J. M. Buisine

Microwave effects in the synthesis of polyethers by phase transfer catalysis

Phase transfer catalysis processes with or without microwave fields are studied. A polycondensation reaction was used as a model, starting from 3,3-bis(chloromethyl)oxetane and various bisphenols. The results obtained show the advantages of microwave fields, concerning the molecular weights for crystalline polymers and the reaction time for all types of structures.

The application of the photopyroelectric method for measuring the thermal parameters of pyroelectric materials

The photopyroelectric calorimetry, in the standard (back) configuration, is applied in order to measure the thermal parameters of some pyroelectric materials. It is demonstrated that the method is able to simultaneously measure the thermal diffusivity and effusivity of a pyroelectric material. The information is obtained via a frequency scan of the amplitude or the phase of the pyroelectric signal; the measurements need no calibration. A combined amplitude-phase procedure, at a single frequency, leads to the same results. In the mean time, if the thermal parameters of the pyroelectric sensor are known, one can get the thermal effusivity of a sample acting in the experimental cell as a substrate. Investigations and theoretical simulations were performed on a well known pyroelectric material, LiTaO3, with various liquid substrates.

Photopolymerization kinetics and phase behaviour of acrylate based polymer dispersed liquid crystals

Polymer dispersed liquid crystal (PDLC) materials were prepared by a polymerization induced phase separation (PIPS) process using UV radiation. The samples were obtained from the liquid crystalline component E7 and a blend of monofunctional (2-ethylhexyl acrylate) and difunctional (1,6-hexane diol diacrylate) monomers as precursors of the matrix. Polymerization energetics and thermodynamic properties of PDLC materials were studied by differential scanning calorimetry. Photopolymerization kinetics and phase behaviour of the PDLC are presented and discussed as a function of UV polymerization conditions (UV light intensity, UV time exposure and isothermal curing temperature).

Equilibrium phase diagram of polystyrene and 8CB

The experimental equilibrium phase diagram of a mixture of linear polystyrene of molecular weight Mw = 44,000 g/mol and 4-cyano-4′-n-octyl-biphenyl (8CB) is established. The three transitions smectic A-nematic, nematic-isotropic, and isotropic-isotropic are observed. The first two are observed both by optical microscopy and differential scanning calorimetry (DSC) while the isotropic-isotropic transition could be seen only via optical microscopy. Two series of samples with the same compositions were independently prepared and yielded consistent results both by microscopy and DSC. Measurements of sample compositions with less than 50 weight % of 8CB were influenced by the vicinity of the glass transition temperature (Tg) of the polymer in the mixture. This quantity is also determined by DSC as a function of composition. A single Tg is observed, which decreases with composition of the LC. Other thermodynamic quantities such as the enthalpy variations of LC in the nematic-isotropic transition and the fraction of LC contained in the droplets are also considered.

Thermophysical and Electro-Optical Characterization of Nematic Liquid Crystal / Acrylate Systems

Abstract New Polymer Dispersed Liquid Crystal (PDLC) materials were prepared by a Polymerization Induced Phase Separation (PIPS) mechanism using U.V. radiation. The samples are obtained from the liquid crystalline mixture E7 and a blend of monofunctional (2-Ethyl Hexyl Acrylate) and difunctional (1,6-Hexane Diol DiAcrylate) monomers as precursors of the matrix. The thermodynamic properties of both cured and uncured samples were studied by Differential Scanning Calorimetry and Optical Microscopy. The optical transmission of selected PDLC films has been measured as a function of frequency and amplitude of the applied voltage for several film thicknesses.

Analysis of the photopyroelectric signal for investigating thermal parameters of pyroelectric materials

The photopyroelectric signal is analyzed in order to simultaneously obtain the thermal diffusivity and effusivity of pyroelectric materials. Two different experimental configurations are described and compared in terms of accuracy and sensitivity. The information is obtained via a frequency scan of the amplitude or the phase of the pyroelectric signal. The methods have been used for the measurement of thermophysical properties of a lead–titanate–zirconate ceramic sample.

Liquid crystalline polymers—VI. A study of the thermal behavior of some ternary copolyethers, containing 4,4′-dihydroxyazobenzene, bisphenol a and 1,1-bis(4-hydroxyphenyl)ethane

Abstract Copolyethers based on 3,3-bis(chloromethyl)oxetane and several bisphenols [4,4′-dihydroxyazobenzene, bisphenol A and 1,1-bis-(4-hydroxyphenyl)ethane] were synthesized. The influence of chain geometry and flexibility on the possible appearance of liquid crystalline behavior was investigated. Polymer properties were studied using 1 H NMR spectroscopy, DSC, optical microscopy in polarized light and theoretical conformational analyses. The optimal mesogen/nonmesogen ratios, correlated with the flexibility of the chain and anisotropic behavior was established.

Study of thermal parameter temperature dependence of pyroelectric materials

A simplified photopyroelectric configuration that allows the determination of the temperature-dependent thermal parameters of pyroelectric materials is described. The procedure is based on a combination of phase and amplitude signal data obtained at a single frequency. Experimental results obtained on the thermal parameters of LiTaO3 single crystal are presented.

Thermophysical behaviour of monofunctional acrylate and liquid crystal systems

Abstract The phase behavior of monofunctional acrylate and low molecular weight nematic liquid crystals (LC) is considered. Systems involving the monomeric 2-ethylhexylacrylate (2-EHA), the eutectic LC mixture known as E7 and the 4-cyano-4′-n-pentyl-biphenyl (5CB) are investigated. A similar investigation is performed on mixtures involving a polymer poly-2-EHA with molecular weight Mw=48,000 g/mol and both LCs. The experimental phase diagrams are established using polarized optical microscopy and analyzed using a theoretical formalism which combines the Flory–Huggins theory of isotropic mixing and the Maier–Saupe theory of nematic order. The results lead to characterization of the miscibility of E7 and 5CB with monomeric and analogous polymeric 2-EHA systems.

Experimental observation of multicritical points with TGB phases on a pure compound

Abstract High pressure studies for the n = 12 homologues of the 3-fluoro-4((R) or (S)-methylheptyloxy)-4′-(4-alkyloxy-3-fluorobenzoyloxy) tolanes series have been performed by thermobarometric analysis. The phase sequence presents two Twist Grain Boundary phases denoted TGBA and TGBC. The pressure-temperature phase diagram has been determined; it exhibits a SC∗ phase, and an induced phase identified as SA which does not exist under atmospheric pressure. The TGBA temperature stability domain versus pressure first increases, then rises to a maximum and finally disappears. Such phase behaviour leads to an experimental observation of two multicritical points SC∗-SA-TGBA and SA-TGBA-N∗ for the pure compound under high pressures. This result confirms the existence of singular points previously predicted by the Renn-Lubensky theory.