Jaroslav M. Ilnytskyi

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.

Modelling elasticity and memory effects in liquid crystalline elastomers by molecular dynamics simulations

We performed molecular dynamics simulations of a liquid crystal elastomer of side-chain architecture. The network is formed from a melt of 28 molecules each having a backbone of 100 hydrocarbon monomers, to which 50 side chains are attached in a syndiotactic way. Crosslinking is performed in the smectic A phase. We observe an increase of the smectic–isotropic phase transition temperature of about 5 degrees as compared to the uncrosslinked melt. Memory effects in liquid crystalline order and in sample shape are well reproduced when the elastomer is driven through the smectic–isotropic transition. Above this transition, in the isotropic phase, the polydomain smectic phase is induced by a uniaxial load. Below the transition, in a monodomain smectic A phase, both experimentally observed effects of homogeneous director reorientation and stripe formation are reproduced when the sample is stretched along the director. When the load is applied perpendicularly to the director, the sample demonstrates reversible deformation with no change of liquid crystalline order, indicating elasticity of the two-dimensional network of polymer layers.

Adsorption of ions on surfaces modified with brushes of polyampholytes

We apply density functional theory to study adsorption of ions, treated in the framework of the restricted primitive model (RPM), on surfaces modified by tethered polyampholytes. The residual electrostatic contribution to the free energy functional is approximated by using the approach proposed by Wang et al. [J. Phys.: Condens. Matter 23, 175002 (2011)] for simple nonuniform RPMs systems. Our research concentrates on the problems how the distribution of the charges within chains of polyampholytes changes the selectivity of adsorption of ions species, the structure of the surface layer, and its electric properties.

Reorientation Dynamics of Chromophores in Photosensitive Polymers by Means of Coarse‐Grained Modeling

We study the photoisomerization of azobenzene chromophores embedded into a polymer matrix by using coarse-grained simulations. Two types of beads are considered: t- and c-beads, which are rich in trans and cis isomers, respectively. Simulations combine deterministic (molecular dynamics) and stochastic (random-type switching) parts. The ratio between the characteristic times for photoinduced reorientation and for orientation relaxation is tuned to be of the order found in experiments. The essential features of the phenomenon: 1) the existence of a stationary state, and 2) anisotropic distribution of the orientations of t-beads (orientation hole-burning effect), are reproduced. We study population dynamics of c-beads and the strength of the orientation hole burning, depending on the illumination wavelength and its intensity. The form of the reorientation potential of the mean force acting on the t-beads is analyzed and its use is validated.

How does the scaling for the polymer chain in the dissipative particle dynamics hold

We performed a series of simulations for a linear polymer chain in a solvent using dissipative particle dynamics to check the scaling relations for the end-to-end distance, radius of gyration and hydrodynamic radius in three dimensions. The polymer chains of up to 80 beads in explicit solvent of various quality are studied. To extract the scaling exponent \nu, the data are analyzed using linear fits, correction-to-scaling forms and analytical fits to the histograms of radius of gyration distribution. For certain combinations of the polymer characteristics and solvent quality, the correction-to-scaling terms are found to be essential while for the others these are negligibly small. In each particular case the final value for the exponent \nu was chosen according to the best least-squares fit. The values of \nu obtained in this way are found within the interval \nu=0.55-0.61 but are concentrated mostly around 0.59, which is very close to the best known theoretical result \nu=0.588. The existence of this interval is attributed both to the peculiarities of the method and to the moderate chain lengths being simulated. Within this shortcoming, the polymer chain in this kind of modeling is found to satisfy the scaling relations for all three radii being considered

Stationary states and spatial patterning in an SIS epidemiology model with implicit mobility

By means of the asynchronous cellular automata algorithm we study stationary states and spatial patterning in an SIS model, in which the individuals are attached to the vertices of a graph and their mobility is mimicked by varying the neighbourhood size q. Here we consider the following cases: q is fixed at certain value; and q is taken at random at each step and for each individual. The obtained numerical data are then mapped onto the solution of its version, corresponding to the limit q→∞. This allows for deducing an explicit form of the dependence of the fraction of infected individuals on the curing rate γ. A detailed analysis of the appearance of spatial patterns of infected individuals in the stationary state is performed.

Complex phase behavior of a fluid in slits with semipermeable walls modified with tethered chains

We study the phase behavior of a two-component fluid in a pore with the walls modified by tethered chains. The walls are completely permeable for one component of the fluid and completely impenetrable for the second component. The fluid is perfectly mixed in a bulk phase. We have found that depending on the details of the model the fluid undergoes capillary condensation inside the pore and wetting and layering transitions at the outer walls. Moreover, we have found transitions connected with the change of symmetry of the distribution of chains and fluid inside the pore.

Local and cluster critical dynamics of the 3d random-site Ising model

We present the results of Monte Carlo simulations for the critical dynamics of the three-dimensional site-diluted quenched Ising model. Three different dynamics are considered, these correspond to the local update Metropolis scheme as well as to the Swendsen–Wang and Wolff cluster algorithms. The lattice sizes of L=10–96 are analysed by a finite-size-scaling technique. The site dilution concentration p=0.85 was chosen to minimize the correction-to-scaling effects. We calculate numerical values of the dynamical critical exponents for the integrated and exponential autocorrelation times for energy and magnetization. As expected, cluster algorithms are characterized by lower values of dynamical critical exponent than the local one: also in the case of dilution critical slowing down is more pronounced for the Metropolis algorithm. However, the striking feature of our estimates is that they suggest that dilution leads to decrease of the dynamical critical exponent for the cluster algorithms. This phenomenon is quite opposite to the local dynamics, where dilution enhances critical slowing down.

Photo-induced deformation of azobenzene polymers: theory and simulations

A microscopic theory is developed to describe light-induced deformation of azobenzene polymers of different chemical structures: uncross-linked low-molecular-weight azobenzene polymers and cross-linked azobenzene polymers (azobenzene elastomers) bearing azobenzene chromophores in their strands. According to the microscopic theory the light-induced deformation is caused by reorientation of azobenzene chromophores with respect to the electric vector of the linearly polarized light, E. Theoretical calculations of the order parameter of short azobenzene molecules (oligomers) affected by the light show that the sign of the light-induced deformation (expansion / contraction along the vector E) depends strongly on the chemical structure of the oligomers. The conclusion of the theory about different signs of the light-induced deformation of low-molecular-weight azobenzene polymers is in an agreement with performed series of molecular dynamics simulations. Using the microscopic theory it is shown that cross-linked azobenzene polymers demonstrate the same light-induced deformation (expansion / contraction) as their low-molecular-weight analogues, i.e. polymers consisting of short azobenzene molecules whose chemical structure is the same as chain fragments of the elastomers.

Computer Simulation of Side-Chain Liquid Crystal Polymer Melts and Elastomers

Liquid crystalline polymers combine the optical activity and mesophase formation of liquid crystals with the visco-elastic properties of polymers. The synergy and interplay between both give rise to a number of phenomena, namely: memory effects, photo-induced deformations, formation of surface relief gratings, etc. Modelling of these effects should retain relevant features of the inter-particle interaction potentials and, simultaneously, be able to address the relatively large length scale on which these phenomena take place. A reasonable compromise between the two can be achieved by employing the so-called semi-atomistic force-field modelling, in which relevant groups of atoms are united into single sites with appropriately parameterised interaction potentials. Such simulations, performed by means of the molecular dynamics method, are considered in this chapter for the particular case of side-chain liquid crystal polymers (SCLCPs). The first part of the chapter is focused on the phase behavior of SCLCPs, covering existing experimental data and computer simulation studies. We consider in detail two particular architectures, the weakly- and strongly-coupled SCLCPs, respectively. The former category has a flexible backbone and long side chain (spacer) of ten monomers, and exhibits isotropic as well as poly- and monodomain smectic phases. The latter exhibits no ordered liquid crystalline phase, but a disordered glass-like low temperature phase. The structure of SCLCPs in these phases is characterised via the orientational order of the mesogens, as well as metric properties of backbones and side-chains, their respective orientational order, relaxation times and diffusion anisotropy. The second part of the chapter extends the analysis to the case of the reaction of SCLCPs to external perturbation. In particular, we assume the mesogens to represent the azobenzene chromophores, and the photo-isomerization of the latter under suitable illumination is reduced to the effect of reorientation of the trans-isomers, followed by the deformation of the polymer subsystem. The coupling between the chromophores and polymer backbone plays the vital role here, yielding contraction (extension) of the SCLCP volume element along the polarization vector for the case of weakly (strongly) coupled SCLCPs. This result, obtained by means of computer simulations, explains the experimentally observed differences in opto-mechanical behavior of azobenzene polymers with different molecular architectures. Finally, in the third part of the chapter, we consider the effect of crosslinking on the coupling between the liquid crystallinity and the mechanical state of the SCLCP. Crosslinking is performed in the smectic-A phase of a weakly coupled SCLCP to form a liquid crystal elastomer. We reproduce the effect of stabilisation of the smectic phase, as well as the memory effects in liquid crystalline order and in sample shape, when the elastomer is driven through the smectic–isotropic transition. Above this transition, in the isotropic phase, the polydomain smectic phase is induced by a uniaxial load. Below the transition, in a monodomain smectic-A phase, both experimentally observed effects of homogeneous director reorientation and stripe formation are reproduced when the sample is stretched along the director. When the load is applied perpendicularly to the director, the sample demonstrates reversible deformation with no change of liquid crystalline order, indicating elasticity of the two-dimensional network of layers.