The influence of orientation on the glass transition of semicrystalline polyethylene was investigated using Brillouin spectroscopy. Orientation was achieved by drawing the sample to 6.55 times its original length. The temperature range covered was 100-300 K. By performing the second logarithmic derivative of the longitudinal sound velocity vs. temperature graph the glass transition is found by a pronounced minimum. In the isotropic sample it takes place at Tg = 234.8±1.7 K. This curve nearly coincides with that found from longitudinal sound velocity in chain direction of the oriented sample. Perpendicular to the chains no effect shows up. As is discussed, thermal expansion can be ruled out as an explanation of this strong anisotropic effect.
We present a new design of a Fabry-Pérot interferometer for Brillouin spectroscopy. Emphasis is placed on a quick change of the free spectral range without losing alignment, the possibility of switching the multipass arrangement from 3 to 5 pass and back, and finally on a simple and inexpensive construction. Besides this the new interferometer shows very good long term stability and is easy to handle. These features were not available before in this combination. The performance of the instrument will be demonstrated.
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