No abstract

Exploring the chance to convert biowaste into a valuable resource, this study tests the potential role of humic acids (HA), a class of multifunctional compounds obtained by oxidative decomposition of biomass, as physical agents to improve gelatin’s mechanical and thermal properties. To this purpose, gelatin–HA aqueous samples were prepared at increasing HA content. HA/gelatin concentrations changed in the range 2.67–26.67 (wt/wt)%. Multiple techniques were employed to assess the influence of HA content on the gel properties and to unveil the underlying mechanisms. HAs increased gel strength up to a concentration of 13.33 (wt/wt)% and led to a weaker gel at higher concentrations. FT-IR and DSC results proved that HAs can establish noncovalent interactions through H-bonding with gelatin. Coagulation phenomena occur because of HA–gelatin interactions, and at concentrations greater than 13.33 (wt/wt)%, HAs established preferential bonds with water molecules, preventing them from coordinating with gelatin chains. These features were accompanied by a change in the secondary structure of gelatin, which lost the triple helix structure and exhibited an increase in the random coil conformation. Besides, higher HA weight content caused swelling phenomena due to HA water absorption, contributing to a weaker gel. The current findings may be useful to enable a better control of gelatin structures modified with composted biowaste, extending their exploitation for a large set of technological applications.

We investigated the morphological transitions of aqueous solutions of Pluronic F68 induced by temperature and polymer concentration by means of rheological and x-ray measurements. We adopted a rheological method to evaluate the characteristic equilibrium temperatures associated with the transition from unimers to spherical micelles and from disordered spherical micelles to a body-centered cubic phase. We used the transition temperatures to build the phase diagram water/F68. Based on a paracrystalline model for hard spheres, we extracted the characteristic size of the micellar core as a function of temperature from small-angle x-ray scattering (SAXS) data. We compared the microstructural information obtained via SAXS with the rheological response, and we developed a consistent link between the microstructural evolution of the system and the macroscopic flow properties.

Exploring opportunities for biowaste valorization, herein, humic substances (HS) were combined with gelatin, a hydrophilic biocompatible and bioavailable polymer, to obtain 3D hydrogels. Hybrid gels (Gel HS) were prepared at different HS contents, exploiting physical or chemical cross-linking, through 1-ethyl-(3-3-dimethylaminopropyl)carbodiimide (EDC) chemistry, between HS and gelatin. Physicochemical features were assessed through rheological measurements, X-ray diffraction, attenuated total reflectance (ATR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and scanning electron microscopy (SEM). ATR and NMR spectroscopies suggested the formation of an amide bond between HS and Gel via EDC chemistry. In addition, antioxidant and antimicrobial features toward both Gram(−) and Gram(+) strains were evaluated. HS confers great antioxidant and widespread antibiotic performance to the whole gel. Furthermore, the chemical cross-linking affects the viscoelastic behavior, crystalline structures, water uptake, and functional performance and produces a marked improvement of biocide action.

Gelatin gels are known to be nonequilibrium systems, because of the continuous growth and rearrangements of physical junctions, even in the solidlike state. Establishing a relationship between the relative degree of cross-linking and macroscopic elasticity would be crucial in understanding, modeling, and predicting the transformation processes of gelatin solutions. Performing rheological experiments on a distinct gel structure, with a definite number of cross-links, is, however, a challenging task. In isothermal conditions, indeed, the density of physical cross-links changes indefinitely, and network evolution cannot be arrested. Inspired by the inverse quenching technique applied in the past to semicrystalline polymers, we here apply an unusual thermal history to an aqueous solution of gelatin in the semiconcentrated regime (6.67%w pig-skin gelatin), in order to freeze the system in a metastable condition for a time sufficiently long to perform a rheological characterization. The solution, initially kept in the sol state at 60°C, is rapidly cooled below gelation temperature, and isothermal gelation is started at 10°C. After soaking at this low temperature for a given time, the sample is rapidly heated (inverse quenching) up to a value in the range 24–29 °C, where kinetics is monitored. If the waiting time at low temperature and the inverse quenching temperature are suitably chosen, sample elasticity will remain stationary for a relatively large time window, and rheological experiments can then be reliably performed.