Polymers chain fragments mobility

The dimethyl ester of this acid in solution shows a quantum efficiency photochemical products. On the other hand, when the same acid is copolymerized with a glycol to form a polymeric compound with molecular weight 10,000 the quantum yield drops by about two orders of magnitude, 0.012. The reason for this behavior appears to be that when the chromophore is in the backbone of a long polymer chain the mobility of the two fragments formed in the photochemical process is severely restricted and as a result the photochemical reactions are much reduced. If radicals are formed the chances are very good that they will recombine within the solvent cage before they can escape and form further products. Presumably the Norrish type II process also is restricted by a mechanism which will be discussed below. 

The investigation of structural dynamics of CP is particularly topical in connection with the establishment of correlation between local intramolecular mobility and the reactivity of chain fragments. It has been established that groups located in the most mobile parts of the polymer chain exhibit the greatest reactivity [48], The chemical heterogeneity in relationship to local mobility is particularly... 

This was qualitatively shown in investigations of conformational behaviour and intramolecular mobility (IMM) of cholesterol-containing polymers in dilute solutions as of a function of solvent quality 134-136,185-l88) and temperature. Polarization luminescence provides one of the most fruitful methods for the evaluation of IMM l75,176). The method permits to get direct information about rotational mobility of the macromolecule as a whole, as well as about the mobility of the main chains and side branches. This is achieved via the attachment to macromolecules of so called luminescent markers (LM) — anthracylacyloxymethane groups in the case reported. Below are shown the chain fragments with LM which give information on the mobility of main chains (LM-1) and of side groups (LM-2) ... 

Small molecule-polymer "moiety" reactions. Groups formed on the backbone of polymer chains (such as ROO peroxy, RCO keto, RO alkoxy, etc.) do react with "small molecule" species, including oxygen and some radical fragments. The effect of the polymer medium apparently is to reduce the bimolecular collision (reaction) rates by a factor of about 10 2 relative to fluid solution rates, due to the reduced mobility of the chains. 

Use has also been nude of the fact that chain fragments within a crosslinked polymer have restricted mobility. Under certain conditions active species attached to die polymer can thus be effectively isolated from each other at relatively high concentrations, providing the advantages of high dilution and specificity along with rapid kinetics. In other cases, properties of die backbone itself such as polarity, pore size and chirality were utilized to achieve unique reactions, the polymer providing a specifrc microenvironment for the reaction. These aspects of PRs have been extensively reviewed (6.7). 

The fractal dimension D of a chain fragment between the points of topological fixing (entanglements, clusters, crosslinks) is an important structural parameter, which controls the molecular mobility and deformability of polymers. Crucial factors accounting for the use of the dimension D are clearly defined limits of variation (1super-molecular structure of the polymer. It should be emphasised that all fractal relations contain at least two variables. 

Mass fluxes are expressed with an effective mass transport coefficient and a concentration difference between the polymeric phase and the external environment. It is assumed that only monomers are able to leave the matrix because of the low mobility of chain fragments. The adopted kinetic scheme is one of reversible polycondensation + W <-> P + P. Polymer... 

Ductile deformation requires an adequate flexibility of polymer chain segments in order to ensure plastic flow on the molecular level. It has been long known that macromoleculai- chain mobility is a crucial factor decisive for either brittle or ductile behavior of a polymer [93-95]. An increase in the yield stress of a polymer with a decrease of the temperature is caused by the decrease of macromoleculai chain mobility, and vice versa the yield stress can serve as a qualitative measure of macromolecular chain mobility. It was shown that the temperature and strain rate dependencies of the yield stress are described in terms of relaxation processes, similarly as in linear viscoelasticity. Also, the kinetic elements taking pai-t in yielding and in viscoelastic response of a polymer are similar segments of chains, part of crystallites, fragments of amorphous phase. However, in crystalline polymei-s above their glass transition temperature the yield stress is determined by the yield stress required for crystal deformation... 

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