Origin and influence of water-induced chain relaxation phenomena in chitosan biopolymers

Effect of solvent acids on the microstructure and corrosion resistance of chitosan films on MAO-treated AZ31B magnesium alloy

This study evaluated the effect of solvent acids on the structure and corrosion resistance performance of chitosan (CS) film on MAO-treated AZ31B magnesium (Mg) alloy. Initially, CS solutions were prepared in four solvent acids: acetic acid (HAc), lactic acid (LA), hydrochloric acid (HCl), and citric acid (CA). The CS films were subsequently deposited on MAO-treated AZ31B Mg alloy via a dip-coating technique. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction analysis (XRD), Fourier-transform infrared spectroscopy (FT-IR), contact angle measurement, and atomic force microscopy (AFM) were employed to characterize the surface and cross-sectional morphology as well as chemical composition. Furthermore, the samples were subjected to potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) tests to assess their resistance against corrosion in simulated body fluid (SBF). These results indicated that the CS film prepared with LA exhibited the lowest surface roughness (Ra = 31.2 nm), the largest contact angle (CA = 98.50°), and the thickest coating (36 μm). Additionally, it demonstrated superior corrosion protection performance, with the lowest corrosion current density (Icorr = 3.343 × 10-7 A/cm2), highest corrosion potential (Ecorr = -1.49 V), and highest polarization resistance (Rp = 5.914 × 104 Ω·cm2) in SBF. These results indicated that solvent acid types significantly influenced their interactions with CS. Thus, the structure and corrosion protection performance of CS films can be optimized by selecting an appropriate solvent acid.

Chiral Nanostructured Glycerohydrogel Sol-Gel Plates of Chitosan L- and D-Aspartate: Supramolecular Ordering and Optical Properties

A comprehensive study was performed on the supramolecular ordering and optical properties of thin nanostructured glycerohydrogel sol-gel plates based on chitosan L- and D-aspartate and their individual components in the X-ray, UV, visible, and IR ranges. Our comparative analysis of chiroptical characteristics, optical collimated transmittance, the average cosine of the scattering angle, microrelief and surface asymmetry, and the level of structuring shows a significant influence of the wavelength range of electromagnetic radiation and the enantiomeric form of aspartic acid on the functional characteristics of the sol-gel materials. At the macrolevel of the supramolecular organization, a complex topography of the surface layer and a dense amorphous-crystalline ordering of polymeric substances were revealed, while at the nanolevel, there were two forms of voluminous scattering domains: nanospheres with diameters of 60-120 nm (L-) and 45-55 nm (D-), anisometric particles of lengths within ~100-160 (L-) and ~85-125 nm (D-), and widths within ~10-20 (L-) and ~20-30 nm (D-). The effect of optical clearing on glass coated with a thin layer of chitosan L-(D-)aspartate in the near-UV region was discovered (observed for the first time for chitosan-based materials). The resulting nanocomposite shape-stable glycerohydrogels seem promising for sensorics and photonics.

Formation, structure, properties of chitosan aspartate and metastable state of its solutions for obtaining nanoparticles

Chitosan (200 kDa) dissolution in an aqueous solution of L-aspartic acid, physicochemical properties and features of the resulting chitosan salt were studied by conductometry, potentiometry, viscometry, turbidimetry, IR and NMR spectroscopy, and X-ray diffractometry. Chitosan aspartate is a water-soluble hydrated polymorph exhibiting properties of a cationic polyelectrolyte with an effective macromolecular coil radius 60-75 nm. The specific conductivity, dielectric constant, viscosity and pH of the chitosan - L-aspartic acid - water system change over time after preparation due to counterion-polycation association to form ion pairs, multiplet structures, and their subsequent aggregation. As a result, nanoparticles (40-90 nm) are formed after ~24 h, microparticles (0.6-1.4 μm) are after ~48 h, and precipitation occurs after 72-96 h. The precipitated phase is a water-insoluble chitosan salt with a developed system of H-bonds and high crystallinity degree. Chitosan nanoparticles have high biocompatibility and the ability to accelerate the proliferative activity of epithelial cells. HYPOTHESIS: Ion pairs and multiplets are formed in the chitosan - L-aspartic acid - water system due to counterion association, which leads to phase segregation of the polymer substance at the level of nanoparticles and microparticles.

Polydopamine-assisted grafting of chitosan on porous poly (L-lactic acid) electrospun membranes for adsorption of heavy metal ions

In this study, a versatile method for the manufacturing of chitosan-grafted porous poly (L-lactic acid) (P-PLLA) nanofibrous membrane by using polydopamine (PDA) as an intermediate layer has been developed. P-PLLA fibres were electrospun and collected as nano/micro fibrous membranes. Highly porous fibres could serve as a substrate for chitosan to adsorb heavy metal ions. Moreover, PDA was used to modify P-PLLA surface to increase the coating uniformity and stability of chitosan. Due to the very high surface area of P-PLLA membranes and abundant amine groups of both PDA and chitosan, the fabricated membranes were utilized as adsorbent for removal of copper (Cu²⁺) ions from the wastewater. The adsorption capability of Cu²⁺ ions was examined with respect to the PDA polymerization times, pH, initial metal ion concentration and time. Finally, the equilibrium adsorption data of chitosan-grafted membranes fitted well with the Langmuir isotherm with the maximum adsorption capacity of 270.27 mg/g.

Optical activity anisotropy of chitosan-based films

The optical activity [α] of films of chitosan and chitosan acetate was studied. [α] depended on the orientation angle θ of the film sample around the light beam. The angular dependences [α] = f(θ) (indicatrices) were processed to extract the constant term [α]₀and four harmonics ([α]_i, Fourier's series) determined by structure elements with the corresponding symmetry: the amorphous (isotropic) phase of chitosan ([α]₀), irregular-shaped structures ([α]₁), rod-shaped ones in the film plane ([α]₂), helical ones located perpendicular to the film surface ([α]₃), and crystalline structures ([α]₄). The contribution of individual supramolecular structures to the overall optical activity of chitosan was estimated: [α]₂ and [α]₄ predominated for the fresh chitosan acetate films, while [α]₄ did for the chitosan ones. The results of the numerical decomposition of indicatrices are in satisfactory agreement with X-ray diffractometry data. The observed regularities predetermine prospects for obtaining stereoselective chiro-optic chitosan-based materials with novel functional properties.

Rheological and structural properties of complex arabinoxylans from Plantago ovata seed mucilage under non-gelled conditions

Two gel-forming arabinoxylan (AX) fractions with very similar linkage composition and molecular weight distributions have been isolated from Plantago ovata seed mucilage. Both isolated fractions have distinct gel properties attributed to differences in intermolecular hydrogen bonding. This study probes the effect of hydrogen bonding on molecular interactions of P. ovata AX fractions under non-gelled conditions achieved using a mild hydrogen bonding inhibitor, 0.2 M KOH. Chain conformation, relaxation dynamics and interactions between AX molecules are investigated using a combination of rheological techniques and small angle X-ray and neutron scattering. The scattering data confirm similar molecular dimensions and chain rigidity for both fractions, while showing distinct patterns of molecular interactions which result in the formation of a self-associated polymer network. The relationship between molecular associations, chain relaxation time and entanglement behaviour of P. ovata AX solutions are corroborated though the analysis of flow profiles and small amplitude oscillatory shear rheology.

Novel gigahertz frequency dielectric relaxations in chitosan films

Molecular relaxations of chitosan films have been investigated in the wide frequency range of 0.1 to 3 × 10(9) Hz from -10 °C to 110 °C using dielectric spectroscopy. For the first time, two high-frequency relaxation processes (in the range 10(8) to 3 × 10(9) Hz) are reported in addition to the low frequency relaxations α and β. These two relaxation processes are related to the vibrations of OH and NH2/NH3(+), respectively. The high-frequency relaxations exhibit Arrhenius-type dependencies in the temperature range 10 °C to 54 °C with negative activation energy; this observation is traceable to hydrogen bonding reorientation. At temperatures above the glass transition temperature (54 °C), the activation energy changes from negative to positive values due to breaking of hydrogen bonding and water loss. Upon cooling in a sealed environment, the activation energies of two relaxation processes are nearly zero. FTIR and XRD analyses reveal associated structural changes upon heating and cooling. These two new high-frequency relaxation processes can be attributed to the interaction of bound water with OH and NH2/NH3(+), respectively. A plausible scenario for these high-frequency relaxations is discussed in light of impedance spectroscopy, TGA, FTIR and XRD measurements.