Chapter 5
Vibrational Up-Pumping in Polymorphic
Materials
5.1 Introduction
Due to the flexibility of molecules and the low directionality of their weak intermolecular interactions, molecules can adopt a variety of packing arrangements upon
crystallisation. Hence, the same compound can exist in different crystal forms, or
polymorphs. This phenomenon is widespread across molecular materials, with Walter
McCrone famously stating that “every compound has different polymorphic forms
and the number of forms known for a given compound is proportional to the time and
energy spend in research on that compound” [1]. Different polymorphic forms can be
obtained during crystallization, for example by controlling the speed, temperature or
pressure under which nucleation occurs [2, 3]. Alternatively, polymorphs can interconvert upon heating or application of external pressure [4, 5]. Polymorphic forms
are known to exhibit drastically different physical properties. This phenomenon has
therefore been strongly monitored by the pharmaceutical industry, where polymorphs
can exhibit, for example, different solubilities [6] and compressibilities [7]. Hence,
a drug composition that is prepared based on one polymorph can behave differently
than that based on another polymorphic form.
Energetic materials (EMs) are also highly prone to polymorphism. Some of the
most well-known EMs exhibit rich polymorphism. For example, RDX is known
to exist in at least two polymorphic forms under ambient conditions, the α- and
β-forms, and two additional forms can be obtained under hydrostatic pressures, γRDX at ca. 5.2 GPa [8] and δ-RDX at ca. 17.8 GPa. A fifth form, -RDX, has also
been identified on simultaneous application of heat and pressure [9]. Other wellknown EMs, such as CL-20, TNT, ammonium perchlorate and ammonium nitrate,
also exhibit rich polymorphism. As polymorphic transformations often occur when
a material is exposed to extremes of temperature and pressure, their occurrence is
crucial to understanding the detonation pathways of EMs.
Despite knowledge of their existence, most EM literature does not explicitly
consider polymorphic modifications during impact testing. A notable exception is
HMX, Fig. 5.1. Under ambient conditions HMX exists in the monoclinic β-form. The
© Springer Nature Switzerland AG 2020
A. A. L. Michalchuk, Mechanochemical Processes in Energetic Materials,
Springer Theses, https://doi.org/10.1007/978-3-030-56966-2_5
157
Vibrational Up-Pumping in Polymorphic
Materials
5.1 Introduction
Due to the flexibility of molecules and the low directionality of their weak intermolecular interactions, molecules can adopt a variety of packing arrangements upon
crystallisation. Hence, the same compound can exist in different crystal forms, or
polymorphs. This phenomenon is widespread across molecular materials, with Walter
McCrone famously stating that “every compound has different polymorphic forms
and the number of forms known for a given compound is proportional to the time and
energy spend in research on that compound” [1]. Different polymorphic forms can be
obtained during crystallization, for example by controlling the speed, temperature or
pressure under which nucleation occurs [2, 3]. Alternatively, polymorphs can interconvert upon heating or application of external pressure [4, 5]. Polymorphic forms
are known to exhibit drastically different physical properties. This phenomenon has
therefore been strongly monitored by the pharmaceutical industry, where polymorphs
can exhibit, for example, different solubilities [6] and compressibilities [7]. Hence,
a drug composition that is prepared based on one polymorph can behave differently
than that based on another polymorphic form.
Energetic materials (EMs) are also highly prone to polymorphism. Some of the
most well-known EMs exhibit rich polymorphism. For example, RDX is known
to exist in at least two polymorphic forms under ambient conditions, the α- and
β-forms, and two additional forms can be obtained under hydrostatic pressures, γRDX at ca. 5.2 GPa [8] and δ-RDX at ca. 17.8 GPa. A fifth form, -RDX, has also
been identified on simultaneous application of heat and pressure [9]. Other wellknown EMs, such as CL-20, TNT, ammonium perchlorate and ammonium nitrate,
also exhibit rich polymorphism. As polymorphic transformations often occur when
a material is exposed to extremes of temperature and pressure, their occurrence is
crucial to understanding the detonation pathways of EMs.
Despite knowledge of their existence, most EM literature does not explicitly
consider polymorphic modifications during impact testing. A notable exception is
HMX, Fig. 5.1. Under ambient conditions HMX exists in the monoclinic β-form. The
© Springer Nature Switzerland AG 2020
A. A. L. Michalchuk, Mechanochemical Processes in Energetic Materials,
Springer Theses, https://doi.org/10.1007/978-3-030-56966-2_5
157