Investigating the role of compression rates in pressure induced polymerization of crystalline acrylamide using ab initio molecular dynamics

Varying the rate at which pressure is applied on a crystal is known to yield different pressure induced polymorphic structures in experiments. In this work, we explore the effect of pressure increase rate on pressure-induced polymerization in crystalline acrylamide, using a density functional theory based approach. Room temperature constant pressure ab initio molecular dynamics performed with two different compression rates- 0.4 GPa/ps and 2 GPa/ps - revealed very different structural evolutions of the system. While both rates ultimately yielded a metastable 1-dimensional polymer at pressures beyond 64 GPa, rapid compression resulted in many disordered polymers with unanticipated linkages at lower pressures. The mechanisms leading to polymerization as well as the structure and electronic properties of the various polymer polymorphs obtained in the two compression routes are described. Our simulations suggest a hierarchical route for the pressure induced polymerization of solid acrylamide towards the thermodynamically favorable Pol-I.