Fourier transform mechanical spectroscopy FTMS

The evolution of visco-elastic properties in non-Newtonian fluids exhibiting time-dependent rheological changes is a matter of wide scientific interest, particiflarly so in systems undergoing gelation. The gel-point, where a three-dimensional network structure is established, may be identified rheologically by the establishment of a eharacteristic frequency dependence of the dynamic moduli, and an associated frequency independent loss tangent [Winter and Chambon, 1986]. 

This criterion for gel-point detection, and the non-equilibrium nature of systems imdergoing gelation, requires that data be obtained rapidly over a wide range of frequeney, prompting the development of a frequeney multiplexing technique known as Fourier Transform Mechanical Spectroscopy, FTMS, which allows the measurement of G at several frequencies simultaneously, rather than eonseeutively, as in a conventional test [Holly et al., 1988]. 

The teelmique, iiutially developed to measure visco-elastic properties in the curing of polymers [Malkin etal., 1984], has been applied to gels (In and Prud homme, 1993) and model visco-elastic fluids [Davies and Jones, 1994]. In a variation of the teehnique, dynamic mechanical properties are determined using the Fourier transform of pulsed deformations [Vratsanos and Farris, 1988]. 

Experimental times for determining dynamic properties depend on a material s inherent time-dependent behavionr and single-point measnrements must span a time period equal to that over which the sample can respond to the imposed stress or strain. This defines a measurement time, over which 

The procedure for performing FTMS measurements using a controUed-stress instrument differs only slightly from conventional test procedures. The test is configured in the same manner as in a time sweep but in this case the selected frequency acts as the fundamental from which further harmonics are selected, with each harmonic frequency being an integer multiple of the fundamental frequency. 

---

For the continuous mode, we utilized a dynamic rheological technique, Fourier transform mechanical spectroscopy (FTMS) (77,75), which provided a powerful method for monitoring, simultaneously, the evolving dynamic moduli at several frequencies during the course of photo-cross-linking. In this technique, an oscillatory strain, y, was applied to the sample, such that... 

---