Thermal degradation kinetics and isoconversional analysis of biodegradable poly (3-hydroxybutyrate)/organomodified montmorillonite nanocomposites

Abstract

Poly(3-hydroxybutyrate) (PHB)/organically modified clay nanocomposites were prepared by the melt mixing method and were characterized using wide-angle X-ray diffraction. Their thermal degradation kinetics was investigated using thermogravimetric analysis at various heating rates. Further kinetic analysis was performed using isoconversional methods and the invariant kinetic parameters method was used to estimate the so-called ‘true’ kinetic parameters, i.e. the pre-exponential factor, A and the activation energy, E, as well as the reaction model. It was found that intercalated structures are formed and the thermal stability of the material is improved by the addition of the nano-filler. From the isoconversional analysis, it was found that the activation energy does not vary significantly with the degree of degradation denoting degradation in one step with similar values for pure PHB and for all nanocomposites. Using the invariant kinetic parameters method, it was found that the model that best describes the experimental data was that of Sestak–Berggren's with f(a) = αn(1 − α)m, where the value of n is always larger than m and is increasing with the amount of the nano-filler. The value of the ‘true’ activation energy was found to be about 100 kJ mol−1 for all nanocomposites and the pre-exponential factor for PHB was estimated equal to 5.35 × 109 min−1. Finally, the values of the kinetic rate constant k were found to decrease with the amount of the nano-filler up to 3 wt%, while for amounts larger than 3 wt% k increased reaching a value greater than that of pure PHB for the 10 wt% nanocomposites