Nonuniform and Uniform Deformations of Stretched Nematic Elastomers

The process of stretching driven director-rotation in nematic elastomers (NEs) depends on the angle of the stretching direction relative to the initial director (theta(0)). The NEs stretched nearly normal to the initial director (theta(0) >= 88 degrees) exhibit necking deformation accompanied by the inhomogeneous process of director rotation where the director rotates faster in the necked portion than that in the non-necked portion. The necking deformation observed here is a phenomenon resulting from the mechanical instability in the NEs stretched normally to the initial director other than the formation of stripe domains observed earlier. The inhomogeneous process of director rotation is investigated by the polarized optical and FT-IR microscopy. In contrast, the NEs undergo the uniform quasi-pure shear deformation accompanied by the uniform director-rotation toward the initially biased direction, when stretched at a slightly but sufficiently oblique angle to the initial director (theta(0) = 86 degrees). The semisoft elasticity theory successfully explains the elongation dependence of the rotation angle of director, tensile stress and Poisson's ratio in the uniform process of director rotation.