Ben Norder

All-aromatic liquid crystal triphenylamine-based poly(azomethine)s as hole transport materials for opto-electronic applications

We have explored the opto-electronic properties of a new series of hole-transport materials based on main-chain triphenylamine-based poly(azomethine)s. 4,4′-Diaminotriphenylamine (TPA) was polymerized under benign conditions with either terephthalaldehyde (TPA-14Ta), 2,5-thiophenedicarboxaldehyde (TPA-25Th) or 1,3-isophthalaldehyde (TPA-13Iso) to yield polymers with an Mn of 5700–16 000 g mol−1. Despite the non-linear, or ‘kinked’, backbone geometry, all polymers form lyotropic solutions in chloroform and this liquid crystal (nematic) ordering could be maintained in the solid film after spin casting. All polymers exhibit high glass-transition temperatures (Tg > 250 °C) and display outstanding thermal stabilities, i.e. 5% wt loss in excess of 400 °C under nitrogen. The HOMO and LUMO energy levels of these polymers were in the range of 5.0–5.3 and 2.4–3.3 eV below the vacuum level, respectively. Introduction of a thiophene heterocycle (TPA-25Th) resulted in a material with a low optical band-gap of approximately 2.0 eV, whereas TPA-14Ta and TPA-13Iso showed optical band gaps of 2.3 and 2.6 eV, respectively. A photovoltaic device based on a TPA-25Th/PCBM blend (1 : 3) showed an EQE of 20% at 500 nm. Under simulated sunlight, the device gives an open-circuit voltage of 0.41 V, a short-circuit current of 1.23 mA cm−2 and a fill factor of 0.24, leading to a power conversion efficiency of 0.12%.

Microphase separation and rheology of a semicrystalline poly(ether‐ester) multiblock copolymer

A microphase separation transition (MST) of a thermoplastic elastomer based on soft segments of poly(tetra methylene oxide) and hard crystalline segments of poly(tetra methylene terephthalate) has been studied by means of rheological measurements, differential scanning calorimetry (DSC), and wide-angle X-ray scattering (WAXS), showing that the MST is entirely caused by melting/crystallization, and that no separate segmental mixing/demixing transition is involved. DSC and WAXS measurements show that melting starts at 190°C, leading to crystal reorganization effects up to above 200°C, and that a gradual decrease in crystallinity occurs from below 210°C up to 224°C, above which temperature no crystals are left. Rheological measurements reveal a wide MST(207-224°C) upon heating, which coincides perfectly with the melting range. From this coincidence together with the Maxwell fluid behavior directly following the MST, it is concluded that melting leads to a one-phase liquid, and that no separate segmental mixing transition occurs. Similar results are obtained upon cooling, indicating that crystallization is the driving force for phase separation and that no separate segmental demixing step precedes crystallization. The wide MST implies a large processing window over which the rheological properties change from highly elastic, with a distinct yield stress, to normal pseudoplastic, enabling application in preparation of structured blends.

Synthesis of magnetic noble metal (nano)particles.

Noble metal particles can be made strongly ferromagnetic or diamagnetic provided that they are synthesized in a sufficiently strong magnetic field. Here we outline two synthesis methods that are fast, reproducible, and allow broad control over particle sizes ranging from nanometers to millimeters. From magnetometry and light spectroscopy, it appears that the cause of this anomalous magnetism is the surface anisotropy in the noble metal particles induced by the applied magnetic field. This work offers an elegant alternative to composite materials of noble metals and magnetic impurities.

Stability of co-continuous polystyrene/poly(ether–ester) blends in shear flow

Abstract Co-continuous morphologies in a blend of poly(ether–ester) and polystyrene break up into a droplet/matrix morphology when subjected to a shear flow in a rheometer with a cone–plate configuration and are therefore not stable under shear. It is shown that the capillary number at these flow conditions is too small to maintain stable extended structures that are necessary for a co-continuous morphology. Co-continuity at such flow conditions is limited to the point of phase inversion, contrary to the range of co-continuity usually found after extrusion.

Can morphological transitions in fibrils drive stiffness of gels formed by discotic liquid crystal organogelators

Organogels formed by a new thermotropic discotic liquid crystal in water/ethanol mixtures show two types of gel with very different levels of elastic modulus depending on the gelation temperature. In the pure material, transitions from a discotic columnar (Colh) to a plastic crystal (PK) to a crystalline (K) phase were observed. The same kinds of transitions seem to take place within the gel fibrils driving the gel’s stiffness. A harder gel formed at lower temperatures, is composed of partly crystalline gelator fibrils, whereas scattering data of the gelator molecules in the soft gel, formed at higher temperatures, indicate a fairly low ordering, reflecting the modest mechanical properties of these gels. The precise structure of the soft gel fibril is very difficult to establish but comparison to the liquid crystal gelator in bulk and in “wet phases” suggests that they may be formed by “wet” columnar fibrils.

Influence of the flow history on stress growth and structure changes in the thermotropic liquid crystalline polymer Vectra B950

The structure changes in the start-up flow of the thermotropic liquid crystalline polymer Vectra 8950 are probed by performing transient experiments after various flow histories. The shear and normal stress growth curves of a squeezed sample and of a randomly oriented sample show a pronounced overshoot at low strains, whereas the stress growth curve of a sample pre-sheared until steady state shows a gradual increase. This first peak is associated with the re-orientation of the director into the shearing plane. All stress transients show a second broad maximum at large strains that results from the generation of a steady defect network. The effect of varying the relaxation period after pre-shearing is reflected in the appearance of two peaks in the subsequent stress growth curves. One of these peaks shifts linearly with re laxation period and the other is more or less fixed in position. The orientation of the molecules during steady shear flow is on average in the flow direction. Intermediate orientation levels may exist in the transient depending on the amount of strain. The material is able to maintain the flow-induced orientation distribution for a long time after cessation of flow. This is reflected in a similar fashion in the initial magnitudes of the stresses and the dynamic moduli after various preshear strains. Moreover, the dynamic moduli decrease with time after cessation of steady shear flow, indicating that the orientation increases during relaxation.

Mechanical and thermal properties of polymer micro- and nanocomposites

Abstract We have analyzed the thermal conductivity and the tensile modulus of composite materials within the framework of the Halpin-Tsai and Lewis-Nielsen models. The parameter linking thermal conductivity and tensile modulus together is the shape factor of the (nano)filler. Model analysis based on experimental data shows that particle aggregation into a weak mechanical network may be required to achieve good correlation between thermal conductivity and the Young’s modulus when analyzing the data within the framework of a single model and requiring the same value of the shape factor. We believe this approach will make quantitative analysis of nanocomposite thermal properties possible.

Elucidation of the orientational order and the phase diagram of p-quinquephenyl.

The orientational order in the nematic phase of p-quinquephenyl, the pentamer of p-phenylene, has been determined by means of birefringence measurements and by wide-angle X-ray scattering (WAXS). The experimentally determined order parameters are compared with the temperature-dependent order parameter predicted by the Maier-Saupe theory. The order parameters derived from the birefringence at different temperatures in the nematic phase of p-quinquephenyl were in excellent agreement with the Maier-Saupe predictions. The values calculated from the azimuthal profiles derived from WAXS measurements were significantly lower than those determined by the birefringence measurements, especially at higher temperatures. For the birefringence measurements, alignment of the director is achieved by using polyimide alignment layers, whereas director alignment for the WAXS experiments was achieved by a magnetic field. We assign the low overall order parameters that were measured by WAXS to a lower macroscopic orientational order. In addition, upon reinvestigating the mesophase behavior of p-quinquephenyl, a monotropic smectic A phase has been observed upon cooling at 390 °C, just before the crystallization temperature is reached.

Transformational kinetics in liquid crystal polymers and differential scanning calorimetry calibration

Liquid crystalline polymers (LCPs) are defined by their instantaneous liquid crystal phase transitions, as compared with set heating/cooling rates in differential scanning calorimetry (DSC). This is well accepted for the case of main-chain (MC) LCPs. It is confirmed here for the case of two side-group (SG) LCPs as well. The DSC temperature calibration is usually based on melting of metals like indium and zinc and hence only possible on heating. Being instantaneous, LC transitions are as such very appropriate for DSC temperature calibration. This is demonstrated for the case of two SG-LCPs: one with a side group including an ester group, and one with a nitrostilbene side group. Transition kinetics were studied upon heating and cooling with DSC rates in the range of 2.5–50 K min−1. After room temperature storage, the SG-LCPs studied showed considerable thermal effects due to physical ageing. These endothermic effects could easily be discerned from the instantaneous LC transitions and appeared to be thermally activated, with apparent activation energies depending on the amount of preceding physical ageing: on reheating after melting and cooling about 475 kJ mol−1, while after long-time storage at room temperature 280 kJ mol−1 is obtained.

Correction to "synthesis of magnetic noble metal (nano)particles".

Langmuir 2011, 27 (12), 7783−7787. DOI: 10.1021/la105051v ■ INTRODUCTION Recently, some of us reported on the production of magnetic noble metal nanoparticles. It was claimed, that when noble metal nanoparticles were synthesized in a sufficiently strong magnetic field, they would acquire (ferro)magnetism. The obtained magnetism was reported to be equivalent to that of iron if present at about 2% of the total mass. This amount of iron was orders of magnitude larger than what was actually detected with the techniques used. We here report on new experimental results that were obtained while trying to enhance the yield of magnetic material.