Abstract

The performance properties of many polymers are dominated by the molecular alignment induced during processing by the elongational flow component, that is, the diagonal component of the velocity gradient tensor. Here we discuss two idealizations of this flow-microstructure interaction. We first summarize our recent studies of biaxial nematic patterns and phase transitions (Forest and Wang, 1998a) of lyotropic liquid crystalline polymers (LCPs) in response to imposed elongational flows. We show axisymmetric biaxial nematic patterns coexist with homogeneous biaxial patterns characterized by Rey (Rey, 1995). All biaxial patterns in unidirectional stretching flows are unstable, whereas planar stretching induces candidate stable biaxial nematic patterns. Non-homogeneous biaxial patterns have potential core defects, and two special director alignments are selected by imposing a finite-pressure condition along the axis of flow symmetry. The role of director dynamics in response to flow perturbations is described, and in particular new, elongation-induced, director instabilities are revealed, extending seminal stability and phase transition results (Khokhlov and Semenov, 1982; See et al., 1992; Bhave et al, 1993; Rey, 1995; Hu and Ryskin, 1992). We then examine two special solution families of the governing equations for free surface, axisymmetric, cylindrical LCP fiber flows, following analogous Newtonian results of Segur et al. (Segur et al., 1997). The cylindrical free surface only accommodates uniaxial orientation, and from these ideal transient solutions we analyze the linearized stability of the coupled flow and microstructure in the LCP thin-filament approximation (Forest et al, 1997).

This content is only available via PDF.
You do not currently have access to this content.