DYNAMIC STUDY OF GELATIN GELS BY CREEP MEASUREMENTS

We have investigated the dynamical properties of gelatin gels using cr eep measurements. A commercial apparatus (Carrimed CSL500) was modifie d in order to increase the deformation of the gel and to take advantag e of the inertia of the system. When a step stress is applied, the ver y first response of these materials is an oscillating strain owing to a coupling of the high elasticity of the gelatin gels and the inertia of the apparatus. From these damped oscillations, we have extracted th e elastic and loss moduli as a function of frequency, which allows us to widen the frequency range (toward high frequencies) of measurement. After subtraction of the oscillations, we have obtained the complianc e funtion from which, using Ferry's formalism, we can calculate the re laxation time distribution function over a very large time range (10(- 3)-10(4) s). We show that the dynamics of gelatin gels is governed by two very different characteristic times. We interpret the faster relax ation time as relaxation at the scale of the gel network mesh-size, wh ile the slower time we assign to relaxations involving the lifetime of the gelatin gel crosslinks. It is now possible to use creep measureme nts as an alternative to the forced oscillatory function determination , as the same data can be obtained but, more quickly, and over a large frequency range. This gives us more indication of the gel's structure (gel network behaviour, kinetics of ageing) than all the laborious me thods previously necessary.