Determination of Local Fiber Texture in Liquid Crystal Polymer by Electron Diffraction
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Wide angle X-ray scattering (WAXS) insufficiently determines the local orientation of TLCP molecules. WAXS averages over the entire area of the irradiated sample. Standard laboratory diffractometers can sample from areas on the order of 600-750im in diameter or more.4' 5'6 X-ray synchrotron radiation sources have been used to achieve better spatial
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Figure 1-repeat units of hydroxybenzoic acid (HBA) and hydroxynaphthoic acid 171
Mat. Res. Soc. Symp. Proc. Vol. 559 ©1999 Materials Research Society
resolution.7'8 Probes on the order of 1-5 jim have been generated using synchrotron sources and microfocus technologies. Smaller probe sizes are needed to accurately detect morphological changes associated with spatially dependent phenomena such as skin-core effect in TLCP fibers, however. The spatial resolution necessary to detect small changes in polymer morphology can be achieved using the transmission electron microscope (TEM). Unlike X-rays, electrons may be focused to probe sizes ranging from sub-nanometer to several microns in diameter. Over the past decade there have been substantial technical developments in digital electron-sensitive cameras, digital microscope control, quantitative electrondose control, and digital data acquisition which can now be exploited to enable the development of quantitative microstructural mapping methods based on spatiallyresolved electron scattering. This paper describes the ongoing development of one
such technique using a digital field-emission scanning transmission electron microscope (TEM/STEM) to quantify variations in microdomain textures of meltprocessed TLCP fibers. EXPERIMENT 4-Hydroxybenzoate (HBA) and 2-hydroxy-6-naphthoate (HNA) fibers approximately 10-15 pým in diameter in both the as-drawn and heat treated states were provided by Ticona, Inc. (Summit, New Jersey). Heat treated fibers are prepared by annealing drawn fibers between the glass transition temperature and 286 °C, the temperature below which the material enters becomes a mesogenic phase. Fibers were mounted in epoxy and cut by room temperature microtomy into sections approximately 80-100 nm thick. The fibers were cut longitudinally to acquire a cross section of the skin and core regions. The sections were placed on standard TEM copper grids with holey carbon support films. A Philips CM20 FEG transmission electron microscope (TEM) was used to acquire diffraction data. The effect of electron irradiation was quantified by measuring the reduction of intensity in the diffraction pattern due to loss of crystallinity. Diffraction patterns were acquired as a function of electron dose at several doses to monitor the effects on the morphology. A diffraction pattern taken from specimen amorphized by extreme dose was subtracted from each of these patterns to remove the non-Bragg intensity. Electron optics provides the necessary spatial resolution to study morphology on sub-micron length scales. As shown infigure 2, the standard two-lens condenser system in a TEM enables electrons to be fo
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