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Phase Behavior of Polystyrene-^/ocA:-poly(2-vinylpyridine) Copolymers in a Selective Ionic Liquid Solvent t4 s T tJ Justin M . Virgili, Alexander Hexemer, John A. Pople, Nitash P. Balsara,*' ' Rachel A. Segalman*^'* and t ^Department of Chemical Engineering, University of California, Berkeley, California 94720, Materials Sciences and^ Advanced Light Source and Energy and Environmental Technologies Divisions, Lawrence Berkeley National Laboratory, Berkeley, California 94720, and Stanford Synchrotron Radiation Laboratory, Menlo Park, California 94025 n Received March 5, 2009; Revised Manuscript Received May A B S T R A C T : T h e phase behavior o f poly(styrene-Woc/c-2-vinylpyridine) c o p o l y m e r solutions i n an i m i d a - z o l i u m bis(trifluoromethane)sulfonamide ([Im][TFSI]) i o n i c l i q u i d has been studied using small-angle X - r a y scattering ( S A X S ) and optical transmission characterization. T h r o u g h scaling analysis o f S A X S data, we demonstrate that the [Im][TFSI] ionic liquid behaves as a selective solvent t o w a r d one o f the blocks. W e observe lyotropic a n d thermotropic phase transitions that correspond qualitatively to the phase behavior observed i n block c o p o l y m e r melts and block copolymer solutions i n molecular solvents. I n a d d i t i o n , we have studied the thermal properties o f block copolymer solutions i n the i o n i c liquid using differential scanning calorimetry a n d wide-angle X - r a y scattering. W e observe distinct c o m p o s i t i o n regimes corresponding to the change i n the block copolymer's glass transition temperature, T , with respect to the concentration o f polymer i n ionic l i q u i d . A t high block copolymer concentrations, a s a l t - l i k e regime corresponding to an increase i n the block copolymer T is observed, while at intermediate block copolymer concentrations, a solvent-like regime corresponding to a decrease i n the b l o c k c o p o l y m e r T is observed. A n unusual thermal transition consisting o f crystallization and subsequent melting o f the ionic l i q u i d is observed at the lowest block copolymer concentration characterized. g g s observed temperature-dependent scaling behavior o f the char- acteristic d o m a i n spacing i n polystyrene-Woc/c-poly(isoprene) (SI) copolymer solutions i n selective solvents. This observation was attributed to the temperature dependence o f the solvent/ block copolymer segment interaction parameter, leading to changes i n solvent partitioning between the polystyrene and poly(isoprene) microphases as temperature was varied. H a n l e y et al. have shown that thermotropic properties o f mixtures o f SI copolymers and diethyl phthalate arise from the tem- perature dependence o f solvent selectivity. Alexandridis et a l . have conducted a thorough investigation o f the phase behavior o f poly(ethylene oxide-Wocfc-propylene oxide-Woc/c-ethylene oxide) P E O copolymers (Pluronic) i n water, a solvent that is selective toward the poly(ethylene oxide) ( P E O ) b l o c k . One o f the observations of Alexandridis et al. was the strong temperature dependence o f micelle formation in dilute Pluronic/water mix- tures. The temperature dependence o f block copolymer/solvent interactions, however, is not universal across all systems. F o r example, Hadjuk et al. observed weak temperature dependence o f solvent/polymer interactions i n concentrated solutions o f an asymmetric PEO-Wocfc-poly(ethyl ethylene) copolymer i n water, despite strongly thermotropic interactions between P E O and w a t e r . These findings suggest that block copolymer composition and the local structure o f water molecules surround- ing the P E O segments play an important role i n determining the phase behavior o f copolymer solutions containing P E O blocks. Introduction Ionic liquids are a novel class o f solvents composed entirely o f ions which exhibit exceptional physiochemical properties, such as nonflammability, negligible vapor pressure, high ionic conductivity, and electrochemical stability. The outstanding electrochemical properties o f ionic liquids have led to studies involving their use i n lithium battery electrolytes, ' fuel c e l l s , and dye-sensitized solar c e l l s . While the need for introducing ionic liquids into a solid supporting matrix for these applications has been recognized, the combination o f block copolymers and ionic liquids results i n a new class o f functional materials with intricate structure o n the nanometer s c a l e . T h e structure is created by block copolymer self-assembly, while the functionality is obtained by exploiting the properties o f ionic liquids. Block copolymer/ionic liquid systems enable the design o f materials whose ion-conducting and mechanical properties can be sepa- rately optimized. Effects resulting from the addition o f both selective and , nonselective molecular solvents to block copolymers o n both the self-assembled nanostructure and the thermodynamic dri- ving forces affecting self-assembly have been extensively investi- gated using both e x p e r i m e n t s and t h e o r y . A t fixed temperature, the addition o f solvents to diblock copolymers results i n a rich variety o f lyotropic phase t r a n s i t i o n s . ' T h e temperature dependence o f interactions between the two blocks, and interactions between the blocks and the solvent, lead to an additional set o f thermotropic phase transitions. L a i et a l . Water-containing polyelectrolyte membranes are o f interest for fuel cell a p p l i c a t i o n s , ' and the incorporation o f nano- structure has recently been shown to significantly improve water uptake, while maintaining mechanical s t a b i l i t y . ' Incorpora- tion o f ionic liquids into polymer membranes is also o f interest Corresponding authors. E-mail: nbalsara@berkeley.edu (N.P.B.), segalman@berkeley.edu (R.A.S.).

Phase Behavior of Polystyrene-block-poly(2-vinylpyridine) Copolymers in a Selective Ionic Liquid Solvent