Molecules (Jun 2019)

Pressure-Induced Polymorphism of Caprolactam: A Neutron Diffraction Study

  • Ian B. Hutchison,
  • Craig L. Bull,
  • William G. Marshall,
  • Andrew J. Urquhart,
  • Iain D.H. Oswald

DOI
https://doi.org/10.3390/molecules24112174
Journal volume & issue
Vol. 24, no. 11
p. 2174

Abstract

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Caprolactam, a precursor to nylon-6 has been investigated as part of our studies into the polymerization of materials at high pressure. Single-crystal X-ray and neutron powder diffraction data have been used to explore the high-pressure phase behavior of caprolactam; two new high pressure solid forms were observed. The transition between each of the forms requires a substantial rearrangement of the molecules and we observe that the kinetic barrier to the conversion can aid retention of phases beyond their region of stability. Form II of caprolactam shows a small pressure region of stability between 0.5 GPa and 0.9 GPa with Form III being stable from 0.9 GPa to 5.4 GPa. The two high-pressure forms have a catemeric hydrogen-bonding pattern compared with the dimer interaction observed in ambient pressure Form I. The interaction between the chains has a marked effect on the directions of maximal compressibility in the structure. Neither of the high-pressure forms can be recovered to ambient pressure and there is no evidence of any polymerization occurring.

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