Pressure-induced polymorphism of caprolactam : a neutron diffraction study

Hutchison, Ian and Bull, Craig L. and Marshall, William G. and Urquhart, Andrew J. and Oswald, Iain D.H. (2019) Pressure-induced polymorphism of caprolactam : a neutron diffraction study. Molecules, 24 (11). ISSN 1420-3049

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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 and we have observed two new high pressure solid forms. 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 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 the 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.