Side-chain rotation, polymer

Figure 1.7. Mobility of hydrocarbon chain. The diagram shows mobility of a polymer chain composed of carbon atoms attached by single bonds with hydrogen side chains. Rotational freedom of the single carbon-to-carbon bonds allows hydrogen atoms to rotate freely about the backbone.

Schaefer and coworkers, in another chapter in this text, used 1h - 13(j dipole-dipole "line shapes" obtained in a very clever way to investigate rotational motion of the aromatic rings in polystyrene. The method used a WAHUHA pulse sequence to decouple proton-proton dipolar interactions, cross polarization to enhance signal acquisition and an overall sampling technique synchronous with the sample rotation. The dipole-dipole interaction was mapped in rotational sideband spectra obtained from 16 "normal" CP/MAS spectra. The method, though somewhat involved, provided a measure of dipole-dipole line-shapes which can be interpreted in terms of side-chain rotation in the polymer. 

Photophysical and photochemical processes in polymer solids are extremely important in that they relate directly to the functions of photoresists and other molecular functional devices. These processes are influenced significantly by the molecular structure of the polymer matrix and its motion. As already discussed in Section 2.1.3, the reactivity of functional groups in polymer solids changes markedly at the glass transition temperature (Tg) of the matrix. Their reactivity is also affected by the / transition temperature, Tp, which corresponds to the relaxation of local motion modes of the main chain and by Ty, the temperature corresponding to the onset of side chain rotation. These transition temperatures can be detected also by other experimental techniques, such as dynamic viscoelasticity measurements, dielectric dispersion, and NMR spectroscopy. The values obtained depend on the frequency of the measurement. Since photochemical and photophysical parameters are measures of the motion of a polymer chain, they provide means to estimate experimentally the values of Tp and Tr. In homogeneous solids, reactions are related to the free volume distribution. This important theoretical parameter can be discussed on the basis of photophysical processes. 

The reason why only the fluorinated ester drops spread over one of the fluorinated coatings can be explained by mutual attraction promoted by the structural and chemical affinity of the two fluorine-containing organic compounds. It had been established [5] that the fluorinated side chains of polymer S are sterically less closely packed and have more freedom of rotation than those of polymer A they may thus permit some lateral penetration of the closely related fluoro chains of the glutarate ester, whereas polymer A with its tight array of side chains is impervious to such penetration. The fluorinated ester exerts a slight solvent action on polymer S, but not on polymer A, when the polymer is totally immersed in the ester for periods longer than 10 days at ambient temperature, or 2 days at 50°C. 

FIGURE 4.11 Wire frame image of PFOM monomer, energy minimized (a) polymer with 4 monomer units, energy minimized (b) and polymer with 4 monomer units, having side chains rotated about the C-O bonds, allowing the polar ester groups to more closely approach the surface (c). 

We have added a companion option to PBUILD, PRANDOM which eases considerably the problem of finding good conformations of a polymer segment. PRANDOM automatically selects all of the polymer backbone and/or side chain bonds and will randomly select rotations for each bond. In a few minutes, one can not only build a polymer fragment, but also set up a Monte-Carlo search of its conformational space. However, even this cannot solve the problems for large models (pentamer or larger), again due to the number of bonds to be rotated. 

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