The effect of ionic crosslinking on thermal properties of hydrogel chitosan membranes

Ionically crosslinked one‑component chitosan (Ch), two‑component chitosan/sodium alginate (Ch/NaAlg) and chitosan/pentasodium tripolyphosphate (Ch/TPP) membranes as well as three‑component chitosan/sodium alginate/pentasodium tripolyphosphate (Ch/NaAlg/TPP) membranes were prepared. Formation of ionic interactions between Ch and NaAlg and/or TPP was confirmed by FTIR analysis. An effect of a crosslinking process on thermal properties of hydrogel membranes was examined using TG/DTG/DTA and DSC methods. Changes in the number of thermal degradation steps of the studied hydrogel membranes were observed. Considering the temperature at which thermal degradation starts as a criterion of the thermal stability, it was concluded that the thermal stability increases according to the series: Ch/NaAlg ≈ Ch/NaAlg/TPP ≈ Ch/TP

Influence of crosslinking process conditions on molecular and supermolecular structure of chitosan hydrogel membrane

Unmodified (Ch) and ionically crosslinked with sodium tripolyphosphate (Ch/TPP) chitosan membranes were prepared. Various crosslinking conditions (pH, crosslinking time) were applied. Differences in membrane molecular structure was examined using FTIR spectroscopy. Scanning electron microscopy (SEM) coupled with Energy Dispersive X-Ray (EDX) Spectrometer as well as atomic force microscopy (AFM) were used for estimation of an effect of crosslinking conditions on supermolecular structure of chitosan membrane. Strong effect of pH of crosslinking TPP solution on crosslinking agent distribution inside the membranes and roughness of membrane surface was found. Differences in membrane morphology prior and after crosslinking results from differences in crosslinking density

Mechanism of water diffusion into noncrosslinked and ionically crosslinked chitosan membranes

Ionically crosslinked chitosan membranes were prepared and their swelling behaviour was analysed. Low‑molecular pentasodium tripolyphosphate (TPP) and/or high‑molecular sodium alginate (NaAlg) were used as ionic crosslinkers. Dynamic swelling of noncrosslinked (Ch) as well as singly (Ch/TPP, Ch/NaAlg) and doubly (Ch/NaAlg/TPP) crosslinked chitosan membranes was performed in buffered aqueous solutions of various pH to determine the mechanism of water diffusion into these hydrogels. It was stated out that mechanism of water diffusion into studied chitosan hydrogel membranes depends both on pH of swelling solution as well as on the type of membrane (type of crosslinker). The water transport mechanism was determined to be: i) Fickian or ‘Less Fickian’ diffusion, ii) non‑Fickian diffusion (anomalous diffusion) and iii) Case II or Super Case II diffusion

Structural and swelling properties of hydrogel membranes based on chitosan crosslinked with glutaraldehyde and sodium tripolyphosphate

Chemically and physically crosslinked chitosan hydrogel membranes were obtained by treatment of chitosan (Ch) with glutaraldehyde (GA) and with GA and subsequently with sodium tripolyphosphate (TPP). The formation of covalent and ionic crosslinks between Ch, GA and ionic TPP was confirmed by FTIR spectroscopy. The studies of dynamic swelling behaviour of synthesized membranes in buffer solutions of different pH (1.0, 7.4, 9.0) indicated that the swelling process obeyed a second-order kinetics. Values of an apparent swelling rate constant for Ch/GA and Ch/GA/TPP membranes were of the same order for acidic and neutral swelling media but they increased for alkaline solutions

State of water in noncrosslinked and crosslinked hydrogel chitosan membranes – dsc studies

Modified chitosan hydrogel membranes were prepared using glutaraldehyde (GA) and sodium citrate (NaCIT) as crosslinking agents. Molecular and supermolecular structure analyses of unmodified and modified chitosan membranes have been conducted by FTIR and X-ray spectroscopy. FTIR results showed covalent and ionic crosslinks formation between chitosan (Ch) and GA or simultaneously Ch, GA and NaCIT. The state of water in noncrosslinked and crosslinked chitosan membranes were analysed by differential scanning spectroscopy (DSC). Three types of water in hydrogel membranes were found: non-freezing bound water, freezing bound water and freezing free water, while there were variations in the amount of non-freezing bound water in these polymers. The effect of ionic crosslinking on water state, mainly on the nonfreezing water content, was discussed

Effect of ionic crosslinking on density of hydrogel chitosan membranes

Ionically crosslinked chitosan membranes were prepared and their density was determined using modified pycnometer method. High molecular compound: sodium alginate (NaAlg) and low molecular compounds: pentasodium tripolyphosphate (TPP) and trisodium citrate (CIT) were used as ionic crosslinkers of chitosan. As a reference liquid n-heptane solution was chosen. It was found out that density of modified chitosan membranes is strongly affected by the type of ionic crosslinker and membrane preparation condition

Thermal degradation of double crosslinked hydrogel chitosan membranes

The thermal degradation behaviour of uncrosslinked and crosslinked chitosan membranes were studied by means of dynamic thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) over the temperature range 25-500ºC in nitrogen atmosphere. Modified chitosan membranes were prepared using a crosslinking method based on covalent crosslinking of chitosan with glutaraldehyde and subsequent ionic crosslinking with sodium citrate, sulfuric acid, sulfosuccinic acid and tripolyphosphate, respectively. Chemical structure of modified chitosan membranes before and after their thermal degradation was characterized by FTIR spectroscopy. Both TGA and DSC experiments as well as spectral results (FTIR spectra of thermal degradation residues) indicated some differences in the mechanism of thermal degradation of uncrosslinked and crosslinked chitosan membranes

Application of pervaporation and osmotic membrane distillation to the regeneration of spent solutions from the osmotic food dehydration

Results of pervaporation (PV) of sucrose and calcium chloride spent solutions were presented. Additionally, osmotic membrane distillation (OMD) of sucrose solutions was investigated. It was found that the regeneration of spent sucrose solution for the reuse is possible by using PV or OMD processes. However, OMD process produces another spent stream i.e. CaCl2. Pervaporation membranes showed fluxes in the range of 0.5 - 0.9 kg m^-2 h^-1 in contact with 40° Brix sucrose solution, whereas OMD water permeate fluxes were in the range of 4 - 5 kg m^-2 h^-1 for the same feed concentration. Two different hybrid processes were suggested: i) pretreatment followed by OMD reconcentration of spent sucrose solution and independently PV for CaCl2 regeneration; ii) membrane pretreatment (MP) followed by PV of sucrose solution. Based on the experimental results, the membrane areas for both systems were calculated and compared. MP-PV system seems to be a better solution for the spent mixtures management