engleski

Is the thermosalient effect without phase transition possible?

Routine measurements, such as evaluation of the crystals’ habit under hot-stage microscope, can become very thrilling if the crystals suddenly start to jump around as if dancing to some inaudible music. This is actually a real effect called thermosalient effect. Thermosalient materials, or more colloquially called jumping crystals, are the ones that during heating/cooling undergo an energetic polymorphic phase transition which is so sudden and abrupt that the crystals are ballistically projected to heights of several hundred times larger than their own dimensions. Such single crystals provide an impressive display of the conversion of thermal energy into mechanical work and are thus excellent candidates for the production of active machines, such as “smart” medical devices or implants, artificial muscles, biomimetic kinetic devices, electromechanical devices, actuators, materials for electronics and heat- sensitive sensors [1- 2]. Our latest study shows a new kind of thermosalient effect in scopolamine bromide. As in most thermosalient systems, uniaxial negative thermal expansion seems to be the most likely candidate for the driving force behind this phenomenon. But one thing in which scopolamine bromide differs from other reported thermosalient materials is the absence of phase transition. In all the other thermosalient systems, thermosalient effect is explained by the accumulation of stress in the crystal lattice which is explosively released during the phase transition (which usually takes place on the timescale of less than a millisecond) thus causing the crystal locomotion. As a comparison, results of the study of thermosalient behavior of oxitropium bromide are presented as well. These two systems have very similar molecular structures, the only difference being that one ethyl group is replaced by methyl group in the case of scopolamine bromide. Both compounds have medical uses, oxitropium bromide is used as a bronchodilator, whereas scopolamine bromide used to prevent nausea and vomiting caused by motion sickness. They also both exhibit thermosalient effect. And this is where the difference stops. As already mentioned, In the case of oxitropium bromide thermosalient effect is caused by the topotactic phase transition during which unit cell drastically changes. On the other hand, surprisingly, scopolamine bromide does not seem to show any phase transition at all, but yet, its crystals are also joyfully jumping around during the heating or cooling of sample. Thermosalient effect is definitely propelled by different mechanisms in these two systems, thus demonstrating and confirming the full complexity and mystery of this phenomenon.

molecular crystals ; thermosalient materials ; thermal axpansion ; crystal polymorphic phase transition

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