Linear polymers, chain conformation

Mate and Novotny [42] studied the conformation of 0.5-13 nm thick Z-15 on a clean Si (100) surface by means of AFM and XPS. They found that the height for PFPE molecules to extend above a solid surface was no more than 1.5-2.5 nm, which was considerably less than the diameter of gyration of the lubricant molecules ranging between 3.2-7.3 nm. The measured height corresponds to a few molecular diameters of linear polymer chains whose cross-sectional diameter is estimated as 0.6-0.7 nm. The experimental results imply that molecules on a solid surface have an extended, flat conformation. Furthermore, they brought forward a model, as shown in Fig. 28, which illustrates two... 

The solution properties of dendrigraft polybutadienes are, as in the previous cases discussed, consistent with a hard sphere morphology. The intrinsic viscosity of arborescent-poly(butadienes) levels off for the G1 and G2 polymers. Additionally, the ratio of the radius of gyration in solution (Rg) to the hydrodynamic radius (Rb) of the molecules decreases from RJRb = 1.4 to 0.8 from G1 to G2. For linear polymer chains with a coiled conformation in solution, a ratio RJRb = 1.48-1.50 is expected. For rigid spheres, in comparison, a limiting value RJRb = 0.775 is predicted. 

Polysilane-based nanostructured composites were synthesized by the inclusion of poly(di-w-hexylsilane) (Mw = 53,600) into mesoporous, Si-OH-rich silica with a pore size of 2.8 nm.81 Two PL bands are observed for the composite. A narrow band at 371 nm, assigned to a PDHS film on a quartz substrate is blue shifted by 20 nm, a shift attributed to the polymer being incorporated into the pores.82 The size of the monomeric unit of the PDHS is about 1.6 nm, so only one polymer chain can be incorporated into a mesopore with a diameter of 2.8 nm. The narrow PL band at 350 nm is due to the reduction of the intermolecular interactions between polymer chains. This narrow PL band at 350 nm is assigned to the excited state of the linear polymer chain.81 Also, a new broad band of visible fluorescence at 410 nm appeared, which is assigned to localized states induced by conformational changes of the polymer chains caused by its interaction with the silanol (Si-OH) covered pore surface. Visible luminescence in nanosize PDHS is observed only when the polymer was incorporated in hexagonal pores of 2.8 nm and is not seen for the polymer incorporated into cubic pores of 2.8 nm diameter or hexagonal pores of 5.8 nm diameter. 

The influence of chain conformation on mobility is clearly seen in the data for PDA-4BCMU. The mobility is highest for isolated, extended linear polymer chains produced at low concentration in the monomer crystal matrix and falls dramaticallyfor the disordered chains in a yellow solution, but is slightly... 

An LCAO (linear combination of atomic orbitals) local-density functional approach was used to calculate the band structures of a series of polymer chain conformations unsubstituted polysilane in the all-trans conformation and in a 411 helical conformation, and all-trans poly(dimethylsilane). Calculated absorption spectra predict a highly anisotropic absorption for the all-trans conformation of polysilane, with the threshold absorption peak arising strictly from polarizations parallel to the chain axis. The absorption spectrum for the helical conformation is much more isotropic. Results for the dimethyl-substituted polysilane chain suggest that the states immediately surrounding the Fermi level retain their silicon-backbone a character upon alkyl-group substitution, although the band gap decreases by I eV because of contributions from alkyl substituent states both below the valence band and above the conduction band to the frontier states. 

Consider a linear polymer chain with N monomers of length b, restricted to the air-water interface (two-dimensional conformations). Repeat the Flory theory calculation and demonstrate that the size R of the chain as a function of the excluded area a per monomer (two-dimensional analogue of excluded volume v) is... 

The special structure of polymer molecules that distinguishes them from other species is their long, flexible chain structure. To describe this situation, let us first consider an isolated polymer chain and then extend the results to ensembles of chains, that is, to the bulk polymer. An isolated linear polymer chain is capable of assuming many different conformations. Because of... 

So far we were concerned with a single linear flexible polymer chain and studied its conformational properties in different environments. We now show how the critical behavior of two interacting long flexible linear polymer chains can also be studied using a lattice model of two interacting walks. 

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. 

The conformation of any polymer is affected by a number of factors, including the polymer architecture and the solvent affinity. In the case of polyelectrolytes, an additional factor is present charge [42,43]. In solution, whereas an uncharged linear polymer chain is usually found in a random conformation (theta solvent), a linear polyelectrolyte will adopt a more expanded, rigid-rod-like conformation due to the coulomb repulsion (the charges on the chain will repel each other) (Scheme 4a). 

If a polymer is built from strictly difunctional monomers, the result is a linear polymer chain. A scale model of a typical linear polymer molecule made from 0.5-cm-diameter clothesline would be about three meters long. This isn t a bad analogy The chains are long, flexible, essentially one-dimensional structures. The term linear can be somewhat misleading, however, because the molecules don t necessarily assume a geometrically linear conformation. 

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