Crystallization of bifonazole and acetaminophen within the matrix of semicrystalline, PEO-PPO-PEO triblock copolymers.

The morphology and microstructure of crystalline drug/polymer solid dispersions could influence their physical stability and dissolution performance. In this study, the drug crystallization mechanism within PEG, PPG, and poloxamer matrix was investigated, and the resultant microstructure of various solid dispersions of acetaminophen (ACM) and bifonazole (BFZ) in the aforementioned polymers was characterized by differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide/small-angle X-ray diffraction (WAXD/SAXS). With a stronger molecular interaction with the PEG segments, ACM decreased the crystallization onset temperature and crystallinity of PEG and poloxamers much more than BFZ. The stronger molecular interaction and better miscibility between ACM and PEG also induced a more defective lamellar structure in the ACM solid dispersions compared with that in the BFZ systems, as revealed by DSC and SAXS investigation. Observed under polarized optical microscopy, PEG, PPG, and poloxamer could all significantly improve the crystallization rate of ACM and BFZ, because of the largely reduced Tg of the solid dispersions by these low Tg polymers. Moreover, when the drug loading was below 60%, crystallization of BFZ in PEG or poloxamer occurred preferably along the radial direction of PEG spherulite, rather than the perpendicular direction, which was attributed to the geometric restriction of well-ordered polymer lamellar structure in the BFZ solid dispersions. Similar phenomena were not observed in the ACM solid dispersions regardless of the drug loading, presumably because ACM could diffuse freely across the perpendicular direction of the PEG spherulite, through the well-connected interlamellar or interfibrillar spaces produced by the defective PEG lamellar structure. The different drug-polymer interaction also caused a difference in the microstructure of polymer crystal, as well as a difference in drug distribution within the polymer matrix, which then synergistically facilitated a "confined crystallization" process to reduce the drug crystallite size below 100 nm.