Influence of synthesis conditions on thermal release of palm oil as liquid core filled in polymeric nanoparticles

Abstract

After synthesis of poly(styrene-co-maleimide) nanoparticles with 70 mass% encapsulated palm oil, by an imidization reaction of the poly(styrene-co-maleic anhydride) copolymer with different concentrations of ammonium hydroxide, the thermal release properties of palm oil were studied under various temperatures and heating times. The altered reaction conditions lead to progressively higher imide contents, resulting in better thermal stability and retarded release profiles. Based on differential scanning calorimetry, the release of oil is associated with an exotherm reaction, while the glass transition phenomena may be partially masked due to the complex structure of the oil and organic phase. The isothermal scans show different rates of oil release followed by a post-imidization reaction. From dynamic mechanical analysis, the thermal release can be followed by changes in visco-elastic properties and a shift in maximum loss factor. The thermal release profiles after heating for 2 min to 6 h at 125–250 °C were identified from FTIR to Raman spectroscopy, following variations in oil-related and imide-related absorption bands. The amount of exposed oil progressively decreases at higher ammonium hydroxide concentrations, while the oil release is stabilized at temperatures below the glass transition temperature and linear oil release starts at temperatures above the glass transition temperature. The kinetics and mechanism of oil release can be described by the release rate and diffusion exponent of the Korsmeyer–Peppas model. Therefore, it was concluded that the diffusion mechanisms are closely related to the softening properties of the polymer, with linear time-dependent release (Fick diffusion) or more immediate release (anomalous diffusion) stimulated by softening of the polymer matrix.