Structurally colored thermally stable

This new structure is thermally stable (infusible). Also, it has been reported that during stabilization, CH2 and CN groups disappear while C=C, C=N and = C-H groups form. At the same time the color of precursor fiber changes gradually and finally turns black when carbonized [36]. Research shows that optimum stabilization conditions lead to high modulus carbon fibers. Too low temperatures lead to slow reactions and... 

In addition to a block copolymer, a microcapsule was made from suspension interfacial polycondensation between diacid chloride having aromatic-aliphatic azo group and aliphatic triamine [70,71]. The capsule was covered with a crosslinked structure having an azo group that was thermally stable but sensitive to light so as to be applicable to color photoprinting materials. 

The first bismuthonium ylide reported by Lloyd is a thermally stable colored substance, but the literature lacks details of its characterization. Several bismuthonium ylides containing a cyclic a,a -dicarbonyl or a,a -disulfonyl framework have been isolated as stable crystalline solids, and 4,4-dimethyl-2,6-dioxo-l-triphenylbismuthoniocyclohexane has been characterized structurally by X-ray crystallographic analysis, where the bismuth atom possesses a distorted tetrahedral geometry and interacts with one of the carbonyl oxygen atoms [90JCS(P1)3367]. The Bi-Cyude bond (2.156 A) is a bit shorter than the Bi-Cph bond (2.21-2.22 A), suggesting little or no double bond nature of the bismuth and ylidic carbon bond (Fig. 3.5). The ylidic carbon of this class of stabilized ylide appears at 8 100-113. 

Preparation of 1,1 -biheterole systems containing arsenic, antimony, and bismuth has been important in understanding thermochromic behavior in terms of structure and bonding. Their interesting solid state structures help explain why some show color transitions, while others do not (see Section 2.16.11.2). The coupled products are sometimes very air-sensitive, but can be isolated by low temperature crystallization for nonpolar solvents or by sublimation. The biarsoles, bistiboles, and bibismoles tend to be thermally stable, while the bibismolanes usually decompose at room temperature. In this section, the different methods by which these compounds may be prepared and isolated are examined. 

The POAC-Z)-PSt copolymer seemed not to be very thermally stable, because the orange color of the OAC gradually faded out over room temperature, although the micellar structure was maintained even after the color disappeared. However, below 0°C the micellar solution retained the orange color for several hours. 

Although elimination reactions are clearly degradation, leading to color and char formation, they can have advantages. Conjugated sequences and ladder structures confer thermal stability on the polymers, forming stable chars. This is the basis of the preparation of carbon fibers by pyrolysis of polyacrylonitrile. It is also one reason for the use of plasticized PVC in electrical insulation in a... 

We can easily see that the structures of beta-pinene and dipen-tene resins are quite different. The polymer chain of a beta-resin is more extended and flexible than that of a dipentene resin. Thus, from a consideration of the structures of the polymer repeating units, dipentene resins should exhibit a lower viscosity than beta-pinene resins at equal degree of polymerization. Actually, the viscosity of dipentene resins also has a greater dependence on temperature it is reduced to a greater extent than a comparable beta-resin. Dipentene resins are also more compatible with ethylene-vinyl acetate copol3nners. The cloud point, obtained in a compatibility test using a 10 20 20 blend of resin wax ethylene-vinyl acetate, is about 90°C and can be compared with about 175°C for a beta-resin. Thus, these resins are preferred over beta-pinene resins for hot melt adhesives. Since formulations employing dipentene resins reach a compatible cloud-free liquid state at a lower temperature, less oxidation is likely to occur. Dipentene resins have also been found to be more color stable than beta-pinene resins, which probably reflects the presence of fewer olefinic sites where oxidation can occur. Dipentene resins also tend to be thermally stable because of their multiple strand structure. 

The biscyclooctatetraene complexes have similar properties, are colored, decompose when heated above 160°C under vacuum, and do not sublime even at 1 X 10 mm. However, they are thermally stable when heated to 360 C in sealed tubes under one atmosphere of argon. They are insoluble in CHCI3, CCI4, and hexane, and are moderately soluble in THF and dioxane. The molecular structure of the cerium complex has been determined (Hodgson and Raymond, 1972b) and is shown in fig. 25.24. The important feature is the symmetric coordination of the two CgHi rings in a 7r-fashion. The molecular symmetry is very close to Dgd-... 

The photochromism of the spiropyran depends on the structure of heterocyclic parts, the medium such as solvent or plastic films, temperature, and light energy. Though the actual mechanisms may be more complex, a simple photochromic behavior in the spiropyrans is illustrated in Scheme 1. Initially, a spiropyran is excited by photoirradiation, and then a cisoid isomer arises after dissociation of the C—O bond. Finally, the cisoid form changes to the thermodynamically stable transoid form. The equilibrium between the cisoid and transoid forms largely depends on the substituent groups. The reversal of the colored form to the colorless spiropyran occurs by thermal or photochemical energy. More detailed mechanisms will be described in Section 1.2.1.6. 

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