Polymer degradation, rate-determining

The rate of polymer degradation was determined by measuring the intrinsic viscosity of the initial and exposed samples in chloroform. The molecular weight (Mw) was calculated from the equation (3) ... 

Pigments can also have an appreciable effect on thermal stability. Thus, Table 9.1 shows the increasing rate of polymer degradation, as determined by failure time measurements as the test temperature is raised from 125 °C to 155 °C. 

Thermal stabilities of modified PVC samples acet-oxylated to varying degrees (reaction temperature 46°C) were determined [45]. Rate of thermal dehydrochlorination at 1% degradation was taken as a measure of thermal stability. The log of the degradation rate is plotted against the acetate content of the polymer in Fig. 2. 

Chain degradation in turbulent flow has been frequently reported in conjunction with drag reduction and in simple shear flow at high Reynolds numbers [187], Using poly(decyl methacrylate) under conditions of turbulent flow in a capillary tube, Muller and Klein observed that the hydrodynamic volume, [r ] M, is the determining factor for the degradation rate in various solvents and at various polymer concentrations [188], The initial MWD of the polymers used in their experiments are, however, too broad (Mw/Iiln = 5 ) to allow for a precise... 

Such studies have shown that it is the chemical structure and composition that determine whether or not synthetic polymers are biodegradable. On the other hand, the precise rate at which a synthetic polymer will degrade is determined by the specific morphology of the article into which the polymer has been fabricated. 

For degradable polymers, the next step is to determine when and how the polymer needs to degrade. Are there preferred degradation products Does degradation need to be fast or slow What environmental factors may contribute to the degradation rate - moisture, sunlight, microbes, mechanical stresses, or cyclic weathering ... 

The rate of weight loss of polymer A was determined in vacuum and in air. The activation energy of degradation in vacuum calculated from this... 

The ability of two geminate radicals to escape from each other and thereby avoid radical-radical recombination is a key mechanistic feature that determines the rate of polymer degradation (Scheme 10). As shown in the discussion, the existence of a photochemically generated radical pair containing... 

In some cases, several of these processes occur simultaneously, depending on the sample size, the heating rate, the pyrolysis temperature, the environment, and the presence of any additives. Although polymer degradation schemes can be greatly altered by the presence of comonomers, side-chain substituents, and other chemical constituent factors, the ultimate thermal stability is determined by the relative strengths of the main-chain bonds. Many additives and comonomers employed as flame retardants are thermally labile and as a result the thermal stability of the polymer system is reduced. In order to reduce the observed effects of the flame-retardant additives on the thermal stability of the polymeric materials, more thermally stable and hence inherently flame-resistant polymers are of increasing interest. 

For multiple species in three dimensions, these simultaneous differential equations for heat and mass transport are very complex and require numerical solution [6,7]. As a result of this complexity, we will discuss only simple geometries in the balance of this chapter and determine the rate limiting steps for these geometries. But, first, we present a detailed discussion of polymer degradation reactions. 

Some kinetic models for thermal or catalytic polymer degradation have been proposed. The commonly used approach is first-order kinetics to investigate the characteristics of degradation (Equation 9.1). In this approach at first the weight loss curve of polymers during the decomposition is determined, and overall rate constants are calculated... 

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