Folded chain concept of fiber structure

Colloid and Polymer Science - The fringed fibril concept of fiber structure is suported by the high longitudinal and smaller lateral tensile strength and elastic modulus, but one experiences...     

Tensile failure of crystalline polymers

In the creep experiment the brittle fracture of the unoriented semicrystalline polymers at very small and very high tensile load with the intermediate ductile region may be explained by the competition between crazing and shear band formation during the microcrack growth phase. The former type of microcrack growth leads to brittle fracture while the latter type yields necking which transforms the original lamellar structure into the final fibrous structure. The actual fate of the strained sample depends on the growth time of the craze, t_g, and of the shear band formation time, t_s. If t_g<t_s, the material will break in a brittle manner, and if t_g > t_s, the material will deform plastically. The failure of the fibrous material seems to occur when the ratio between the average distance and diameter of the microcracks reaches a value about 3. The microcracks seem to form primarily at defects of the microfibrillar structure, i.e., at the ends of microfibrils where the axial connection of subsequent crystal blocks through the amorphous layers by a great many taut tie molecules is either completely interrupted or at least drastically reduced. The stress concentration resulting from the opening of these defects into microcracks may rupture also some of the adjacent microfibrils. Such nucleation and subsequent lateral growth of the microcrack ruptures the taut tie molecules in its path. The ruptured molecules yield free radicals which can be monitored by electron spin resonance.     

Synthesis of new functionalized imidazo[2,1‐b]thiazoles and thiazolo[3,2‐a]pyrimidines

5‐Oxo‐5H‐[1,3]thiazolo[3,2‐a]pyrimidine‐6‐carboxylic acid ( 4 ), and 6‐methylimidazo[2,1‐b]thiazole‐5‐carboxylic acid ( 17 ) were reacted with amines 6a‐i by the reaction with oxalyl chloride and N, N‐di methyl‐formamide as a catalyst into primary and secondary amide derivatives 7‐14 and 19‐22. From compound 24 N,N'‐disubstituted ureas 26, 27 and perhydroimidazo[1,5‐c]thiazole 29 derivatives of imidazo[2,1‐b]thiazole were prepared. By nmr analysis of compound 29 , the existence of two stereoisomers resulting from both optical, due to centre of chirality at C7′a, and conformational isomerism, due to restricted C5? N6′ bond rotation were proved.     

Synthesis and transformations of some quinolines and 1H-pyrido[3,2-c]azepines containing amino acid units

The synthesis of two complex acetyl azides 5 and 12 having quinoline or pyrido[3,2-c]azepine ring bound to the acetyl group through the ring nitrogen, and their further transformations into the corresponding carbamates 6 and 13 , isocyanates 7 and 14 and 1-unsubstituted heterocyclic compounds 8 and 15 are described. The later compounds were debenzoylated on heating in concentrated sulfuric acid to give the corresponding products 9 and 16 .