Polymer - borate hybrids films - theoretical and experimental dеtermination of the structure

: Transparent organic-inorganic hybrid films with composition (wt%) PVA(54,5): PEG(40): B 2 O 3 (5,5) is obtained by the sol-gel process and solution casting method at ambient temperature. Two different precursors of boron aqua solutions of boric acid (H 3 BO 3 ) and ethanol solution of trimethyl borate (СН 3 О) 3 В) are used for the preparation of hybrid solutions. The effect of starting boron solution type on the structure of obtained materials are studied by infrared spectroscopy (IR). It was found that degree of cross-linking and type of bonding in the hybrid structure are depended on by the functionality of borate units. From experimental and calculated spectral data it can be concluded that the best cross-linked hybrid structure is obtained from the aqua solution of H 3 BO 3 at pH 10, with a predominant amount of polyborate ions involved in the B-O-C ester complexes. By combined experimental and computational study, a probable mechanism of boron-containing organic-inorganic network are proposed.


Introduction
In the recent years, there is an increasing interest to produce polymeric and hybrid hydrogel materials with improved properties by blending existing polymers and cross-linking agents. The main reason for intensive research in this area is the new and different properties of these materials, which the traditional composites and conventional materials do not have [1][2][3][4][5]. For reasons of environmental problems, it is significant to develop biodegradable polymeric materials by modification of the traditional and non-degradable polymers. Available commercial biodegradable polymers are mainly limited to aliphatic polyesters, polyethers, for example, PEG, polyvinyl alcohol, and native polysaccharides [6]. Polymeric hydrogel networks based on PVA/PEG and hybrid are broadly studied because they have many realised and potential applications, especially in the field of biomaterials such as contact lenses, real-time immunoassay, tissue engineering matrices, drug delivery systems, acid-resistant biomembrane [1][2][3][4][5][6][7][8][9].
PEG one of the most widely used polymers possessing properties such hydrophilicity, a solubility in water and organic solvents, non-toxicity, and absence of antigenicity and immunogenicity, which allow using for many biomedical and cosmetic applications [1,2,5,[8][9][10]. Poly(vinyl alcohol) (PVA) is a water-soluble polymer, employed in practical applications due to its excellent chemical resistance, processing facilities, non-toxicity, biodegradability, physicochemical properties, and most importantly good film forming by solution casting [1][2][3][4][5][9][10][11][12][13]. Addition of these it is immiscible or partially miscible with most polymers, even with acrylic polymers. The presence of a second polymer, or/ and crosslinking nanostructure can perform a significant change in the structure and properties of the PVA-hydrogel.
Boric acid B(OH)3 is a weak Lewis acid and in aqua and have been widely investigated, diluted solutions B(OH) -4 and B(OH)3 species are presented simultaneously. At boron concentration 185 higher than about 0.025M and pH 7-10, due to the shifting of acid-base equilibrium various polyborate anions are formed [14]. It is known crosslink effectiveness of borate ions so that H3BO3 is relatively well dissolved in polyvalent alcohols PVA, PEG, polysaccharides to form organic-inorganic hydrogels [11][12][13][15][16]. This present paper preparation of PVA/PEG/Boron hybrid films, morphological and structural study by SEM and FTIR. In this work, we use a combined approach including computational modelling and the interpretation of experimental vibration spectra to determine a probable mechanism of crosslinking for obtained boron containing hybrid network.

Experimental and Theoretical calculations
Transparent polymer-borate hybrid films of mass ratio PVA/PEG/B2O3 = 10/7/1 are obtained by the sol-gel process, and solution casting method at an ambient temperature referred to in our previous papers [17][18][19]. Ground-state fully optimised geometries; analytical vibrational frequencies were obtained using Density Functional Theory (DFT). Calculated IR spectra are presented and discussed without scaling. The calculations were run by using the Becke three-parameter Lee-Yang-Parr (B3LYP) hybrid exchange-correlation functional coupled with 6-31g(d) basis set. All calculations were performed with Gaussian 09 package.
The absorption bands of initial polymers with that of hybrid films obtained by different boron precursors are compared in Table 1.
Table1. Infrared vibrational frequencies of PVA, PEG and hybrid PVA-PEG-B2O3 films obtained by different boron precursors and pH.
The main changes in the spectra are associated with characteristic frequencies of OH groups on PEG and H3BO3. Hydrogen-bonded and ester type cross-linking between inorganic and organic compound is observed. In Figure 4 can be seen that absorption bands of the hybrid film obtained on the aqueous solution of H3BO3 are shifted in compared those of PEG. The displaced of absorption bands 2908 cm -1 (νs СН2), 1655 cm -1 (bending mode of H2О), (1105 cm -1 C-O-C of esters) and 590 cm -1 (bending of ОH) to higher frequencies is a result of hydrogen-bonded interaction between O and H atoms from the polymers, boron species and aqueous molecules. The absence of bands in region 2280-1950 cm -1 and shifting from 3400 cm -1 to 3370 cm -1 are related to replacing of intramolecular hydrogen bonds presented of all precursors with intermolecular [23,24]. Shifted to the lower wave number frequencies bands 1425 cm -1 , 1335 cm -1 , 1290 cm -1 corresponding to the vibrations of C-OH of PEG may be related to the interactions with BO3 (1500-1200 cm -1 ), while band appearing of 845 cm -1 and already discussed at 1105 cm -1 to the framework vibrations of C-H and ester bonds with BO4 (1200-850 cm -1 ). The polymerborate esters bonds may expect in an interval 1400-1000 cm -1 , where are occurred absorptions bands of C-OH, C-O-C, BO3 and BO4 groups. Characteristic frequency of B-O-C bonds in the literature occurs at 1030 cm -1 [11][12][13]. The changes observed in the IR spectrum of the film obtained from an aqua solution of H3BO3 at pH=10 are related to higher BO4/BO3 ratio. The bands concerning BO4 and BO3 structural groups are more shifted to lower frequencies accordingly (1095 cm -1 , 920 cm -1 , 840 cm -1 ) and (660 cm -1 , 610 cm -1 ). Furthermore simultaneously increase of intensities of bands 1100 cm -1 and decrease in region 1425-1250 cm -1 are observed. The IR spectra of films obtained from ethanol solution of (CH3O)3B are similar, resulting in hydrolysis rate ( Figure 5). If the synthesis is approved without pH adjustment (рН=5) in the precursor solutions almost no hydrated boron ester and the structure is based on hydrogen bonding network and minimal quantity BO4-complexes. 189 On the base of spectral results and above analyses in Figure 6 are supposed following polymerborate structures taking part in the hybrid network and computer simulated IR spectra are made.
Experimental absorption bands concerning PEG-borate complexes are in a good correlation with the simulated IR spectra. Therefore, we suppose that PVA-PEG-B2O3 network is based on initial crosslink between the borate units and PEG to the formation of PEG-borate esters followed by hydrogen bonding them with long chains of PVA.

Conclusions
Transparent PVA/PEG/Boron hybrid films are obtained by solution casting method. Surface morphology of hybrid films analysed by SEM is revealed homogeneous, microporous surface, with larger single surface pores and free of crystallites. Results of the FTIR analysis showed that borate species existing in the precursors are depended on pH that determines the BO4/BO3 ratio, a cross-linking, final morphology and a structure of a hybrid network. The best cross-linked hybrid structure is obtained from an aqueous solution of H3BO3 at pH=10, with the major amount of polyborate ions. A probable mechanism based on the formation of a hybrid network involving PEG-borate ester complexes entanglement through the hydrogen bonds of PVA is supposed.