Determination of viscoelastic and rheo-optical material functions of water-soluble cellulose derivatives

Abstract

Cellulose derivatives in aqueous solutions are mainly used in applications that exploit their specific solution properties as thickeners, binding agents, emulsifiers and stabilisers. They are also used for their pseudoplastic and thixotropic behaviour, their water retention and film formation capacity and their surface activity, to name but a few of their properties. Because of there versatile properties cellulose derivatives are used in a wide variety of applications like food- pharma- cosmetic- and other convenience products as well as in the textile, paper, construction and engineering industry.If new applications are to be found and existing ones improved, it is absolutely essential to determine the viscoelastic and rheological material functions of the solution system so that they can be employed during processing and in the subsequent use of a product.However, before the rheological material functions can be employed, they have to be precisely correlated with the molecular structural parameters such as molar mass, particle size and the molar mass distribution, as well as the chemical structure (DS, MS, distribution of substituents etc.). This review article therefore intends to show that it is only correlation of molecular parameters with the rheological material functions that allows structure–property relationships to be established, which then make it possible to describe the flow behaviour of cellulose derivative solutions in detail.Here the simplest structure–property relationship of the individual molecule in dilute solution, the already widely used [η]–M relationship, is tabulated for a large number of polymer–solvent systems. In the meantime some RG–M relationships have also been determined for cellulose derivatives; since the introduction of ultrasonic degradation for generating homologous molar mass series, the determination of further RG–M and [η]–M relationships has been possible with relatively simple resources.However, structure–property relationships have also been established for the more complex flow behaviour of more highly concentrated solutions starting from systems of ideal, synthetic polymer standards.The zero-shear viscosity, which is not exceeded in many types of process, can be described with the η0–M–c relationship. However, for pseudoplastic flow, it was also possible to determine the viscosity yield quantitatively over the entire range of shear rates with the aid of the η–M–c–γ̇ relationship.Because these relationships take account of the solution structures as a function of the molecular structural parameters, they also enable the flow behaviour to be predicted in problems — of particular relevance to engineering applications — concerning solutions of cellulose derivatives in aqueous media, which are frequently dissolved in a non-ideally molecularly disperse state. In this way the η0–[η]–c and η–[η]–c–γ̇ relationships can also be used for systems containing aggregates and associates.The dependence of the flow behaviour on the chemical structural parameters is also influenced by external conditions (temperature, salt and surfactants), as illustrated in Sections 3.3.3 and 3.4.5. In this case, far-reaching conclusions were drawn about the dependence of the viscoelastic properties upon the substitution pattern (η–DS relationships) and the type of substituents.Comparison of rheological shear and oscillation experiments enables supramolecular structures to be detected in non-molecularly disperse solutions and, in particular, provides an opportunity for detecting energetic, intermolecular interactions. The material functions from the oscillation experiment enable the elastic properties of a cellulose derivative solution to be determined in the relaxed state. A comparison with the elastic normal stresses of the flow process enable engineering applications to be optimized, such as injection processes or thread formation, the rheological characteristics of which are primarily determined by the elastic component.For some years it has been possible to monitor rheo-optical material functions, such as flow birefringence and flow dichroism, in order to determine the orientation of polymer segments and aggregates in a flow field. As a result, it is not only possible to monitor overall rheological behaviour of cellulose derivatives but also the flow behaviour at the molecular level. One special feature of this method is that it becomes possible to detect the orientation and shear stability of associated superstructures in isolation from the components dissolved in a molecularly disperse state. However, in addition to the superstructures, a molecular description of the disentanglement behaviour in the power-law range of the flow curve is also made possible by means of the rheo-optical investigation methods.