Thermal Degradation Kinetics of Poly (3-hydroxybutyrate)/Cellulose Nanocrystals based Nanobiocomposite

Abstract

Polyhydroxybutyrate (PHB)/ Cellulose nanocrystals (CNCs) nanobiocomposites are prepared by solvent exchange cum solution casting technique at various loading fractions. The effects of CNC loading on dispersion in polymer matrix are studied. Acid hydrolysis of cellulose pulp from bamboo (Bambusabalcooa) yields crystalline rod shaped CNCs having width in the range of 10-20 nm and length being 300-400 nm. Morphological and X-ray diffraction (XRD) studies revealed improved interfacial adhesion of PHB with hydroxyl groups on CNC surface at a threshold loading of 3 wt%. Thermogravimetric analysis (TGA) showed that thermal stability of the nanobiocomposites (PHB/ CNC) slightly improved at 3 wt% CNC loading compared to pristine PHB. Further, kinetic analysis of the PHB/CNC nanobiocomposites at different loadings are investigated using isoconversional methods to predict the kinetic triplet. Kinetic parameters predicted from isoconversional methods using Ozawa Flynn Wall (OFW) and Kissinger Akahira Sunose (KAS) models showed that activation energy does not significantly vary with the degree of degradation, revealing that overall degradation follows a single step mechanism. The predicted activation energy values from both OFW and KAS models are in the range of 100-130 kJ/mol The activation energy values are high at higher CNC loadings, showing enhancement in thermal degradation rate due to agglomeration of CNCs. Thermal degradation phenomenon is further studied using Coats Redfern method considering phase boundary controlled models, first order reaction model and power law model. Overall investigation and comparison of the kinetic parameters led to the conclusion that thermal degradation mechanism of PHB/CNC nanobiocomposites followed phase boundary controlled first order reaction models (contracting volume) with random chain scission mechanism