Investigation of GaN heteroepitaxial layers by means of the kossel effect technique

GaN heteroepitaxial layers grown on spinel substrates have been investigated by the Kossel effect technique. The excitation of GaK_α X-rays inside the crystal lattice was carried out either by means of 1.25 MeV protons or 40 KeV electrons. Both methods give similar results concerning the values of the lattice parameters and the sign of polarity. Following the proton irradiation a lattice expansion normal to the surface occurs. A comparison of our results with those of other authors shows that the growth direction is the same for {111} spinel and {0001} sapphire substrates.     

Low band-gap oligomers and polymers

The relationship between electronic structure and chemical stability is discussed for three different classes of low band-gap oligomers and polymers. In oligo- and poly[n]acenes as well as in poly-(arylenemethide)s, the low band-gap character is connected with a high chemical reactivity due to an energetically favored rearomatization of olefinic or quinonoid substructures, respectively. For double-stranded oligo- and poly(rylene)s, the stability is dramatically increased. Therefore, rylenes appear especially promising in the field of low band-gap materials.     

Molar mass determination by end group analysis - amino group determination in aliphatic and partially aromatic polyamides with 1-fluoro-2,4-dinitrobenzene in benzyl alcohol

End group analysis is the most important absolute method of obtaining number-average molar mass values for polymers of the step-growth polymerization type. A great number of methods are available which primarily differ in the detection of the end group concerned. Special chemical methods based on the formation of covalent derivatives of the end groups (e.g. introduction of UV/VIS chromophores) are important if there is a need to demonstrate, with the aid of gel permeation chromatography, that damage to the polymer (e.g. onset of cross-linking) has occurred. In the case of fibre-forming polycondensation products, the applicability of end group analysis is often restricted by the lack of suitable reagents and solvents. This is also true of the determination of amino end groups in polyamides by means of 1-fluoro-2,4-dinitrobenzene, as is already contained in the German Standards specification (DIN 54 274) - albeit only for polyamide 6 and 6,6. This contribution describes a modified version of the 1-fluoro-2,4-dinitrobenzene method. The analytical principle and practical procedure correspond to those of DIN 54 274, but the solvent used for the reaction with the reagent is benzyl alcohol, and that for the photometric evaluation is 1,1,1,3,3,3-hexafluoro-2-propanol. The scope of the method is thus increased. Examples of applications for this method are commercial products based on aliphatic polyamides (6; 11; 12; 6,6; 6, 12), partially aromatic polyamides (2,4,4-trimethylhexamethylenediamine, terephthalic acid; copolyamides with bis-(4-aminocyclohexyl)propane(2), isophthalic acid, ω-aminododecanoic acid) and multicomponent systems with the abovementioned polyamides (bicomponent fibres, blends). It is shown that large differences may arise between the value determined by titration and the actual amino end group content. In addition, the presence of secondary amino groups can be demonstrated in polyamides of the Trogamid T type.