Philippe Gramain

Thermotropic liquid crystals from N-alkylpyridinium halides ω-substituted with a mesogenic group

Abstract A series of N-alkylpyridinium halides ω-substituted with a 4-methoxybiphen-ylyloxy mesogenic group were synthesized and characterized. The molecules of these compounds contain three distinct parts: a flexible aliphatic chain, a rigid polarizable aromatic core, and a positively charged pyridinium ring associated with a negatively charged counterion. Their thermotropic liquid-crystalline behaviour was studied by differential scanning calorimetry and optical microscopy. Three types of smectic mesophase, namely A, B, and E, were identified by X-ray diffraction. Their structure consists of single layers of upright molecules laterally arranged head to tail. Segregated from the non-ionic parts of the molecules, the ionic end groups are set in double layers with the oppositely charged species facing each other and equally distributed between the two sub-layers. For the ordered smectic E phases, the anions are arranged in rows along the rectangular two dimensional unit cell diagonals with the pyridinium r...

Dissolution kinetics of human enamel powder: I. Stirring effects and surface calcium accumulation

Abstract The kinetics of dissolution of human enamel powder were studied at 37°C and constant pH in the range 3.7–6.5 by continuous recording of proton uptake and calcium release. The effects of stirring and the behavior of the enamel powder during the first stages of dissolution were analyzed. It was shown that the dissolution rate was controlled at low stirring speed by a diffusive process which became a solid-like controlled process at higher stirring speed. Moreover, in any case, a tendency of interfacial calcium accumulation was observed in parallel with a progressive slowing of the dissolution rate. The higher the pH, the more important the accumulation. These observations suggest that an interfacial calcium-and/ or phosphate-rich layer is rapidly formed on the crystal surface. This “by-product” layer reduces considerably the rate of dissolution of hydroxyapatite. At high stirring, the dissolution rate is limited by the diffusion of calcium and/or phosphate ions through this layer, whereas at low stirring, the rate is limited by the diffusion of calcium or phosphate in the Nernst layer adjacent to the surface deposited layer.

Poly(vinylidene fluoride)/poly(vinyl alcohol-co-vinyl acetate) blends : 1. compatibility study by differential scanning calorimetry (DSC)

Abstract Blends of poly(vinylidene fluoride) (PVDF) and some poly(vinyl alcohol–co-vinyl acetate) copolymers (PVAAc) with various molecular weights and acetate contents are studied by differential scanning calorimetry. First, two sets of blends of PVDF with two different PVAAc at various concentrations are analysed. Then, 50/50 (wt/wt) blends of each copolymer with PVDF are studied. Even though the glass transitions of both polymers are not observable, the PVDF melting and crystallization data give some indications about these blends: (1) the amorphous phases of both polymers seem to be, in any case, immiscible; (2) the crystallization of PVDF is modified by mixing with the copolymers (PVAAc), the most crystalline of them (with low acetate contents) improving its crystallinity ratio.

Synthesis and thermal properties of poly[N-(monomethoxypolyethylene oxide)-4-vinylpyridinium salt]s

A new type of cationic polyelectrolyte with poly(ethylene oxide) (PEO) side-chains has been prepared by the spontaneous polymerization of 4-vinylpyridine in the presence of PEO monotosylates or bromides. This very easy anionic polymerization method gives fully PEO-grafted poly(vinylpyridinium) salts of high molecular weight which are soluble in water and many organic solvents. The synthesis, characterization, and thermal properties of a series of polysalts are presented. Five monomethoxypolyethylene glycols of different molecular weights and functionalized by tosylation or bromination were used in the quaternization-polymerization of 4-vinylpyridine. These polymers present good thermal stability up to 260°C in air and decomposition takes place in two steps with the first dequaternization reaction. The effects of the PEO length and the nature of the counter-ion on the melting and glass transition temperatures have been studied. It is shown that the pyridinium structure reduces the tendency of the grafted PEO chains to crystallize. Moreover, all of these polysalts have low T gs in spite of the rigid backbone and the presence of ionic charges.

Polymérisation radicalaire vivante du mma en présence de dérivés piperidino-dithiocarbamates comme photoiniferters

Radical polymerizations of methyl methacrylate were carried out in the presence of photoiniferters (RDC) containing a piperidino-dithiocarbamate group. The effects of the structure of the RDC on the photopolymerization in the presence or absence of a transfer agent such as tetraethylthiuram disulfide (TD) were investigated. The influence of the presence of a transfer agent (TD) on the living character and the control of the polymerization were also discussed.

Effect of smectogenicity of the ionic groups on the thermotropic liquid crystalline behavior of pyridinium and poly(4-vinylpyridinium) salts quaternized with mesogenic groups

In order to demonstrate the important smectic power of the ionic functions present in mesogenic molecules, a series of N-alkylpyridinium bromides ω-substituted with (4-cyanobiphenylyl)oxy or [4-(2-methyl-1-butoxy)biphenylyl]oxy mesogenic group and their analogous 4-vinylpyridinium polymers were synthesized and characterized. The liquid crystalline behavior was studied by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. Smectic mesophases, namely A, B, and E, were identified for the low molecular weight compounds, whereas smectic A and E mesophases were identified for the analogous polymers. Both structures were found to be very similar. They consist of single layers of upright molecules laterally arranged head-to-tail; the polymer backbone is inserted in between the layers. The monolayer smectic ordering observed in spite of the presence of the interacting cyano and chiral groups demonstrates the prevailing effect of the electrical interactions upon the structural organization.

Influence of backbone tacticty on thermotropic properties of side chain liquid crystal polymethacrylates

In order to study the tacticity effect of the backbone on the thermotropic properties of side-chain liquid crystalline (SCLC) polymers, the synthesis of four polymethacrylates with different tacticities and bearing methoxy biphenyl mesogenic groups was carried out. Homopolymers were obtained upon photopolymerisation at low temperature of two mesogenic methoxy biphenyl monomers, with and without ethylene oxide spacer. Methacrylic acid copolymers were obtained by grafting the same mesogenic groups onto 92% syndiotactic or atactic polymethacrylic acids. It is demonstrated that an increase of 11% of the content of syndiotactic triads compared to an atactic structure, does not modify the liquid crystal behavior of homopolymers. Indeed, smectic OB, A, E and nematic phases were identified as for similar atactic samples. On the other hand, with both copolymers, the liquid crystalline behavior disappears to the advantage of crystalline phases independently on the backbone configuration. The results confirm that mesogenic interactions govern the structural behavior of such polymethacrylate polymers.

Inhibition of dissolution of hydroxyapatite powder by adsorbed anionic polymers

Abstract The effect of the presence in solution of neutral or anionic hydrosoluble polymers on the kinetics of dissolution of hydroxyapatite powder (HAP) is analysed at a constant pH of 5 and under well-controlled conditions. Four polymers are studied. It is shown that the polymers that are able to be adsorbed induce a decrease in the rate of dissolution which strongly depends on the chemical structure of the polymers. Chemical groups with the ability to bind calcium ions, such as phosphate and phosphonate groups, are particularly efficient. The results are interpreted with the use of a recently proposed apatite dissolution model which assumes the formation, at the apatite interface, of an ionic semipermeable layer produced by the adsorption of calcium ions. According to this model, adsorbed polymers increase the ionic capacity of the interface.

Single-ion and salt conductor polymer electrolytes based on poly(4-vinylpyridine) quaternized with poly(ethylene oxide) side chains

A new type of single-ion conductor with fixed cation was synthesized by spontaneous anionic polymerization of 4-vinylpyridine in the presence of short polyethylene oxide (PEO) chains as alkylating agents. These comblike polymers have low Tgs and are amorphous with the shorter PEOs. Their conductivities are unaffected by the nature of the anion (Br−, ClO4−, and tosylate) and are controlled by the free volume and the mobility of the pendant cation. By comparison of the results at constant free volume, it is shown that the charge density decreases with the increasing length of pendant PEO demonstrating that PEO acts only as a plasticizing agent. Best conductivity results (σ = 10−5 S cm−1 at 60°C) are obtained with PEO side chains of molecular weight 350. With this sample, the conductivity in the presence of various amounts of added salt (LiTFSI) was studied. A best value of 10−4 S cm−1 at 60°C is obtained with a molar ratio EO/Li of 10. It is shown that, over the range of examined concentrations (0.2–1.3 mol Li kg−1), the reduced conductivity σr/c increases linearly with increasing salt concentration showing that the ion mobility increases continuously. Such behavior is quite unusual since in this concentration range a maximum is generally observed with PEO systems. To interpret this result and by analogy with the behavior of this type of polymer in solution, it is proposed that the conformation of these polymers in the solid state is segregated with the P4VP skeleton more or less confined inside the dense coils surrounded by the PEO side chains. Under the influence of the increasing salt concentration, this microphase separation vanishes progressively: The LiTFSI salt exchanges with the tosylate anions and acts as a miscibility improver agent.

Preparation of polystyrene-co-divinylbenzene microspheres with controlled sizes by polymerization of stabilized microsuspensions

Abstract The preparation in two steps of polystyrene- co -divinylbenzene microbeads with controlled sizes in the range 2–20 μm diameter has been studied. A stable microsuspension of the monomers is first achieved in water at room temperature and under high shear using different steric stabilizers with or without co-stabilizers. Then, the system is polymerized using free radical initiators to form microbeads in high yield (85%). The conditions to control the characteristics of the microsuspensions have been studied. It has been shown that the microsuspensions when properly stabilized can be polymerized without noticeable change of their size and size distribution, forming microbeads free of adhering emulsion particles. However, a study of the evolution of the size and size distribution of the particles during polymerization shows that the percentage of particles with higher diameters increases slowly during the polymerization in spite of the rapid gelling process. This suggests a mechanism in which the small amount of monomers dissolved in the water phase is preferably adsorbed by those larger particles probably more poorly protected by the steric stabilizer than the finer beads.