Block Copolymers: Past Successes and Future Challenges

Block copolymers represent a subject of broad current research emphasis across the full spectrumofmacromolecular chemistry and physics, ranging from development of new synthetic strategies and molecular architectures to application of advanced theoretical and computational methods. Almost fifty years after the preparation of the first laboratory samples by living anionic polymerization, scientific interest in these materials continues to grow, as does the global market for block copolymer materials. The aim of this article is to provide an individual perspective on this particular facet of macromolecular science, touching both on historical aspects of the field as well as promising future directions. In so doing, I also hope to convey some sense of, and justification for, the excitement that drives academic research in this arena. At the same time this is not intended as a thorough review, and space limitations preclude discussion of many interesting topics. It will be helpful to introduce some terminology at the outset. Block copolymers containing two distinct monomers A and B have various architectures, such as linear diblock (AB), triblock (ABA), pentablock (ABABA), multiblock or segmented copolymers (AB)n, and star diblocks (AB)nX. When a third ingredient is added, linear ABC, ACB, and BAC triblocks can be prepared, and also three-armed stars. The total degree of polymerization (in terms of a common reference segment), N, is proportional to molecular weight, and the composition of the copolymer is expressed in terms of the volume fractions of the blocks fA(1⁄4NA/N), fB, etc. The thermodynamic interaction between any two dissimilar monomers is given by the dimensionless interaction parameter wAB (or simply w), which is inversely proportional to temperature. It is worth noting an important distinction between the ‘‘theorist’s w’’ and the ‘‘experimentalist’s w’’. The former quantity is a strictly energetic quantity, defined in the spirit of regular solution theory by the exchange energy required to interchange two dissimilar monomers, divided by thermal energy kT. The latter quantity is a parameter obtained by fitting some experimental observations to a theoretical relation. As the experimental system is likely to violate one or more assumptions inherent in the theory, the resulting w values reflect the influences of the various non-idealities present in addition to the expected exchange energy contribution. The central thesis of this paper is that the future importance of block copolymers will be founded on two already recognized aspects of their behavior: access to exquisite control over self-assembly, and advanced, ‘‘non-scalar’’ application of the resulting nanostructures. Block copolymers are the pre-eminent self-assembling materials, for the following five reasons: