Semi-crystalline Particles Research Articles

Molecular mobility study of amorphous and crystalline phases of a pharmaceutical product by thermally stimulated current spectrometry

Two crystalline forms and the amorphous state of irbesartan, a pharmaceutical drug chosen as a model, were analyzed by Thermally Stimulated Current (TSC) spectroscopy, a powerful technique currently used in polymer science to investigate the molecular dynamics of heterogeneous and complex materials. Whereas no specific dielectric response was noted for the B crystalline form, the A form of irbesartan exhibited molecular motions localized inside its channel structure. The dynamics involved in the dielectric glass transition of amorphous samples followed a compensation law characteristic of highly cooperative relaxation processes. Concerning the amorphous content in physical mixtures, a calibration curve and a limit of detection (2.5%) were established. The limit of detection could be improved by optimizing the TSC experimental parameters. The amorphous sample recrystallized at a single temperature was interpreted by the “idealized one‐state model” defined here to describe systems composed of identical semicrystalline particles in which amorphous and crystalline phases are independent of each other (i.e., no chemical and physical interaction between the two phases). Therefore, the idealized one‐state model may be simulated by a two‐state model, which is representative of the two‐phase model. Other samples recrystallized through a complex annealing stage were explained by the classical one‐state model in agreement with the three‐phase model used to describe bulk semicrystalline systems. These results demonstrate that, as for polymers, the semicrystalline state of pharmaceutical drugs should not be considered as a single state but as a more complex system that can be described as an idealized one‐state model or a one‐state model depending on the applied thermal treatment. These results give a new view that should be taken into account in the development of amorphous pharmaceutical drugs and formulations.

The Effect of Dry Grinding on Antigorite from Mulhacen, Spain

AbstractAlteration of the crystal structure of Mulhacén antigorite caused by dry, vibration grinding was investigated by X-ray diffraction (XRD), infrared spectroscopy (IR), thermal analyses (TG), grain-size distribution, and transmission and analytical electron microscopy (TEM, AEM). Grinding for 1 min reduces particles to a size ideal for IR and TG. With prolonged grinding, XRD and electron diffraction patterns showed that the crystal structure was affected mainly along the c axis, causing a partial loss of crystallinity. TG analyses revealed that vibration grinding modified mineral dehydration, accelerating the dehydroxylation process and transforming the structural OH to adsorbed water in the resulting matrix. IR spectra and AEM showed that grinding affected the tetrahedral sheet to a lesser extent than the octahedral sheet. Partial release of Mg by preferential destruction of the octahedral sheet after 10 min grinding produced an increase in the Si/Mg ratio in semi-crystalline particles, whereas the amorphous material product after 120 min showed the same composition as the initial antigorite. TEM and grain-size distribution results revealed that grinding led to a general decrease in particle size at the beginning of the experiment followed by the agglomeration of ultrafine particles as grinding proceeded.

Reaction-induced phase separation in poly(butylene terephthalate)-epoxy systems: 2. Morphologies generated and resulting properties

Abstract Poly(butylene terephthalate) (PBT) was used as a semicrystalline modifier of epoxy-aromatic diamine formulations in concentrations ranging from about 3 wt% to 8 wt%. The epoxy monomer was based on diglycidylether of bisphenol A (DGEBA) and the diamines were either 4,4′-methylenebis [3-chloro 2,6-diethylaniline] (MCDEA) and 4,4′-diaminodiphenylsulfone (DDS). Using conversion-temperature transformation diagrams developed in part 1, thermal cycles were selected to generate different morphologies. In the case of PBT-DGEBA-DDS systems, phase separation in the course of reaction led to a random dispersion of spherical particles (sizes in the range of 1 μm), rich in PBT. Small and wide angle X-ray scattering, carried out in situ , during cure, revealed that the dispersion of spherical particles was produced by a nucleation-growth mechanism and that crystallization took place after phase separation. A completely different morphology, characterized by a distribution of large and irregular semicrystalline particles, was produced by crystallization before reaction. However, both types of morphologies introduced a small increase in the critical stress intensity factor. The main toughening mechanism was crack bridging produced by highly drawn thermoplastic particles. On the other hand, PBT-DGEBA-MCDEA formulations were cured at temperatures high enough to avoid crystallization of PBT during reaction. In this case, the PBT remaining dissolved in the matrix did not introduce any toughening effect.