Compression Rate Dependence and Hierarchical Mechanism in the Pressure-Induced Polymerization of Acrylamide: Insights from Ab Initio Molecular Dynamics

Varying the rate at which pressure is applied to a crystal is known to yield different pressure-induced polymorphic structures in experiments. In this work, we investigate the effect of pressure increase rate on pressure-induced polymerization in crystalline acrylamide, using room temperature constant pressure ab initio molecular dynamics simulations. Simulations performed with two different compression rates revealed very different structural evolutions of the system at lower pressures. Fast (nonequilibrated) pressure increase yields disordered (polymer) structures with unanticipated linkages for pressures up to 67 GPa. On the other hand, slow (quasi-static) pressure increase gives no new structures until 64 GPa. At pressures greater than 67 GPa, both pathways converge toward an ordered 3-dimensional polymer through a hierarchical mechanism involving 1-dimensional polymeric intermediates. The structural and electronic details of the mechanisms leading to polymerization are discussed.