Tamara Borisova

Comparative dielectric and TSDC studies of molecular mobility in liquid-crystalline side-chain poly(methacrylate)

Abstract The molecular mobility of a LC side-chain polymethacrylate (PM6) was carried out by dielectric spectroscopy and by thermally stimulated discharge current (TSDC) methods. Six relaxation processes γ2, γ1, β, β1, α, and δ at successively increasing temperatures were observed in PM-6. For each of them, a molecular interpretation was proposed. There is a good correlation between the peak temperature positions obtained from the global TSDC spectrum at an equivalent frequency and those obtained from the dielectric method. The TSDC thermal windowing experiments for PM-6 show the high ability to resolve the overlapping processes at low temperatures. Dielectric and TSDC methods giving different details of the observed relaxation processes, allow us to create the overall picture of molecular mobility in the system under investigation. These two methods revealing the differences and similarities of the obtained results could be considered as complementary to each other.

Dielectric spectroscopy and low temperature molecular dynamics of thermotropic side-chain polymethacrylates with side halogen terminal groups

Local forms of molecular mobility of LC side-chain halogen-containing polymers (P-Hal) where Hal is F, CI, Br, or J, with caproyloxyphenyl benzoate mesogenic groups were studied by dielctric relaxtion spectroscopy (DRS) and by thermally stimulated depolarization currents (TSDC) techniques. For all halogen-containing polymers investigated herein three relaxation ranges of dipole polymerization were detected at tempearture below T g , β, γ 1 and γ 2 process in the order of decreasing temperature. The molecular mechanism of these processes correspond to the mesogen motion about the long axis, to the spacer motion, and to the mobility of the mesogen end part, respectively. For all P-Hal, the kinetic characteristics for each of the β, γ 1 and γ 2 processes can be described by a single curve in the Arrhenius plot, log F max = φ(1/T). In other words, the local molecular mobility in the series P-F, P-CI, P-Br, and P-J does not changes with halogen type.

Cooperative modes of molecular mobility in liquid crystalline comb‐like oligoacrylates and oligomethacrylates with 4‐cyanoazobenzene side groups

Cooperative modes of molecular mobility in comb-like liquid crystalline oligoacrylates and oligomethacrylates containing mesogenic 4-cyanoazobenzene side groups and spacers of 3, 5, 7, 9, 10, and 11 CH 2 groups were studied by means of dielectric spectroscopy. Near T g the existence of two cooperative relaxation processes denoted as α and δ was revealed. The δ-relaxation is connected to the motion of mesogenic groups with respect to the short mesogenic axis and the α-relaxation is connected to the motion of mesogenic groups with respect to the long axis and with the segmental mobility of the main chains. The temperatures of α- and δ-transitions decrease with increasing spacer length. The liquid-crystalline structure formation increases main chain rigidity and temperatures of the α- and β-transitions. For these oligomers aligned by an external electric field or by mechanical shear, the planar or homeotropic orientation of mesogenic groups is established. The choice of orientation conditions makes it possible to obtain the predetermined order parameters.

Molecular dynamics of liquid crystalline side-chain oligoacrylates and oligomethacrylates with 4-cyanazobenzene side groups. Influence of external fields on dielectric properties

Abstract Dielectric behavior of several smectic side-chain oligoacrylates and Oligomethacrylates with 4-cyanazobenzene mesogenic side groups has been investigated. Below the glass transition temperature two relaxation regions of dipole relaxation (the γ1 and the [Btilde] processes) caused by local mobility were detected. The molecular mobility of the γ1 process increases with spacer length but in the case of the [Btilde] process the mobility is not changed. In the LC state, near Tg transition, two cooperative processes, α- and β, related to the reorientation of transverse (μ⊥) or longitudinal (μ|) components of the dipole moment of the mesogenic group, respectively, were observed. The orientation in external electric or mechanic fields leads to the establishment of planar or homeotropic orientation of mesogenic side groups.

Cooperative forms of molecular mobility in liquid-crystalline side-chain polymers

Abstract Dielectric behavior of LC side-chain polymers with oligomethylene spacers and mesogenic side groups with different structures has been investigated. Near Tg transition, two cooperative relaxation processes, α and δ, were observed. Relaxation times (τ), activation energies (U), and transition temperatures for the α and β processes were determined from logτ=φ(1/T) dependences. In the LC state these dependences are approximated by straight or curved lines. If the δ process is observed both in the LC and in the isotropic state, the logτ=φ(1/T) dependences are approximated by two lines with a break near Tcl. Kinetic parameters of the α and δ processes are mainly dependent on main chain structure, while the intensities are determined by parallel and perpendicular components of the dipole moment of mesogenic group.

Dielectric relaxation and molecular dynamics of liquid crystalline side-chain oligoacrylates with 4-cyanazobensene side mesogenic groups in external electric or mechanical fields

Dielectric relaxation and molecular mobility have ben investigated over the frequency range 60Hz-1MHz between 160 degrees C and 150 degrees C for smectic side-chain oligoacrylates with 4-cyanazobenzene mesogenic side groups and methylene spacers of different length. The studied oligomers were oriented by electric or mechanic fields. In the range of subglass temperatures two dielectric processes were observed - the (gamma) 1 and the (beta) . The (gamma) 1 process reflects the local motion of the spacer groups and the (beta) process is connected with the local motion of the mesogenic moieties. the molecular mobility of the (gamma) 1 process increases with the spacer lengthening but at the same time in the case of the (beta) process the mobility is not changed. In LC state near Tg transition, the temperature-frequency dependencies of dielectric losses show two cooperative processes, the (alpha) - an the (delta) , related to the reorientation of the transverse or longitudinal components of the dipole moment of the mesogenic group, correspondingly. The preliminary orientation in external electric or mechanic fields leads to the establishment of planar or homeotropic orientation of the side mesogenic groups. The order parameters of oriented films were calculated. It was shown the planar or homeotropic orientation of mesogenic groups did not influence on relaxation times of the observed dielectric processes.