Polymers, Liquids and Colloids in Electric Fields

The following sections are included:

• Foreword

• Preface

• Contents

This chapter surveys aspects of patternings that occur in a wide array of physical systems due to interacting combinations of dipolar, interfacial, charge exchange, entropic, and geometric influences. We review well-established phenomena as a basis for discussion of more recent developments. While the materials of interest range from bulk inorganic solids and polymer organic melts to fluid colloids and granular suspensions, we note that often there are unifying principles behind the various modulated structures, such as the competition between surface or line tension and dipolar interaction in thermally reversible systems; their properties can be understood by free-energy minimization. In other cases, the patterns are determined by dissipative forces. In all these systems the patterning is modulated by the application of force fields. Another common feature of these disparate systems is that a phase diagram often emerges as a convenient descriptor. We also mention a number of interesting technological applications for certain of the systems under review.

A Ginzburg-Landau theory is presented on polar binary mixtures containing ions. It takes account of electrostatic, solvation, image, and amphiphilic interactions among the ions and the solvent molecules. The ion distributions and the electric potential are calculated around an interface with finite thickness in equilibrium. The surface tension is increased for hydrophilic ion pairs, but is decreased for hydrophilic and hydrophobic ion pairs. Introducing the amphiphilic interaction, we also treat ionic surfactants, which aggregate at an interface and reduce the surface tension. A mesophase with periodic composition and ion modulations emerges for sufficiently large asymmetry between the cationic and anionic solvation strengths. Also, among ions, there arise long-range attractive interactions in the Ornstein-Zernike form, which are mediated by the composition fluctuations. Under strong solvation conditions, they can dominate over the Coulomb interaction in the range shorter than the correlation length. In the presence of three ion species, the ion distribution can be very complex.

The influence of an external uniform electric field on the critical mixing temperature is still a subject of a debate. In this chapter the available experiments and theoretical expectations will be presented and discussed.

The interaction of an electric field with matter has been studied since 19th century. Extensive experiments have shown that an electric field can act on dielectric materials exerting an electrostatic force. Because the electrostatic interactions are relatively strong and long-ranged, they can be used to control structures on length scales that are difficult to achieve by any other means. There are techniques available to pattern materials to obtain features that are below the diffraction limit of light. Here some advances in the use of electric fields to achieve this end are reviewed, that demonstrate the simplicity and elegance of this approach.

In this chapter we discuss the basic principles of electrowetting in equilibrium conditions as well as two examples of dynamic electrowetting. We show that the electrowetting phenomenon is caused by the balance between the electrostatic stresses – the Maxwell stress – acting on the drop surface and the surface tension forces, which can be viewed as a reduction of the effective solid-liquid interfacial tension on a large scale. With respect to dynamics, we show that AC electric fields substantially reduce the contact angle hysteresis typically encountered on any solid surface in ambient air. For electrowetting in an ambient oil environment, we discuss the displacement of the oil by the aqueous drop that is moving under the influence of the electric field. Furthermore, we explain the basic principles used for the operation of various electrowetting-driven microfluidic devices, in particular lab-on-a-chip systems.

The following sections are included:

• Introduction

• Fundamental Aspects of Phase Separation
  • Non-reacting systems
  • Reacting systems

• Phase Separation of Polymer Blends Induced by Photo-chemical Reactions
  • Significance of photochemical reactions
  • Design of hierarchical morphology by using competing interactions in polymeric systems
  • Controlling phase separation of polymer blends by using temporal and spatial modulation
    • Morphology design by irradiation with spatial modulation
    • Morphology design by using irradiation with temporal modulation
    • Spatially graded morphology designed by using strong light intensity
    • Morphology with an arbitrary distribution of characteristic length scales designed by photochemical reactions: The computer-assisted irradiation (CAI) method
    • Reversible phase separation of polymer blends driven by two UV wavelengths: Effects of reaction-induced deformation on morphology

• Concluding Remarks

• Acknowledgments

• References

Basic electrostatics and some less familiar thermodynamics is reviewed, and useful Claussius-Clapeyron-like equations are derived. They are applied to predict the form of phase diagrams of two systems. The first is a bulk system of body-centered-cubic phase which undergoes a phase transition either to a hexagonal or disordered phase on the application of an electric field. The second is a surface film of cylindrical phase which can be oriented perpendicular to the substrate by the application of a field.

In the first part of this chapter, we discuss the origin of the electric field driven alignment of block copolymer nanostructures in solution. We followed the reorientation kinetics of various model block copolymer solutions exposed to an external electric d.c. field. The characteristic time constants follow a power law indicating that the reorientation is driven by a decrease in electrostatic energy. Moreover, the observed exponent suggests an activated process in line with the expectations for a nucleation and growth process. When properly scaled, the data collapse onto a single master curve spanning several orders of magnitude both in reduced time and in reduced energy. In the second part, we show that electric d.c. fields can be used to tune the characteristic spacing of a block copolymer nanostructure with high accuracy by as much as six percent in a fully reversible way on a time scale in the range of several milliseconds. We discuss the inuence of various physical parameters on the tuning process and study the time response of the nanostructure on the applied field. A tentative explanation of the observed effect is given based on the anisotropic polarizabilities and permanent dipole moment of the monomeric constituents.

We review the transition pathways found between different block copolymers structure-types, upon confinement in a thin film and application of an uniform external electric field. The method considered is the dynamic density functional theory (DDFT), which is a dynamic version of self-consistent field theory. We disregard the screening due to free ions and consider a dielectric mechanism to study the transitions. The structure types vary from spheres to lamellae. The transition dynamics depends on the degree of phase separation, and takes detours across phase boundaries depending on the position with regard to the phase boundary.

The following sections are included:

• Index