Microstructure design in mixed biopolymer composites
Introduction
Biopolymers and mixtures of biopolymers are used in many industries (e.g. Food, Pharmaceutical, Paint and Personal Care) to impart specific flow behaviours, textures, appearances and, where required, tactile and oral properties to products. As a consequence of this extensive use, many academic and industrial scientists have investigated their behaviour. Most of the studies however, have dealt with the equilibrium behaviour and the small deformation properties of the materials.
Thus far, there have only been limited studies in biopolymer mixtures of how the process of phase separation is related to the kinetics of molecular ordering and gelation (Alevisopoulos et al., 1996, Aymard et al., 2000, Clark et al., 1999, Loren and Hermansson, 2000, Loren et al., 2001, Owen and Jones, 1998). There have also been relatively few studies of the large deformation and failure response of materials formed in this manner, and of how this is influenced by the phase microstructure (Brownsey et al., 1987, Frith and Norton, 2000, Langley and Martin, 1994). Such microstructures may be, in turn, manipulated by the application of flow fields as the system is going through molecular ordering, gelation or phase separation events in order to produce materials with tailored properties (Wolf et al., 2001, Wolf et al., 2000).
All of these issues are of considerable importance to the production and properties of foods, hence, in this paper, we present an overview of recent and current work in the area. We also consider the subject of future studies that are required if we are truly to be capable of designing the microstructure and properties of mixed biopolymer composites.
Section snippets
Materials and methods
As this text is an overview of work in the area of mixed biopolymers, results are discussed that are based on studies of a variety of materials and experimental techniques. Detailed accounts of these are given in the publications referred to in the main text. In addition, a brief outline is given below of the materials and techniques discussed, along with the corresponding references.
Phase separation in biopolymer mixtures.
It has long been known that when biopolymers are mixed they often phase separate to form domains that are rich in one polymer and poor in the other (Beijerinck, 1910). Although this is entropically unfavourable, the enthalpy term is advantageous, as ultimately the molecules prefer to have neighbours of similar structure to ones that are dissimilar. This is, of course, particularly true when, as the temperature is lowered, the molecules start to interact and order, since interactions between
Conclusions
We have discussed some of the recent progress that has been made in the study of mixed biopolymer solutions and gels. Much of this work has focussed on the kinetics of the demixing process, the interface properties in such systems, and how these affect the bulk material properties of the mixtures. Notable advances include the following.
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Measurements can now be made of the interfacial tension in phase separated systems. The value obtained is three orders of magnitude below those observed for
Acknowledgements
The authors would like to thank T. Foster, P. Knight, S. Pomfret, K. Plucknett, B. Wolf, M. Williams, L. Lundin, V. Normand, M. Butler, M. Heppenstall-Butler and A. Clark for many helpful discussions and for kindly allowing their data to be reproduced here.
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