Switching miscibility: How to make polymer blends mix with electricity

Original paper: Jumping In and Out of the Phase Diagram Using Electric Fields:
Time Scale for Remixing of Polystyrene/Poly(vinyl methyl ether)
Blends


Many consumer products, such as clothes and food packaging, are made of blends of polymers, long molecules consisting of repeating chemical units. The attractiveness of using blends of different polymers arises from the engineers’ desire to combine multiple unique properties of each individual polymer, such as transparency, stretchability, and breathability, into a seamless whole. However, different polymers are not necessarily miscible, a term scientists use to describe whether two materials mix at the molecular level. Miscibility isn’t a one-and-done kind of deal: scientists and engineers have known for years how to make polymer blends mix by careful temperature control. What if there were conditions other than temperature to achieve polymer blend miscibility? This may ultimately help in industrial processing of polymer blends. In this week’s paper, Professors Annika Kriisa and Connie B. Roth from Emory University in Atlanta, Georgia, explore the mixing dynamics of two polymers by using a strong electric field.

fig_1_bill_003
Figure 1. Miscibility diagram of a hypothetical polymer blend consisting of polymers A and B. The x-axis is the fraction of polymer A in the blend (Composition φ) and the y-axis is the temperature of the blend (T). The curves represent the temperature above which the blends are immiscible without an electric field (black curve) and with an applied electric field (blue curve). The presence of the electric field increases the miscibility of the blend (higher transition temperature) at a given fraction of polymer A. (Image adapted from original paper.)

Before we dive into the meat of the paper, it’s important to know how temperature affects the miscibility of a polymer blend. The black curve in Figure 1 is a representative miscibility diagram of two blended polymers, which shows the temperature at which a polymer blend transitions from being miscible (below the black curve) to immiscible (above the black curve) as a function of the fraction of one polymer (denoted Composition φ) of the blend itself. The polymer blend is considered more miscible if the miscibility curve is shifted upwards, so that the blend turns immiscible at a higher temperature (see blue curve in Figure 1).

Kriisa and Roth wanted to explore how the application of an electric field influences the mixing dynamics of polystyrene (PS) and poly(vinyl methyl ether) (PVME) polymers. You may be quite familiar with these materials: PS is the formal name of styrofoam, the main component in plastic cups, and PVME is typically used in glues and adhesives. In the past, Kriisa and Roth studied the effect of electric fields in blends of these materials, and found that the electric field enhances polymer blend miscibility: an electric field raises the temperature at which a PS/PVME blend becomes immiscible, similarly to the blue curve in Figure 1 [1]. What interested the authors the most in this today’s paper was the dynamics of mixing; in other words, how quickly do immiscible blends remix once they are exposed to an electric field?  And what can we learn about the factors governing the remixing process?

fig2_bill_003
Figure 2. Switching the miscibility of polystyrene/polyvinyl ethyl ether (PS/PVME) polymer blends as a function of time through the application of an electric field (E). The red curve is the intensity of the fluorescence of a molecule attached to polystyrene, which decreases with time. The blue curve is the imposed electric field, which is repeatedly switched on and off. (Image adapted from original paper.)

The authors showed that the dynamics of mixing a PS/PVME blend is highly sensitive to the application of an electric field. They demonstrated this by examining a PS/PVME blend at the temperature four Kelvin higher than the temperature at which it becomes immiscible. They repeatedly switched on and off an electric field regularly, causing the blend to switch from being miscible to immiscible (see blue curve in Figure 2). To determine how well mixed the blend was, they measured the intensity of the light emitted by a fluorescing molecule, which was chemically attached to the PS molecules (see red curve in Figure 2). When PS and PVME are fully mixed, the fluorescence intensity decreases to 0. After switching on the electric field, the blend starts mixing immediately, showing a high sensitivity to the presence of the electric field.

fig3_bill_003
Figure 3. Remixing timescale (τ) as a function of temperature (T) and applied electric field (E). The black symbols correspond to absence of electric field, red to electric field at E =12.8 MV/m, and blue at E=13.0 MV/m. Ts (shown by the dashed lines) is the temperature at which the blend becomes immiscible at the given electric field. The remixing timescales follow the same black curve, showing that they are largely independent of E. (Adapted from original paper.)

The authors repeated this experiment for a variety of temperatures and electric field strengths. From the fluorescence curves, they extracted the remixing timescale or the time it takes for the blend to remix, as shown in Figure 3. The black symbols correspond to absence of electric field, while the red correspond to E = 12.8 MV/m and the blue to 13.0 MV/m. One may notice that the time it takes for the polymer blend to remix is largely independent of the electric field strength at a given temperature, since all remixing timescales (τ) follow the same black curve. Thus, the authors concluded that the rate of remixing is not affected by the electric field.

In short, Kriisa and Roth showed that the dynamics of remixing polymer blends are sensitive to electricity. They found that immiscible blends immediately begin to remix when exposed to an electric field and that the time it takes for the blend to completely remix is independent of the field’s strength. From an industrial perspective, this shows that the miscibility of polymer blends can be influenced by factors other than temperature. An important advantage is that an electric field can be applied uniformly and instantaneously, whereas changes in temperature take time to propagate through materials. Thus, engineers may be able to instantly tune the miscibility of polymer blends using electric fields; a discovery that may lead to future technological advances in devices and materials whose properties would be quickly ‘’switched’’  through electricity.

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[1] Kriisa, A.; Roth, C. B. “Electric Fields Enhance Miscibility of Polystyrene/Poly(vinyl methyl ether) Blends.” J. Chem. Phys. 2014, 141, 134908.

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