Flexible units

More precise control over the emission color may be achieved by employing equimolar quantities of a fc/.v-phosphonium salt, e.g., 67 and a dialdehyde containing a flexible unit sueh as 66. This approach has been exploited by a number ol researchers 98-102[. The emission wavelength of these materials is in the blue te blue-green region of the spectrum (470-495 nm). 

Fig. 16. Sketch of selected chain in the amorphous layers of LPE, subject to localized constraints of the motion (O) due to the interaction with neighboring chains. Through these constraints the chain is divided into flexible units. A selected deuteron 2H on unit j is shown for convenience Fig. 17. Fractions of flexible units in the amorphous layers of LPE as obtained from the analysis of 2H NMR line shapes displayed in Fig. 15...

The number of the constraints to chain mobility shown in Fig. 16 decreases with increasing temperature, reflecting the increase of the free volume. From a comparison of the spectra in Fig. 15 with line shapes calculated for flexible chains on a diamond lattice 23 (one can infer that the average length of the flexible unit increases from 3-5 bonds at room temperyture to about 10-15 bonds at 380 K. Our model thus can quantitatively explain the gradual increase of the number of conformations accessible to the chain motion. The earlier XH wide line data 72 are also in accord with our findings. 

In 1970 the Poison Prevention Packaging Act was passed. It provides for packaging designed and constructed to be difficult for young children to open within a reasonable time and that is not difficult for adults to use properly. Establishing child-resistance effectiveness in a flexible unitized package is considerably more complex and, in certain circumstances, more difficult than doing so with a child-resistant cap on a bottle [21]. 

If the backbone as well as the side chains consist of flexible units, the molecular conformation arises out of the competition of the entropic elasticity of the confined side chains and the backbone [ 153 -155]. In this case, coiling of the side chains can occur only at the expense of the stretching of the backbone. In addition to the excluded volume effects, short range enthalpic interactions may become important. This is particularly the case for densely substituted monoden-dron jacketed polymers, where the molecular conformation can be controlled by the optimum assembly of the dendrons [22-26,156]. If the brush contains io-nizable groups, the conformation and flexibility may be additionally affected by Coulomb forces depending on the ionic strength of the solvent [79,80]. 

As in the case of nylons, the flexibility of PUs is increased as the number of methylene groups is increased, and the rigidity is related to the number of stiffening groups, such as phenylene groups in the chain. As the number of methylene units increases (Table 14.6), the Tm decreases. The Tm generally increases as the flexible units are replaced by nonflexible units, such as phen-ylenes and piperazines. Thermal and physical properties of aliphatic PUs are shown in Table 14.7. 

The most striking result of this study is the observation that by incorporating even very small numbers of flexible units, the crystallizability of the polycarbonate is increased considerably. 

With a mixture of HMDA with various amounts of dimethylhexamethy-lene diamine (DMHMDA), the crosslink density is determined by the relative amount of diamine with respect to the secondary diamine, but now the role of secondary diamine is to extend the mesh of the network by inserting flexible units into the chains between crosslinks. 

As refineries have evolved, distillation has remained the prime means by which petroleum is refined (Speight, 1999). Indeed, the distillation section of a modem refinery is the most flexible unit in the refinery since conditions can be adjusted to process a wide range of refinery feedstocks from the lighter cmde oils to the heavier, more viscous cmde oils. However, the maximum permissible temperature (in the vaporizing furnace or heater) to which the feedstock can be subjected is 350°C (660°F). The rate of thermal decomposition increases markedly above this temperature if decomposition occurs within a distillation unit, it can lead to coke deposition in the heater pipes or in the tower itself with the resulting failure of the unit. 

An independent producer like the U.S.-based Stepan is in a position to provide a complete range of anionic, cationic, and nonionic agents because it has flexible units in four areas of the United States as well as one in southeastern France in Voreppe. 

Polymra-s Comprising Both Rigid and Flexible Units or Sequences... 

Mesomorphic behavior has been observed in a number of random copolymers consisting of two kinds of units, one rigid and rodlike and the other flexible , in the sense of offering a variety of conformations and, hence, capable of disrupting perpetuation of the axis of the rigid unit. The p-hydroxybenzoate (PHB) unit is prototypal of a rigid unit it has often been incorporated with ethylene terephthaiate (ET) as the flexible unit present as the minor constituent (0.3-0.4 mole fraction). 

Further theoretical studies by Floryconcerned the clarification of phase transitions for the systems with a definite distribution of macromolecules over the length and also the analysis of equilibrium for a model system composed of macromolecules in which rigid blocks are separated by flexible units. An example of such systems are copolyesters exhibiting the thermotropic transition into the liquid crystalline state in the absence of a solvent... 

Thermotropic liquid crystalline polymers with alternating rigid and flexible units along the backbone are a class of polymers which have been studied extensively over the years. The rigid units are usually aromatic moieties connected to each other by a variety of functional groups while the flexible units are most commonly methylene or... 

Further studies on the detailed structure of the polymers in this series are in progress and will be reported shortly. Synthesis of higher homologues of this series and low molecular weight model compounds is also being pursued in an effort to determine the role of the rigid and flexible units in influencing the formation of liquid crystalline phases. 

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