Kinetics for Inhibited Polymorphic Transition of HMX Crystal after Strong Surface Confinement

The kinetics and the tuning mechanisms for the polymorphic transitions remain poorly understood. In this work, the mechanisms for the inhibited β → δ polymorphic transition of 1,3,5,7-tetranitro-1,3,5,7-tetrazocane (HMX) crystal after strong surface confinement in poly(dopamine) (PDA) were studied thoroughly by nonisothermal and isothermal kinetics. The activation energy for the polymorphic transition of HMX was about 400 kJ mol^-1, which was almost independent of the conversion extent. The transition process followed the three-dimensional growth of nuclei (A3) model. As for HMX@PDA, the activation energies increased from 100 to 620 kJ mol^-1 with increasing conversion from 0 to 1.0. The transition process could be divided into two partially overlapped stages, i.e., the first transition step follows some mechanism between the first-order reaction model and the two-dimensional diffusion (D2) model and the second step of HMX@PDA approaches the two-dimensional nucleation and nucleus growth (A2) model. It has been demonstrated that nucleation was the rate-limiting step during the polymorphic transition. The PDA coating significantly decreased the polymorphic transition rate, especially for the nucleation process. In particular, it was decreased by 10⁸ times compared to the uncoated HMX. Based on the density functional theory calculation and systematic characterizations, the inhibition of this transition was attributed to the strong interactions between the polymer chain of PDA and the function groups on the surface of HMX crystal, which blocked the formation of δ-nuclei at the crystal surface.