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The processing temperature of thermotropic liquid crystalline polymers (LCPs) can be reduced by increasing the flexibility of the backbone, simultaneously decreasing the relaxation time and resulting in a decrease in the mechanical properties in the processed polymer. Here, we report on the development of thermotropic LCPs with low processing temperatures, an increased relaxation time, and excellent mechanical performance via a versatile two-step synthesis. These LCPs can be processed at temperatures below 200 degrees C into injection-molded parts which have a mechanical performance comparable to that of commercially available grades with Young's modulus and a tensile strength over 12.5 GPa and 200 MPa, respectively. These LCPs are synthesized using carboxylic acid-terminated thermotropic prepolymers that are produced via a standard acidolysis polycondensation at 220 degrees C, a temperature that allows incorporation of monomers having lower thermal stability. Subsequently, the prepolymers are subjected to a chain-extension reaction with bis(2-oxazoline)s in a reactive extrusion step. The facile two-step method provides excellent control over the chemical composition, molecular weight, and chain architecture of the resulting polymer. As the molecular weight of the LCPs increases, they display interesting and uncharacteristic viscoelastic behavior. The viscoelastic response of the LCPs changes from that of an unentangled melt, typically observed in rigid LCPs, toward a physically constrained melt indicated by the appearance of a crossover point with an increase in molar mass. Such a response is normally not observed in commercial thermotropic LCPs even with an increase in molar mass. We attribute the change in the viscoelastic response to the chain-extension reaction. This fundamental change in the state of the nematic melt greatly increases the relaxation timescales of both the interchain orientation and the nematic texture, providing further evidence toward their interrelation. The change in the relaxation mechanism of the nematic phase, with increasing molar mass, is followed by time-resolved X-ray diffraction and optical microscopy.

Molecules and atoms at material interfaces have properties that are distinct from those found in the bulk. Distinguishing the interfacial species from the bulk species is the inherent difficulty of interfacial analysis. For organic photovoltaic devices, the interface between the donor and acceptor materials is the location for exciton dissociation. Dissociation is thought to occur via a complex route effected by microstructure and the electronic energy levels. The scale of these devices and the soft nature of these materials create an additional level of difficulty for identification and analysis at these interfaces. The transmission electron microscope (TEM) and the spectroscopic techniques it incorporates can allow the properties of the donor-acceptor interfaces to be revealed. Cross-sectional sample preparation, using modern focused ion beam instruments, enables these buried interfaces to be uncovered with minimal damage for high resolution analysis. This powerful instrument combination has the ability to draw conclusions about interface morphology, structure and electronic properties of organic donor-acceptor interfaces at the molecular scale. Recent publications have demonstrated these abilities, and this article aims to summarise some of that work and provide scope for the future.

Yarns of different inherent viscosities, in the range 0.6-1.1 dL/g, spun in industrial plants, and drawn at room temperature to obtain mesomorphic samples, have been characterized. The evolution from the mesomorphic form toward the triclinic crystalline form has also been studied by combined differential scanning calorimetry (DSC), dynamic mechanical analyses (DMA), and accurate wide angle X-ray diffraction experiments. The DMA analysis of the mesomorphic samples allows better resolution of the glass transition and crystallization phenomena, which are superimposed in the DSC scans. The degree of molecular orientation in the mesomorphic samples, and the temperature of crystallization from the mesomorphic form (60-80-degrees-C), are essentially independent of the polymer molar mass. (C) 1994 John Wiley & Sons, Inc.

The effect of chain length on the helix orientation of alpha-helical diblock copolypeptides in Langmuir and Langmuir-Blodgett monolayers is reported for the first time. Amphiphilic diblock copolypeptides (PLGA-b-PMLGSLGs) of poly(alpha-L-glutamic acid) (PLGA) and poly(gamma-methyl-L-glutamate-ran-gamma-stearyl-L-glutamate) with 30 mol% of stearyl substituents (PMLGSLG) of various block lengths were studied. The tilt angle between the helices and the substrate-normal decreases upon increasing the transfer pressure coincident with "double brush" formation. The hydrophobic block length strongly affects the maximum surface chain density and thereby the helix orientation of the diblock copolypeptides. Increasing the degree of polymerization of the hydrophobic block (DP(PMLGSLG)) results in an increase in the helix tilt angle tentatively attributed to the off-axis interactions of the unscreened peptide dipoles between the parallel aligned alpha-helices. In those cases where the alkyl side chains surround the PMLGSLG helices, the smallest helix tilt angle of 29 degrees for the PMLGSLG block and 67 degrees for the PLGA block were obtained for the diblock copolymer of DP(PLGA) 37 and DP(PMLGSLG) 24. For smaller DP values of the hydrophobic block, in particular the diblock copolymer having DP(PMLGSLG) 11, the long alkyl side chains are partially expelled from the brush layer and the alpha-helices of the PMLGSLG block are oriented nearly perpendicular to the interface.