Effect of the Cross-Linking with Calcium Ions on the Structural and Thermo-Mechanical Properties of Alginate Films
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Effect of the Cross-Linking with Calcium Ions on the Structural and Thermo-Mechanical Properties of Alginate Films
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Turner K. Vidal Urquiza1, O. Perales Pérez2, and M. Gálvez Saldaña3 University of Puerto Rico, Department of Mechanical Engineering, P.O. Box 9045, Mayagüez, P.R. 00681-9045-9000 2 University of Puerto Rico, Department of Engineering Science & Materials, P.O. Box 9044, Mayagüez, P.R. 00681-9044 3 University of Puerto Rico at Mayagüez, Department of Physics, Mayagüez P.R. 00980
ABSTRACT The present research discusses the thermal and structural properties of calcium alginate films synthesized by cross linking precursor sodium alginate films with Ca2+ ions. Raman and FTIR spectroscopy analyses evinced the interchange of Na ions with Ca ions. Both techniques revealed a shift of the COO- vibration modes; the symmetrical COO- peak of calcium alginate exhibited a shift towards higher wavenumber (~14 cm-1) from 1415 cm-1 to 1429 cm-1 in the Raman spectra. TGA and TMA analyses performed under a nitrogen atmosphere revealed that increasing the Camolarity and immersion time resulted in an increase in the glass transition temperature (Tg) and the strain deformation of the Ca-alginate film. The increase in Tg can be attributed to the Cacross-linking that could restrict the molecular response to temperature change whereas the increased strain could be due to the enhanced entrapment of water between the molecular chains of the polymer. INTRODUCTION Alginate, produced mainly from brown algae, is a polysaccharide composed of 1000–3000 building units linked together in an alternately stiff and flexible chain. The sequential structure and composition of alginate vary considerably not only among different species but also among different parts of the same algae [1]. At the molecular level, alginate is a salt of alginic acid, which is a family of unbranched binary copolymers of (1→4) – linked β-D – mannuronic acid (M) and α-L – guluronic acid (G) residues [2]. M-Alginate based materials, where ‘M’ can be Mg, Ca, Sr, Ba or Na , can be used as bio-polymeric coatings or drug delivery platforms when synthesized as films, membranes, or stable emulsions [3]. The strong affinity of alginates to divalent cations, including alkaline and heavy metals [1] [4], is responsible for the strength and flexibility of a sea weed host [4]. Alginates interact with a variety of mono- and poly-valent cations through several mechanisms including electrostatic and ionic interactions, covalent-like bonding, coordination, and redox reactions [5] [6]. Specifically, alginate cross-linking with Ca+2 ions is conducive to strong and insoluble films. In turn, the strong affinity of the alginate-based structure for divalent and trivalent cations explains its potential use as a biocompatible and environmentally friendly adsorbent for toxic metal ions present in polluted water streams. The performance of the calcium alginate films will be dependent not only on the chemical and structural properties but also on the resulting mechanical strength and stiffn
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