Polydisperse block copolymers: Don't throw them away

Block copolymers are a fascinating class of macromolecules that can self-assemble to form nanostructured morphologies, are products of commerce, hold great potential for nanomaterial synthesis, and have been widely studied by the polymer science community. The synthesis and evaluation of these hybrid macromolecules commenced, in large part, with the discovery of living anionic polymerization. This polymerization protocol allowed not only for the synthesis of block copolymers by simple sequential addition of monomers, but also resulted in the production of polymers with very narrow molecular weight distributions as measured by the familiar polydispersity index. These narrow molecular weight distribution materials, in particular poly (styrene)-b-poly(isoprene), were the "models" for much of the work aimed at understanding the phase behavior of block copolymers in general. There was and still is a natural tendency to make an inseparable connection between the formation of narrow molecular weight distribution block copolymers and their ability to self-assemble into a range of intricate microstructures. As a result, the sentiments of the community have generally been that narrow molecular weight distribution materials were a necessary requirement when exploiting or studying block copolymer self-assembly. I for one was certainly of the mindset "monodisperse good; polydisperse bad" when it came to block copolymers. I'm not sure what exactly constitutes a polydisperse block copolymer, but when I made block copolymers with polydispersity indices greater than about 1.25, I was inclined to throw them away. I certainly don't feel that way any more. As everyone knows, anionic polymerization is only one of a variety of controlled polymerization methods for making block copolymers. Nowadays, controlled metal-catalyzed and free radical polymerization methods are ever present, and scads of block copolymer structures have been made with these techniques. However, it turns out that a good fraction of the new methods available do not result in materials with quite the levels of monodispersity achievable by anionic polymerization; examples are easy to find. Of course, block copolymer science marches on with these "not quite so model" materials. However, the historical bias against more polydisperse materials still exists. In fact, one can find comments to this effect in the contemporary literature. In my view, we should not be wary of polydisperse block copolymers. On the contrary, we should view polydispersity or distribution in molecular weights and/or block compositions as another "knob" that can be used to control phase behavior. (In fact, Leibler anticipated the influence of polydispersity on block copolymer phase behavior in his seminal work in 1980!1) If you buy into this premise, an important issue arises: how and why does polydispersity in block copolymers impact their overall behavior? More than 25 years ago Hadziiaonnou and Skoulios addressed a related issue in an article that explored binary mixtures of monodisperse di- and triblock copolymers.2 In that article, they conclude that in these inherently polydisperse blends "the quality of the organization and the sharpness of the interfaces are not affected by the mixing." Since then, there has been much work, both experimental3 and theoretical,4 on the phase behavior of binary blends of block copolymers. The upshot of these efforts has been that the binary blends can be treated using "a one-component approximation"4 provided the components in the blend do not differ too much in molecular weight or composition. In another related set of studies, multicomponent blends of block copolymers with similar overall molecular weights but quite different compositions were explored in an effort to decouple molecular weight distribution and composition distribution.5 In that work, block copolymers with polydispersity indices up to 1.7 exhibited well-defined lamellar phases with appreciable long-range order. Beyond that level of polydispersity macrophase separation was apparent. Collectively, these blending of block copolymer studies demonstrate that blends with considerable polydispersity self-assemble into discrete order morphologies and give structures that are nearly identical to those observed in their monodisperse "one-component" systems. Moreover, blending of block copolymers can be a useful protocol for dictating structure thus lending credence to the idea that polydispersity (at least as it is manifest in block copolymer blends) should be included in the block copolymer scientist's toolbox. My interest in this area was piqued by a article on the self-assembly of a set of polydisperse poly(styrene)-b-poly(acrylic acid) diblocks.6 The block copolymers reported in that study were prepared by controlled radical polymerization and exhibited polydispersity indices greater that 2.1 in all cases; the size exclusions chromatograms shown were extremely broad and not indicative of the more familiar model materials. Nonetheless, these blocks gave small-angle X-ray scattering patterns that were consistent with highly organized morphologies and showed no evidence for macrophase separation. Unlike the blends described above, the polydispersity of these materials was a direct result of the polymerization chemistry, and the polymers could best be described as having continuous distributions of molecular weights and block compositions. This work immediately suggested to me that polydisperse block copolymers should not be relegated to the bin. Last year, the self-assembly of another set of higher-molecular weight acrylic block copolymers prepared by controlled radical polymerization was reported.7 The polydispersity indices of these block copolymers ranged from 1.6 to 2.4. Again, macrophase separation was not observed and these materials generally gave organized microstructures although with a limited degree of long-range order. Intrigued by the observations of Bendejacq et al.,6 we recently explored the self-assembly of a block copolymer system with tunable polydispersity in one block.8 To do this we took advantage of the equilibrium nature of the ring-opening polymerization of lactide. This enabled the production of block copolymers with constant average composition but controlled polydispersity in one block. Here again, even with polydispersity indices of up to 2.0 in the poly(lactide) block, self-assembly of these materials resulted in well-ordered morphologies as evidenced by small-angle X-ray scattering. Moreover, changing polydispersity at constant composition could lead to changes in the observed ordered-state symmetry; three block copolymers each with about 33% poly(lactide) gave either lamella, gyroid, or cylinders depending on the poly(lactide) polydispersity index. We discussed how chain length distribution influences interfacial curvature and domain spacing in detail, and contemporary theory on the phase behavior of polydisperse block copolymers supports these results and interpretations.7, 9-11 Polydisperse block copolymers are here to stay. Given the practical utility of controlled radical polymerizations for the formation of block copolymers, there is no question that "not so model" block copolymers are going to be ever present in tomorrow's pallet of hybrid macromolecules. As such, the community should be motivated to systematically explore the influence of polydispersity on the phase behavior, processing, and practical implementation of block copolymers. The behavior of AB diblock copolymers is really only the starting point. How does block polydispersity influence compatibilization of polymer blends? How do discrete distributions (i.e., blends) compare to more continuous distributions in molecular weight? When does macrophase separation become an issue? How will polydispersity influence the phase behavior of more practically relevant ABA triblocks?12 Will polydispersity influence long-range order in, for example, thin film templates? Given the multitude of complex structures that ABC triblock terpolymers can adopt, these materials represent fertile ground for studying the influence of polydispersity on complex phase behavior. With answers to these questions, synthetic polymer chemists will be motivated to reproducibly prepare block copolymers with controlled levels of polydispersity as this will provide another tactic to tune the behavior of these hybrid macromolecules. In the end, we should accept that polydisperse block copolymers will become more and more common and prepare ourselves for how distributions in chain length and composition influence their overall behavior. So, if you've ever made a block copolymer with a broad molecular weight distribution, don't throw it away. Send it to me instead.