Hydration and dehydration induced changes in porosity of starch microspheres

Characterization and tuning of the porosity of amorphous starch materials are important for many applications, including controlled release of encapsulated proteins. The porosities of these materials in dry and hydrated states can have different physicochemical origins and properties. Here, porosities of dry crosslinked starch microspheres and their hydration-induced transformations were characterized by small angle X-ray scattering, scanning electron and optical microscopies, thermogravimetric analysis, sorption calorimetry, nitrogen sorption, and helium-pycnometry. The analyses revealed that dry microspheres consist of porous cores with pore diameters below 100 nm and shells which appeared to be denser but contained wider pores (100–300 nm). The outer crust of the microspheres shell is non-porous, which restricts diffusion of nitrogen, water, and ethanol. Partial hydration triggered an irreversible collapse of dry porosity at 12 wt% water. Further hydration resulted in interfacial changes and promoted wet porosity, related to characteristic distances between polymer chains.

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Materials

Non-crosslinked maltodextrin (Glucidex 9) was kindly provided by Roquette (France). Degradable starch microspheres (DSM) of four different types were manufactured and provided in a dry state by Magle Chemoswed (Malmö, Sweden), Table 1. Starch microspheres were produced from acid hydrolyzed potato starch employing a water-in-oil emulsion technique and using epichlorohydrin as a crosslinking agent. Briefly: a solution of epichlorohydrin in toluene was added to an alkaline aqueous solution of hydrolyzed starch. Emulsification was performed at a specific concentration, temperature, duration, and stirring rate to obtain the desired starch droplet size. Once formed, the droplets were fixed by adding the crosslinking agent to the mixture. The same amount of the crosslinking agent per unit mass of starch was used for starch microspheres of types B, C and D. The formed microspheres were then purified through an extensive washing scheme, before being dried under vacuum at elevated temperature. Prior to further analysis, the microspheres were additionally dried at room temperature in vacuum with 3 Å molecular sieves for a 24 h period. DSM-A particles were partly described previously (Wojtasz et al., 2016). The degradable starch microspheres of types B, C and D differ from DSM-A primarily in the degree of cross-linking. The cross-linking degree in DSM-A particles, unlike in DSM-B, DSM-C and DSM-D, was found to be insufficient to provide adequate robustness to enable the microspheres to maintain their integrity and recover their initial state after a hydration-drying cycle (Wojtasz et al., 2016).

Ramūnas Digaitis, Peter Falkman, Viveca Oltner, Lars-Erik Briggner, Vitaly Kocherbitov,
Hydration and dehydration induced changes in porosity of starch microspheres,
Carbohydrate Polymers, 2022, 119542, ISSN 0144-8617,
https://doi.org/10.1016/j.carbpol.2022.119542.