Melt Diffusion in Model Liquid Crystalline Polyiers
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MELT DIFFUSION IN MODEL LIQUID CRYSTALLINE POLYIERS E. HALL*, C.K. OBER*, E.J. KRAMER*, R.H. COLBY**, J.R. GILLMOR** AND G. GALLIt *Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853-1501 "**CorporateResearch Laboratories, Eastman Kodak Company, Rochester, NY, 14650-2110 tDepartment of Chemistry and Industrial Chemistry, University of Pisa, Pisa, Italy
ABST]RACT Diffusion of model main-chain and side-chain liquid crystalline polymers was studied in the nematic phase. The concentration of deuterium labeled polymer vs. depth in a thick matrix layer was profiled with Forward Recoil Spectrometry to determine diffusion coefficients. In each system, the tracer diffusion coefficient was measured as a function of temperature, and correlated both with phase transitions determined by thermal analysis and with melt viscosity data. INTRODUCT[ON Our research into the diffusion of liquid crystalline (LC) polymers involves developing and studying model systems for the most technologically important LC polymer architectures. The polymers investigated here are model materials in that they exhibit both nematic phases at temperatures where they are thermally stable and solubility in THF enabling molecular weight characterization. Thermotropic LC engineering resins now commercially available have main-chain architectures, i.e. mesogenic groups within the backbone of the polymer. Applications of LC polymers in opto-electronic devices rely more on side-chain architectures wherein mesogenic groups are attached to a flexible polymer backbone. Each class of LC polymer has unique characteristics resulting from their differing architectures. In the main-chain polymers, nematic ordering will have a direct effect on polymer conformations and hence on dynamics. In contrast, the side-chain LC polymers may exhibit organization and orientation of the mesogenic groups with much less influence on the polymer backbone and hence its dynamics. We would therefore expect striking differences in diffusion and melt flow behavior between these two classes of LC polymers. Described in this paper are diffusion measurements made on representatives of both classes of LC polymers and their corresponding rheological behavior.
Mat. Res. Soc. Symp. Proc. Vol. 248. @1992 Materials Research Society
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EXPERIMENTAL Polymer
Models
The model main-chain polymer studied was a polyether based on 4,4'dihydroxy-a-methylstilbene (DHMS) mesogenic core copolymerized with a second mesogenic core, 2,2'-dimethyl-4,4'-dihydroxyazoxybenzene (DMAz) [1]. DHMS and DMAz (80:20 ratio) were copolymerized with dibromononane to produce $8Z2-9. Figure 1 illustrates the mesogenic structures used. Molecular weights from gel permeation chromatography (GPC) calibrated against polystyrene standards were found to be twice as large as ones from light scattering [2]. Thus molecular weights for the polyethers reported in Table I were determined by GPC recalibrated with the light scattering data. The model side-chain polymer was composed of methoxyazobenzene
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