bode, et al.. Polymorphism of even saturated carboxylic acids from n-decanoic to n-eicosanoic acid.The polymorphism of normal saturated even carboxylic acids from n decanoic to n eicosanoic acid is discussed. Seven crystal modifications, including polymorphs and polytypes, were identified and fully characterized by the combination of calorimetric measurements (DSC) at atmospheric and high pressures, X ray powder diffraction, FT IR spectroscopy and scanning electron microscopy (SEM). All seven crystal forms, including polymorphs and polytypes, are observed at room temperature. Forms A 2 and A super are triclinic, form C is monoclinic and forms E and B show both a monoclinic and an orthorhombic polytype. The triclinic modifications A 2 and A super predominate for acids up to n tetradecanoic acid (C 14 H 27 O 2 H). The orthorhombic and the monoclinic forms predominate for acids from n hexadecanoic (C 16 H 31 O 2 H) up to n eicosanoic acid (C 20 H 39 O 2 H). When the temperature is increased, all the crystal modifications transform irreversibly to the C form. In the first part of this paper, cell parameters for the different forms are given, the observed temperatures and enthalpies of the transitions are reported and the stability of the different forms is discussed. In the second part, we state the main contribution of each technique for the identification and interpretation of the polymorphism of even numbered carboxylic acids.

The presence of surface-induced crystal structures is well known within organic thin films. However, the physical parameters responsible for their formation are still under debate. In the present work, we present the formation of polymorphic crystal structures of the molecule dihexyl-terthiophene in thin films. The films are prepared by different methods using solution-based methods like spin-coating, dip-coating and drop-casting, but also by physical vapour deposition. The thin films are characterised by various X-ray diffraction methods to investigate the crystallographic properties and by microscopy techniques (atomic force microscopy and optical microscopy) to determine the thin film morphologies. Three different polymorphic crystal structures are identified and their appearance is related to the film preparation parameters. The crystallisation speed is varied by the evaporation rate of the solvent and is identified as a key parameter for the respective polymorphs present in the films. Slow crystallisation speed induces preferential growth in the stable bulk structure, while fast crystallisation leads to the occurrence of a metastable thin-film phase. Furthermore, by combining X-ray reflectivity investigations with photoelectron spectroscopy experiments, the presence of a monolayer thick wetting layer below the crystalline film could be evidenced. This work gives an example of thin film growth where the kinetics during the crystallisation rather than the film thickness is identified as the critical parameter for the presence of a thin-film phase within organic thin films.

A method for the preparation of uniaxially oriented thin films of terthiophene (20–50 μm thick) is introduced. It relies on the crystal growth with a Bridgman-type process that decouples nucleation and growth phenomena. An effective thermal gradient of 6–11.6 °C/mm has been used in which the sample (terthiophene powder deposited on either glass or fluorinated glass substrates) is displaced from a hot zone to a cold zone at a constant rate of 2.5–5 μm/s. The size and orientation of crystals have been investigated by polarized optical microscopy and X-ray diffraction measurements. A coexistence of two polymorphs of terthiophene has been observed, but optimal gradient conditions enabling the selective crystallization of only the room temperature stable polymorph have been found. Terthiophene films deposited on fluorinated glass substrates and crystallized using the thermal gradient technique show a stronger tendency to polymorphism and random orientation of crystallites for all gradient conditions tested. The monoclinic unit cell (a = 15.410 Å, b = 5.709 Å, c = 26.052 Å, β = 97.77°) of the room temperature phase orients its ab plane parallel to the substrate. Pole figures demonstrate the growth of uniaxially aligned crystals with the [100] and [−100] directions along the gradient axis. Finally, a tentative explanation for this peculiar in-plane orientation is given based on crystal morphology calculations.

A detailed structural study of the bulk and thin film phases observed for two potential high performance organic semi conductors has been carried out. The molecules are based on [1]benzothieno[3,2 b]benzothiophene (BTBT) as conjugated core and octyl side groups, which are anchored either symmetrically at both sides of the BTBT core (C 8 −BTBT−C 8 ) or nonsymmetrically at one side only (C 8 −BTBT). Thin films of different thickness (8−85 nm) have been prepared by spin coating for both systems and analyzed by combining specular and grazing incidence X ray diffraction. In the case of C 8 −BTBT−C 8 , the known crystal structure obtained from single crystal investigations is observed within all thin films, down to a film thickness of 9 nm. In the case of C 8 −BTBT, the crystal structure of the bulk phase has been determined from X ray powder diffraction data with a consistent matching of experimental and calculated X ray diffraction patterns (Rwp = 5.8%). The packing arrangement of C 8 −BTBT is similar to that of C 8 −BTBT−C 8 , that is, consisting of a lamellar structure with molecules arranged in a "herringbone" fashion, yet with lamellae composed of two head to head (or tail to tail as the structure is periodic) superimposed molecules instead of only one molecule for C 8 −BTBT−C 8 . As for C 8 −BTBT−C 8 ,we demonstrate that the same phase is observed in bulk and thin films for C 8 −BTBT whatever the film thickness investigated.

A convenient method to induce face-on orientation of an alkoxy phtalocyanine discotic mesogen is described. The alignment is imposed by the confinement of the discotic thin films with a top sacrificial polymer layer that is easily removed by washing with a selective solvent, after thermal annealing. Thin films have been characterized by optical and atomic force microscopy, UV-Vis absorption spectroscopy, and grazing incidence wide angle X-ray scattering. The data converge in showing the central role of the sacrificial layer in promoting alignment with the planar molecules orienting parallel to the substrate in an essentially homeotropic arrangement over large lateral length scales and the persistence of this desirable alignment after removal of the layer.