Modeling of the properties of plasticized poly(3-hydroxybutyrate) as a function of aging time and plasticizer content

Abstract

Poly(3-hydroxybutyrate) (PHB) is a biodegradable polymer synthetized by microorganisms from renewable feedstocks, and is a promising alternative for the replacement of polymers from non-renewable sources. However, this polymer undergoes physical aging, becoming brittle. The plasticization of PHB has been widely reported in the literature as a strategy for overcoming this problem. In this work, we analyze the effects of the amount of plasticizer and time on the extent of aging, by evaluating the changes in the mechanical, dynamic mechanical and thermal properties of formulations of PHB and triethyl citrate (TEC) at room temperature. Formulations with a variable mass fraction of TEC were prepared by melt extrusion, followed by the injection molding of specimens. The extent of aging of the formulations at room temperature was evaluated by analyzing specimens at a predetermined frequency for up to three months. The magnitude of the variation in their properties was larger at the beginning, and underwent exponential decay with aging time. Aging resulted in embrittlement of the formulations, the loss of the capacity to dissipate mechanical energy, and significant changes in the relaxation spectra. The effects of aging on impact resistance were more pronounced as the TEC concentration increased. The dynamic mechanical behavior suggested phase separation in formulations richer in plasticizer, induced by secondary crystallization, which expels TEC from the crystalline structure. A linear model was used to describe and to predict the effects of aging and plasticizer content on the mechanical properties of the formulations.