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42372-G7
Blanketing Thermodynamic Interactions in Conducting Polymers

Rachel A. Segalman, University of California (Berkeley)

The ability to self-assemble soft materials on a nanometer lengthscale is a key enabling step for applications ranging from plastic photovoltaics to cell signaling. Block copolymers have received a great deal of attention for their ability to self-assemble on this lengthscale, 1, 2 however now classical phase diagrams and models only apply to idealized systems with randomly coiled chains. Nanoscale control and patterning of functional block copolymers presents a new challenge due to non-idealities in molecular conformation and mixing interactions that are present in these materials. In a large class of polymers the primary or secondary bonding structure induces rigid chain behavior; typical rod-like polymers include helical proteins and semiconducting polymers with rigid π-conjugated backbones. A number of stunning and intriguing phases had been observed in rod-coil systems, but prior to our investigation no complete understanding of the thermodynamic parameters driving this self-assembly process and no predictive phase diagram of polymeric rod-coil systems had emerged. Fundamental understanding of the controlling thermodynamics in rod-coil block copolymer systems could only arise from a model system with accessible phase transitions. We contributed a risky hypothesis that protecting the rod from enthalpic interactions with the coil by coating the rod with short sidechains similar in composition to the coil will decrease the strength of interaction. We prepared and studied a model rod-coil block copolymer of poly(2,5-diethylhexyloxy-phenylene vinylene)-block-polyisoprene (PPV-b-PI).3, 4 The 2-ethylhexyloxy side groups attached to the PPV backbone provide a thermodynamic “sheath” increasing the similarity between the rod and the coil and presumably decreasing the enthalpic contributions to the free energy of mixing. As a result, our well-defined model system is weakly segregated but has large conformational asymmetry and blocks much longer in length than the domain spacing. The experimentally accessible phase transitions in this weakly segregated system have allowed us a unique opportunity to generate one of the first equilibrium phase diagrams for a model, polymeric rod-coil system and rigorously compare to theoretical predictions. 3-6 In our system, at low temperatures, the separation of the rod from the coil leads to lamellar and hexagonal domains on the 10nm lengthscale. The rod-rod interaction, µN, remains strong at high temperatures,7 even after the blocks begin to mix resulting in a phase-mixed nematic phase. Only when the coil is longer than the rod, does this nematic phase become less favorable and the nematic-isotropic transition begins to deviate from that of the pure homopolymer (220°C). The pinching off of the nematic region with increasing coil fraction suggests that a direct isotropic to lamellar transition would be observed in a more strongly-segregated system.4 Many applications of these semiconducting block copolymers rely on films less than 200nm in thickness. We have found that the rigid nature of the chains contributes to kinetic trapping of grain boundaries and unusual nanostructures in this regime.6 The ACS-PRF G grant has led to a successful NSF-CAREER award for R. Segalman and 5 publications . It has funded the experimental efforts of a graduate student, Bradley Olsen, whose salary was paid by a graduate student fellowship (Hertz).

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