Wim H. de Jeu

Abstract: A review is given of the wide variety of predictions that results from a Landau-type of description of the nematic-isotropic phase transition. This includes a discussion of the nature of the order parameter and of the various types of possible phases, of the influence of external fields, and of the effect of inclusion of spatial variations of the order parameter. The various predictions are compared with the available experimental results. It is concluded that there is still no clear picture about the nature of the singularity near the nematic-isotropic phase transition. Though the assumption of classical (mean-field) critical behaviour seems to be incorrect, there is no conclusive proof which alternative applies.

Abstract: The origin of the occurrence of induced smectic phases (ISP) in binary mixtures of liquid crystals is discussed. First the experimental evidence is reviewed, which leads to a division in strong complexing (electron–donor–acceptor complex as indicated by a charge–transfer band) and weak complexing, where only indirect evidence is available. Within the mean field approximation a theoretical model is given in which the influence of complex formation on the nematic–smectic‐A transition temperature is investigated for ideal orientational order. The ISP mechanism is attributed to the dispersive interactions between the complexes. However, in principle in a two‐component mixture without complexing also ISP can be obtained due to interactions involving permanent dipoles. From fits to experimental phase diagrams and from estimates of parameters it follows that the first mechanism is more likely. Three nonstandard groups of phase diagrams with characteristic maxima of minima are predicted. The location of the extrema is calculated as a function of complex concentration and effective, intermolecular interactions. The model correctly describes presently existing, very rich, experimental data.

Abstract: 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. Decreasing the temperature simultaneously triggers off a crystallization of PCL and microphase separation between the PCL and PEO blocks. Coupling and competition between microphase separation and crystallization results in a morphology of PEO spheres surrounded by PCL partially crystallized in lamella. Further decreasing temperature induces the crystallization of PEO spheres, which have a preferred orientation due to the confinements from hard PCL crystalline lamella and from soft amorphous PCL segments in different sides. The final morphology of this highly asymmetric block copolymer is similar to the granular morphology reported for syndiotactic polypropylene and other (co-) polymers. This implies a similar underlying mechanism of coupling and competition of various phase transitions, which is worth further exploration

Abstract: The birefringence of mixtures of homologues from the terminally non-polar series of p,p′-dialkylazoxybenzenes has been studied. The birefringence is used as a probe of the orientational order parameter, which is coupled to the translational (smectic) order parameter. The tricritical nematic to smectic-A point which results from this coupling is found at a McMillan ratio TAN/TNI= 0.977. This is similar to the values reported for terminally polar systems and is higher than predicated by theory. The critical exponent for the variation of the order parameter across the SA–N phase transition obeys the expected trend.

Abstract: A theoretical description is proposed for recently observed antiferroelectric smectic-A phases. Order parameters are introduced that retain information on the head-tail asymmetry of the constituent molecules and minimum ingredients are given for an averaged model potential. The calculations show that for high orientational order the antiferroelectric smectic-A structure is the best compromise between dipole repulsion and dispersive attraction. The resulting phase diagrams compare favorably with the experimental situation.

Abstract: As their name indicates, liquid crystals simultaneously exhibit some characteristics common to both ordinary isotropic liquids and solid crystals. This ambivalence is also found in the effects of surfaces on these systems which lead to a great diversity of phenomena. These phenomena are reviewed focusing on nematic liquid crystals which have the simplest structure among the many existing types and which have been the most extensively studied. Three main kinds of effects can be distinguished. The first concerns the perturbation of the liquid crystalline structure close to the surface. Beyond this transition region, the bulk liquid crystalline structure is recovered with an orientation which is fixed by the surface: this phenomenon of orientation of liquid crystals by surfaces is the so-called anchoring. Finally, close to bulk phase transitions, critical adsorption or wetting can occur at surfaces as is also seen in isotropic systems.

Abstract: This article reviews the physics of colloidal dispersions in nematic liquid crystals as a novel challenging type of soft matter We first investigate the nematic environment of one particle with a radial anchoring of the director at its surface Three possible structures are identified and discussed in detail; the dipole, the Saturn-ring and the surface-ring configuration Secondly, we address dipolar and quadrupolar two-particle interactions with the help of a phenomenological theory Thirdly, we calculate the anisotropic Stokes drag of a particle in a nematic environment which determines the Brownian motion of particles via the Stokes–Einstein relation We then turn our interest towards colloidal dispersions in complex geometries where we identify the dipolar configuration and study its formation Finally, we demonstrate that surface-induced nematic order above the nematic-isotropic phase transition results in a strongly attractive but short-range two-particle interaction Its strength can be controlled by temperature and thereby induce flocculation in an otherwise stabilized dispersion

Abstract: The roots of the chemistry of conventional rod-like liquid crystals are briefly considered The low-molecular-weight liquid crystals are analysed in terms of rigid groups (rod-like and disc-like ring systems), flexible groups, polar groups and short, linking segments On the basis of this analysis, the liquid-crystalline compounds actually existing can be classified into 48 types, which are illustrated with the aid of examples The relationships of low-molecular-weight and polymeric liquid crystals are briefly indicated Finally, from the presented material, some general conclusions are drawn

Abstract: The self-assembly of block copolymers (BCPs) in the presence of crystallization as the second driving force is reviewed, for BCPs in the bulk, thin films, single crystals and micelles. The crystallization of semicrystalline BCPs in the bulk is introduced briefly and the unique morphologies of semicrystalline BCPs at various levels due to crystallization are discussed. The thin film morphologies shown by crystalline BCPs are summarized in terms of the factors affecting the relative strengths of various driving forces. Special attention is paid to the thin film morphologies of functional BCPs containing crystalline poly(3-alkylthiophene) and perylene bisimide units. The single crystal morphologies of semicrystalline BCPs are also presented. Finally, the micellar morphologies of BCPs with a semicrystalline core are reviewed. The controlled and living growth of crystalline micelles, which is the unique characteristic of such micelle, is then discussed.