Chain mobility restriction

Yang et al. have also observed an enhancement of the residues of polymer-alumina nanocomposites after calorimeter tests on several polymers (PS, PMMA, and PC). In contrast, they have also found that the reduction in peak HHR happened at a very low loading level of nanometric alumina. This phenomenon has been explained by a catalyzing effect of alumina. As a matter of fact, the presence of alumina may accelerate the decomposition of polymers by a catalyzing effect, as well as enhancing the thermal stability by radical trapping and chain mobility restriction [39]. 

When a chain with M= 200,000 g/mole is linked to other chains at four points, the average molar mass between cross-links, M., amounts to 40,000. The mass of one unit is 4x12 + 6x1 =54 g/mole so the number of units between cross-links is about 740. At the glass-rubber transition no whole chains obtain free mobility, as a result of the entanglements, but chain parts of 30 to 100 monomer units. The chemical cross-links, therefore, hardly contribute to the restriction in chain mobility the increase in Tg will, therefore, be negligible. 

Polymers with a high cross-link density do not melt. A small degree of cross-linking restricts chain mobility and increases the Tg values. Partly crystalline polymers may be flexible at temperatures above the Tg and below the T... 

The presence of two substituents on every alternate carbon atom restricts chain mobility, so polyalkyl methacrylates are less flexible than the corresponding polyalkyl acrylates. Also, the presence of an alpha alkyl group increases (compared with the corresponding polyalkyl acrylates) the stability of polyalkyl methacrylates to light and chemical degradation,... 

It was shown, on the one hand, that gum-filler interactions are associated with the immobilization of a certain amount of rubber on the surface or inside the carbon black aggregates, and, on the other hand, that the corresponding bound or occluded rubbers play important roles in the reinforcement process due either to a restriction of elastomer chain mobility in the vicinity of the filler or to an increase of the effective volume of the latter. What are now the effects exerted by a filler on the stress-strain behavior and the modulus of cured rubbers ... 

Transamidation is an important process in the melt phase for polyamides because it is usually the process by which an equilibrium molecular weight distribution is reestablished and, in the case of the melt blending of two or more polyamides to form a copolymer, it is the process by which randomization of the individual monomers along the chain is effected. In the solid phase, chain mobility is restricted and equilibrium in either case often is not achieved. 

Yet another substitution that restricts the conformational freedom of amino acids is the introduction of a double bond at the Ca and atoms. In addition to the obvious effect that a, /3-unsaturation has on the side-chain mobility, this modification also affects the conformation of the backbone. The effects of Ca-Cp unsaturation have been explored experi-... 

Control of this property is possible by controlling the structure of the polymer chain. Monomers with bulky side groups restrict chain mobility and thus raise the glass transition temperature. The composition of copolymers and the ratio of polymer blends often are determined by the desired glass transition temperature of the final product. 

Finally, the NMR and the dynamic mechanical study show that two regions are present in filled silicone rubbers above the Tg, which differ significantly in local chain mobility immobilized chain units adsorbed at the filler surface and mobile chain units outside the adsorption layer. The local chain motions outside the adsorption layer are similar to those for unfilled rubbers. Chain motions in the adsorption layer however are strongly restricted. The frequency of chain motions in the adsorption layer at 300 K is comparable to the fi-equency of chain motions in a crosslinked PDMS containing 3-4 elementary chain units between network junctions [26]. 

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