Mustapha Benmouna

Electro-optical properties of polymer-dispersed liquid crystals

This paper deals with the electro-optic (EO) properties of polymer-dispersed liquid crystals (PDLCs). Several systems are considered to analyze the effects of preparation conditions and film characteristics on the EO response functions. Particular emphasis is put on systems based on mixtures of the commercial compound ASX-95, the difunctional acrylate tripropylene diacrylate and the eutectic mixture of low-molecular-weight liquid crystals E7. Other systems are considered to assess the influence of monomer functionality on EO properties using for example the trifunctional glycerylpropoxytriacrylate. Various modes of preparing PDLCs are considered based on the mechanism of polymerization-induced phase separation using either electron-beam (EB) or UV radiation curing processes. The dose is changed in both techniques to improve film strength and determine which method leads to the best samples in terms of EO response functions. Other important parameters, such as film thickness, composition and applied voltage, are also considered to evaluate the impact on these functions. The article focuses on a comparison of EO performances of films elaborated by exposure to EB and UV radiations. Under similar conditions, one definitively finds a net superiority of the former technique. In addition this technique does not require any photoinitiator and leads to a higher conversion of the monomeric compounds, i.e. higher mechanical strength and less severe aging conditions.

Application of random phase approximation to the dynamics of polymer blends and copolymers

Abstract The dynamics of melts of homopolymer mixtures and copolymers is studied with RPA. The first cumulant and the zeroth-order time moment of the measured dynamic scattering function S(q,t) are expressed-in terms of their counterparts in the non-interacting system of bare chains. The qualitative behaviour of these quantities as function of the wave number q and the interaction parameter χ F are obtained using Rouse dynamics for bare chains, and the results are presented graphically as a guide to the interpretation of dynamic scattering experiments on such systems. The q -dependent threshold for spinodal decomposition in the case of copolymers, and the variation of the growth rate of the mean response with q in the unstable regime are also discussed qualitatively in both systems.

On the phase behavior of blends of polymers and nematic liquid crystals

The phase behavior of mixtures of polymers and nematic liquid crystals (LC) is investigated. Two types of systems are examined. The first one deals with blends in which the polymer is made of linear chains. In this case, a systematic study of the effects of various parameters on the phase diagrams is performed. In particular, it is shown how increasing the polymer size and/or the LC molecule size increases the miscibility gap of the mixture. It also reduces the region where a single nematic phase is observed in the presence of a tiny amount of polymer. Likewise, the relative effects of the isotropic and the nematic interaction parameters on the phase diagrams are examined. The second part of this investigation deals with blends involving crosslinked polymers. Here, substantial differences are observed as compared to the case where the polymer components are made of linear chains. These differences are illustrated by showing the phase diagrams in similar conditions for both blends. Unlike the case of a linear polymer matrix, it is observed that the single nematic phase and the nematic-isotropic spinodal branches are absent from the phase diagram of crosslinked polymers. This results into significant distortions of the phase diagram. In order to highlight all these effects, examples representing hypothetical blends are considered. These examples are chosen for illustration of the results in which the choice of numerical parameters is made consistently with the existing values in the literature which makes comparison with published data possible.

Cyclic Polymers in Good Solvents

This paper deals with the effects of excluded volume interactions on the thermodynamic and structural properties of cyclic polymers in good solvents. Several thermodynamic properties are discussed with particular emphasis on excluded volume interactions and their effects on the radii of gyration and the form factors. An empirical model describing the mean square distance between two points along a cyclic polymer is proposed and its predictions compared with those of other models. Comparison between cyclic and linear chain properties highlights the effects of chain closure under good solvent conditions.

Phase equilibrium of poly(n-butyl acrylate) and E7

The experimental equilibrium phase diagram of mixtures of linear poly(n-butyl acrylate) of molecular mass Mw = 112000 g mol-1 and the low molecular mass LC mixture E7 has been established using polarized optical microscopy and light scattering techniques. The diagram is found to be reminiscent of an upper critical solution temperature system. Two independent series of samples with the same composition were studied, yielding consistent results. A region of nematic and isotropic coexisting phases and a region of a single isotropic phase were identified in the composition-temperature phase diagram. The results were analysed within a theoretical model combining the Flory-Huggins lattice theory for isotropic mixing and the Maier-Saupe theory for nematic ordering. Interestingly, no region of isotropic coexisting phases was observed in our experiments. This is probably due to the fact that the nematic interaction overwhelms the isotropic interaction in the region where (I + I) coexisting phases could appear. A preferential solubility of certain constituents of the LC mixture in the polymer could possibly be a reason for this behaviour.

The zero average contrast condition: Theoretical predictions and experimental examples

Abstract This paper deals with mixtures of homopolymers and copolymers in solvents under the zero average contrast condition. This condition is chosen in order to decouple the correlations due to polymer composition fluctuations from those due to polymer concentration fluctuations. This makes the measurement of these correlations much easier using techniques based on static light scattering, photoncorrelation spectroscopy, small angle neutron scattering, neutron spin echo or X-ray scattering. In the first part of the paper, a simple theoretical framework is developed to present the guidelines for the interpretation of the scattering data. In the second part, some experimental examples are discussed using the scattering techniques mentioned above. Data obtained from various systems involving alike (deuterated and ordinary) homopolymers and diblock copolymers as well as unalike species are considered. An example of mixtures of polyelectrolytes in aqueous solutions is also discussed within the framework of the zero average contrast condition.

Equilibrium phase behavior of polymer and liquid crystal blends

A theoretical framework describing the equilibrium phase behavior of polymers and liquid crystals is presented. Linear and crosslinked polymers are considered, and complexities found in the phase properties of systems involving crosslinked networks are higlighted. Effects of the rubber clasticity parameters in the elastic free energy are found to induce substantial distortions in the phase diagram. The Flory-Huggins interaction parameter which governs the miscibility of the mixture in the istropic state is assumed to be independent of the polymer architecture and modeled either by using a function of temperature only or temperature and composition. The thermodynamic description of the ordered domains is made according to the Maier-Saupe theory for nematic order and its extension to include other ordering properties. In particular, the smectic-A order is described according to the generalization of the Maier-Saupe theory proposed by McMillan. In the presence of nematogens, the coupling leads to quite different phase properties. In the strong coupling limit, a wide single nematic phase is found. In the weak coupling, the miscibility gap is much wider. These mixtures are described with mean-field theories of nematogen first developed by Brochard et al. and later extended by Kyu et al. This theoretical formalism has been applied successfully to analyze data obtained on several systems including linear and crosslinked polymer networks, smetic and nematic low molecular weight liquid crystals (LMWLC), and nematogen mixtures.

Polymerization-induced Phase Separation: Phase Behavior Developments and Hydrodynamic Interaction

The process of polymer synthesis based on polymerization-induced phase separation (PIPS) is revisited from the theoretical point of view. Cahn–Hilliard–Cook theories for spinodal decomposition are adapted to describe the kinetics of phase separation and deduce the time-resolved scattering function, while the double reaction model is used to describe the kinetics of polymerization. Coupling of these two kinetics is provided by the Carothers equation relating the fraction of reacted monomers to the degree of polymerization at time t, denoted N(t). It is argued that the approach to criticality is governed by a critical parameter, χc, that is different from the usual parameter for spinodal decomposition, χs, deduced from the second derivative of the free energy. While the latter parameter depends on the reciprocal degree of polymerization N−1(t), the former one depends on its time integral. This leads to significant consequences on the phase behavior developments during the PIPS process. Hydrodynamic interactions are found to speed up the emergence of instability modes. Although the qualitative trends remain similar to those of the Rouse dynamics, important quantitative changes are found due to the long-range viscous flow effects.

Light scattering from acrylate-based polymer dispersed liquid crystals: theoretical considerations and experimental examples

A light scattering (LS) study made using acrylate-based polymer dispersed liquid crystals (PDLCs) is presented. The polarized component IVV is measured for blends of a polyacrylate and the liquid crystal (LC) E7 at several compositions. Only the off-state configuration of the droplets with no external fields is considered here. These composites consist commonly of micron-sized nematic LC droplets dispersed in a solid polymer matrix. Theoretical expressions for the scattered intensities in the case of isotropic and anisotropic spherical droplets are given both in the Rayleigh-Gans approximation (RGA) and in the anomalous diffraction approximation (ADA). Series of VV and VH components of the scattering intensities are calculated using the models of Meeten, Stein and coworkers. The model calculations are compared with the light scattering data. This comparison enables us to extract information on the size and the shape of droplets assuming that the size distribution is uniform and that the scattering is due ...

Phase diagrams of poly(dimethylsiloxane)/E7 mixtures

Abstract The phase behavior of blends of poly(dimethylsiloxane) and the eutectic mixture of liquid crystals E7 is investigated. Experimental phase diagrams are established by polarized optical microscopy for two systems with widely different polymer molecular weights. Surprisingly, we find a very slight loss of miscibility of the mixture as a result of an increase of the polymer molecular weight by almost an order of magnitude. This unexpected phase behavior is in contrast with the results reported so far on other systems.