Polymer chain conformation effect

Counterion effects similar to those in ionic chain copolymerizations of alkenes (Secs. 6-4a-2, 6-4b-2) are present. Thus, copolymerizations of cyclopentene and norbomene with rhenium- and ruthenium-based initiators yield copolymers very rich in norbomene, while a more reactive (less discriminating) tungsten-based initiator yields a copolymer with comparable amounts of the two comonomers [Ivin, 1987]. Monomer reactivity ratios are also sensitive to solvent and temperature. Polymer conformational effects on reactivity have been observed in NCA copolymerizations where the particular polymer chain conformation, which is usually solvent-dependent, results in different interactions with each monomer [Imanishi, 1984]. 

The authors prefered the latter explanation. They argue that the difference in the dielectric constant of the medium, when a hydrocarbon rich mixture is compared with that rich in sulfur dioxide, affects the ratio of gauche to trans conformations of the polymer chain. Such effects were observed in other systems, e.g. in 1,2 dichloroethane (38). Hence,... 

Even when the azobenzene chromophores are incorporated into the polymer backbone, the dipole moment increase of azobenzene residues by photoirradiation can also induce a change in polymer chain conformation. The solution viscosity of poly(dimethylsiloxane) containing azobenzene residues in the main chain decreased upon ultraviolet irradiation, and the effect was attributed to the trans to cis photoisomerization [9]. The photodecrease of the viscosity depended on the polarity of the solvent. It was 24% in non-polar heptane, but negligible in polar dichloroethane. 

Very little work has appeared in the literature which deals with blends in which the component materials can cocrystallize. It is generally believed (16.17) that a requirement for cocrystallization is that there must be a close matching of the polymer chain conformations and of crystalline dimensions. Also, some level of miscibility should exist between the two polymers and the crstallization kinetics cannot be very different. Certainly, in the case of liquid crystalline polymers, in general, these requirements would be expected to be met. Some of our recent work (8) has suggested, however, that not all liquid crystal polymers do cocrystallize. The present work suggests that in certain cases it may be possible to achieve this effect. 

It is important to understand how the energy arising from these numerous contacts affects the conformations of a real polymer chain. The effective interaction between a pair of monomers depends on the difference... 

Vaia and Giannelis [19] emphasized that the main fundamental aspects differentiating nanocomposites from conventional composites are their vast interfacial areas per unit volume and the nanoscopic dimensions between the nanoelements. The presence of many chains at interfaces means that much of the polymer is really interphase-like instead of having bulk-like properties. Furthermore, the polymer chains are quite often confined between the surfaces of nanoplatelets which are closer to each other than the radius of gyration of a chain. Both adjacency to a nanoplatelet surface and confinement between such surfaces clearly modify the thermodynamics of polymer chain conformations and the kinetics of chain motions. These two factors may potentially also modify the effective mechanical properties of the polymer. Such... 

The freely jointed model assumes implicitly that two elements of the same molecule, possibly remote along the chain, can occupy the same position in space at the same time. In real polymer chains conformations in which this exists are impossible. Each segment of a real polymer chain exists within a volume that excludes all other segments. The number of such forbidden conformations that must be excluded is greater for the more compact arrangements with smaller values of r. The net effect of such long-range interaction is to expand the actual chain dimension (r ) over its unperturbed dimensions, by an expansion coefficient defined by Equation 12.13 ... 

The IR characteristic bands at 1230/1250 cm of stereoregular polyacrylonitrile (PAN) are dependent on not only the configuration, but also on the conformation through the arrangements of the configurations on the polymer chain. The effect of configuration and conformation on the IR results was determined to be a function of the film preparation conditions (180). 

The conformation of polymer chains (in effect, their size) in solution is an important characteristic of a system in solution and at an interface. The detailed analysis of polymer conformations is a very complex process requiring powerful computer facilities. A simpUfied treatment based on random-flight (or random-walk) statistics allows for the estimation of chain dimensions to a degree adequate for most practical situations. 

An example of the effect polymer chain conformation can have on high-resolution solid-state C CPMAS/DD NMR spectra is presented for the semi-crystalline polymer polyethylene (PE) in Figure 5.11. These spectra [20] were recorded with Torchia s CF-T pulse sequence [21], as illustrated, which... 

The study of the conformational behavior of polypeptides has intrinsic interest in a complex and challenging theoretical and experimental problem. There are also strong biological implications inasmuch as there are many known naturally occurring polypeptides, both linear and cyclic, with potent effects as hormones, toxins, antibiotics, and ionophores. It is very probable that these functions are closely related to the polymer chain conformations. Investigations of model polypepetides have been extremely useful for the interpretation of conformational behavior and for the elucidation of the interaction between proteins and other macromolecules. The conformational structures of polyproline and related compounds have been of particular interest due to the important role L-proline plays in effecting protein structure. 

These factors are not acting independently and have been seldom analysed. The structure and molecular weight of a polymer affect the solubility of polymer and consequently the solvent effects. The polymer chain conformation which is certainly a function of solvent and temperature will then influence both intra- and inter-polymer interactions. 

More recent MM studies have permitted more complete geometric relaxation to release steric strain and moderate the magnitudes of steric effects. Quite recently, full transition state geometric relaxation including polymer chain conformational sampling has been achieved, and stereo- and regiodifferentiation events comparable in magnitude to experiment have been computed. ... 

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