The Effect of pH on the Viscoelastic Response of Alginate-Montmorillonite Nanocomposite Hydrogels

Abstract

Ionically cross-linked alginate hydrogels are used in a wide range of applications, such as drug delivery, tissue engineering, and food packaging. A shortcoming of these gels is that they lose their strength and degrade at low pH values. To develop gels able to preserve their integrity in a wide range of pH values, Ca-alginate-montmorillonite nanocomposite gels are prepared, and their chemical structure, morphology, and mechanical response are analyzed. As the uniformity of nanocomposite gels is strongly affected by concentrations of MMT and CaCl_2, it is revealed that homogeneous gels can be prepared with 4 wt.% MMT and 0.5 M CaCl₂ at the highest. The viscoelastic behavior of nanocomposite gels in aqueous solutions with pH = 7 and pH = 2 is investigated by means of small-amplitude compressive oscillatory tests. It is shown that Ca-alginate-MMT nanocomposite gels preserve their integrity while being swollen at pH = 2. The experimental data are fitted by a model with only two material parameters, which shows that the elastic moduli increase linearly with a concentration of MMT at all pH values under investigation due to formation of physical bonds between alginate chains and MMT platelets. The presence of these bonds is confirmed by ATR-FTIR spectroscopy. The morphology of nanocomposite gels is studied by means of wide-angle X-ray diffraction, which reveals that intercalation of polymer chains between clay platelets increases the interlayer gallery spacing.

Keywords: alginate; hydrogel; mechanical properties; montmorillonite; nanocomposite.

Figure 1

Storage modulus E′ (circles) and loss modulus E″ (squares) versus frequency f for alginate–MMT hydrogels cross-linked with (A) 4 wt.% of MMT and 0.5 M CaCl₂ and (B) 4.5 wt.% of MMT and 1.5 M CaCl₂ in aqueous solutions with pH = 7. Each figure shows observations on two samples (black and purple lines) with identical preparation conditions.

Figure 2

FTIR spectra of alginate, MMT, and Ca-alginate/MMT hydrogels with 1, 3, and 4 wt.% MMT.

Figure 3

XRD patterns of pure MMT (red) and Ca-alginate/MMT nanocomposite hydrogel (green).

Figure 4

Storage modulus E′ (circles) and loss modulus E″ (squares) versus frequency f for alginate–MMT gels with (A) 0, (B) 1, (C) 3, and (D) 4 wt.% of MMT in aqueous solutions with pH = 7 and pH = 2.

Figure 5

(A) Storage modulus E′ and (B) loss modulus E″ at frequency f = 1 Hz versus concentrations of MMT in aqueous solutions with pH = 7 and pH = 2. Circles: experimental data. Solid line: their approximation by a linear function.

Figure 6

(A) Schematic presentation of the electrostatic links between the positively charged edge of MMT and the negatively charged alginate chains at pH = 7, and (B) hydrogen bonds between the OH groups of MMT and protonated COOH groups of alginate chains at pH = 2. (C) By lowering the pH from 7 to 2, the weak “egg-box” structure is broken; consequently, the electrostatic connection between MMT and alginate chains is broken.

Figure 7

Storage modulus E′ (◦) and loss modulus E″ (•) versus frequency f. Symbols: experimental data in small-amplitude oscillatory tests at room temperature on alginate–MMT nanocomposite gels with various concentrations of MMT swollen in water with pH = 7. (A) 0, (B) 1.0, (C) 3.0, (D) 4.0 wt.% MMT. Solid lines: results of numerical analysis.

Figure 8

Storage modulus E′ (◦) and loss modulus E″ (•) versus frequency f. Symbols: experimental data in small-amplitude oscillatory tests at room temperature on alginate–MMT nanocomposite gels with various concentrations of MMT swollen in water with pH = 2. (A) 1.0, (B) 3.0, (C) 4.0 wt.% MMT. Solid lines: results of numerical analysis.

Figure 9

(A) Elastic modulus E and (B) coefficient κ versus MMT concentration. Symbols: treatment of experimental data at room temperature on alginate–MMT nanocomposite gels swollen in water with pH = 7 and pH = 2. Solid lines: results of numerical simulation.