Crystallization and transformation behavior of triacylglycerol binary mixtures forming molecular compounds of POP/OPO, POP rac-PPO, and POP sn-PPO

Abstract: Fat-based soft materials generally contain different triacylglycerol (TAG) species; hence, TAG mixtures’ phase behaviors determine their important physical properties. TAG binary mixtures typically exhibit eutectic, monotectic, solid solution, and molecular compound (MC) forming phases; among them, the molecular interactions and the crystallization kinetics controlling the formation processes of MC-phases remain unexplored. We used thermal analysis, X-ray diffraction, and optical microscopy to examine the crystallization and transformation kinetics of the three MC-forming 1:1 binary TAG mixtures of POP/OPO, POP/rac-PPO, and POP/sn-PPO. In the POP/OPO mixture, the MC_POP/OPO crystals of the most stable β form with double chain length stacking (β-2L) were crystallized by simple cooling treatments. In the POP/rac-PPO mixture, the β-2L form of MC_POP/rac-PPO was crystallized only by cooling at a rate of 0.1 °C/min, while metastable β’-2L formed at rates of 0.5 °C/min and 2.0 °C/min; then β’-2L melted and partially transformed into the β form during heating. The POP/sn-PPO mixture showed different and significantly intricate polymorphic behaviors; a single component of sn-PPO crystallized in β’ with triple chain length stacking (β’-3L) during cooling. This was followed by crystallization of POP and MC_POP/sn-PPO of the β’-2L forms at slow rates of 0.1 °C/min and 0.5 °C/min; the α-2L forms were cooled at 2.0 °C/min. During heating, the metastable β’ or α forms melted and MC_POP/sn-PPO transformed into β form before melting. Observations of isothermal crystal growth rates by optical microscopy revealed that sn-PPO β-3L crystals grow faster than the other single TAGs or MCs, particularly above ~18 ^°C. Differences in crystallization kinetics between sn-PPO, POP, and MC_POP/sn-PPO may cause separate crystallization while the POP/sn-PPO mixture cools.