Marina Saphiannikova

Layered Double Hydroxide Based Polymer Nanocomposites

Nanocomposites based on polymers and inorganic filler materials not only create enormous interest among researchers because of their unique way of preparation and properties, but also promise development of new hybrid materials for specific applications in the field of polymer composites. The present article deals with the application of a relatively new class of inorganic materials, namely layered double hydroxides (LDHs), as nanofiller for synthesizing polymer-based nanocomposites. LDHs are mixed metal hydroxides of di- and trivalent metal ions crystallized in the form similar to mineral brucite or magnesium hydroxide (MH) with the incorporation of interlayer anionic species. Several procedures for the synthesis of LDHs, their organic modification, and the synthesis of polymer/LDH nanocomposites are discussed in detail with reference to work done in very recent years. The potential of LDHs, especially magnesium and aluminum-based LDHs (Mg–Al LDH) as nanofillers for the polymer matrix has been investigated. The important aspects in characterizing such hybrid materials (i.e., morphological analysis and melt rheological behavior) have been reported in detail to understand the nature of LDH particle dispersion and its influence on the melt flow behavior of the nanocomposites. The specialty of LDHs as nanofiller is their thermal decomposition behavior, which makes them potential flame retardants for polymers. This aspect has been reported in detail in the case of polyethylene-based systems, where the flame retarding efficiency of organically modified Mg–Al LDH alone and also in combination with conventional flame retardants has been discussed.

Microscopic Theory of Light-Induced Deformation in Amorphous Side-Chain Azobenzene Polymers

We propose a microscopic theory of light-induced deformation of side-chain azobenzene polymers taking into account the internal structure of polymer chains. Our theory is based on the fact that interaction of chromophores with the polarized light leads to the orientation anisotropy of azobenzene macromolecules which is accompanied by the appearance of mechanical stress. It is the first microscopic theory which provides the value of the light-induced stress larger than the yield stress. This result explains a possibility for the inscription of surface relief gratings in glassy side-chain azobenzene polymers. For some chemical architectures, elongation of a sample demonstrates a nonmonotonic behavior with the light intensity and can change its sign (a stretched sample starts to be uniaxially compressed), in agreement with experiments. Using a viscoplastic approach, we show that the irreversible strain of a sample, which remains after the light is switched off, decreases with increasing temperature and can disappear at certain temperature below the glass transition temperature. This theoretical prediction is also confirmed by recent experiments.

Opposite photo-induced deformations in azobenzene-containing polymers with different molecular architecture: Molecular dynamics study

Photo-induced deformations in azobenzene-containing polymers (azo-polymers) are central to a number of applications, such as optical storage and fabrication of diffractive elements. The microscopic nature of the underlying opto-mechanical coupling is yet not clear. In this study, we address the experimental finding that the scenario of the effects depends on molecular architecture of the used azo-polymer. Typically, opposite deformations in respect to the direction of light polarization are observed for liquid crystalline and amorphous azo-polymers. In this study, we undertake molecular dynamics simulations of two different models that mimic these two types of azo-polymers. We employ hybrid force field modeling and consider only trans-isomers of azobenzene, represented as Gay-Berne sites. The effect of illumination on the orientation of the chromophores is considered on the level of orientational hole burning and emphasis is given to the resulting deformation of the polymer matrix. We reproduce deformations of opposite sign for the two models being considered here and discuss the relevant microscopic mechanisms in both cases.

Linear viscoelastic analysis of formation and relaxation of azobenzene polymer gratings.

Surface relief gratings on azobenzene containing polymer films were prepared under irradiation by actinic light. Finite element modeling of the inscription process was carried out using linear viscoelastic analysis. It was assumed that under illumination the polymer film undergoes considerable plastification, which reduces its original Youngs modulus by at least three orders of magnitude. Force densities of about 10(11) N/m3 were necessary to reproduce the growth of the surface relief grating. It was shown that at large deformations the force of surface tension becomes comparable to the inscription force and therefore plays an essential role in the retardation of the inscription process. In addition to surface profiling the gradual development of an accompanying density grating was predicted for the regime of continuous exposure. Surface grating development under pulselike exposure cannot be explained in the frame of an incompressible fluid model. However, it was easily reproduced using the viscoelastic model with finite compressibility.

Theory of light-induced deformation of azobenzene elastomers: Influence of network structure

Azobenzene elastomers have been extensively explored in the last decade as photo-deformable smart materials which are able to transform light energy into mechanical stress. Presently, there is a great need for theoretical approaches to accurately predict the quantitative response of these materials based on their microscopic structure. Recently, we proposed a theory of light-induced deformation of azobenzene elastomers using a simple regular cubic network model [V. Toshchevikov, M. Saphiannikova, and G. Heinrich, J. Phys. Chem. B 116, 913 (2012)]. In the present study, we extend the previous theory using more realistic network models which take into account the random orientation of end-to-end vectors of network strands as well as the molecular weight distribution of the strands. Interaction of the chromophores with the linearly polarized light is described by an effective orientation potential which orients the chromophores perpendicular to the polarization direction. We show that both monodisperse and polydisperse azobenzene elastomers can demonstrate either a uniaxial expansion or contraction along the polarization direction. The sign of deformation (expansion/contraction) depends on the orientation distribution of chromophores with respect to the main chains which is defined by the chemical structure and by the lengths of spacers. The degree of cross-linking and the polydispersity of network strands do not affect the sign of deformation but influence the magnitude of light-induced deformation. We demonstrate that photo-mechanical properties of mono- and poly-disperse azobenzene elastomers with random spatial distribution of network strands can be described in a very good approximation by a regular cubic network model with an appropriately chosen length of the strands.

Photosensitive response of azobenzene containing films towards pure intensity or polarization interference patterns

In this paper, we report on differences in the response of photosensitive azobenzene containing films upon irradiation with the intensity or polarization interference patterns. Two materials are studied differing in the molecular weight: an azobenzene-containing polymer and a molecular glass formed from a much smaller molecule consisting of three connected azobenzene units. Topography changes occurring along with the changes in irradiation conditions are recorded using a homemade set-up combining an optical part for generation and shaping of interference patterns and an atomic force microscope for acquiring the kinetics of film deformation. In this way, we could reveal the unique behavior of photosensitive materials during the first few minutes of irradiation: the change in topography is initially driven by an increase in the azobenzene free volume along with the trans-cis isomerization, followed by the mass transport finally resulting in the surface relief grating. This study demonstrates the great potential of our setup to experimentally highlight puzzling processes governing the formation of surface relief gratings.

Atomic force microscopy inspection of the early state of formation of polymer surface relief gratings

The process of surface relief grating formation was inspected by atomic force microscopy after short-pulse exposure with counter-rotating circularly polarized laser light of 488 nm on a polymer film containing an azobenzene side-chain homopolymer (pDR1M, TG=129 °C). During light inscription, the grating formation was probed by time-resolved visible scattering with red laser light. The efficiency of grating formation depends on the pulse length of blue light exposure. The shortest pulse length of 2 s did not create a permanent surface relief. After 5 s, a speckled surface modification starts rising and the surface relief becomes more and more uniform with a sinusoidal shape for longer exposure. The experimental findings reveal the individual addressing of azobenzene side groups by the actinic light providing a local lateral force via molecular trans-cis and cis-trans isomerization which subsequently causes grating formation.

Temperature Dependent Analysis of Grating Formation on Azobenzene Polymer Films

The temperature dependence of surface relief grating formation was studied using continuous and pulse like exposure. Surface relief gratings were inscribed on amorphous azobenzene polymer thin films using a holographic pattern of circularly polarized light at wavelength equal to 514 nm in a vacuum chamber to avoid the hot air turbulence. The efficiency of the surface relief grating formation was probed using a He-Ne laser of wavelength 633 nm by monitoring the first order diffraction peak (I1) as well as the specularly reflected intensity (Is). Under continuous exposure permanent grating formation was observed up to a temperature of about 100°C only. The same was found under pulse like exposure but grating still exists as long as the actinic light is on. Above 100°C it relaxes entirely after switching the light off. This elastic component disappears at about 115°C, i.e., close to the glass transition temperature. Our findings can be interpreted by the competition between light-induced ordering of azobenzene side chains and temperature induced disorder. Because the accumulated stress within the polymer decreases with temperature, permanent grating formation can only be observed when the light-induced stress is above the yield stress.

Molecular tracer diffusion in thin azobenzene polymer layers

Translational diffusion of fluorescent tracer molecules in azobenzene polymer layers is studied at different temperatures and under illumination using the method of fluorescence recovery after photobleaching. Diffusion is clearly observed in the dark above the glass transition temperature, while homogeneous illumination at 488nm and 100mW∕cm2 does not cause any detectable diffusion of the dye molecules within azobenzene layers. This implies that the viscosity of azobenzene layers remains nearly unchanged under illumination with visible light in the absence of internal or external forces.

Time and temperature dependence of surface relief grating formation in polymers containing azobenzene groups with different dipole moment

We present the results of time and temperature dependent deformation of two azobenzene materials with different dipole moment during the process of surface relief grating formation. The effect of chromophore orientation on grating formation in these films was studied in situ using continuous and pulselike exposure techniques at various temperatures up to the glass transition temperature of the respective polymer material. The results obtained for high dipole moment pDR1M and low dipole moment pMEA were compared in terms of the rate of the grating formation and related to the quantities as relaxation of the chromophores in the absence of the inscribing light and induced stress due to the chromophore orientation. The parameters obtained from the simulation data can be well explained considering viscoelastic material model. The experimental findings can be explained by assuming the interaction of azochromophores with polarized light leading to the orientation anisotropy of the azobenzene macromolecules, ther...