Polymer polymorphs from crystalline acrylamide with high pressure: an ab initio investigation

Varying the rate of hydrostatic pressure on a material is known to give rise to different polymorphic structures of the material. Here, we present a first-principles density functional theoretic (DFT) study of the effect of changing the compression rate on the polymerization of crystalline acrylamide, using static optimization (0 K) as well as ab initio molecular dynamics (AIMD) at room temperature (RT). Several polymer structures are found to be accessible at 0 K, with the structure obtained depending on the optimization method (rate). The polymer obtained by slow compression at 23 GPa is the global minimum, while metastable polymers were obtained by rapid compression. Detailed RT AIMD simulations with slow compression (SC) at 0.4 GPa/ps and with rapid compression (RC) at 2 GPa/ps, confirmed that the polymorphs obtained depend on the compression rate as a reduction in the polymerization pressure by almost 25 GPa was observed by RC, as compared to SC. Also, we see a tendency for RC to give polymers lacking order as opposed to SC where pure polymers are obtained. The structures and electronic properties of these interesting polymer polymorphs are described. The detailed mechanisms leading to polymerization are elucidated in terms of hydrogen bond re-orientation and a topochemical parameter.