Philippe AUROY

Paris Sud XI University, Physics, Faculty Member

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A liquid crystal device comprising anchoring means on at least one IMPROVED confining plate giving a degenerate orientation memoryless

by Georges Durand and Philippe AUROY

Determination of the Structure of Polyelectrolyte Brushes

Macromolecules, 1999

We have prepared polyelectrolyte brushes on silicon blocks in two steps: first, we graft polystyr... more We have prepared polyelectrolyte brushes on silicon blocks in two steps: first, we graft polystyrene (PS) chains terminated by a trichlorosilane group, and then, we convert the grafted PS to poly(styrenesulfonate, sodium salt) by using a soft sulfonation reaction. The grafted layers are fully characterized by ellipsometry and infrared spectroscopy. Using neutron reflectivity, we determine the interfacial density profile of the polyelectrolyte brushes so formed in aqueous solutions. In pure water, the chains are strongly stretched because of the electrostatic repulsion between charged monomers; the mean thickness of the brush is proportional to the chain length and does not depend on the grafting density. Upon addition of salt, two regimes are observed. As long as the salt concentration is less than the inner concentration of counterions in the brush φci, the added electrolytes have no effect. On the other hand, if the salt concentration exceeds cci, the polyelectrolyte layer shrinks as the ionic strength increases but never collapses.

$Figure 2. Diagram of states of a polyelectrolyte brush in the coordinates ionic strength vs grafting density. ps is the volume fraction of added salt, and o is the grafting density.$

$Figure 5. (a) Open circles: reflectivity data of a PSSNa brush (N = 310, D = 22.0 A, a = 0.62 + 0.08) in D20. The lines are the best fits corresponding to the reflectivity calculated with a model of 3 (solid line) or 10 layers (dashed line). (b) Similar monomer volume fraction profiles given by both fits.$

$Figure 8. (a) Monomer volume fraction profile for various sulfonation rates a. The PSSNa brushes have the same polymerization index N = 310 and similar grafting density. (b) Variation of the mean thickness h (A) as a function of a1”.$

$Figure 9. Comparison of the monomer yolume fraction profile of a PSSNa brush (N = 310, D = 22.0 A, ao = 0.62 + 0.08) in D.O (dotted line; same data as in Figure 5) with a Gaussian (solid line).$

$Figure 10. Monomer volume fraction profile of a brush without, consolidation (solid line characteristics: N = 310, D = 22.0 A, and a = 0.62 + 0.08 with the same data as in Figure 5) and a brush after consolidation, of similar characteristics (dashed line characteristics: N = 310, D = 21.2 A, anda = 0.60 + 0.08).$

$Figure 11. (a) Monomer volume fraction profile for various distances between anchoring points D. The PSSNa brushes have the same polymerization index N = 310 and the same sulfonation rate a = 0.60 + 0.08. (b) Variation of h/a’? as a function of D.$

$Figure 12. (a) Reflectivity curve of a PSSNa brush in D2O for N = 63. (b) Same curve as in part a but for N = 1100. (c) Monomer volume fraction profile of PSSNa brushes of different chain length in D20. (d) Variation of h/a’? as a function of the polymerization index N.$

$Figure 13. (a) Reflectivity curve of a poly(styrenesulfonate tetramethylammonium salt) brush (characteristics: N = 300, D = 21.9 A, and a = 0.40 + 0.10). (b) Distribution of tetramethylammonium counterions (corresponding to the data shown in part a compared to the monomer volume fraction of a poly(styrenesulfonate sodium salt) brush (characteristics: N = 310, D = 21.0 A, and a = 0.45 + 0.10).$

$Figure 15. (a) Reflectivity curve of a PSSNa brush in D20 and in 5 mol/L NaCl solution. The brush characteristics are as follows: N = 310, D = 27.0 A, and a = 0.60 + 0.08. (b) Same curve as in part a but in the representation R/Rr vs k. (c) Monomer volume fraction profile of the same brush as in part a for various NaCl concentrations in the strong screening regime. (d) Variation of the mean thickness / as a function of the added salt concentration Ms.$

$Figure 17. (a) Monomer volume fraction profile of a poly- (styrenesulfonate calcium salt) brush for various CaCl, con- centrations (in the strong screening regime). The brush has the following characteristics: N = 310, D = 27.0 A, and a = 0.60 + 0.08. (b) Comparison between the effect of adding NaCl or CaCl, on the brush monomer volume fraction profile.$

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Colloidal Structures from Bulk Demixing in Liquid Crystals

Langmuir, 2004

This experimental paper deals with phase separations of binary mixtures composed of a continuous ... more This experimental paper deals with phase separations of binary mixtures composed of a continuous liquid crystal phase and an isotropic dispersed phase. In contrast to isotropic binary mixtures, the investigated mixtures do not lead to a full phase separation but to a self-ordering of colloidal particles, as reported earlier (Loudet, J. C. et al. Nature 2000, 407, 611). We present here further aspects of such phase separations which include the kinetics of the phase separation, the origin of the formation of dislocation-like patterns, the influence of surfactants, chiral additives, and temperature on the formed colloidal structures. The present results show that (i) the dislocations in chain arrays can be seen as kinetically frozen defects, (ii) temperature can be used to control the size of the domains formed upon demixing, (iii) a slight change in surface chemistry, via the addition of surfactants, profoundly alters the formed colloidal structures, and (iv) chiral additives allow the formation of unique helical pearl chains which reflect the symmetry of the liquid crystal phase they are embedded in.

format_quoteMonodisperse droplets evolve into ordered arrays of chains, remaining stable for months, contrasting with typical fluid behavior.format_quote

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Synthesis of Poly(styrene sulfonate) Brushes

Journal of the American Chemical Society, 2001

Anomalous adhesion in adsorbed polymer layers

The European Physical Journal B, 1998

Anomalous adhesion behaviour observed in the polydimethylsiloxane/heptane/silica system is report... more Anomalous adhesion behaviour observed in the polydimethylsiloxane/heptane/silica system is reported. This behaviour is characterised by the infrequent appearance of one or more elastic minima which occur in addition to the ever-present primary adhesion. The resulting elastic force is attributed to loops connecting the tip to the substrate and can be described by two models; a worm-like chain, and a freely jointed chain. Both models give similar values for the characteristic segment length of around 0.24 nm and indicate that the chains have been extended by between 90 to 95% of their chain contour length before desorption. In some cases multiple adhesions were observed between tip and substrate and have been used to suggest a method for determining the distribution of adsorbed polymer loops.

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Rhéologie de suspensions colloidales de nanoparticules chevelues

Nous avons etudie les proprietes rheologiques de suspensions de nanoparticules constituees d'... more Nous avons etudie les proprietes rheologiques de suspensions de nanoparticules constituees d'un coeur de silice de rayon 15 nm entoure d'une couronne de polymeres greffes de 35 nm d'epaisseur. Dans les regimes de concentration dilue et semi-dilue, le comportement trouve est intermediaire entre celui d'une suspension de spheres dures et celui d'une solution de polymeres. Les resultats ont ete interpretes en terme de spheres molles ; les couches de polymere peuvent partiellement se comprimer. Nous avons egalement compare nos resultats aux comportements observes pour les solutions de polymeres en etoiles.

Local structure and density profile of polymer brushes

Physical Review Letters, 1992

Brushes of grafted polystyrene have been studied using the classical small-angle neutron scatteri... more Brushes of grafted polystyrene have been studied using the classical small-angle neutron scattering techniques. The results show unambiguously that the interface, in good solvent, has essentially a parabolic shape, with a small exponential tail. Furthermore, the local structure is similar to that of a semidilute polymer solution with a typical blob size equal to the distance between grafting sites. Mean-field and scaling theories therefore account for these results, within their own range of validity.

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Thin Film Instability Induced by Long-Range Forces

Langmuir, 1999

Dispersion forces are present everywhere. Their importance, however, is largely neglected because... more Dispersion forces are present everywhere. Their importance, however, is largely neglected because directly at a surface or at an interface they are mostly weak compared with specific interaction of short range. Here, we show that these forces are nonetheless extremely relevant and may have drastic consequences on the stability of thin films. We demonstrate that a force (per unit area) of <1 Pa is capable of "destroying" 100 nm (!) thick films, even if they are "glued" to the substrate by end-grafted polymers. We present the temporal evolution of different morphologies of unstable thin liquid polymer films caused by destabilizing intermolecular forces.

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Silica particles stabilized by long grafted polymer chains

Journal of Colloid and Interface Science, 1992

Silica particles grafted with long polydimethylsiloxane chains have been prepared. A progressive ... more Silica particles grafted with long polydimethylsiloxane chains have been prepared. A progressive grafting process was used, allowing transfer of the beads from their initial polar medium to various organic solvents. They are revealed to be model colloids whose properties are driven by the structure of the grafted polymer layer. Depending upon the grafting density, two basic kinds of particles may be distinguished, exhibiting either coulombian or steric interactions.

format_quoteWG particles can't disperse in poor solvents; SG particles behave differently, indicating solvent quality's critical role in colloidal stability.format_quote

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Characterization of the brush regime for grafted polymer layers at the solid-liquid interface

Physical Review Letters, 1991

Using small-angle neutron-scattering techniques, the scaling laws between the thickness of a graf... more Using small-angle neutron-scattering techniques, the scaling laws between the thickness of a grafted polymer layer h, its graft density a. , and the molecular weight of the grafted chains M have been determined for polydimethylsiloxane chains end grafted on porous silica. The observed laws h = Mcr in bad solvent and h = Mo' ' in good solvent provide the first experimental evidence of the "brush" regime, where the grafted chains, confined by their neighbors, are stretched normal to the surface.

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Hollow Glass Product Marking Data Matrix Indelible

Polymeres greffes a l'interface solide-liquide

Nous avons etudie experimentalement les couches de polydimethylsiloxane greffe en extremite sur u... more Nous avons etudie experimentalement les couches de polydimethylsiloxane greffe en extremite sur une surface de silice. Nous avons caracterise les structures locales de ces couches greffees par diffusion de neutrons aux petits angles. A forte densite de greffage, la couche interfaciale en bon solvant, a le meme comportement local qu'une solution semi-diluee de polymere. De plus, les chaines sont fortement etirees. En non solvant, la couche est homogene et dense, d'epaisseur bien definie. Les structures globales de ces couches greffees peuvent etre decrites par des lois d'echelle. Nous avons determine l'influence de la densite de greffage. Celle-la est imposee par la concentration de la solution de polymere dans laquelle on procede au greffage. A faible densite, les chaines sont juxtaposees, sans interaction mutuelle. Leur conformation est fonction de la qualite du solvant et de la nature de la surface. A plus forte densite, differents regimes peuvent etre identifies. ...

A liquid crystal device comprising anchoring means on at least one IMPROVED confining plate giving a degenerate orientation memoryless

Wetting of Polymer Brushes by a Nematogenic Compound

Physical Review Letters, 1998

Les muscles artificiels

Chain segment ordering in lamellar sublayers of block copolymers: A NMR study

The European Physical Journal E

The chain segment dynamics in the bulk lamellar phase of polystyrene-polydimethylsiloxane (PS-PDM... more The chain segment dynamics in the bulk lamellar phase of polystyrene-polydimethylsiloxane (PS-PDMS) block copolymers has been probed by NMR. The experiments were performed on a PS-PDMS diblock and on a PS-PDMS-PS triblock with twice the molecular weight. In the diblock, at room temperature, the PDMS block segments undergo uniaxial reorientations around the normal to the lamellae. In the triblock, the reorientational motions exhibit a lower degree of symmetry: deviations from a uniaxial dynamics are observed. Such a behaviour originates in the anchorage of both PDMS chain ends into the PS glassy layers.

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Synthesis and liquid crystal alignment properties of new cinnamate-based photocrosslinkable polymers

Http Dx Doi Org 10 1080 026782900202796, Aug 6, 2010

ABSTRACT Because optical buffing can achieve aligning patterns with different azimuthal angles of... more ABSTRACT Because optical buffing can achieve aligning patterns with different azimuthal angles of the liquid crystal directors, photoalignment appears to be a very hopeful technique for designing complex LCD structures. Until recently, most photopolymers provided low anchoring and/or low tilt angles which are unsuitable for industrial applications. In this work, four new photocrosslinkable polymers based on biphenyl and naphthalene moieties have been prepared and characterized. These photopolymers have been irradiated with linearly polarized UV light, and liquid crystal cells have been made. The tilt angles generated by oblique irradiation have been measured as well as the anchoring energies. Finally, the stability of the liquid crystal alignment properties have been studied.

Polymeres greffes a l'interface solide-liquide

Http Www Theses Fr, 1990

Alumina titanate porous structure

Honeycomb Catalyst Substrate and Method for Producing Same

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