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

Pressure Induced Topochemical Polymerizationof Solid Acryalmide Facilitated by Anisotropic Response of Hydrogen Bond Network

The pressure induced polymerization of molecular solids is an appealing route to obtain pure, crystalline polymers without the need for radical initiators. Here, we report a detailed density functional theory (DFT) based study of the structural and chemical changes that occur in defect free solid acrylamide, a hydrogen bonded crystal, when it is subjected to hydrostatic pressures. Our calculations predict a polymerization pressure of 94 GPa, in contrast to experimental estimates of 17 GPa, while being able to reproduce the experimentally measured pressure dependent spectroscopic features. Interestingly, we find that the two-dimensional hydrogen bond network templates a topochemical polymerization by aligning the atoms through an anisotropic response at low pressures. This results not only in conventional C-C, but also unusual C-O polymeric linkages, as well as a new hydrogen bonded framework, with both NH... O and C-H...O bonds.</p