Chain conformations in solution

PDHS Structures in Solution. The determination of the chain conformation of polysilylenes in solution, particularly the conformations at temperatures just above or below the low-temperature thermochromic transition, is of great interest. NMR spectroscopy is one of the most useful techniques for probing chain conformation in solution (2i), and NMR is especially effective because of the large sensitivity of the carbon chemical shift to bond conformation (22). Silicon nuclei are also very sensitive to chain conformation, but a good correlation between silicon chemical shift and bond conformation has not been established yet. Unfortunately, both of these nuclei suffer from low sensitivity, primarily because of their low natural abundance. In contrast, protons have an essentially 100% natural abundance, but compared with the carbon or silicon chemical shift, the proton chemical shift is not very sensitive to bond conformation. Efforts to use NMR to probe the low-temperature dilute-solution conformation of the polysilylenes have been unsuccessful thus far. The diflSculty is that PDBS and PDHS precipitate from solution in 20-30 min after cooling through the thermochromic tran-... 

Although much has been learned about the structures of polysilylenes, a tremendous amount of work remains before a full understanding of these materials is developed. The microstructure of the polymers can be studied directly by solution NMR spectroscopic techniques. The determination of the chain conformation in solution is diflScult, particularly at low temperature. Light-scattering techniques may be able to establish the solution dimensions of the polysilylenes through the low-temperature thermochromic transition. The chain conformation in the solid state can be established by X-ray and electron difiraction methods. Solid-state Si NMR spectroscopy can become... 

A model of chain conformations in solution has been studied numerically on a computer. The polymer chains less than fully flexible were considered. Each chain was represented by relatively rigid groupings of base units called compact bundles, intercalated with sequences of base units called extended bundles. Several parameters characterizing the chains were varied. Thus, consequences of the model were found from the point of view of the exchange interaction energy, polymer concentration, number of segments in a bundle, molecular mass and temperature. Experimental evidence supporting conclusions from our model as well as the model itself is reviewed. 

Computers are used to study polymeric materials, to provide better processing of these materials, and also to make possible better use of the products. In the present work we study consequences of a model of chain conformations in solution. The computer is used to find out how varying a parameter characterizing the chain affects the chain conformations, hehavior and properties. Several parameters describing the chain are so varied in turn. The study at the molecular level is related to macroscopic behavior of polymer-containing liquid phases. 

The addition of electrolytes to a solution decreases the viscosity of a fresh polymer solution as will be discussed in the next section. High temperatures less than the thermodegrading conditions (less than 70-100°C depending on the polymer) cause a drop in the viscosity. This is also a reversible change associated with the change in the conformation of the chains in solution. The temperature effect on solution viscosity will be discussed. The effects of salt, temperature, severe shear, and aging will be discussed from the standpoint of viscosities and chain conformations in solution. First of all, the factors causing temporary viscosity losses will be presented. 

Winnik, M. A. (1977). Photochemical process of hydrocarbon chain conformation in solution, Acc. Chem. Res., 9 34. 

The study of the chain conformation in solution of PPA and poly(2-octyne) is of interest for two reasons. First, these two polymers differ as the PPA solution is red while the poly(2-octyne) solution is transparent, so that if it exists, any relation between the chain stiffness and the n electron delocalization may be expected to have a statistical length larger for PPA than for the poly(2-octyiie). The second reason is that PPA and polystyrene have almost identical monomer unit structures, the unique difference being the conjugated backbone structure of the PPA. 

Polymer chain conformation in solution Figure 17.1 Illustration of the separation of polymer molecules by size exclusion. 

The morphological features of nanostructured films have been correlated with the chain conformations in solution [13], for which a dummy atom model (DAM) was used. The AFM images in Figure 9.5 represented an off print of the solution conformation when molecules of emeraldine base POEA (POEA-EB) were adsorbed on the substrate. The size of the globules is comparable to the aggregates of polymer molecules in solution. Significantly, the adsorption mechanisms and film properties for polyanilines are altered by a mere change in pH [14], which has been exploited in sensors and other applications [15]. 

These studies have been eictended to analogous polymers 7.5 (Eq. 7.2) with bulky adamantylmethyl groups attached to the bipyridyl substituents [11]. Viscosity measurements in the presence of added salt showed the intrinsic viscosity of 7.5 ([rj] 23 mL g ) to be approximately twice that of 7.3 and 7.4, which is indicative of a much more extended chain conformation in solution. 

Experimental findings show that the diameter of electrospun fibers is dependent on the solution concentration and the polymer chain conformation in solution. Thus, the Berry number is defined by the following equation as indicated in Section 1.6. 

A different approach has been proposed to improve carrier transport and to increase emission efficiency by exploiting conformational properties of conjugated polymers indifferent solvents. MEH-PPV in tetrahydrofuran (THF) gives rise to coils that partially suppress interchain interactions. A different situation occurs in chlorobenzene (CB) solutions where a more extended conformation is detected resulting in an efficient aggregates formation [150,151]. Different polymer chain conformations in solutions are likely retained in films spun from them and then exploited to improve LED performances. Since their operation is a complicated balance of transport and emission properties, a multilayer structure with MEH-PPV films spun from different solvents could improve OLED performances. Schwartz and coworkers exploited different conformations of MEH-PPV in CB and THF... 

A comparison of the asparagine and glutamine side-chain conformation in solution and crystal has been done for hen egg-white lysozyme by Higman et The analysis of Dhn and Dhh RDCs allows to state that some of the side chains show a well-defined position, whereas others show a range of orientations. 

We now turn our attention to hairy rods consisting of a conjugated polymer backbone, in which case the hairy-rod concept offers possibilities to achieve processible, i.e., soluble or fusible, electroactive materials. It allows one to control the chain conformation in solution [154] and to obtain improved charge transport in the self-organized bulk phase [155]. Here the discussion will still be limited to covalently bonded side chains. In the next section, the extension to supramolecular hairy rods will be considered. 

The studies on optical activity of relatively simple vinyl type polymers allow one to draw interesting conclusions on chain conformations in solution, independently of molecular weight, at least for macromolecules containing more than about 20 monomeric units [2]. 

Heparin and i-carrageenan (segments) should have relatively stiff backbones with an essentially regular structure, and therefore liable (in principle) to assume ordered chain conformations in solution. ... 

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