates, 1996). This added complexity in the case of block copolymers leads to a variety of interesting phenomena, all of which can be controlled and fine-tuned using synthetic chemical approaches, which enables the creation of a new class of functional materials. Many of the interesting properties of block copolymers arise from the chemical interactions between different blocks of the polymer. In a traditional, molecular chemistry sense, unfavorable interactions between two molecules, such as water and oil, lead to repulsions and, if the forces are strong enough, phase separation. However, in a block copolymer, even though certain blocks may have unfavorable interactions with each other, they cannot simply separate like water and oil, because they are all part of the same macromolecule. Rather than driving phase separation processes such as those found in water-oil mixtures, the relative strengths of chemical interactions in block copolymers drive a process known as self-assembly, in which the blocks fold, twist, and Unlike regular polymers, which are long chains of the same repeating monomer unit, block copolymers are assembled from “blocks” of different monomers, creating a large macromolecule with varying chemical composition at different points along the chain (Matsen and Bates, 1996). systems, highlighting their versatility and widespread use. In 1907, scientist Leo Baekeland completed the first successful synthesis of a polymer, Bakelite, which found use in an immense amount of commercial products such as brake pads and electrical insulation, and was coveted for its high resistance to heat and chemical corrosion. The creation of Bakelite ushered in the age of modern plastics, which resulted in the creation of many useful polymeric materials such as Nylon, Kevlar, and Teflon. Today, while attempts to synthesize simple polymers continue, significant research effort is directed towards understanding the properties and applications of block copolymers. Unlike regular polymers, which are long chains of the same repeating monomer unit, block copolymers are assembled from “blocks” of different re-organize into a more favorable three-dimensional structure that situates chemically similar blocks next to each other (Matsen and Bates, 1996). Due to the complexity of self-assembly processes, they are best understood using a familiar analogy to highlight the atomic-scale driving forces that result in self-assembly. Each of the blocks in a block copolymer can be thought of as one person in a large gathering of ten thousand or more people. This gathering, however, has the odd caveat that every person must hold the hands of two random people, effectively linking everyone into one large chain. Due to the random selection of two adjacent people, it is completely possible, and in fact likely, that two people holding hands do not know each other at all. After some time, the large chain of people would re- 10 • B erkeley S cientific J ournal • S ynthetics • S pring 2014 • V olume 18 • I ssue 2
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