Enthalpy-Entropy Compensation in Polyester Degradation Reactions

Adam Al-Mulla

doi:10.1155/2012/782346

Abstract

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 N2at 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 N2is 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 logk3T1versus logk3T2, and (b) Krug et al. linear regression of ΔHversus 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.

Highlights

In this paper the kinetic degradation parameters of polyethylene terephthalate (PET), polytrimethylene terephthalate (PTT), and polybutylene terephthalate (PBT) were estimated using the CoatsRedfern method for two different weight loss regions ranging from 2–8% (Zone I) and 8–40% (Zone II)
Kinetic studies carried out on similar compounds with a correctly chosen mechanism function f (α) (α is the weight fraction of material decomposed at temperature T and time t) exhibit a linear relationship between the logarithm of the preexponential factors and activation energies known as the compensation effect [1,2,3,4,5]
The Coats-Redfern equation was used to calculate the values of activation energy and the preexponential factor (A) for the nonisothermal degradation of PET, PTT, and PBT in

Summary

Introduction

Kinetic studies carried out on similar compounds with a correctly chosen mechanism function f (α) (α is the weight fraction of material decomposed at temperature T and time t) exhibit a linear relationship between the logarithm of the preexponential factors and activation energies known as the compensation effect [1,2,3,4,5]. The changes of Gibbs energy (ΔG), enthalpy (ΔH), and entropy (ΔS) 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, f (α). Two types of reaction models, the first order f (α)1 = (1 − α) and the second order f (α)2 = (1 − α) , are used for the thermal degradation studies of polymers

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Conclusion