Filtern
Erscheinungsjahr
Dokumenttyp
- Zeitschriftenartikel (18)
Sprache
- Englisch (18)
Schlagworte
- Polymorphism (18) (entfernen)
Organisationseinheit der BAM
The unusually fast crystallization of the organic compound nifedipine has been investigated in detail using different solutions as precursors for the formation of the initial glassy nifedipine films. Starting from these amorphous films the crystallization process was investigated by time-resolved light microscopy at different temperatures around Tg of nifedipine. In all studied cases (acetone, acetonitrile, ethyl acetate, dichloromethane, tetrahydrofuran) the measured mean crystallization rates are, on average, 10000 times higher than rates reported in previous studies. Such high rates cannot be explained by a classical diffusional crystal growth mechanism of nifedipine. Instead, nifedipine shows a strong propensity for diffusionless growth. Solvent assisted or solvent induced preordering of the glassy modification is suggested to be the most probable driving force behind these fast crystallization processes. The nifedipine crystallization rates can be controlled not only by specific solvent–molecule interactions but also by temperature. Below 40 °C, the temperature dependence of the rates is generally small. Above 40 °C, a slowing down of the crystallization rates with increasing temperatures indicates a kinetic competition between different polymorphs. The molecular dynamics of nifedipine molecules in the amorphous phase as well as the relaxation times are studied by dielectric measurements. The dielectric data corroborate the inherently strong propensity of nifedipine to crystallize diffusionlessly under all conditions.
Quinaldine (2-methyl quinoline) is a liquid at room temperature, which can be supercooled to reach finally the glassy state. By heating the glass above the glass transition temperature Tg = 180 K the sample performs two subsequent transitions into, likewise, dielectrically active phases. Thus, the reorientational relaxations of these phases as well as the kinetics of the phase transitions can be tracked in a highly resolved way by dielectric spectroscopy. X-ray diffraction analysis clearly shows two structurally different crystalline phases in addition to the supercooled liquid. Calorimetric measurements support the notion of first order phase transitions, occurring irreversibly in the supercooled regime, and suggest that the intermediate crystalline phase is metastable, too. Analyzing the quite distinct dielectric relaxation strengths, we discuss the possible nature of the two crystalline phases. Additionally, a very similar behavior to quinaldine is observed for 3-methyl quinoline, indicating a broad field of polymorphism among the quinoline derivatives.
An unusually fast crystallization of the organic glass former nifedipine has been observed. The crystallization process, starting from an amorphous film to crystalline material, was investigated by time resolved Raman microspectroscopy. The crystallization rates of the initially crystallizing metastable β-form are four orders of magnitude higher than those of previous studies.
Crystallization processes under different conditions are of fundamental interest in chemistry, pharmacy, and medicine. Therefore, we have studied the formation of micro- and nanosized crystals using water-caffeine (1,3,7-trimethyl-1H-purine-2,6(3H,7H)-dione) solutions under ambient conditions as a relevant model system. When droplets of an aqueous caffeine solution evaporate and eventually dry on surfaces (glass, polystyrene, and polyester), stable coffee tabletop rings with a perimeter of typically 3 mm are formed after 20 to 50 min. Using a micro focus X-ray beam available at the BESSY µSpot-beamline, the fine structure of different caffeine needles can be distinguished. Unexpectedly, both crystal modifications (α- and β-caffeine) are present, but locally separated in these rings. Furthermore, AFM studies reveal the presence of even smaller particles on a nanometer length scale. To eliminate influences of surface irregularities from the crystallization process, acoustic levitation of liquid samples was employed. Such levitated droplets are trapped in a stable position and only surrounded by air. The solvent in an ultrasonically levitated drop evaporates completely, and the resulting crystallization of caffeine was followed in situ by synchrotron X-ray diffraction. In this case, the diffraction pattern is in accordance with pure α-caffeine and does not indicate the formation of the room temperature polymorph β-caffeine. Hence, our investigations open new vistas that may lead to a controlled formation of cocrystals and novel polymorphs of micro- and nanocrystalline materials, which are of relevance for fundamental studies as well as for pharmaceutical and medical applications.
Poly(ester urethane) with varying polyester chain length: Polymorphism and shape-memory behavior
(2013)
The swelling, viscoelastic, and mechanical behavior of phase-segregated poly(ester urethane) (PEU) block copolymers, composed of 4,4'-methylenediphenyl diisocyanate, 1,4-butanediol as a chain extender, and crystallizable poly(1,4-butylene adipate) (PBA) with molecular weights between 1330 and 4120 g mol-1, are investigated. Wide-angle X-ray scattering (WAXS) is employed to study the overall PEU crystallinity, which increases from 8.6 to 13.6% at higher PBA contents. The existence of two crystalline, polymorphic PBA phases, a thermodynamically stable α phase and a metastable β phase, is confirmed by further WAXS measurements. Calorimetric and thermomechanical investigations give evidence for controllable PBA polymorphic behavior. The crystallization conditions, like the cooling rate, affect the emerging polymorphic mixture, whereas the storage conditions either promote or inhibit the polymorphic (β to α) transition. The introduced concepts represent a new approach for gaining control over programmable thermoresponsiveness, which may be transferable to other shape-memory polymers with polymorphic switching segments.
Mechanochemistry offers a unique opportunity to modify and manipulate crystal forms, often providing new products as compared with conventional solution methods. While promising, there is little known about how to control the solid form through mechanochemical means, demanding dedicated investigations. Using a model organic cocrystal system (isonicotinamide:glutaric acid), we here demonstrate that with mechanochemistry, polymorphism can be induced in molecular solids under conditions seemingly different to their conventional thermodynamic (thermal) transition point. Whereas Form II converts to Form I upon heating to 363 K, the same transition can be initiated under ball milling conditions at markedly lower temperatures (348 K). Our results indicate that mechanochemical techniques can help to reduce the energy barriers to solid form transitions, offering new insights into controlling polymorphic forms. Moreover, our results suggest that the nature of mechanochemical transformations could make it difficult to interpret mechanochemical solid form landscapes using conventional equilibrium-based tools.
Carbamazepine (CBZ) is known for its variety of anhydrous and hydrous polymorphs. Herein, a thorough analysis of the crystallization behavior of CBZ is presented. The influence of the solvent and the concentration on the crystallization behavior under different environmental conditions is investigated by combined in situ XRD and Raman spectroscopy measurements. Crystallization studies conducted via conventional crystallization in glass tubes and in levitated droplets using an acoustic levitator indicate a dependence of the crystallization process from solvent and surface.
A combination of two analytical methods, time-resolved X-ray diffraction (XRD) and Raman spectroscopy, is presented as a novel tool for crystallization studies. An acoustic levitator was employed as sample environment. This setup enables the acquisition of XRD and Raman data in situ simultaneously within a 20 s period and hence permits investigation of polymorphic phase transitions during the crystallization process in different solvents (methanol, ethanol, acetone, dichloromethane, acetonitrile). These real time measurements allow the determination of the phase content from the onset of the first crystalline molecular assemblies to the stable system. To evaluate the capability of this approach, the setup was applied to elucidate the crystallization process of the polymorphic compound nifedipine. The results indicate the existence of solvent-dependent transient phases during the crystallization process. The quality of the data allowed the assignment of the lattice constants of the hitherto unknown crystal structure of the β-polymorph.
The crystallization of nifedipine was studied by means of synchrotron-X-ray diffraction, single-crystal X-ray structural analysis, and Raman spectroscopy. The results of slow evaporation (24 h in minimum) using dimethyl sulfoxide (DMSO) are presented. Since fast crystallization processes (typically minutes) in different solvents always led to the final formation of the thermodynamically most stable α-polymorph of nifedipine, we observed a novel pseudo-polymorph due to slow crystallization from DMSO. The single-crystal X-ray structure of the solvated species nifedipine·DMSO (1:1) is reported for the first time. In addition, the crystallization process on surfaces was followed by means of light microscopy and environmental scanning electron microscopy (ESEM) coupled with energy-dispersive X-ray spectroscopy (EDS) analysis. Different diffractions pattern and Raman spectra were observed for crystals grown from stock solution and those obtained by drying the solution on soda lime silicate surfaces.
The transformation of amorphous precursors into crystalline solids and the associated mechanisms are still poorly understood. We illuminate the formation and reactivity of an amorphous cobalt phosphate hydrate precursor and the role of water for its crystallization process. Amorphous cobalt phosphate hydrate nanoparticles (ACP) with diameters of ∼20 nm were prepared in the absence of additives from aqueous solutions at low concentrations and with short reaction times. To avoid the kinetically controlled transformation of metastable ACP into crystalline Co₃(PO₄)₂ × 8H₂O (CPO) its separation must be fast. The crystallinity of ACP could be controlled through the temperature during precipitation. A second amorphous phase (HT-ACP) containing less water and anhydrous Co₃(PO₄)₂ was formed at higher temperature by the release of coordinating water. ACP contains approximately five molecules of structural water per formula unit as determined by thermal analysis (TGA) and quantitative IR spectroscopy. The Co²+ coordination in ACP is tetrahedral, as shown by XANES/EXAFS spectroscopy, but octahedral in crystalline CPO. ACP is stable in the absence of water even at 500 °C. In the wet state, the transformation of ACP to CPO is triggered by the diffusion and incorporation of water into the structure. Quantitative in situ IR analysis allowed monitoring the crystallization kinetics of ACP in the presence of water.