Permeability of A Liquid Crystalline Epoxy
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PERMEABILITY OF A LIQUID CRYSTALLINE EPOXY Jianxun Feng and Elliot P. Douglas* Department of Materials Science & Engineering, University of Florida, PO Box 116400, Gainesville, Florida 32611 ABSTRACT This paper presents a comparison of moisture permeation in liquid crystalline and conventional epoxy systems. The permeability is obtained using a dynamic method. It is found that both epoxy systems exhibit Fickian behavior. The liquid crystalline epoxy network exhibits higher barrier properties for moisture transport than the conventional epoxy network. The efficient chain packing within the smectic mesophase of the liquid crystalline thermoset (LCT) is suggested as the main factor for this difference. The stoichiometry has a large effect on the moisture permeation. The diffusion coefficient decreases monotonically with increasing amine/epoxide functional ratio. The permeability (P) and solubility coefficient (S) reach a minimum at a functional ratio of one. The mechanism of the permeation is described in terms of the two-phase morphology present and hydrogen bonding between absorbed water and the network. INTRODUCTION Liquid crystalline thermosets (LCTs) possess the advantages of both liquid crystals and high performance thermosets. Because the molecules are arranged with a one or two dimensional positional regularity and are crosslinked by covalent bonds to form three-dimensional networks, they have advantages over conventional thermosets with regards to both processing and properties. Even though much research has been conducted on the synthesis [1-7], orientation [2,3,8-10], mechanical properties [1-3,6,9-13], and thermal properties [1-3,7,11,14] of LCTs, the permeability of water through liquid crystalline thermosets has remained unclear [15]. The aim of the present work is to gain insight into the transport properties of LCTs by comparing a conventional epoxy and a liquid crystalline epoxy. EXPERIMENT Materials. The liquid crystalline epoxy monomer, 4,4’-diglycidyloxy-α-methylstilbene (DOMS), and a commercial, non-LC epoxy monomer of similar molecular weight, diglycidyl ether of bisphenol A (DGEBA) (DER383), supplied by The Dow Chemical Company, were selected for comparison. Sulfanilamide (SAA), purchased from Aldrich Chemical Co., was used without further purification. DOMS was synthesized in our lab as described elsewhere [16]. The EEW of DER383 was supplied by Dow, while the EEW of DOMS was measured in our lab. The chemical structures are shown in Figure 1. DOMS or DER383 was first melted at 150 °C in a convection oven. The curing agent, sulfanilamide, was then added to the resin. This mixture was periodically stirred over the next 15 CC9.1.1
minutes to dissolve all the sulfanilamide into the resin. Once all the sulfanilamide had dissolved, the mixture was degassed and then poured onto a preheated aluminum plate. Sufficient degassing is critical to obtain bubble-free films. After about 10 minutes, a second aluminum plate was placed on top of the first one. The curing cycle followed this procedure: after 4 ho
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