Radiation synthesis and characterization of xanthan gum hydrogels

Highlights

• Preparation of xanthan gum (XG) hydrogels by ionizing Radiation.

• The influences of paste like state, acetylene gas and CCl₄ on the radiation induced gelation of XG.

• Preparation and characterization of P(AAcNa)/XG semi-IPNs by gamma rays.

Abstract

In this study, xanthan gum (XG) based hydrogels were prepared at a paste like state, in the presence of acetylene gas and CCl₄ through γ-irradiation. Sodium polyacrylate/XG semi-interpenetrating networks [P(AAcNa)/XG semi-IPNs] were also prepared and characterized. It was determined that XG could not undergo high gelation when irradiated at a paste like state. Relatively higher gelation was obtained in the presence of CCl₄. Irradiations carried out in the presence of acetylene resulted in formation of insoluble structures and viscosity of XG increased almost threefold up to a certain dose. Hydrogel structures with superabsorbent character and highest gelation were obtained with P(AAcNa)/XGsemi-IPNs. Swelling and absorbency under load (AUL)tests showed that P(AAcNa)/XG semi-IPNs have as much fluid absorption capacity as commercially available super absorbent polymers (SAPs). Basic network structural parameters of the P(AAcNa)/XG semi-IPNs were also calculated and characterized by mechanical and rheological tests. It was determined that the addition of XG to the P(AAcNa) structure greatly enhanced the mechanical strength of the hydrogels while fluid absorption capacities were adequately high for a novel XG based superabsorbent hydrogel.

Introduction

Thanks to their many advantages such as being soft and elastic, being permeable to body fluids and drug molecules, today hydrogels are used in a wide variety of fields such as wound dressing materials, controlled drug delivery and release systems, artificial tissue, etc. and their development continues rapidly. In recent years, hydrogels of natural-based polymers, specifically polysaccharides, have developed increasing interest due to their unique characteristics such as inherent biocompatibility, biodegradability, non-toxicity and renewability as well as their unique abilities to improve properties of hydrogels (Kume et al., 2002; Yoshii et al., 2003; Lee et al., 2005; Varshney, 2007; Zhao et al., 2013).

As it is known, hydrogels can be synthesized using either by chemical or ionizing radiation techniques. In the last few decades, ionizing radiation has been well known and widely used for modification and processing of polymeric materials through cross-linking, degradation, grafting and curing. Besides the economic benefits, radiation processing of polymers also has many technical and environmental advantages, such as: low pollution and low energy consumption, single step modification and sterilization without chemical additives, etc. which makes it a green technology and attracts the attention of many researches in recent years(Rosiak, 1991; IAEA, 2004; Chmielewski et al., 2005). On the other hand, it has long been well known that polysaccharides such as cellulose, starch, chitin, chitosan and their water-soluble derivatives undergo degradation when exposed to ionizing radiation in their dilute aqueous solution or in solid state (Charlesby, 1955; Kume et al., 1982; Tabata et al., 1991; Yoshii, 2004). However, since 2003 various methods have been developed to obtain hydrogels from natural polymers using high energy radiation. One of these methods suggested by Yoshii et al. (Yoshii et al., 2003) for the first time. They showed that water-soluble polysaccharide derivatives, such as carboxymethylcellulose (CMC), carboxymethylstarch (CMS), carboxymethylchitin (CMCT) and carboxymethylchitosan (CMCTS) are crosslinked when irradiated at highly concentrated aqueous solutions (more than 10% in paste-like state).

Another method for cross-linking of natural polymers using ionizing radiation was suggested by Ramnani et al. (Ramnani et al., 2004). By using this method, crosslinked chitosan was synthesized using gamma rays and electron beam in the presence of carbon tetrachloride as sensitizer (Ramnani et al., 2004; Ramaprasad et al., 2008).

γ-radiation induced crosslinking of natural polymers in the powder form in the presence of a mediating gas (generally an alkyne gas) has been reported by Al-Assaf et al. Globular structures such as arabinogalactan proteins, branched chain polysaccharides such as pullulan and dextran, chemically modified structures such as CMC and gelling polysaccharides as pectin and carrageenan were successfully modified. Using this method, it was shown that the molecular weight could be increased to the point where insoluble structures are formed, and thereafter, with further absorbed dose, a hydrogel state results (Al-Assaf et al., 2006; Al-Assaf et al., 2007).

In addition to these, since the mechanical strength of hydrogels based on natural polymer is poor, the hydrogels obtained from natural-synthetic polymer mixtures is another method that is extensively studied by researchers (Sperling, 1994; Lee et al., 2000; Myung et al., 2007; Liu et al., 2009; Reddy et al., 2009; Sun et al., 2012).

Mixtures of natural-synthetic polymers generally form semi-interpenetrating network structures, which are formed by the cross-linking of the synthetic polymer to form the network and the natural polymer to be entangled in this cross-linked structure and being a part of the network structure (Sperling, 1994). In our previous study, using this method, we synthesized semi-IPNs with high mechanical strength by radiation induced polymerization and crosslinking of partially neutralized acrylic acid (sodium acrylate, AAcNa) in the presence of locust bean gum (LBG), a galactomannan type polysaccharide. By controlling the degree of neutralization (DN) of AAc, the amount of crosslinker, natural polymer to synthetic polymer ratio and the amount of absorbed dose, optimum conditions were determined to prepare novel superabsorbent hydrogels (Şen et al., 2012).

Another widely used method for the preparation of hydrogel systems is grafting of vinyl type monomers i.e. acrylates or methacrylates on the synthetic or natural polymers. Many research studies have been reported in the literature last decades on this field.

Said et al. (2004) reported that the CMC in hydrogel form can be prepared by grafting it with AAc under irradiation with EB in aqueous solution. EB irradiation initiates free radical polymerization of AAc on a backbone of CMC. The product of water radiolysis is helpful for abstracting protons from macromolecular backbones. The irradiation of CMC and the monomer produces free radicals that can combine to form a hydrogel. The authors suggested that such an AAc based hydrogel could be used for the recovery of metal ions such as copper, nickel, cobalt and lead. They also reported the use of hydrogels in skin dressings.

Zhai et al. (2002) were prepared a starch based hydrogel by grafting poly(vinyl alcohol) (PVA). Starch was first dissolved in water to form a gel-like solution, then added to a PVA solution, heated at 90 °C for 30 min and continuously stirred to form a homogeneous mixture. The investigation showed that there was a grafting reaction between PVA and starch molecules in addition to the cross-linking of the PVA molecules caused by irradiation. It was found that amylose of starch was an important reactive component, which also governed the properties of the starch/PVA blend hydrogel.

Cai et al. (2005) were prepared thermo and pH sensitive hydrogels from chitosan and N-isopropylacrylamide (NIPAAm) using graft copolymerization of chitosan. The grafting percentage and grafting efficiency was shown to increase in proportion with the monomer concentration and the total absorbed dose.

A new approach to grafting on polysaccharides has recently been proposed by Barsbay and Güven (2009). It is based on the combination of radiation initiation and controlled free radical polymerization techniques. The main advantage of this method is that the graft chains are of uniform length. This allows the preparation of surfaces with desired, tunable and well-defined properties. This approach has been used, for instance, to graft styrene and sodium 4-styrene sulphonate on cellulose (Barsbay et al., 2007; Barsbay et al., 2009).

The other hydrogel preparation techniques by using radiation processing technology are reviewed in details for different applications in the Radiation Chemistry of Polysaccharides book of IAEA (Abad, 2016; Şen, and Hayrabolulu, 2016).

Many polysaccharide type natural polymers have been extensively studied to be crosslinked using ionizing radiation with the methods mentioned above. However, there are no comprehensive studies of the interaction of XG with ionizing radiation, cross-linking and preparation of hydrogels of XG using ionizing radiation. XG is a polysaccharide which is widely employed in food, pharmaceutical, cosmetics and other industries due to its properties such as high stability, high viscosity and biocompatibility (Mandala et al., 2004; Kulkarni et al., 2008; Turabi et al., 2008). Recently, we investigated the effect of ionizing radiation on XG in powder form and its diluted aqueous solutions (Şen et al., 2016; Hayrabolulu et al., 2018). In this study, hydrogels containing xanthan gum were prepared by determining the most suitable method, irradiation dose rate, absorbed dose, optimum polymer concentration and type and concentration of chemicals that will assist crosslinking. The synthesized hydrogels were then characterized by swelling, absorbency under load (AUL), mechanical and rheological tests.