Michel Dumon

Thermodynamic analysis of reaction-induced phase separation in epoxy-based polymer dispersed liquid crystals (PDLC)

The Flory-Huggins theory for the free energy of mixing in isotropic phases, in conjunction with the Maier-Saupe theory for phase transition of a nematic liquid crystal, was applied to a diepoxide-diamine-liquid crystal blend in pre- and post-gel stages. The analysis was performed taking the distribution of polymeric species into account at any conversion level. In the post-gel stage, the elastic energy contribution was included in the free energy expression. Cloud-point and shadow curves, characteristic of isotropic-isotropic and nematic-isotropic equilibria, were generated in temperature versus composition coordinates covering the whole conversion range. Experimental results for the isotropic-nematic transition could be reproduced by numerical simulation, using an interaction parameter inversely proportional to temperature and decreasing with conversion. The thermodynamic analysis can be used to control the morphologies of polymer dispersed liquid crystals.

Polymer-dispersed liquid crystals based on epoxy networks: 1. Effects of the liquid crystal addition on the epoxy-amine reaction

Abstract The kinetics of the epoxy-amine (diglycidylether of bisphenol A (DGEBA) and polypropylene oxide diamine D-400) reaction was studied with or without a liquid crystal additive (E7) using different concentrations. The neat system was found to obey the second-order kinetic model fairly well with a reactivity ratio n , i.e. the ratio of the rate constants for primary to secondary amines, equal to 1. The conversion at the gel point, x gel , was found at the theoretical value 0.57 for such a system (diepoxide-aliphatic diamine). In contrast, the rate of the epoxy-amine reaction underwent a dramatic decrease upon addition of the liquid crystal. This retarding effect is not only due to the dilution effect. In addition, the gel point was always observed at a higher conversion than for the neat matrix ( x gel ≥ 0.60), implying that the reactivity ratio n is strongly decreased. We suggest reasons accounting for such a retarding effect in view of the interactions of the liquid crystal with the epoxy-amine reaction mechanism or intramolecular reactions. After curing in a homogeneous phase (100°C), the polymer-dispersed liquid crystal systems were cooled down, which induced the phase separation of the liquid crystal-rich phase.

Hydroxyl‐terminated oligomers crosslinked by alkoxysilane sol‐gel or polyurethane chemistries: A comparison

The structures of in situ generated clusters and the level of physical interactions in two types of networks differing from the chemistry of crosslinking were studied by means of small-angle X-ray analysis and dynamic mechanical spectroscopy. For the first type of networks, the crosslinks result from the hydrolysis and condensation of ethoxysilane endgroups, thus generating silicon-rich dispersed phase. In the second case, the crosslinks result from the formation of urethane units by introducing a triol. In the two cases, different types of soft segment precursors having different polarities are considered. α, ω-hydroxyl-terminated oligomers of hydrogenated polybutadiene or polycaprolactone or a polyester from oleic acid are used. The miscibility of the soft-segment chains with the relatively polar crosslinks is the most important parameter for understanding the morphology and the mechanical behavior of such materials. The main difference obtained from the SAXS analysis and DMS experiments is that the silicon-rich clusters appear to be stiffer and well separated in comparison with the trimethylolpropane-urethane crosslinks. In addition, in the case of silica clusters generated in situ, the phase separation plays an important role. In the polycaprolactone-based systems, the formation of clusters is mainly governed by the nature and the reactivities of the functional groups. As a consequence, the clusters are more fractal-like. The mechanical behavior, i.e., the mechanical losses and the high-temperature behavior, is discussed as a function of the existing interactions and the concentration of elastically active network chains in the different types of networks considered.

Phase separation kinetics of an SA liquid crystal in a reactive epoxy-amine PDLC system : an X-ray diffraction study

A Polymer Dispersed Liqued Crystal composite based upon a smective liquid crystal and reactive crosslinkable epoxide-amine monomers was studies. The composite material is obtained by a polymerization induced phase separation process. The polymerization and the phase separation kinetics are with monitored. The polymerization induced phase separation is detected and quantiatively monitored by X-ray diffraction. The normalized X-ray intensity is used to calculate a phase separation rate and to define the phase separation kinetics which is comppared to the polymerization rate and to the polymerization kinetics. The rate difference accounts for changes in morphology.

Polymer Dispersed Liquid Crystals Based on Epoxy Networks: Transformations and Phase Separation Phenomena During Polycondensation

Abstract A diepoxy-diamine/liquid crystal mixture (50/50 wt%) is studied in the course of the crosslinking step-polymerization at different isothermal reaction temperatures or with a two temperature cure cycle. Depending on the reaction temperature, the systems undergoe phase separation either isothermally, when the reaction temperature is far below the nematic-isotropic (N-I) temperature of the neat LC, or on cooling partially reacted systems when the reaction temperature is above the N-I temperature of the neat LC. Phase separation is characterized simultaneously by two glass transition temperatures (Tg ), one nematic-isotropic transition (T N-1) and the appearance of nematic droplets as determined by differential scanning calorimetry (DSC) or polarized optical microscopy (POM). Thus, temperature-polymerization conversion phase diagrams are presented in the form of the evolution of Tg s and of T N-1 of the LC-rich phase. Furthermore, we attempted to use the calorimetric data to calculate the mass fract...

Tuning morphologies of thermoset / thermoplastic blends Part 1: Kinetic modelling of epoxy-amine reactions using amine mixtures

Abstract Different epoxy amine networks based on a mixture of diamine hardeners have been studied and their kinetic behaviour was modelled in order to understand the global and particular behaviour of each diamine in such blends. 4,4’-Diaminodiphenylsulfone (DDS) and 4,4’-methylenebis-(3-chloro-2,6- diethylaniline) (MCDEA) were used for this study by varying the DDS molar ratio from 0% to 50%. The determined kinetic model allowed us to calculate the composition for each diamine of the reactive epoxy-amine system in the whole range of epoxy conversions. Then the influence of 10% thermoplastic additive was checked and it was found to be negligible in the kinetic behaviour.

Tuning morphologies of thermoset/thermoplastic blends - Part 2: Phase separation of poly(vinyl methyl ether), PVME, using amine mixtures as thermoset hardeners

Abstract Part 2 of the study investigates morphologies of epoxy thermoset / thermoplastic blends obtained with formulations of the thermoset hardeners. The thermosetting matrices are composed of one epoxy resin crosslinked by a mixture of two aromatic diamine hardeners, namely MCDEA+DDS or MDEA+DDS (DDS is 4,4’-diaminodiphenylsulfone, MCDEA is 4,4’-methylenebis-(3-chloro-2,6- diethylaniline), MDEA is 4,4’-methylenebis-(2,6-diethylaniline). The blends are made at a fixed concentration of thermoplastic (poly(vinyl methyl ether), PVME, 10wt%) whereas three different cure temperatures are chosen and the matrix composition is varied by the ratio of DDS. DDS is a PVME-insoluble diamine whereas MDEA or MCDEA are PVME-soluble diamines. The domain size of the PVME nodules is tuned from a few micrometers down to sub micron sizes, typically between 50 and 80 nm. We attempt to explain this evolution thanks to the difference in the reactivity of the amines with the epoxy oligomer coupled to the difference in miscibility of each amine towards PVME. The kinetic study developed in Part 1 of this work is used to calculate the concentrations of each amine along the reaction time and their rate of incorporation in the polymer network. The latter parameter is shown to be the limiting factor for obtaining a nano phase separation.

Effect of molar mass of an epoxy oligomer on the phase separation in epoxy based polymer dispersed liquid crystals

Polymer dispersed liquid crystals based on epoxy-amine crosslinked matrices and a nematic liquid crystal, E7, have been studied over the course of polymerisation, i.e. as a function of the polymerisation conversion. The influence of epoxy oligomer molar mass on the initial temperature-concentration and the temperature-conversion phase diagrams has been investigated.An increase of the epoxy oligomer molar mass greatly reduces the initial liquid crystal solubility and brings the cloud points to earlier polymerisation conversions, which have been quantified. Thus the phase separation is markedly enhanced.The temperature-conversion phase diagrams have been characterised at two isothermal polymerisation temperatures for one liquid crystal composition (50wt.). These diagrams (isotropic-nematic and nematic-isotropic transition temperatures) are shown to obey master curves when the epoxy molar mass is varied.Finally, the size of the liquid crystal droplets is shown to decrease when the epoxy molar mass increases. This effect is mainly due to the viscosity increase resulting from the oligomer mass increase. Viscosity measurements were made at intervals during polymerisation.

Polymer-dispersed liquid crystals: liquid crystal/polymer composites as electro-active materials

Polymer Dispersed Liquid Crystals (PDLC) are materials formed by dispersions of liquid crystal (LC) droplets in a polymer matrix (1)• These materials are electro-optical active films which can be switched from a light scattering state (off state) to a transparent state (on state) by the application of a voltage (2)• The embedded mesophase within the droplets can be nematic (N), cholesteric (N*), chiral smectic C (SC*), or antiferroelectric (SCA*) (2)