Tuning thermal release kinetics of soy oil from organic nanoparticles using variable synthesis conditions

Abstract

The thermal release properties of soy oil from poly(styrene-co-maleimide) nanoparticles containing 50wt% encapsulated oil have been quantified as a function of temperature and time. The effects of different synthesis conditions on the thermal stability of the nanoparticles and their oil release have been evaluated, i.e., by gradually increasing the amount of ammonium hydroxide used for the imidization of poly(styrene-co-maleic anhydride). First, the intrinsic thermal properties of the oil-filled nanoparticles were analysed by differential scanning calorimetry, which revealed an exothermal reaction related to the oil release and a suppression of the glass transition that may be masked owing to the complex structure of the hybrid nanoparticles. The isothermal scans showed different rates of oil release after a post-imidization reaction. The oil release was better followed by dynamic mechanical analysis, which illustrated changes in visco-elastic properties expressed by the maximum in the loss factor that related to the amount of released oil. Depending on the amount of ammonium hydroxide, the oil started to release below the glass transition temperature at various rates. Thermal release profiles of the oil were quantified by infrared and Raman spectrocopy after heating for 2min to 6h at 125 to 250°C, based on variations in oil-related and imide-related absorption bands. The oil release increased below and above the glass transition temperature, following a parabolic trend, and progressively decreased at higher ammonium hydroxide concentrations, in parallel with higher imide content and changes in imide conformation. The kinetics and mechanism of the oil release can be described by the Korsmeyer–Peppas model, suggesting a dominating diffusion mechanism that is influenced by further imidization of the polymer matrix during heating.