Interplay Between the Crystal and Liquid Crystalline Ordering of iPP and Carbon Nanotube Composites under Melt-Shear
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Interplay Between the Crystal and Liquid Crystalline Ordering of iPP and Carbon Nanotube Composites under Melt-Shear Georgi Georgiev1,2, Michael B. McIntyre1, Robert Judith2, Erin A. Gombos1, and Peggy Cebe2 1 2
Department of Natural Sciences, Assumption College, Worcester, MA 01609, U.S.A. Department of Physics and Astronomy, Tufts University, Medford, MA 02155, U.S.A.
ABSTRACT Carbon nanotubes (CNTs) exhibit liquid crystalline order and their nematic director couples to the one of low molecular weight liquid crystals. Here we explore the interactions between CNTs and the smectic liquid crystal phase of a polymer and the possibility for a similar coupling in this system. Isotactic Polypropylene (iPP) and iPP/CNTs nanocomposites were made in solution with varying CNT concentrations and hot pressed into 50-100µm thick films. The pure iPP and iPP/CNT films were then sheared at one rotation per second in the melt state. Shearing continued as the temperature was decreased from 200°C to 145°C to induce a smectic liquid crystal phase. The sheared samples were analyzed using polarized optical microscopy, Two Dimensional Microscopic Transmission Ellipsometry (2D-MTE) and Two Dimensional Wide Angle X-Ray Scattering (2D-WAXS). During shearing we detected a sudden increase of birefringence at 151°C in the samples, higher than the iPP crystallization temperature, indicating liquid crystalline ordering. The samples were then crystallized at 135°C for 30 minutes. We measured anisotropic 2D-WAXS patterns of the samples that contained CNTs, indicating strong ordering of the crystals. Upon reheating, we measured birefringence at temperatures higher than the melting endotherm for the iPP crystals, using polarized microscopy, which indicates that some smectic order still persists in the samples, even after crystallization and complete melting of all crystals. Our results indicate that CNTs couple to the smectic phase of iPP, improve its order upon shearing and the crystals created after the formation of the oriented smectic phase are strongly aligned parallel to the direction of shearing. INTRODUCTION Isotactic polypropylene (iPP) has been known to exhibit a smectic phase under shear stress [1]. This smectic phase acts as a precursor to an oriented fibrillar crystal structure along the direction of shear flow [1]. The effects of multi-walled carbon nanotubes (MWCNTs) on the formation of this smectic phase have not been well studied. It has been shown that MWCNTs align under shear stress in polymer melts [2]. It has also been shown, that CNTs accelerate the crystallization kinetics of iPP [3]. We prepared iPP/CNT nanocomposites from solution with CNT concentrations ranging from 0.01 to 5% per weight by co-precipitation from a non-solvent. Films were made by compression molding. It is known that CNTs form liquid crystal phase in their lyotropic solutions [4] and interact strongly with liquid crystal molecules [5-7]. We are exploring whether the CNTs interact in a similar way with polymer liquid crystalline structures as with small mol
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