Degradation solvent interaction parameter

Below a critical concentration, c, in a thermodynamically good solvent, r 0 can be standardised against the overlap parameter c [r)]. However, for c>c, and in the case of a 0-solvent for parameter c-[r ]>0.7, r 0 is a function of the Bueche parameter, cMw The critical concentration c is found to be Mw and solvent independent, as predicted by Graessley. In the case of semi-dilute polymer solutions the relaxation time and slope in the linear region of the flow are found to be strongly influenced by the nature of polymer-solvent interactions. Taking this into account, it is possible to predict the shear viscosity and the critical shear rate at which shear-induced degradation occurs as a function of Mw c and the solvent power. 

Several authors have attempted to corrolate the degradation rate with such solvent parameters as osmotic coefficient [35], viscosity [36-38] and the Flory Huggins interaction parameter, % [39,40] - a low % value indicates a good solvent in which the polymer is expected to exhibit an open conformation (as opposed to coiled) and therefore is more susceptible to degradation (Fig. 5.15). 

At PVC s degradation in solution, one of the basic reasons of change of the process kinetic parameters is the nueleophilic activation of PVC s dehydrochlorination reaction. The process is described by mechanism [1-3]. Thus, there is a linear dependence between PVC s thermal dehydrochlorination rate and parameter of solvent s relative basicity BIcm (Fig. 1) [1-3]. (The value B/cm is evaluated by shift of a characteristic band OH of phenol at A, = 3600/cm in an IR-spectrum at interaction with the solvent [4]). It is essentially important that the rate of PVC s dehydrochlorination in the solvents with relative basicity B > 50/cm was always above, than the rate of PVC s dehydrochlorination without the solvent, while when B < 50/cm, PVC s desintegration rate was always less, than at it s destruction without the solvent. The revealed dependence = (B) is described by the equation ... 

Equation (1) turns into an Eq. (2) if to take into account that the PVC s degradation rate is determined not only by parameter of relative basicity of the solvent B, but also by its concentration in a solution (C, mol PVC/1), as well as by degree of pol5mier-pol5mier interaction (degree of macromolecules stmcturizarion in a solution AC = /C-C /, where C - concentration of a begirming of PVC maeromolecules association in a solution) ... 

Chemical environments decrease the integrity of polymers by two mechanisms - physical and chemical means. The primary effect is physical or solvent effect while chemical effect or degradation occurs in a minority of cases. Physical effects are mainly functions of polymer and solvent strucmres. A number of predictive tools have been developed which are helpfiil-though less than perfect. They include a number of solubility parameters such as Hildebrand and Hanson systems. Solubility parameters are discussed later in this chapter. Other parameters, such as polarity (or lack thereof) of the polymer and solvent, can be used as rough estimators of interaction between these materials. 

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