(200c) Suppression of the Orthorhombic Network Phase in Poly(Isoprene-B-Styrene-B-Ethylene Oxide) (Iso) Triblock Copolymers through the Introduction of Polydispersity

The phase behavior of block copolymers has been studied intensely for several decades, both theoretically and experimentally. Despite this long history, relatively little attention has been focused on understanding the impact of polydispersity. Presumably, this has been a consequence of the living synthetic methods commonly employed for block copolymer synthesis that naturally produce polydispersities < 1.1. Nevertheless, the importance of polydispersity was noted as early as 1980 (1) when Leibler stated: ?Much more important than fluctuation effects are those effects resulting from the polydispersity of the copolymer blocks?it seems that even a small polydispersity of the sample may be crucial.? Since 1980, there have been only a few experimental studies which have aimed to address this issue. We have recently studied (2) functional organolithiums that can be used to prepare α-hydroxy polystyrene with polydispersity indices ranging from ~1.1-1.6. Here we used 3-triisopropylsilyloxy-1-propyllithium (TIPSOPrLi) to prepare monomodal α-hydroxy poly(styrene-b-isoprene) diblocks containing a polystyrene block with M_w/M_n = 1.31 or 1.44. More than 15 ISO triblocks were synthesized from these two parent diblocks along the f_I = f_S isopleth. These studies (3) of polydispersity effects are unique in that they address the impact of polydispersity in the S block which is forced to bridge between the I and O domains. Surprisingly, we did not observe the orthorhombic network morphology which has been observed in monodisperse analogs. These results clearly illustrate the important role that polydispersity can play in the phase behavior of block copolymers and are a demonstration of the delicate nature of the free energy balance governing morphological behavior.

(1) Leibler, L. Macromolecules 1980, 13, 1602-1617. (2) Meuler, A.J.; Mahanthappa, M.K.; Hillmyer, M.A.; Bates, F.S. Macromolecules 2007, 40, 760-762. (3) Meuler, A.J.; Ellison, C.J.; Evans, C.M.; Hillmyer, M.A.; Bates, F.S. Macromolecules, submitted.