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 experimentally known to yield different pressure induced polymorphic structures. Herein, we explore the effect of pressure increase rate on pressure induced polymerization in crystalline acrylamide, using a density functional theory based approach. While quasi-static compression at 0 K stabilizes a 3-dimensional topochemical polymer, Pol-I, at 23 GPa, rapid compression optimizations suggest the presence of multiple polymeric intermediates in the system. Room temperature ab initio molecular dynamics performed with two different compression rates - 0.4 GPa/ps and 2 GPa/ps - revealed very different structural evolution 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 at lower pressures with unanticipated linkages. 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. While large kinetic barriers delay the formation of the thermodynamically favored polymer Pol- I, our simulations suggest a hierarchical route for the pressure induced polymerization of solid acrylamide towards the thermodynamically favorable Pol-I.