Magnetic Orientation of Polymers A Novel Technique of Controlling Birefringence
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EXPERIMENTAL DETAILS Polymer samples used were iPS (Mw: 400,000) purchased from Scientific Polymer Products, Inc. and PET (Mw: ca. 10,000) obtained from Asahi Chemical Ind. Co. Ltd, and PET homopolymer ([η]=1.05) and PET copolymer ([η]=0.8, cyclohexadimethanol 3.3 mol%) obtained from other sources. Pellets (or powder) of these polymers were hot-pressed, followed by quenching to obtain amorphous films. These films were subjected to the isothermal melt cry stallization in an Oxford superconducting magnet (6T), as well as outside the magnet for the sake of comparison, for various periods of crystallization time, and quenched to obtain samples for the wide angle X-ray diffraction (a MAC Science MXP system) and optical measurements. In-situ measurement of magnetic birefringence was carried out on a home-built optical apparatus [4] under the crossed polar condition. DISCUSION Figure 1 shows a general scheme of heat treatment necessary to attain magnetic orientation. The quenched amorphous film is heated from room temperature up to the maximum heating temperature (Tmax), which is above the melting point Tm, and kept for a while and then cooled down below Tm, followed by isothermal crystallization at the crystallization temperature, Tc. In the figure, the initial stage of crystallization is designated as “induction period”, in which no crystal formation is observed by means of X-ray diffraction. We believe that partially ordered structures, most likely liquid-crystalline-like structures, occur transiently in this period . FT-IR spectroscopic study [8] of melt crystallization of PEN has provided the evidence to support this assumption. It was reported therein that there is conformation change in the ethylene glycol part of the repeating unit of PEN during the induction period: the decrease in amorphous gauche and the increase in amorphous trans. In-situ magnetic birefringence measurements for iPS and PET are shown in figure 2. In the present optical setup, the transmitting light intensity is given: Tmax
I 2~sin 2 (πd∆n/λ)
(1)
Tm
INDUCTION PERIOD
Tc CRYSTALLIZATION RT TIME
Figure 1. General scheme of heat treatment used for magnetic orientation. Tm: melting point; Tmax: maximum heating temperature; Tc: crystallization temperature.
where d is the sample thickness, ∆n is birefringence, and λ is the wavelength of the He-Ne laser. If there is a large change in ∆n, the transmitting light intensity might oscillate according to equation 1. This oscillation is clearly observed in figure 2 for the measurement carried out in the magnet. The decay of the intensity at a later stage of crystallization is attributed to the loss of intensity due to scattering by crystallites. The intensity behavior observed in the magnet is entirely different from that observed outside the magnet. This clearly demonstrates that magnetic orientation is occurring. 0.4 (b)
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