Positron annihilation studies of 4-n-butyl-4′-isothiocyanato-1,1′-biphenyl

Abstract

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.

Positron annihilation in liquid crystals

“…The intensity of this component is lower than 10% and increases to about 20% during the transition. A similar conclusion was drawn in the case of 4TCB crystal phase for which I 3 is even lower, i.e., it increases from 0% to 5% with temperature before the transition and then to 21% during the transition [28]. The value of  3 between 1.8 ns and 1.9 ns is in accordance with the size of the molecular vacancy for 4TCB crystal lattice.…”

“…The SmE phase of nTBC have been extensively investigated and new models of the SmE lamellar structure with two types of sublayers consisting of aromatic cores and alkyl chains of molecules has been proposed [30,31]. This is connected to the previous reports concerning molten state of the alkyl chains in the SmE phase which has been confi rmed by thermodynamic, structural, and spectroscopic studies including PALS results [28,32,33]. Formation of Ps bubbles in mobile liquid-like regions of hydrophobic alkyl chains was also found for the racemic triple chain amphiphilic diol 1 that exhibits hexagonal columnar and cubic mesophases [34].…”

Positron beam and RBS studies of thermally grown oxide films on stainless steel grade 304

“…PAS (positron annihilation spectroscopy) is a sensitive method for detection of structural defects at the atomic scale such as vacancies, vacancy clusters, dislocations, and nanometer-scale voids [13,14]. The basis of this technique is positron-electron annihilation process.…”

Kinetics of Cold Crystallization of 4-Cyano-3-fluorophenyl 4-Butylbenzoate (4CFPB) Glass Forming Liquid Crystal. I. Nonisothermal Process As Studied by Microscopic, Calorimetric, and Dielectric Methods

“…7−9 It is especially true for glass-formers, where the phenomenon of the so-called cold crystallization (cc) occurs on heating a substance. 10−14 For low molecular organic materials, the cc (a process typical for polymers and called recrystallization) has been identified on heating after softening of glasses of isotropic phases, 12−14 nematics, 7,15,16 crystal-like smectics, [7][8][9][10]17 conformationally disordered crystalline phases, 10,18 and ferroelectric phases 9,19 of liquid crystalline materials and also of plastic phases of alcohols. 20,21 In many cases, a metastable crystal is formed (one or more) which transforms on further heating to the stable crystalline phase.…”

“…Studies of phase transitions in liquid crystalline materials revealed complex polymorphism of liquid-like and solid-like phases. − It is especially true for glass-formers, where the phenomenon of the so-called cold crystallization (cc) occurs on heating a substance. − For low molecular organic materials, the cc (a process typical for polymers and called recrystallization) has been identified on heating after softening of glasses of isotropic phases, − nematics, ,, crystal-like smectics, − , conformationally disordered crystalline phases, , and ferroelectric phases , of liquid crystalline materials and also of plastic phases of alcohols. , In many cases, a metastable crystal is formed (one or more) which transforms on further heating to the stable crystalline phase. Research on the kinetics of cold-crystallization in liquid crystalline glass-formers was conducted by polarizing microscopy (POM) along with calorimetric (adiabatic and/or DSC) methods ,, and broadband dielectric spectroscopy (BDS), also performed in real time of the experiment. ,,, …”