TIME RESOLVED X-RAY-DIFFRACTION STUDIES OF THE POLYMORPHIC BEHAVIOR OF TRIPALMITIN USING SYNCHROTRON RADIATION

The polymorphic behaviour of the monoacid triglyceride, tripalmitin was studied by time resolved X-ray diffraction experiments involving synchrotron radiation. Fast crystallization processes and phase transitions can be investigated under conditions identical to those in differential scanning calorimetry (DSC). Upon melting of the α-phase, recrystallization occurs in the β-phase with the intervention of a very unstable β′-phase. During the melting process of the α-phase, the lamellar organization of the molcules (along the c-axis of the crystal cell) remains intact and is only destroyed after melting of the most stable β-polymorph. The melting process of the α-phase is merely restricted to a disordering at the atomic level (in the a-b-plane of the crystal cell). For tripalmitin, the β′-phase can only be prepared if crystallization is performed from the isotropic melt. Although the existence of more than one β′-phase is confirmed, no clear experimental evidence is available to prove unambiguously the existence of two distinct and independent β′-polymorphs. Both β′-phases seem to differ only in crystal perfection and crystallinity. In this context, crystallinity is defined as the degree to which the enthalpy of the unit cell approaches its theoretical minimum. A similar situation is found for the β-phase. Depending on the thermal conditions under which the β-form is crystallized, the β-phase can exhibit large differences in thermal characteristics, which can also be explained in terms of crystal perfection and crytallinity. The polymorphic transitions seem to take place from α to β′ and then further to β, with the transitions being mainly expressed as changes in lateral arrangements of the dimeric unit. During the polymorphic transitions, the change in lateral structure first takes place along the b-axis, followed by a delayed rearrangement along the a-axis of the crytallographic unit cell. © 1990.