直鎖長鎖状第一臭化物の相転移 (第2報) : 偶数炭素数の1-プロモ-<I>n</I>-アルカン (C<SUB>22</SUB>-C<SUB>28</SUB>) の誘電的研究
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Dielectric measurements of four linear long chain bromides, 1-bromodocosane (C<SUB>22</SUB>Br), 1-bromotetracosane (C<SUB>24</SUB>Br), 1-bromohexacosane (C<SUB>26</SUB>Br) and 1-bromooctacosane (C<SUB>28</SUB>Br), were carried out using a transformer bridge at 10 kHz. <BR>Phase transitions were observed on cooling in all the bromides studied here, and the transition points of C<SUB>22</SUB>Br, C<SUB>24</SUB>Br, C<SUB>26</SUB>Br and C<SUB>28</SUB>Br were 30, 35, 42 and 46°C respectively. These transitions were reversible except those of C<SUB>22</SUB>Br and C<SUB>24</SUB>Br. In the measurement with a high rate of temperature change, however, the transition point on heating of C<SUB>24</SUB>Br was observed also. Phase transition phenomena are affected by the rate of temperature change. <BR>Freezing points are on the smooth curve. Transition points are indicated as the linear function of carbon atoms of the samples. On the basis of this result, it is expected that the transition point of 1-bromooctadecane (C<SUB>18</SUB>Br) could be observed on cooling. <BR>Dielectric constants of high temperature phases of these compounds were higher than those of low temperature phases. In the high temperature phase, molecular rotations around the chain axes contribute to the srientation polarization which may be responsible for the high value of dielectric constant. Therefore, these high temperature phases are assumed to be a rotator phase. <BR>Dielectric constants of the high temperature phases on heating were gradually decreased by holding the samples at a constant temperature in these phases, so that the high temperature phase on heating may be metastable. However, the high temperature phase on cooling may be stable, considering the results of similar experiments. The difference between stable phase and metastable one can be explained as follows. The stable high temperature phases on cooling arise from the liquid state in which large degree of freedom of molecular motions are possessed, whereas the low temperature phases in which molecular motions are restricted in some measure change into the metastable high temperature phases on heating.
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