Aligned microstructure of some particulate polymer composites obtained with an electric field

Abstract

Alignment by an electric field was obtained for a variety of particles dispersed in photopolymerizable fluids. The particle shapes studied were irregular, spherical, rhombohedral, rod-like (fibres), and platelet. The sizes ranged from sub-micrometres to tens of micrometres, and the dielectric constants of the particles varied from less than that of the lquid matrix to very much greater than that of the matrix. Polymerization or hardening of the matrix was possible at room temperature, required only a few seconds, and the aligned structures obtained were able to be examined by both light and scanning electron microscopy after fracture or sectioning. Nominally equiaxed particles, containing a statistical proportion of non-equiaxed particles, could be completely aligned at 48 vol% concentration in a fluid having a viscosity of about 2.5 Pa s, but at 57 vol%, the mixture behaved as a paste, and only particle rotation and local rearrangements were possible. The rate of alignment seemed to depend generally on the magnitude of ε1(aβ)2, where ε1 is the relative dielectric constant of the liquid resin, a is the particle radius, and β is the particle dipole coefficient given by (ε2−ε1)/(ε2+2ε1), where ε2 is the relative dielectric constant of the particles. ε1(aβ)2 emphasizes the importance of particle size and the relative unimportance of the particle dielectric constant for alignment, except when ε2≈ε1. Platelets were more rapidly aligned than fibres.