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Connie Roth

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Tuning Miscibility of Polymer Blends with Electric Fields

Miscibility and phase separation of polymer blends has long been studied with the goal of understanding the specific interactions that affect blending.  This is especially important for polymers because large macromolecules limit the entropic contribution to the free energy of mixing allowing even weak interactions to play a significant role in miscibility.  Understanding how chemical structure affects specific interactions in polymer blends is important in developing blending models that can predict miscibility and blend properties from their individual constituents.  We seek a better understanding of blending interactions in polymers and methods to locally tune and control the enthalpic interaction of a blend.

Electric fields are one potential route to tuning the miscibility of polymer blends.  Several key energy technologies such as polymer solar cells and batteries involve polymer blends sandwiched between two electrodes such that the application of electric fields are part of the device's application.  Organic photovoltaics have electrode spacings typically of order only 100 nm so that even a modest applied voltage of 1 V easily leads to electric field strengths of 10 MV/m.

Even though the miscibility of mixtures in the presence of electric fields has been studied since the 1960s, there are conflicting reports on both the magnitude and direction that the phase separation temperature TS can shift in the presence of electric fields.  Theoretical understanding of the phenomenon regarding the free energy contribution of electric fields to the miscibility has been hampered by a lack of experimental data with unambiguously large shifts in TS.
 

Characterization of Phase Separation of Polystyrene / Poly(vinyl methyl ether) Blends Using Fluorescence

      Annika Kriisa, Sung S. Park, and Connie B. Roth, J. Polym. Sci., Part B: Polym. Phys. 2012, 50, 250-256. [Link]

We have characterized for the first time changes in the fluorescence emission spectra of pyrene and anthracene dyes covalently bonded to polystyrene (PS) upon phase separation from poly(vinyl methyl ether) (PVME).  The specific chemical structure of the fluorescent labels is found to affect the measured phase separation temperature TS, with fluorophores covalently attached in closer proximity to the PS backbone identifying phase separation a few degrees earlier.  The sharp increase in fluorescence intensity upon phase separation that occurs for all fluorophores with little change in spectral shape is consistent with a mechanism of static fluorescence quenching resulting from the specific interaction with a nearby quenching molecular unit.  Based on recent work that has identified a weak hydrogen bond occurring between the aromatic hydrogens of PS and the ether oxygen of PVME, we believe a similar weak hydrogen bond is likely occurring between the PVME oxygen and the aromatic dyes providing a local (few nanometer) sensitivity to phase separation.  Our observations indicate that fluorescence is sensitive to the early initiation of phase separation.  In comparing similar dyes, fluorophores in which the dye is covalently bonded closer to the PS backbone denote phase separation on average 3 oC earlier than those fluorophores in which the dye is located further away on a covalent tether.  As the PVME component moves further away from the PS component upon phase separation, local breaking of these weak hydrogen bonds connecting the PVME oxygen with the aromatic rings of PS and the aromatic dyes, leads to the observed increase in fluorescence intensity.  Future work building on the present study will be aimed at quantifying the strength of the weak hydrogen bond in this prototypical miscible blend.
 

Electric Fields Enhance Miscibility of Polystyrene / Poly(vinyl methyl ether) Blends

      Annika Kriisa and Connie B. Roth, J. Chem. Phys. 2014, 141, 134908. [Link]

Using our fluorescence method with trace levels of pyrene-labeled PS, we have determined how the miscibility of PS/PVME blends changes in the presence of uniform electric fields.  We have observed reliable large shifts in the phase separation temperature TS well outside of experimental error.  For electric field strengths of 17 MV/m, we have measured shifts in TS of up to 13.5 ± 1.4 K for a 50/50 PS/PVME mixture, some of the largest absolute shifts every reported.  We have also demonstrated that the effect is completely reversible with the blend returning to the same TS value after the electric field has been removed.  Our shift in TS with electric field strength (ΔTS/E2) showing enhanced mixing in the presence of electric fields is comparable to other experimental studies in the field on small molecule mixtures, but is still much larger than can be explained theoretically.