1. Olga N. Malinkina*1,2, Anna B. Shipovskaya1,2, Alexander V. Sobolev1
Saratov Fall Meeting 2016
ANALYSIS OF ORGANIC-INORGANIC XEROGELS STRUCTURE BASED ON CHITOSAN AND SILICON TETRAGLYCEROLATES BY SEM
Olga N. Malinkina*1,2, Anna B. Shipovskaya1,2, Alexander V. Sobolev1
 1 - Institute of Chemistry, Saratov State University
2 - Research and Education Institution of Nanostructures and Biosystems,
Saratov State University

2. Abstract
Hybrid xerogels based on chitosan and tetraglycerolate silicon were obtained by cold extraction in combination with the cryotreatment.
Scanning electron microscopy shows that the solid phase of the inorganic-organic xerogels is a matrix of agglomerated particles whose sizes increase with the chitosan concentration in the system.

3. Materials The following reagents were used:
Tetraethoxysilane Si(OEt)4 ("EKOS-1" Ltd., RF)
Glycerol C3H8O3 (VEKTON Ltd., RF)
Chitosan С6Н11О4N (CS) with a viscosity-averaged molecular weight of 32 kDa, a degree of deacetylation of 70 mol.%. (Bioprogress Ltd., RF)
D-Ascorbic acid C6H8O6 (АA) (Base №1 chemical reagents Ltd., RF)
Glycolic acid 70% C2H4O3 (GA) (Sigma-Aldrich, USA)
NaCl (NPO EKROS Ltd., RF)
Ethanol (EtOH)
Acetone
CaCl2
Distilled water
All the chemicals used were of analytical grade or higher.

4. Methods Solid phase preparation
In this study we propose a xerogels preparation process of:
Synthesis of organically modified silicon
Chitosan solution preparation
Organic-inorganic precursor
Polymer matrix
“One pot” SOL-GEL SYNTHESIS
hybrid ORGANIC-INORGANIC hydrogel
Solid phase preparation
EXHAUSTIVE COLD EXTRACTION COMBINED WITH CRYOTREATMENT AND RE-EXTRACTION
hybrid ORGANIC-INORGANIC xerogel

5. Synthesis of organically modified silicon
The synthesis of silicon tetraglycerolates was performed by transesterification of tetraethoxysilane in a polyol excess without a catalyst:
Si(OEt)4 + 6 GlyOH ↔ Si(OGly)4 · 2 GlyOH + 4 EtOH↑
The reaction's completeness was monitored using 1H NMR spectroscopy by no signal of –O–C2H5 within 1.1−2.3 ppm.
Chitosan solution preparation
Solutions of CS with concentrations СCS = 2 and 4 wt.% in AA or GA with concentrations Сacid = 2 (4) % were used. The solutions were prepared by dissolving polymer powder in an aqueous solution of acid. The binary salts of ascorbate (glycolate) chitosan were formed.
Synthesis of chitosan hydrogels
To synthesize hydrogels, chitosan and Si(OGly)4·6C3H8O3 solutions were used in weight ratios from 1:1 to 13:1 with/without a low-molecular-weight accelerator (NaCl powder, 1 wt.%). The component concentrations were controlled gravimetrically and were expressed in wt.%. The mixed solutions were stirred carefully until homogeneity (1-2 min) and kept at 20±2°C or 80±2°C under atmospheric pressure for the sol-gel synthesis. The gel point was fixed by the loss-flow time of the system, using the "inverting the tube" method.

6. Hypothesized scheme of the intra- and intermolecular template-precursor interactions in the sol-gel synthesis of Si(OGly)4 in the presence of CS in the GA (AA) media by covalent and ionic interactions between the functional groups of the polymer and the inorganic network of ≡Si-O-Si≡ bonds
Polymer Template
Polysalt of Chitosan
Precursor
Hybrid xerogel
Hybrid hydrogel
Evaporation
exhaustive cold extraction combined with cryotreatment and re-extraction

7. Preparation of the xerogel
The preparation of the xerogel (solid phase) was performed by exhaustive cold extraction combined with cryotreatment and re-extraction. Samples were placed into EtOH (95.6%) for 30 days.
The choice of ethanol was caused by its being a non-solvent (precipitant) for chitosan and silicon glycerohydrogel but a solvent for water, AA or GA, and glycerol. The precipitant was replaced every 10 days.
Then the samples were cryo-frozen in a refrigerator (Sanyo [ultra-low temperature freezer] mdF-U3286S) at – 85°C for 24 h and were re-kept in the precipitator (alcohol/acetone) for 3 days followed by drying in a desiccator (CaCl2) within 24 h.
Surface Morphology
The surface morphology of the samples was evaluated by SEM on a MIRA/LMU scanning microscope (Tescan, Czech Republic) at a voltage of 8 kV and a conductive current of 60 pA.
A 5-nm thickness golden layer was sprayed onto each sample with a magnetron sputtering installation K450X carbon coater (Germany) at a spraying current of 20 mA and a spraying duration of 1 min.

8. Concentration of polymer template increase
SEM images of the solid phase of the organic-inorganic xerogel based on CS-AA and Si(OGly)4· 2GlyОН.
Concentration of polymer template increase
A schematic image of the distribution of globular particles of the branched 3D network of ≡Si–O–Si≡ bonds on the CS matrix.

9. Conclusions
In order to study structural features of the hydrogel solid phase, several samples were treated by exhaustive cold extraction combined with cryotreatment. SEM images show that the morphology of the isolated solid phase of our inorganic/organic hybrid xerogel is represented by a matrix of evenly distributed agglomerated particles. The globular structures on the sample surface exhibit the formation of a 3D network of ≡Si–O–Si≡ bonds. Irrespective of the initial concentration of the components used for the hydrogel synthesis, the agglomerated particle size increases with the chitosan concentration.