Critical assessment of the application of the WLF/free volume theory to the structural properties of high solids systems: A review

In the last ten years or so, attempts have been made to assess the structural properties of high solids foodstuffs using the WLF equation which has found a good degree of general acceptance in the investigation of amorphous synthetic polymers and diluted systems. However, there appears to be a misguided effort to apply the equation in a variety of molecular processes 'as long as it fits'. It should be remembered that the merit of the WLF scheme is invariably associated with the theory of free volume. The theory follows the kinetic contributions to polymer relaxation at the glass transition region, which is a second order thermodynamic process thus signifying a change in state not in phase. Critical application of the combined WLF/free volume theoretical framework to high sugar/biopolymer mixtures using the technique of small-deformation dynamic oscillation yields the rheological glass transition temperature (T-g), the thermal expansion coefficient (alpha (f)) and the fractional free volume (f(g)) at T-g. The physical significance of the rheological T-g lies in providing a threshold above which che free volume effects are superseded by the predictions of the reaction-rate theory. Furthermore, the treatment is capable of resolving the complicated mechanical properties of high solids foodstuffs into one basic function of frequency (time) alone and one basic function of temperature alone. The last section of the article deals with the concept of the monomeric friction coefficient used in the physics of synthetic materials to relate measurable viscoelastic constants to molecular characteristics. It remains to be seen whether a similar advance can be achieved in biological melts and glasses.