Modification of rheological properties of a thermotropic liquid crystalline polymer by melt-state reactive processing Zhenpeng Li a , Paola A. Gonzalez Garza a , Eric Baer c , Christopher J. Ellison a, b, * a Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712, USA b Texas Materials Institute, University of Texas at Austin, Austin, TX 78712, USA c Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH 44106, USA article info Article history: Received 12 March 2012 Received in revised form 27 April 2012 Accepted 6 May 2012 Available online 12 May 2012 Keywords: Vectra A950 reactive processing liquid crystalline polymers abstract Thermotropic main-chain liquid crystalline polymers typically have very low melt viscosity with strong temperature dependence compared to other common thermoplastics. While this is beneficial in some processing applications, such as injection molding, it presents challenges for others, such as coextrusion. In this study, the rheological properties of a thermotropic main-chain liquid crystalline polymer (Vectra A950) were enhanced by melt-state reactive processing with triphenyl phosphite (TPP), which can react with up to three polymer chain-ends through their chain-end functionalities. The influence of processing time and TPP content on the shear viscosity and other important material properties were investigated. Optimal conditions, which increased the shear viscosity by nearly a factor of 20 over the neat polymer, were found to be 4 wt% TPP and 30 min of reaction time at 290  C. Further results from differential scanning calorimetry, wide-angle X-ray diffraction and polarized optical microscopy confirmed that coupling with TPP did not affect the microstructure, melting/crystallization behavior or liquid crystallinity. The stability of TPP- modified samples was also studied at 80  C in air and following melt reprocessing at 290e300  C under N 2 or air. Samples were stable (as measured by shear viscosity) for more than one month at 80  C in air or when reprocessed in N 2 at 290  C for up to 10 min. However, when reprocessed at 300  C in air, the viscosity enhancement was partially reversed due to scission of PeO bonds that were formed during the initial reaction between the polymer chain-ends and TPP. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Liquid crystalline polymers (LCPs) are very attractive high performance materials due to their excellent mechanical, optical, and transport properties [1,2]. As a result, LCPs have been successfully developed as ultra high strength fibers. One of the most famous examples is Kevlar (p-phenylene terephthalamide), whose tensile modulus is in the range of 9e17 Mpsi, while tradi- tional commercial polymer fibers like nylon and PET are only about 0.9 and 1.8 Mpsi, respectively [1]. If crosslinks are introduced, the liquid crystalline networks can display remarkable elastic proper- ties due to the transition from a polydomain to a monodomain structure [3]. Recent studies have found that these LCP networks can be used as shape memory materials through hydrogen bonding or light activation [3,4]. Recent research has also shown that side- chain LCPs can be easier to process, due to their solubility in common solvents, while exhibiting interesting optical properties by their self-assembly leading to potential applications in display devices [5]. Besides their mechanical and optical properties, LCPs have very attractive gas and liquid transport characteristics. Paul and coworkers [6e8] have found that LCPs possess up to 1000 times lower gas permeability than many other conventional polymers, which has been attributed to low gas solubility and diffusivity in these materials afforded by the low free volume of LCPs. Due to their combined barrier properties and extraordinary chemical resistance, LCPs could potentially be widely used in high perfor- mance membrane and packaging applications. Main-chain LCPs commonly have low melt viscosities, which can be beneficial for some melt processes such as injection molding. However, this feature is still a significant challenge in the application of LCPs in many areas. For instance, it is desirable to prepare membranes, packaging film and other devices containing LCPs through multilayer coextrusion [9]. Multilayer coextrusion of two or more polymer layers can be performed most effectively when the melt flow characteristics (i.e., viscosities, elasticities, etc.) of the layer components are closely matched, a similar situation to * Corresponding author. Department of Chemical Engineering, University of Texas at Austin, Austin, TX 78712, USA. Tel.: þ1 512 471 6300; fax: þ1 512 471 7060. E-mail address: ellison@che.utexas.edu (C.J. Ellison). Contents lists available at SciVerse ScienceDirect Polymer journal homepage: www.elsevier.com/locate/polymer 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.05.008 Polymer 53 (2012) 3245e3252