Main chain rotation

The other three polymers have additional rotation angles in the side chains, x and/or x. For poly(3-methyl-1-butene), the minimum was found in the three-dimensional plot. For poly(U-methyl-l-pentene) and poly(methyl methacrylate), the stable conformation of the side chain was first calculated with the fixed main chain conformation corresponding to the (7/2) and (5/1) helices, respectively. The potential energy was calculated against the main chain rotation angles, x and t2, by fixing x and x of the side chain at the values thus obtained. ... 

Figure 2. The dependence of the main-chain rotating-frame relaxation rate, and apparent diffusion coefficients of H2 and CO, on the concentration of TCP in PVC. The relaxation rates were measured at 3U kHz and 26 °C. The diffusion coefficients were measured at 270 cm-Hg and 27 °C.

The validity of this dynamic model can be justified by the fact that in polystyrene and other carbon-chain polymers of the type (—CH2—CHR—) (poly(p-chlorostyrene), poly(methyl acrylate) or poly(vinyl acetate)), containing no methyl groups bonded directly to the main chain, rotational isomerization with 17 5—6 Kcal/mol (in the time interval of r 10 — 10 s in solvents with ij 0.01 P) can occur ... 

Reference has previously been made in Section 2 to the rapid decay of all radical species in the vicinity of the Tg of the polymer. Data for the combined decay rates of all radical species in polybutadiene at various temperatures are shown in Fig. 11. The decay rate at temperatures well below Tg is negligible, but increases rapidly in the range (Tg - 30) K to Tg, where the decay becomes very rapid. The increasing decay rate with temperature in the vicinity of Tg is clearly associated with molecular mobility, particularly main diain rotation. Since the radicals observed arise from main chain scission, the radicals vdli be near or at chain ends. It is possible that the chain ends will become mobile at temperatures just below the temperature at which main chain rotation freely occurs, and this may account for the observed radical decay at temperatures just below Tg. Alternatively, it may be attributable to the general impreciseness of the glass transition temperature. 

Extensive theoretical and experimental works were carried out on local dynamics of polymers in solution and bulk to elucidate the mechanism of conformational transitions [106]. Formerly, it was believed that the most reasonable mechanism for the conformational transitions was a crankshaft-like motion such as the Schatzki crankshaft [117] or three-bond motions [118,119] in which two bonds in a main chain rotate simultaneously. However, recent computer simulations [ 120-128] have revealed many interesting features of conformational transitions of a polymer chain in solutions and melts. 

Mechanical Properties Related to Polymer Structure. Methacrylates are harder polymers of higher tensile strength and lower elongation than thek acrylate counterparts because substitution of the methyl group for the a-hydrogen on the main chain restricts the freedom of rotation and motion of the polymer backbone. This is demonstrated in Table 3. 

Figure 1.6 Diagram showing a polypeptide chain where the main-chain atoms are represented as rigid peptide units, linked through the atoms. Each unit has two degrees of freedom it can rotate around two bonds, its Ca-C bond and its N-Ca bond. The angle of rotation around the N-Ca bond is called phi (cj)) and that around the Co-C bond is called psi (xj/). The conformation of the main-chain atoms is therefore determined by the values of these two angles for each amino acid.

In the case of polar polymers the situation is more complex, since there are a large number of dipoles attached to one chain. These dipoles may either be attached to the main chain (as with poly(vinyl chloride), polyesters and polycarbonates) or the polar groups may not be directly attached to the main chain and the dipoles may, to some extent, rotate independently of it, e.g. as with poly(methyl methacrylate). 

This effect is not simply due to the better packing possible with the branched isomers. The lumpy brcuiched structures impede rotation about the carbon-carbon bond on the main chain, thus giving a stiffer molecule with consequently higher transition temperature. 

The chemical features that prohibit crystallinity are main chain flexibility (e.g., rotation), branching, random copolymers or low inter-polymer chain attraction. Normally, polymers are not miscible with each other and on cooling from the melt will separate into different phases. When miscibility is exhibited, e.g., poly(phenylene oxide) (PPO) and PS, crystallisation does not take place. 

We may return now to the polysaccharides present in the peanut for a brief consideration of the relationship of the other components present in the pectic materials to the araban constituent. All the evidence indicates that the pectic acid portion of the peanut is identical with normal pectic acid and, as was indicated in the previous section, this material, which is very stable to acid hydrolysis and possesses a high positive rotation contains a main chain which is built up of D-galac-turonic acid residues of the pyranose type. If, therefore, the araban associated with the pectic acid had been derived directly from the pectic acid by decarboxylation without intermediate hydrolysis of the poly-galacturonide, the sugar residues in the araban should also be in the pyranose form. The experimental evidence shows clearly, however, that the arabinose residues in araban are furanose in type and it follows that any hypothesis concerning the direct conversion of pectic acid into the araban by decarboxylation is untenable. 

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