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Enthalpy-Entropy Compensation in Polyester Degradation Reactions

DOI: 10.1155/2012/782346

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In an earlier work the author had studied the degradation kinetics of polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polybutylene terephthalate (PBT) under nonisothermal conditions in air and N2 at heating rates of 5, 10, 15, and 20°C/min. In this paper the kinetic degradation parameters of PET, PTT, and PBT were estimated using the Coats-Redfern method for two different weight loss regions ranging from 2–8% (Zone I) and 8–40% (Zone II). A comparative analysis of the enthalpy-entropy compensation effect for these polyesters in air and N2 is presented. A linear relationship was found to exist between entropy and enthalpy values. The following criteria were applied to establish an enthalpy-entropy compensation effect and to check the presence of an isokinetic temperature: (a) Exner’s plot of log versus log , and (b) Krug et al. linear regression of ΔH versus ΔG. By the use of the latter two methods, varying isokinetic temperatures were obtained. These temperatures were not in the range of the experimental work conducted, indicating that these systems do not display compensation phenomena. 1. Introduction Kinetic studies carried out on similar compounds with a correctly chosen mechanism function ( is the weight fraction of material decomposed at temperature and time ) exhibit a linear relationship between the logarithm of the preexponential factors and activation energies known as the compensation effect [1–5]. Several theories and explanations for such compensation behavior have been put forth [6–8]. In the case of thermal decomposition of solids, the existence of the compensation effect permits certain conclusions concerning the decomposition mechanism and thermal characteristics of the compounds under investigation. The changes of Gibbs energy ( ), enthalpy ( ), and entropy ( ) for the degradation reactions can be obtained by studying the kinetics of the thermal decomposition of solid compounds with nonisothermal heating using the thermogravimetric (TG) curves and a correct algebraic expression of the conversion function, . The reaction mechanism for polymer degradation is a very complex chain mechanism that includes initiation, propagation, and termination reactions. Normally, two types of reaction models, the first order and the second order , are used for the thermal degradation studies of polymers. The author has previously reported [5] the kinetic parameters characterizing the degradation of PET, PTT, and PBT in air and N2 using data from nonisothermal thermogravimetry and the calculation procedure of Coats and Redfern

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