Effect of complex flow kinematics on the molecular orientation distribution in injection molding of liquid crystalline copolyesters Stanley Rendon a , Wesley R. Burghardt a, * , Anthony New a , Robert A. Bubeck b , Lowell S. Thomas b a Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60208, USA b Michigan Molecular Institute, Midland, MI 48640, USA Received 21 January 2004; received in revised form 22 April 2004; accepted 26 April 2004 Available online 2 June 2004 Abstract Wide-angle X-ray scattering (WAXS) is used to probe the molecular orientation in steady isothermal complex channel flows (in situ) and in injection molded plaques (ex situ) of a new, low-cost aromatic copolyester based on the mesogen 4,4 0 -dihydroxy-a-methylstilbene (DHaMS). Complex orientation states arise from the competition of inhomogeneous mixed shear and extension in isothermal flows. Slit- contraction flows lead to a significant but temporary increase in the average degree of molecular orientation, suggesting that this polymer is of the ‘shear-tumbling’ type. Conversely, bimodal orientation states are observed in slit-expansion flows, where transverse extension leads to a strong reduction in the average degree of molecular orientation along the flow direction. Similar bimodal orientation states are observed in injection molded plaques, suggesting that these kinematic concepts translate rather directly to the more complex transient non-isothermal case of injection molding. Variations in orientation state induced by changes in plaque thickness may be rationalized by systematic changes in the relative importance of shear and extension. These results suggest a complementary perspective on ‘skin-core’ morphologies in liquid crystalline polymer moldings, and provide a clear conceptual link between more fundamental studies in isothermal flows and structure development during processing. q 2004 Elsevier Ltd. All rights reserved. Keywords: Liquid crystalline polymer; Channel flows; Injection molding 1. Introduction Liquid crystalline polymers (LCPs) offer significant promise as high strength/lightweight engineering materials. Their excellent mechanical properties are derived from the spontaneous ordering of stiff polymer molecules in the liquid state, which is ultimately translated into high molecular orientation in finished products. Furthermore, the superb specific strength and stiffness achievable in LCP fibers have been found to have a strong dependence on the overall degree of molecular orientation [1]. Understanding the effect of processing on molecular orientation, and specifically the role of flow fields, is thus a prerequisite to rational design of processes that exploit and enhance the characteristics of LCPs. The past 15 years have seen dramatic improvements in the fundamental understanding of lyotropic LCPs [2], where it is now possible to predict many aspects of their rheology and structure in shear flows based on a consistent theoretical framework [3]. More recent studies of ‘model’ main-chain thermotropic liquid crystalline polymers (TLCPs) that incorporate flexible spacers in the backbone have provided systematic and reliable rheological data on thermotropic polymers [4–7]. Many aspects of the dynamics and rheology of these more flexible TLCPs now appear to be understood [8–11]; however, commercial TLCPs (typically aromatic copolye- sters) are still poorly understood at any fundamental level. Unfortunately, from a practical perspective, the need for fundamental understanding of flow-orientation relationships is most urgent in commercial TLCPs. Fiber spinning is dominated by uniaxial extension which promotes high levels of molecular orientation, and optimal 1-D mechanical properties, along the fiber axis [1]. Thermotropic LCPs, however, may also be processed by extrusion and injection molding, processes in which competition between shear and 0032-3861/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2004.04.073 Polymer 45 (2004) 5341–5352 www.elsevier.com/locate/polymer * Corresponding author. Tel.: þ 1-847-467-1401; fax: þ1-847-491-3728. E-mail address: w-burghardt@northwestern.edu (W.R. Burghardt).