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*

*Corresponding author for this work

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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.
Original languageEnglish
Article number2174
Issue number11
Number of pages16
Publication statusPublished - 2019


  • High-pressure single-crystal X-ray diffraction
  • High-pressure neutron diffraction
  • Phase transitions
  • Intermolecular interactions
  • Energy frameworks


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