Understanding Structure-Rheology Relationships of Biopolymer Solutions

Abstract

The term biopolymers may be used to describe macromolecules that are obtained from a wide variety of biological sources. In the wake of growing concerns surrounding the use of non-biodegradable synthetic polymers, they serve as ideal replacements for a wide range of engineering applications. However, given the versatility of (charged or uncharged) chemical species that form biopolymers, there are considerably large uncertainties in determining their exact chemical structure. The presence of charged species in biopolymers further complicates matters, as they remain sensitive to parameters such as acidity (pH) and ionic strength (conductivity), thereby challenging the traditional physical models that describe polymer dissolved within specific solvents.

The work presented in this thesis tries to overcome these limitations through an abstract and idealised description of their structure, using both, specific knowledge about the type of charges present within the biopolymer, as well as parameters such as pH and conductivity. Further, intentional changes are made to the pH and conductivity by introducing specific (counter) ions that are known to influence the thermodynamic stability of the biopolymer in solution. These include both, changes to the conformation of the biopolymers coils due to electrostatic interactions, as well as intermolecular interactions between neighbouring biopolymer coils due to the formation of (weak→ strong) physical bonds. Naturally, the accompanying changes to the structural conformations has an predictable influence on the properties of the biopolymer solution, which in this case is captured using rheology, i.e. the deformation of (solid, liquid and viscoelastic) materials due to the application of a load.

Specifically, rheology is used to track changes to the following parameters, the intrinsic viscosity: which is a measure for the size of the dissolved macromolecules, the Herschel-Bulkley consistency index: which is a measure for the viscous dissipation of concentrated biopolymer solutions, and the storage modulus and the yield stress. These latter two parameters collectively provide quantitative insight about the packing of weakly bonded biopolymer gel particles. In turn this information, i.e. changes to the rheological properties of a wide variety of representative biopolymer systems, is used to derive relevant structure-property relationships that may be exploited for engineering applications.

Finally, the collective insight gained by establishing these structure-property relationships is condensed to provide a simple rheology experiment. This simple experiment is practically relevant to ensure the on site quality of novel biopolymer formulations, as well as to aid suitable modifications to the existing extraction protocol.