Morphology Control Of Organic Crystals By Macromolecular Additives And The Mechanism

Crystal morphology is one of the important evaluation indexes of the crystallization process,which significantly affects and even determines the product quality,downstream process efficiency,and end-use efficacy of organic crystal.Additive has been widely studied and applied as one of important methods of controlling crystal morphology in crystallization process.Macromolecular additives have more advantages in morphology modification than small-molecule additives,because of their adjustable molecular weight,abundant interaction sites,and steric effect.In this study,three organic compounds,ethyl vanillin(EVA),nitroguanidine(NQ),and imidacloprid(IMI),were studied to explore the influence mechanism of macromolecular additives on the process of nucleation and crystal growth.Based on this,the morphology control of spherulite and ultrafine crystal in solution was studied.Firstly,according to the classical nucleation theory,the effects of cooling rate,saturation temperature,solvents,and additives on the nucleation kinetics in cooling crystallization of EVA and NQ were investigated by using the modified Sangwal’s model.It was found that the nucleation kinetics depends on the balance between the saturation temperature and crystallization driving force.The dependence of EVA nucleation rate on the solid-liquid interfacial energy of crystal was emphasized.Combined with molecular simulation,it was proved that the change of interface energy with crystallization driving force was due to the solvent effect on the crystal faces.The additive can significantly inhibit nucleation without affecting the thermodynamic solubility.The increase of interfacial energy indicates that the additive inhibits nucleation mainly by adsorbing on the crystal surfaces.Besides,gelatin had the strongest inhibition effect on the nucleation of NQ.Secondly,the influence mechanism of Gelatin,polyvinylpyrrolidone,and hydroxyethylcellulose on the growth process of NQ crystal was further investigated.The molecular dynamics simulation revealed that the asymmetric growth behavior of NQ crystal in the aqueous solution should be attributed to the different interaction strength of the solvent molecules with different crystal faces.The model of additivecrystal face interaction showed that the three additives adsorbed on the crystal faces by van der Waals interaction thus inhibited the diffusion of solute molecules into crystal faces.The inhibitory effect of additives on crystal growth is proportional to the strength of interaction between additive and crystal face and additive concentration,and also is influenced by solvent.Based on the autodeformation mechanism of crystal growth,it was revealed that the noncrystallographic branching was induced by the defects formed by the incorporation of gelatin into the NQ crystal lattice.Then,in order to clarify the spherulitic growth mechanism to better control the spherulite morphology,based on the advantage of gelatin in nucleation inhibition and noncrystallographic branching induction of NQ crystal,the effects of gelatin concentration,supersaturation,and temperature on morphology evolution and microstructure of NQ spherulites in cooling crystallization were investigated.The mechanism of coexistence and transformation between the central multidirectional growth and unidirectional growth of spherulite growth in solution was proposed,while the essential connection between spherulitic growth and the growth symmetry of crystallite at the nucleation stage was established.The bulk density of the spherulitic products increased by more than 3 times.Finally,aiming at improving the broad crystal size distribution(CSD),long-time consuming and intensive energy consumption in the preparation process of ultrafine crystals by physical grinding,the strategy of preparing IMI ultrafine crystal by antisolvent crystallization was designed.In order to obtain the maximum supersaturation,the optimal antisolvent(water)-solvent(DMSO)ratio was determined.Based on the analysis of crystal structure,PVA-2488 was selected as an additive,and its mechanism was revealed by molecular simulation.Multi-factor orthogonal experiments were designed to prove that supersaturation and additive concentration were the main factors affecting CSD and dispersion of the products,respectively.The fusiform-flake-shaped ultrafine crystals of IMI with uniform particle size and good dispersity were successfully prepared by ultrasonication.