STUDIES OF THE MECHANISM OF CRYSTAL-GROWTH OF HEXAMETHYLMELAMINE .2. RATES OF GROWTH OF THE CRYSTAL FACES

The growth rates of single crystals of HMM were studied in solution of methanol and cyclohexane. The relationship between the crystal size (length and breadth) and time (t) was best described using square-root t as the independent variable and fitting the dependence by second or third degree polynomial equations. The first derivative of the latter equations yielded the growth rates of the crystals. The dependence of the solute concentration on time for the methanol and cyclohexane solution followed an exponential equation and a second degree polynomial equation, respectively, from which the supersaturation, sigma, at any time t was calculated. From these results, continuous curves of the fitted growth rates as a function of the fitted sigma were generated. Purely volume diffusion controlled growth rates were also estimated for the crystal faces and shown to be comparable to the measured growth rates. The surface entropy (alpha) factor for different faces of the HMM crystal was also evaluated. The results appear to suggest that there is no single mechanism dominating the entire process of growth for HMM crystals but rather the growth is determined by an interplay between the surface-controlled and volume diffusion-controlled mechanisms.