Substituted Septithiophenes with End Groups of Different Size: Packing and Frustration in Bulk and Thin Films.

We report on three different liquid crystalline compounds with a central septithiophene core and alkylated end groups of strongly increasing bulkiness. In principle, the thiophene cores prefer to pack parallel to optimize their π-π interactions, which becomes sterically impossible for the bulkier end groups. Using X-ray diffraction, we find that the way out of this packing dilemma is toward liquid-crystal phases of higher dimensionality in the order smectic → columnar ↔ bicontinuous cubic.

Thinning and thickening of free-standing smectic films revisited.

We present a theoretical explanation of the remarkable thickness instabilities that occur in free-standing smectic films (FSSF) upon changing the external conditions: i) upon heating the film above the bulk smectic disordering temperature, generally the film does not rupture but instead shows successive layer-by-layer thinning transitions; ii) thickening of FSSF, which occurs within the thermal range of the smectic phase upon local heating. All observations reported so far can be explained on the basis of the Landau-de Gennes theory of the smectic state in combination with nucleation theory. In overheated smectic films (thinning) or locally heated FSSF (thickening) an additional normal tensile force appears due to a change of the mean density of the film.

Monolayer properties of asymmetrically substituted sexithiophene.

We present a structural comparison of monolayers on a SiO2 substrate of two asymmetrically substituted sexithiophenes (6T). Molecule 1 consists of 6T with a branched alkyl chain at one end only and shows a crystalline structure. In molecule 2, the bifunctional 6T has in addition at the other end a linear alkyl chain.

Random disorder and the smectic-nematic transition in liquid-crystalline elastomers.

We report effects of disorder due to random cross-linking on the nematic to smectic-A phase transition in smectic elastomers. Thermoelastic data, stress-strain relations and high-resolution x-ray scattering profiles have been analyzed for two related compounds with a small and a larger nematic range, respectively, each for 5% as well as 10% cross-links. At 5% cross-link density the algebraic decay of the positional correlations of the smectic layers survives in finite-size domains, providing a sharp smectic-nematic transition.

Lyomesophases of C3-symmetrical bipyridine-based discs in alkanes: an X-ray diffraction study.

The importance of the role of alkane solvents in the self-assembly process of pi-conjugated molecules is well recognized but hardly understood. Here we present our results on the X-ray diffraction studies that we conducted to gain insight into the supramolecular structure of mixtures of a bipyridine-based molecule (1) with alkanes. Independent of the alkane used (linear or branched), above x(w) > 0.

Thermotropic phase behavior of trialkyl cyclohexanetriamides.

The thermotropic phase behavior of symmetric cyclohexanetriamides carrying various linear and branched alkyl chains was investigated using calorimetry, microscopy, solid-state NMR, dielectric relaxation spectroscopy, and X-ray scattering techniques. Cyclohexanetriamides carrying C(6) or longer linear alkyl chains formed columnar plastic phases with a pseudocentered rectangular lattice. Those with C(8) or longer alkyl chain also showed a nematic liquid crystalline phase.

Road to disorder in smectic elastomers.

We present a high-resolution x-ray study of the effects of disorder induced by random cross-linking side-chain smectic elastomers. The influence of variation of the concentration and stiffness of the cross-link units on the disruption of the one-dimensional translational order is reported in detail. Precise analysis of the line shape of the quasi-Bragg peaks associated with the smectic layering indicates a transition from algebraic decaying ordering to disorder.

Capillary wrinkling of floating thin polymer films.

A freely floating polymer film, tens of nanometers in thickness, wrinkles under the capillary force exerted by a drop of water placed on its surface. The wrinkling pattern is characterized by the number and length of the wrinkles. The dependence of the number of wrinkles on the elastic properties of the film and on the capillary force exerted by the drop confirms recent theoretical predictions on the selection of a pattern with a well-defined length scale in the wrinkling instability.

Morphology of a highly asymmetric double crystallizable poly(epsilon-caprolactone-b-ethylene oxide) block copolymer.

The morphology of a highly asymmetric double crystallizable poly(epsilon-caprolactone-b-ethylene oxide) (PCL-b-PEO) block copolymer has been studied with in situ simultaneously small and wide-angle x-ray scattering as well as atomic force microscopy. The molecular masses Mn of the PCL and PEO blocks are 24,000 and 5800, respectively. X-ray scattering and rheological measurements indicate that no microphase separation occurs in the melt.

Dynamic heterogeneity in hydrogen-bonded polymers.

We report on neutron spin echo experiments on hydrogen-bonded polymers and compare the experimentally found dynamical structure factor with theoretical predictions. Surprisingly, we find that in the melt phase the expected scaling of the Rouse dynamics is not satisfied. We propose an explanation based upon the large spatial volume occupied by the connecting groups.

Dynamics of fluctuations in smectic membranes.

We present a comprehensive account of the dynamics of layer-displacement fluctuations in smectic liquid-crystal membranes as studied by x-ray photon correlation spectroscopy (XPCS) and neutron-spin echo (NSE). Combining these two techniques at fast relaxation times, three distinct relaxation regimes can be distinguished. For thin membranes, at the specular Bragg position oscillatory relaxation occurs, which transforms for thicker samples into exponential decay.

Hydrophilic elastomers for microcontact printing of polar inks.

A moderately hydrophilic, thermoplastic elastomer (poly(ether-ester)) was investigated as a stamp material for microcontact printing of a polar ink: pentaerythritol-tetrakis-(3-mercaptopropionate). Stamps with a relief structure were produced from this polymer by hot embossing, and a comparison was made with conventional poly(dimethylsiloxane) (PDMS) and oxygen-plasma-treated PDMS. It is shown that the hydrophilic stamps can be used for the repetitive printing (without re-inking) of at least 10 consecutive patterns, which preserve their etch resistance, and this in rather sharp contrast to conventional and oxygen plasma-treated PDMS stamps.

Self-assembly of the perfluoroalkyl-alkane F14H20 in ultrathin films.

Scanning force microscopy on monomolecular films of eicosylperfluorotetradecane, F(CF(2))(14)(CH(2))(20)H, on mica, silicon oxide, or water revealed spontaneous organization to well-defined nanoscopic ribbon and spiral or toroidal superstructures. Whether ribbons or nanospirals were formed depended on the solvent from which the molecular monofilm was cast. Ribbons were observed when a hydrocarbon or a perfluorocarbon solvent was used, e.

Shear-induced smectic ordering and crystallisation of isotactic polypropylene.

Shear-induced smectic ordering and crystallisation of isotactic polypropylene (iPP) has been studied with in-situ small- and wide-angle X-ray scattering. Shear-induced smectic bundles with a periodicity of about 4 nm have been observed at temperatures below as well as above the melting point. This applies to iPP of different molecular weight and from different sources.

Disorder by random crosslinking in smectic elastomers.

We present a high-resolution x-ray study of the effects of disorder due to random crosslinking on the one-dimensional translational ordering in smectic elastomers. At a small crosslink density of about 5%, the elastomer network stabilizes the smectic structure against layer-displacement fluctuations, and the algebraically decaying layer ordering extends up to several micrometers. With increasing concentration of crosslinks, the finite size of these domains is strongly reduced, indicating that disordering takes over.

Shear-induced smectic ordering in the melt of isotactic polypropylene.

We report an unusual shear-induced smectic phase of high-molecular-mass isotactic polypropylene at temperatures well above its melting point. The necessary stiffness in the polymer chain is provided by shear-induced coil-helix transitions. Coupling between the conformational ordering and the density generates the smectic layering.

Surface and bulk elasticity determined fluctuation regimes in smectic membranes.

We report combined x-ray photon correlation spectroscopy (XPCS) and neutron spin echo (NSE) measurements of the layer-displacement fluctuations in smectic liquid-crystal membranes in the range from 10 ns to 10 micros. NSE reveals a new regime, determined by bulk elasticity, in which relaxation times decrease with the wave vector of the fluctuations. XPCS probes slower surface-tension-dominated relaxation times, independent of the wave vector.

Two-stage surface freezing of a single top layer in a smectic-A membrane.

The crystallization of a single liquid top layer of smectic membranes of the compound 4O.8 has been studied with grazing-incidence x-ray diffraction. As this process takes place in two steps, involving an intermediate hexatic smectic-B layer before the final crystalline-B surface structure is reached, it provides a model for melting in two dimensions.

Smectic membranes in motion: approaching the fast limits of x-ray photon correlation spectroscopy.

The dynamics of the layer-displacement fluctuations in smectic membranes have been studied by x-ray photon correlation spectroscopy (XPCS). We report transitions from an oscillatory damping regime to simple exponential decay of the fluctuations, both as a function of membrane thickness and upon changing from specular to off-specular scattering. This behavior is in agreement with recent theories.