Glass Transition Kinetics and Physical Aging of Polyvinylpyrrolidones with Different Molecular Masses

Stability of Rapidly Crystallizing Sulfonamides Glasses by Fast Scanning Calorimetry: Crystallization Kinetics and Glass-Forming Ability

Production and evaluation of the kinetic stability of the amorphous forms of active pharmaceutical ingredients are among the current challenges of modern pharmaceutical science. In the present work, amorphous forms of several sulfonamides were produced for the first time using Fast Scanning calorimetry. The parameters, characterizing the glass-forming ability of the compounds, i.e. the critical cooling rate of the melt and the kinetic fragility, were determined. The cold crystallization kinetics was studied using both isothermal and non-isothermal approaches. The results of the present study will contribute to the development of approaches for producing amorphous forms of rapidly crystallizing active pharmaceutical ingredients.

Predicting self-diffusion coefficients in semi-crystalline and amorphous solid dispersions using free volume theory

The self-diffusion coefficient of active ingredients (AI) in polymeric solid dispersions is one of the essential parameters for the rational formulation design in life sciences. Measuring this parameter for products in their application temperature range can, however, be difficult to realise and time-consuming (due to the slow kinetics of diffusion). The aim of this study is to present a simple and time-saving platform for predicting the AI self-diffusivity in amorphous and semi-crystalline polymers on the basis of a modified version of Vrentas' and Duda's free volume theory (FVT) [A. Mansuri, M. Völkel, T. Feuerbach, J. Winck, A.W.P. Vermeer, W. Hoheisel, M. Thommes, Modified free volume theory for self-diffusion of small molecules in amorphous polymers, Macromolecules. (2023)]. The predictive model discussed in this work requires pure-component properties as its input and covers the approximate temperature range of T < 1.2 Tg, the whole compositional range of the binary mixtures (as long as a molecular mixture is present), and the whole crystallinity range of the polymer. In this context, the self-diffusion coefficients of the AIs imidacloprid, indomethacin, and deltamethrin were predicted in polyvinylpyrrolidone, polyvinylpyrrolidone/vinyl acetate, polystyrene, polyethylene, and polypropylene. The results highlight the profound importance of the kinetic fragility of the solid dispersion on the molecular migration; a property which in some cases might entail higher self-diffusion coefficients despite an increase in the molecular weight of the polymer. We interpret this observation within the context of the theory of heterogeneous dynamics in glass-formers [M.D. Ediger, Spatially heterogeneous dynamics in supercooled liquids, Annu. Rev. Phys. Chem. 51 (2000) 99-128] by attributing it to the stronger presence of "fluid-like" mobile regions in fragile polymers offering facilitated routes for the AI diffusion within the dispersion. The modified FVT further allows for identifying the influence of some structural and thermophysical material properties on the translational mobility of AIs in binary dispersions with polymers. In addition, estimates of self-diffusivity in semi-crystalline polymers are provided by further accounting for the tortuosity of the diffusion paths and the chain immobilisation at the interface of the amorphous and crystalline phases.

The Glass Transition Temperature of Heterogeneous Biopolymer Systems

Biopolymers are abundant, renewable, and biodegradable resources. However, bio-based materials often require toughening additives, like (co)polymers or small plasticizing molecules. Plasticization is monitored via the glass transition temperature versus diluent content. To describe this, several thermodynamic models exist; nevertheless, most expressions are phenomenological and lead to over-parametrization. They also fail to describe the influence of sample history and the degree of miscibility via structure-property relationships. We propose a new model to deal with semi-compatible systems: the generalized mean model, which can classify diluent segregation or partitioning. When the constant k_GM is below unity, the addition of plasticizers has hardly any effect, and in some cases, even anti-plasticization is observed. On the other hand, when the k_GM is above unity, the system is highly plasticized even for a small addition of the plasticizer compound, which indicates that the plasticizer locally has a higher concentration. To showcase the model, we studied Na-alginate films with increasing sizes of sugar alcohols. Our k_GM analysis showed that blends have properties that depend on specific polymer interactions and morphological size effects. Finally, we also modeled other plasticized (bio)polymer systems from the literature, concluding that they all tend to have a heterogeneous nature.

Physical aging of hydroxypropyl methylcellulose acetate succinate via enthalpy recovery

Amorphous solid dispersions (ASDs) utilize the kinetic stability of the amorphous state to stabilize drug molecules within a glassy polymer matrix. Therefore, understanding the glassy-state stability of the polymer excipient is critical to ASD design and performance. Here, we investigated the physical aging of hydroxypropyl methylcellulose acetate succinate (HPMCAS), a commonly used polymer in ASD formulations. We found that HPMCAS exhibited conventional physical aging behavior when annealed near the glass transition temperature (T_g). In this scenario, structural recovery was facilitated by α-relaxation dynamics. However, when annealed well below T_g, a sub-α-relaxation process facilitated low-temperature physical aging in HPMCAS. Nevertheless, the physical aging rate exhibited no significant change up to 40 K below T_g, below which it exhibited a near monotonic decrease with decreasing temperature. Finally, infrared spectroscopy was employed to assess any effect of physical aging on the chemical structure of HPMCAS, which is known to be susceptible to degradation at temperatures 30 K above its T_g. Our results provide critical insights necessary to understand better the link between the stability of ASDs and physical aging of the glassy polymer matrix.