Properties of Dextran

Variations in dextran characteristics (e.g., molecular weight and branching) cause its properties to be different. The main chain of dextran with α-(1→6) bonds adopts a helical shape, which is modified by the presence of branches (α-(1→2), α-(1→3) or α-(1→4)), such that the linear structure of glucan is repeatedly folded.

The solubility and rheological properties of dextran are affected by its molecular weight and branching. The solubility of polymers refers to the interaction of the molecule with water through interactions by hydrogen bridges. Some research asserts that if the dextran molecule were totally linear (without branches), it would be totally soluble, because its hydroxy groups (–OH) would be exposed to interact with water molecules. Other investigations affirm that the greater the number of branches, the greater the solubility of the dextran due to the increase in amorphous areas in the molecule that favor water adsorption and retention. There are even reports that, in general, all low molecular weight polysaccharides have a higher solubility compared to long chain polysaccharides. There is no direct relationship between the characteristics of the molecule and the variation of the properties. However, regardless of the degree of solubility, dextrans are considered soluble EPS due to their ability to incorporate large amounts of water and form hydrogels.

The rheology and viscosity of polymers show their behavior as flow or deformation under an applied force, respectively, which is associated with –OH groups that easily interact with other molecules through hydrogen bonds, which are they break during shear. Generally, the viscosity of dextran is directly related to the concentration and the shear rate, which means that at low concentrations they have a Newtonian behavior (independent of the shear rate) and at high concentrations their behavior is non-Newtonian (or pseudoplastic). Other studies show that the viscosity is also in direct relation to the molecular weight of the dextran, since as one increases, the other increases.

On the other hand, the flexibility of the polymers is determined as a function of the temperature; however, the temperatures vary depending on the intermolecular forces, crystallinity, and the size of the polymer. Linear amorphous polymers have characteristics like glass at low temperatures—that is, little flexibility due to the zero mobility of the polymer chains. With increasing temperature, they tend to become leathery (at the glass transition temperature, Tg), then rubbery and finally melt (at the melting temperature, Tm) . During this transformation process, polymers show their most flexible point. In crystalline polymers, the Tg is high due to intermolecular forces between the polymer chains. In short chain polymers, the Tm is low because the entropy is low, whereas long chains tend to be less mobile with high entropies, so the Tm is high.