Kinetic models for water sorption and a hydrolysis reaction with poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH)

Abstract

Biodegradable polymers, including polyhydroxyalkanoate (PHA), have attracted attention because plastic waste is entering the ocean and causing problems. Controlling the rate of PHA degradation is essential for expanding its application, and fundamental studies on the reaction mechanisms are needed. The purpose of this study is to experimentally verify the mechanisms of water sorption and hydrolysis reactions involving poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBH), a type of PHA, and construct a model. In this study, the water sorption process was measured by near-infrared (NIR) spectroscopy and Fourier transform infrared spectroscopic (FTIR) imaging. From the results, we found that water sorption on PHBH proceeds by non-Fickian diffusion, two kinetic processes involving adsorption and diffusion. The reaction rate of PHBH hydrolysis was determined by gel permeation chromatography (GPC) analysis, and a model was developed to predict PHBH degradation during hydrolysis from the experimental results. Furthermore, we proposed a reaction-diffusion model that combines water sorption and hydrolysis reactions. As a result, the degradation of PHBH plates with different thicknesses can be predicted for this first time.