Dataset Information

Pressure-Induced Polymorphism of Caprolactam: A Neutron Diffraction Study.

ABSTRACT: 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.

SUBMITTER: Hutchison IB

PROVIDER: S-EPMC6600225 | biostudies-literature | 2019 Jun

REPOSITORIES: biostudies-literature

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Pressure-Induced Polymorphism of Caprolactam: A Neutron Diffraction Study.

Hutchison Ian B IB Bull Craig L CL Marshall William G WG Urquhart Andrew J AJ Oswald Iain D H IDH

Molecules (Basel, Switzerland) 20190610 11

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 be ...[more]

PMID: 31185609

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Project description:We report high-pressure neutron powder diffraction measurements of the most hydrated phase in the MgSO4–H2O system, magnesium sulfate undecahydrate, including an analysis of the elastic strain tensor and observations concerning phase changes that occur at high pressure. We have collected neutron powder diffraction data from MgSO4·11D2O (the deuterated analogue of meridianiite), a highly hydrated sulfate salt that is thought to be a candidate rock-forming mineral in some icy satellites of the outer solar system. Our measurements, made using the PEARL/HiPr and OSIRIS instruments at the ISIS neutron spallation source, covered the range 0.1 < P < 800?MPa and 150 < T < 280?K. The refined unit-cell volumes as a function of P and T are parameterized in the form of a Murnaghan integrated linear equation of state having a zero-pressure volume V0 = 706.23?(8)?Å3, zero-pressure bulk modulus K0 = 19.9?(4)?GPa and its first pressure derivative, K? = 9?(1). The structure’s compressibility is highly anisotropic, as expected, with the three principal directions of the unit-strain tensor having compressibilities of 9.6?×?10?3, 3.4?×?10?2 and 3.4?×?10?3 GPa?1, the most compressible direction being perpendicular to the long axis of a discrete hexadecameric water cluster, (D2O)16. At high pressure we observed two different phase transitions. First, warming of MgSO4·11D2O at 545?MPa resulted in a change in the diffraction pattern at 275?K consistent with partial (peritectic) melting; quasielastic neutron spectra collected simultaneously evince the onset of the reorientational motion of D2O molecules with characteristic time-scales of 20–30?ps, longer than those found in bulk liquid water at the same temperature and commensurate with the lifetime of solvent-separated ion pairs in aqueous MgSO4. Second, at ??0.9?GPa, 240?K, MgSO4·11D2O decomposed into high-pressure water ice phase VI and MgSO4·9D2O, a recently discovered phase that has hitherto only been formed at ambient pressure by quenching small droplets of MgSO4(aq) in liquid nitrogen. The fate of the high-pressure enneahydrate on further compression and warming is not clear from the neutron diffraction data, but its occurrence indicates that it may also be a rock-forming mineral in the deep mantles of large icy satellites.

Dataset Information

Pressure-Induced Polymorphism of Caprolactam: A Neutron Diffraction Study.

Publications

Pressure-Induced Polymorphism of Caprolactam: A Neutron Diffraction Study.

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