(390b) Effect of PVP On Nucleation and Crystal Growth of RDX

Polyvinylpyrrolidone (PVP) was found to remarkably affect nucleation and crystal growth in the cooing crystallization of 1,3,5-trinitro-1,3,5-triazinane (RDX) from acetone: firstly, it was found that PVP acts an effective nucleation retarder. In the cooling crystallization of RDX from 30 ¢ªC saturated acetone solution at constant cooling rates of 0.25, 0.5, and 1.0 ¢ªC/min, values of maximum allowable undercooling(ΔT_max) were 7.5, 9.7, and 10.9 ¢ªC, respectively. On the other hand, the values of ΔT_max with PVP K30 of 100 ppm were found to increase to 18.4, 19.6, and 20.7 ¢ªC, respectively. According to well-known empirical relationship of nucleation rate (Nývlt et al.), nucleation order was estimated as 3.5 in the crystallization without additive, but that was 12.0 with PVP, which indicates that the nucleation mechanism is changed. Polar aprotic lactam ring in PVP has strong affinity with polar RDX molecules and therefore PVP is expected to interact with growing RDX cluster. In addition, RDX molecules can be easily adsorbed to any direction and part of PVP, because many branched lactam rings along the backbone of a chain in PVP molecule. As a result, PVP may penetrate into the emerging cluster, and thus disrupt the packing and lattice ordering of the RDX cluster. Secondly, PVP was confirmed as useful tailor-made additive for growth habit modification of RDX crystal. Morphology of RDX crystals obtained from acetone is prism-like with four trapezoid broad faces of {210} planes and eight isosceles triangle side faces of {111} planes. However, when PVP K30 of 100 ppm was added, morphology of RDX crystals was tetragonal bipyramid with eight faces of {111} planes. It indicates that the binding energy of PVP with (111) face of RDX is higher than that with (210) face, and thus the growth of (111) face of RDX was noticeably obstructed by PVP.