Synchrotron SAXS, XRD and FTIR Characterization of Nanostructured

PVA/TEOS Hybrid Cross-linked with Glutaraldehyde

H.S. Mansur1, a, A.A.P. Mansur1, b 1Department of Metallurgical and Materials Engineering, Federal University of Minas Gerais, Rua

Espírito Santo, 35, 30.160.030, Belo Horizonte, Brazil

a hmansur@demet.ufmg.br,

b aapiscitelli@uol.com.br

Keywords: Nanostructure, Polymer, Hybrid, SAXS, Spectroscopy and Hydrogel.

Abstract. Polyvinyl alcohol (PVA) is a unique synthetic biocompatible polymer and it has a large number of hydroxyl groups that can react with many kinds of functional groups. In the present work, nanostructured PVA/TEOS hybrids were characterized by Small-angle X-ray Scattering (SAXS) associated with Fourier Transform Infrared Spectroscopy (FTIR) and X-ray diffraction (XRD) techniques. We have synthesized hybrids based on the reaction of PVA with tetraethoxysilane (TEOS). PVA/TEOS hybrids were also modified in the nanometer-scale by crosslinking with glutaraldehyde (GA) during the synthesis involving hydrolysis and condensation of PVA/TEOS network. FTIR spectra showed major vibration bands associated with organic-inorganic chemical groups present in the hybrid composite PVA/TEOS. XRD coupled to FTIR results have indicated the presence of semi-crystalline domains embedded in amorphous PVA matrix. SAXS results were presented as plots with experimental scattering intensity, I(q), as a function of the modulus of the scattering vector, q. SAXS curves showed quite different trend on vector q with the scattering intensity I(q) corresponding to samples PVA, PVA/GA and PVA/TEOS/GA. Pure PVA sample showed a “knee” type curve, with maximum value q=0.04 Å-1. These results have indicated different nano-ordered disperse phases for PVA, PVA/TEOS hybrid and PVA/TEOS/GA chemically crosslinked hybrid.

Introduction

Poly (vinyl alcohol) is a well-known synthetic polymeric material the biocompatibility of which justifies its inclusion in the class of biomacromolecules. This characteristic comes jointly with a remarkable chemical versatility due to the presence of the hydroxyl moiety, which makes feasible a number of grafting modifications and of cross-linking reactions of the polymer backbone. The unique properties of nanostructured materials have been investigated for the last decade [1-2]. A novel class of materials called organic-inorganic hybrids would combine properties of organic polymers with ceramics. Hybrids and hydrogels can be tailored for several potential applications, as for instance as biomedical materials, carrier for signaling molecules, biomembranes for separation and biosensors, diagnostics, and mostly being used as matrices for drug delivery systems. Chemical crosslinking is a highly versatile method to create and modify polymers, where properties can be improved, such as mechanical, thermal and chemical stability [2].

Experimental

PVA/TEOS/GA Hybrids Synthesis

TEOS (Si(OC2H5)4 > 98%) and GA (25% aqueous) were supplied by Sigma-Aldrich. PVA (CPQ Chemicals, Brazil, 90+ % hydrolyzed, molecular weight of 72,000 g/mol). 96-well polystyrene microplates (Nunc MaxiSorp) were used as molds. Milli-Q deionized water was used in all aqueous solutions (18.0 MΩ). PVA hydrogel was prepared by fully dissolving 5.0g of polymer powder without further purification in 100 ml of Milli-Q deionized water, under magnetic stirring, at temperature of 60ºC ± 2ºC. PVA 5% (wt%) solution was let to cool down to room temperature and the pH was

Solid State Phenomena Vols. 121-123 (2007) pp 855-858Online available since 2007/Mar/15 at www.scientific.net© (2007) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/SSP.121-123.855

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corrected to 2.00 ± 0.05 with 1.0 M HCl. Hybrids derived from PVA and TEOS were synthesized by aqueous route. Under steady stirring, 5.0 ml of TEOS was gently added to previously prepared PVA acid solution at temperature of 25 ºC ± 1ºC. PVA/TEOS solution was poured into molds and allowed to solidify for 24-72 hours. Crosslinked hybrids were prepared by mixing 20.0 ml of PVA/TEOS aqueous solution with 5.0 ml of GA. PVA, PVA/GA and PVA/TEOS/GA thin films samples were also prepared by spreading few droplets of each solution onto microscopy glass slides, allowing then solidifying for 24-72 hours.

PVA/TEOS/GA Characterization

Synchrotron SAXS measurements were performed using the SAS beam line of the National Synchrotron Light Laboratory (LNLS, Campinas, Brazil). Each SAXS pattern corresponds to a data collection time of 900s. Because of the normalization procedure, the SAXS intensity was determined for all samples in the same arbitrary units so that they can be directly compared. FTIR spectroscopy was used to qualitatively investigate the presence of specific chemical groups in the hybrid network. FTIR spectra were obtained within the range between 4000 cm-1 and 400 cm-1 (Perkin-Elmer, Paragon 1000), with diffuse-reflectance accessory mixed with dried KBr Powder and subtracted KBr background. Attenuated total reflectance (ATR-FTIR) was also used to characterize PVA and PVA/TEOS films. X-ray diffraction patterns were obtained using CuKα radiation (Philips PW 1710, λ = 1.54056 Å).

Results and Discussion

We have synthesized the hybrids samples via chemical reaction of organic polymer (PVA) with silicon alcoxide (TEOS) and crosslinked by glutaraldehyde. TEOS hydrolysis and policondensation reactions have occurred into PVA aqueous solution. Optically transparent disk-like samples were produced with average weight of 10 ± 2 mg, 1.0 mm thick, and 3.0 mm diameter. We have used FTIR spectroscopy for the characterization of the hybrid materials and their crosslinking. In Figure 1a, FTIR spectrum of pure PVA is showed. The spectrum clearly reveals the major peaks of PVA due to C-O stretching (ν=1090-1150 cm-1), C-H broad alkyl stretching band (ν=2850-3000 cm-1), and typical strong hydroxyl bands for free alcohol (non-bonded OH stretching band at ν=3600-3650 cm-1), and hydrogen bonded (ν=3200-3570 cm-1) [3-8]. FTIR spectrum of hybrid made of PVA/TEOS is showed in Fig.1b. It can be observed major vibration bands (Si-O-Si, ν=1080 and 450 cm-1; Si-OH,ν=950 cm-1) associated with polysiloxane (TEOS) reactions added to PVA polymer solution [2]. Also, in the frequency range from 3000 cm-1 to 3650 cm-1, mainly related to hydroxyl groups [2,5], a broader band was noted for PVA/TEOS hybrid spectrum (Fig.1b) compared to PVA (Fig.1a). Therefore, introducing of Si-OH and Si-O-Si through hydrolysis and condensation reactions of TEOS [2] has modified PVA semi-crystalline structure. Such broad band observed on FTIR spectrum of PVA/TEOS has also some contribution of physically and chemically water incorporated during the hybrid synthesis via sol-gel aqueous route. These results have clearly indicated that an organic-inorganic hybrid network was achieved based on PVA and TEOS. It can be observed two important peaks at ν=2850 cm-1 and ν=2750 cm-1 of C-H stretching related to aldehydes [5]. GA has acted as crosslinker among polymer chains of PVA and an organic-inorganic covalent binder.

SAXS technique from synchrotron radiation source was used to evaluate the hybrid PVA/TEOS/GA structure at the nano-order level. In order to analyze the SAXS results, a simple two-electron density model was applied. All SAXS results are presented as plots with experimental scattering intensity, I(q), as a function of the modulus of the scattering vector, q.

856 Nanoscience and Technology

Figure.1. FTIR spectra of (a) PVA and (b) PVA with TEOS.

Figure.2. SAXS spectra intensity, I(q), as a function of q of (a) PVA/TEOS/GA; (b) PVA; (c) PVA/GA.

SAXS curves have showed quite different dependence on vector q with the scattering intensity I(q)

corresponding to samples PVA, PVA/GA and PVA/TEOS/GA. Pure PVA sample has shown a “knee” type curve, with maximum value q=0.04 Å-1 (Fig.2a). Such trend can be explained by

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Solid State Phenomena Vols. 121-123 857

assuming a semi-crystalline structure of PVA polymer sample. The average size between domains is determined by assuming spherical entities, with a radius R, forming a compact arrangement. We would have an average distance of 15 nm among PVA nanocrystallites (q=0.04 Å-1; Fig.2a). This result has strong correlation with values reported in recent publications [2]. Hydrogen bonds are very important on stabilizing polymer structures. Therefore, the number of hydrogen bondings present in PVA chain will be maximized, causing all of them to occur in parallel sheets, with strong intermolecular forces, stabilizing polymer crystals. The usual form of such crystal domains is lamellar, occurring in thin plates or sheets. The lamellae size is typically 10-20 nm. In summary, SAXS curve obtained for PVA sample has clearly indicated the formation of nanocrystalline domains. SAXS curve of PVA/TEOS/GA sample (Fig.2c) has presented a typical power-law dependence on vector q [2] when compared to curves of poly (vinyl alcohol) (Fig.2b) and PVA modified with GA (Fig.2b). Such trend is assumed to be caused by breaking most nano-ordered tri-dimensional organic structured previously found in the PVA sample. Due to siloxane hydrolysis and policondensation reactions with PVA aqueous solution, several new chemical covalent bonds have been created (Si-C, Si-OH, Si-O-Si) reducing the hydrogen bonds formation between polymer chains. Briefly, for PVA/TEOS/GA samples, we consider that the network is basically composed of some multi-dispersed nanocrystalline PVA domains embedded into a continuous amorphous organic-inorganic hybrid matrix. XRD spectra (not showed) have indicated the occurrence of semi-crystalline PVA, with typical diffraction at d-spacing, corresponding to 2θ. Namely, PVA has its 100, 101, 101, and 200 crystalline reflections 16.0°, 19.4°, 20.1°, and 22.7°, respectively.

Conclusion

The SAXS, XRD and FTIR spectroscopy characterizations have confirmed that hybrid organic-inorganic materials were successfully obtained based on the combination of PVA and TEOS with glutaraldehyde crosslinked nanometer-scale network.

Acknowledgments

The authors acknowledge CNPq/FAPEMIG/CAPES for financial support on this project. The authors are also grateful for the important contribution from LNLS staff and for synchrotron SAXS facilities.

References

[1] G. Paradossi, F. Cavalieri, E. Chiessi, V. Ponassi, and V. Martorana: Biomacromolecules, Vol. 3 (2002), p. 1255-1262

[2] H. S. Mansur, R. L. Oréfice and A. A.P. Mansur: Polymer, Vol. 45 (2004), p.7193–7202. [3] N.A. Peppas, Makromol. Chemie, Vol. 178 (1977), p.595. [4]. N.A. Peppas: Polymer, Vol. 18 (1977), p.403-408.

[5] John Coates, In Encyclopaedia of Analytical Chemistry, R.A. Meyers; John Wiley & Sons Ltd.: Chichester, 2000; p 10815–10837.

[6] N.A. Peppas and S.L. Wright,: Macromolecules, Vol. 29 (1996), p.8798.

[7] J. D. Thomas, MSc Thesis, Sep 2001, Drexel University, Novel associated PVA/PVP hydrogels for nucleus pulposus replacement, Philadelphia, PA, USA

[8] C.M. Hassan and N.A. Peppas: Adv. Polym. Sci., Vol. 153 (2000), p.37.

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Nanoscience and Technology 10.4028/www.scientific.net/SSP.121-123 Synchrotron SAXS, XRD and FTIR Characterization of Nanostructured PVA/TEOS Hybrid Cross-

Linked with Glutaraldehyde 10.4028/www.scientific.net/SSP.121-123.855

DOI References

[1] G. Paradossi, F. Cavalieri, E. Chiessi, V. Ponassi, and V. Martorana: Biomacromolecules, Vol. 3 2002), p.

1255-1262

doi:10.1021/bm0256247 [2] H. S. Mansur, R. L. Oréfice and A. A.P. Mansur: Polymer, Vol. 45 (2004), p.7193–7202.

doi:10.1016/j.polymer.2004.08.036 [4] . N.A. Peppas: Polymer, Vol. 18 (1977), p.403-408.

doi:10.1016/0032-3861(77)90090-8 [6] N.A. Peppas and S.L. Wright,: Macromolecules, Vol. 29 (1996), p.8798.

doi:10.1021/ma9613392 [2] H. S. Mansur, R. L. Orfice and A. A.P. Mansur: Polymer, Vol. 45 (2004), p.71937202.

doi:10.1016/j.polymer.2004.08.036 [3] N.A. Peppas, Makromol. Chemie, Vol. 178 (1977), p.595. [4]. N.A. Peppas: Polymer, Vol. 18 (1977),

p.403-408.

doi:10.1002/macp.1977.021780228