Effect of Intermolecular and Interfacial Interactions on the Three- and Two-Dimensional Structure and Phase Behavior of Three-Block Liquid-Crystalline Siloxane Derivatives

Abstract

Using several experimental techniques, we have investigated the thermodynamics and the structure of the bulk, monolayers, and multilayers of new three-block organosiloxanes based on the pentyloxy-4′-cyanobiphenyl mesogenic unit. Compared to conventional amphiphiles or smectogens, these molecules contain an additional siloxane moiety whose bulkiness was gradually increased. This enables fine tuning of the steric effect imposed by the siloxane parts and allows determination of the balance between moleculesubstrate and molecule-molecule interactions as well as of the effect of the siloxane end group on the microscopic structure, order, and film homogeneity. We show that the three-and two-dimensional behaviors are highly correlated and are determined by the subtle balance between these interactions. We particularly showed that surface effects induce a three-layer smectic film whatever the bulk state. Various 2D phenomena have been observed such as wetting, mono-and bilayer melting, liquid droplet freezing, several 2D phase transitions, and a tricritical point. These are reversible phenomena and easily controllable via lateral density change, making the present organosiloxane molecules a good model system to investigate in much more detail some challenging physical aspects in two dimensions.

Self‐Assembly at Different Length Scales: Polyphilic Star‐Branched Liquid Crystals and Miktoarm Star Copolymers

“…The reversible formation of well defined single, triple, and even thicker multilayers by compression of monolayers was also reported for amphiphilic liquid crystals with a polar group at only one end, and incorporated rodlike105–110 or disclike anisometric units 111. For these compounds, a predominately vertical or tilted alignment of the rigid cores was observed, whereas bolaamphiphiles with polar groups at both ends adopt the side‐on arrangement in monolayers 112.…”

“…This indicates that the proper combination of rigid rodlike and flexible units is a general approach to fluid, but well defined mono‐ and multilayers. Moreover, depending on the molecular topology, either SmA or SmC‐like structures (aromatic cores perpendicular or tilted to the layer planes)105–108 or structures similar to the Lam phase,28 where the aromatic cores are aligned parallel to the layers, could be obtained. The former type is typical for linear rodlike amphiphiles without lateral chains, whereas the latter has become available with the T‐shaped and X‐shaped polyphiles 103…”

Reversible collapse of the Langmuir films of a series of triphenylsilyl ether-terminated amphiphiles

“…The bulge amphiphile Langmuir film is treated in terms of sublayers, as suggested by Ibn-Elhaj et al [21]. The main rise in surface pressure coincides with the bulge group sublayer reaching a closely packed arrangement near 38 Å 2 /molecule.…”

“…Ibn-Elhaj et al have observed a small entropy increase for the monolayerto-multilayer transition for films derived from three-block organosilane amphiphiles 5AB 2 and 5AB 3 . They ascribe the small entropy increase to a change in water ordering [21]. Plehnert et al have examined a series of mesogenic amphiphiles in which the liquid crystalline and Langmuir film ordering are in competition.…”

“…Ibn-Elhaj et al have reported another amphiphile that does not form crystalline monolayer phases at the air-water interface [21]. Both of these amphiphiles are liquids at room temperature, which reflects the low intermolecular forces within these materials.…”