Natalia A. Nikonorova

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.

The effect of silver nanoparticles on the phase state of comb-shaped liquid crystalline polymers with cyanobiphenyl mesogenic groups

A new approach is proposed for the preparation of a new class of hybrid polymer systems based on comb-shaped LC polymers with cyanobiphenyl mesogenic groups and silver nanoparticles with dimensions ranging from 5 to 54 nm. A correlation between copolymer composition and dimensions of the formed nanoparticles is established. As the concentration of nanoparticles in LC copolymer is increased, the resultant glass transition temperature increases, and the temperature interval of the existence of LC phase is reduced. This behavior is related to the adsorption of cyanobiphenyl and carboxylic polymer groups on the surface of silver nanoparticles. In this case, the conductivity and dielectric permittivity of the composites are also increased.

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.

Molecular Mobility of Molecular Brushes With a Polyimide Backbone and Polymethylmethacrylate Side Chains of Different Lengths

The molecular mobility of polyimide (PI) brushes, i.e., graft-copolymers with a polyimide backbone and PMMA side chains of different lengths, is studied by the dielectric spectroscopy method. Three dipolar polarization regions are found in the temperature range from –150 to 200°C: β-, β1, and α-processes. The β-process is due to a local mobility of polar groups of the PI backbone in the glassy state. Ester groups of polymethylmethacrylate (PMMA) side chains are responsible for two forms of the molecular mobility: a local mobility in the glassy state (β1-process) and a cooperative segmental motion of the skeletons of the PMMA side chains (α-process). The parameters of the β- and β1-processes are weakly dependent on the side chains length. For the α-process, the molecular mobility of the PMMA side chains decreases with increasing their length.

Molecular mobility in comb-shaped copoly(methacrylates) with fluoromethylene and chromophore-containing side chains

The molecular mobility of copoly(methacrylates) with chromophore-containing side groups of various structures has been studied by the dielectric method. The region of relaxation of dipole polarization related to the segmental mobility (α process) has been discovered. At temperatures that are ∼30°C higher than those corresponding to the α process, the relaxation transition reflecting the reorientation of chromophore-containing side groups (δ process) takes place. It is suggested that the occurrence of the δ process is a general phenomenon for such systems. This effect allows one to determine the temperature at which the polarization of polymer films under application of corona discharge should be carried out so that a high level of nonlinear optical properties associated with the macroscopic noncentrosymmetrical orientation of chromophores may be achieved.

Molecular mobility of a comb-shaped copolymethacrylate with chalcone chromophore-containing side groups

The molecular mobility of copolymethacrylate with chromophore-containing chalcone side chains is studied by broadband dielectric spectroscopy. Five regions of dipolar polarization relaxation are identified: namely, γ, β, β1, α, and δ processes. It is shown that γ and β1 processes are related to the local mobility of methylene sequences and ester groups adjoining the backbone, while the α process is associated with the cooperative segmental mobility of the backbone. The β and δ processes are attributed to the mobility of chalcone groups: the local mobility in the glassy state (reorientation relative to the long axis of chromophores) and the cooperative mobility in the rubbery state (reorientation relative to the short axis of chromophores), respectively. The incorporation of 20% chalcone groups does not change the glass-transition temperature but enhances the cooperativity of the α process and intermolecular interactions.

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.

Local forms of molecular mobility in comb-shaped chromophore-containing copoly(methacrylates)

The local forms of molecular mobility of comb-shaped random copoly(methacrylates) have been studied by the method of dielectric spectroscopy. Side branches of these copolymers are composed of alternating chromophore-containing chains and flexible fluoromethylene sequences. Two regions of relaxation of dipole polarization, γ2 and β processes, related to the mobility of end polar groups and the reorientation of chromophore groups relative to their long axes, respectively, have been discovered in the glassy state. The times of relaxation of the γ2 process depend on the ratio between the flexible and rigid parts of the copolymer. In the region of the β process, the molecular mobility appears to be slightly dependent on the chromophore structure and its intensity is determined by the degree of substitution of chromophore groups.