Annika Rautenberg, José Ignacio Vázquez-Olvera, Paul Bühlbecker, Alf Lamprecht
Abstract
Hypothesis: During spray freeze drying (SFD), vitrification, recrystallization, and polymorphic transitions govern particle morphology and mechanical integrity. We hypothesize that with control over freezing temperature, annealing relative to the glass transition of the maximally freeze-concentrated matrix (Tg’), and drying temperature it is possible to modulate ice growth, molecular mobility, and solid-state transformations, thereby enabling the production of advanced materials via freeze-casting droplets with designable properties.
Experiments: A full-factorial design was conducted using lactose, mannitol, and trehalose. Formulations were frozen at temperatures above or below water’s glass transition, annealed under four conditions (no annealing, annealing above or below Tg’, and extended times), and dried either near or above Tg’. Resulting powders were analysed regarding ice formation mechanism, solid state composition and morphological behaviour.
Findings: Mannitol displayed pronounced process-dependent polymorphism, which is driven by annealing and temperature during sublimation. Lactose and trehalose remained amorphous throughout. Annealing above Tg’ promoted viscous flow within the vitrified matrices, producing partial particle fusion and reduced mechanical stability. Variations in recrystallization and Ostwald ripening during annealing systematically altered surface area and pore architecture. This connects annealing to sintering mechanisms, electrical conductivity and mechanical stability of micrometre sized spheres. Across all conditions, SFD consistently yielded spherical, low-density particles, yet the interplay of vitrification and phase transitions generated distinct microstructural outcomes.
Keywords:
Freeze-casting; Polymorphs, annealing; Powder design; Spray-freeze-drying.