Characterization of poly(vinyl alcohol)/poly(ethylene glycol) hydrogels and PVA-derived hybrids by small-angle X-ray scattering and FTIR spectroscopy

Abstract

The purpose of this study is to develop novel poly(vinyl alcohol) (PVA)/poly(ethylene glycol) (PEG) hydrogel blends and PVA-derived organic–inorganic hybrid materials and perform nanostructural characterizations. PVA and PEG hydrogels were prepared by dissolving the polymer in aqueous solution, followed by addition of glutaraldehyde (GA) chemical crosslinker. Hybrids were synthesized by reacting PVA in aqueous solution with tetraethoxysilane (TEOS). PVA/TEOS were also modified in the nanometer-scale by crosslinking with GA during the synthesis reaction. Hydrogels and hybrids were characterized by using small-angle X-ray scattering synchrotron radiation (SAXS) and Fourier transform infrared spectroscopy (FTIR). Thin film samples were prepared for SAXS experiments. SAXS results have indicated different nano-ordered disperse phases for hydrogels made of PVA, PEG, PVA/GA, PVA/PEG. Also, PVA/TEOS and PVA/TEOS/GA hybrids have indicated different X-ray scattering patterns. FTIR spectra have showed major vibration bands associated with organic–inorganic chemical groups present in the hybrid nanocomposites PVA/TEOS and PVA/TEOS/GA. PVA/PEG hydrogels and PVA-derived hybrid materials were successfully produced with GA crosslinking in nanometer-scale network.

Introduction

Recently, the field of material science has witnessed the emergence of both hydrogels and novel class of materials called organic–inorganic hybrids. Hydrogels and hybrid materials are of intensive interest in contemporary material chemistry as these materials have potential applications in biomedical devices, matrices for drug delivery systems, carrier for cells immobilization, carrier for signaling molecules, and bioseparation membranes [1], [2], [3], [4], [5], [6]. The major driving forces behind the intense activities in this area are the new and different properties of these materials, which the traditional composites and conventional materials do not have. Hybrids would combine properties of organic polymers with ceramics. These different components can be mixed at length scales ranging from nanometer to micrometer, in virtually any ratio leading to the so-called hybrid organic–inorganic materials. They are also termed as ‘ceramers’ and ‘ormosils’ (organically modified silicates) or ‘ormocers’ (organically modified ceramics), which are normally nanocomposites [4]. The hybrids having such combined characteristics of organic and inorganic substances promise new high performance or high functional materials to fully exploit this technical opportunity with benefits of the better of the two worlds. On the other hand, hydrogels are three-dimensional, hydrophilic polymeric networks capable of absorbing and retaining different amounts of water or biological fluids. The networks are insoluble due to the presence of chemical crosslinks (junctions, tie-points) or physical crosslinks (crystallites, entanglement), which permit hydrogels to be thermodynamically compatible with water [7], [8], [9]. As a result, in comparison to other synthetic materials, hydrogels resemble nature living tissue closely in their physical properties due to their high water contents and softness, which also contribute to their biocompatibility and biodegradability. PVA and PEG hydrogels have been widely explored as water-soluble polymers for numerous biomedical and pharmaceutical applications due to the advantages of non-toxic, non-carcinogenic and bioadhesive properties [10], [11]. A great variety of methods to establish crosslinking have indeed been used to prepare hydrogels and organic–inorganic nanocomposite systems. Water-soluble polymers with hydroxyl groups (e.g. PVA and PEG) can be chemically crosslinked with several reagents such as glutaraldehyde, succinyl chloride among several others. Fig. 1 shows an illustration of hydrogels produced by physical and chemical crosslinking of polymer chains. Chemical crosslinking is a highly versatile method to create and modify polymers, where properties can be improved, such as mechanical, thermal and chemical stability. Mostly water-soluble polymers have been used as reagents that would undergo physical or chemical crosslinking processes. They can also be blended with other water-soluble polymers and again undergo crosslinking process either physically or chemically [10], [11]. Polymer blends are produced by physical mixing of two or more existing polymers. It is a convenient route to develop new polymeric materials, which combine the properties of more than one existing polymer. This strategy is usually cheaper and less time-consuming than the development of new monomers and/or new polymerization routes. A wide range of material properties can be obtained by merely changing the blend composition. So, hydrogels based on PVA/PEG, with different crosslinked nanostructure, create unique opportunities for controlling biodegradability, pH sensitive drug carriers, and designing tissue engineering scaffolds. Despite of the tremendous advances that have been made in these rapidly growing fields some important challenges have yet to be overcome. Most of these processes take place at nanometric scale due to interactions between the material and the biological system. Therefore, nanoscience and nanoengineering will play a crucial role on understanding and designing novel materials modified for specific functions. The nanostructure of hydrogels and hybrids are very complex and up to now are not properly understood. As a result, the characterization of such polymeric hydrophilic networks and hybrids in nano-order scale would allow researchers to design new systems tailoring their properties for different applications. In order to understand their unusual properties, knowledge about their network is generally required. Because of the nanometric and rather disordered nature of precursors, intermediate materials and final products, their structural characterization is a challenge for material scientists. If the relevant structural features are at a super-atomic level, from 1 to 100 nm, small-angle X-ray scattering (SAXS) is the most broadly used technique [10], [11], [12]. The SAXS synchrotron radiation provides statistical and overall information averaged in a volume in the order of 1 mm³. SAXS beamlines in synchrotron radiation laboratories provide very intense monochromatic X-ray beams that make studies of weak scatterer materials possible and, also, in situ analyses of structural transformations with a high time resolution. Besides providing a high photon flux, the nature of synchrotron radiation emission spectrum allows one to use the effect of anomalous scattering for many useful applications. Fourier transform infrared spectroscopy (FTIR) can be performed in many cases because, it is sensitive on changing the local chemical environment, being an extremely useful complement for scattering investigations.

In summary, poly(vinyl alcohol) and poly(ethylene glycol) hydrogels were produced by glutaraldehyde crosslinking reactions and polymer blending. Also, organic–inorganic hybrids derived from poly(vinyl alcohol) and tetraethoxysilane were synthesized and crosslinked using glutaraldehyde PVA/TEOS/GA. The PVA/PEG hydrogel networks and PVA/TEOS hybrid matrices formed were characterized at the nanosize level by SAXS and FTIR spectroscopy.