Dryzek E, Juszyńska E, Zaleski R, Jasińska B, Gorgol M, Massalska-Arodź M
The H. Niewodniczański Institute of Nuclear Physics PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland.
Phys Rev E Stat Nonlin Soft Matter Phys. 2013 Aug;88(2):022504. doi: 10.1103/PhysRevE.88.022504. Epub 2013 Aug 19.
Positron annihilation lifetime spectroscopy (PALS) measurements were performed between 93 and 293 K in order to study the supercooled smectic-E (Sm-E) phase of 4-n-butyl-4'-isothiocyanato-1,1'-biphenyl (4TCB), the ordered molecular crystal of 4TCB, and the phase transition between the Sm-E phase and the ordered molecular crystal of 4TCB. The phase transition was well reflected in the abrupt increase of the ortho-positronium (o-Ps) lifetime and intensity. The value of the o-Ps lifetime in the Sm-E liquid crystalline phase of 4TCB, i.e., 2.21 ns at room temperature, was explained by the formation of bubbles induced by Ps atoms, which are created due to a liquidlike state of the butyl chains of 4TCB molecules in the Sm-E phase. The temperature dependence of the o-Ps intensity for the supercooled Sm-E phase can be explained by thermal generation of sites where bubbles are formed; an activation energy equal to 0.30±0.02 eV was estimated. This value was compared with the activation energies of molecular motions. The o-Ps lifetime in the ordered molecular crystal was interpreted as originating from the annihilation of o-Ps confined in molecular vacancy-type imperfections in the crystal lattice. The value of the o-Ps pickoff annihilation between 1.8 and 1.9 ns is in accordance with the size of the molecular vacancy for the 4TCB crystal lattice. Its intensity is lower than 5%. The isothermal crystallization of the 4TCB Sm-E phase was observed by PALS. The low-dimensional crystal growth was concluded from the Avrami equation fitted to the time dependence of the o-Ps intensity, which resulted in an Avrami exponent equal to 1.73.
为了研究4-正丁基-4'-异硫氰酸根合-1,1'-联苯（4TCB）的过冷近晶-E（Sm-E）相、4TCB的有序分子晶体以及4TCB的Sm-E相和有序分子晶体之间的相变，在93至293 K之间进行了正电子湮没寿命谱（PALS）测量。该相变在邻正电子素（o-Ps）寿命和强度的突然增加中得到了很好的体现。4TCB的Sm-E液晶相中o-Ps寿命的值，即在室温下为2.21 ns，是由Ps原子诱导形成的气泡所解释的，这些气泡是由于4TCB分子在Sm-E相中的丁基链呈液态而产生的。过冷Sm-E相的o-Ps强度的温度依赖性可以通过气泡形成位点的热生成来解释；估计活化能等于0.30±0.02 eV。将该值与分子运动的活化能进行了比较。有序分子晶体中的o-Ps寿命被解释为源于限制在晶格中分子空位型缺陷内的o-Ps的湮没。1.8至1.9 ns之间的o-Ps拾取湮没值与4TCB晶格的分子空位大小一致。其强度低于5%。通过PALS观察了4TCB Sm-E相的等温结晶。根据拟合o-Ps强度随时间变化的Avrami方程得出低维晶体生长，其Avrami指数等于1.73。
