Frequency-sweeping process

At sufficiently low strain, most polymer materials exhibit a linear viscoelastic response and, once the appropriate strain amplitude has been determined through a preliminary strain sweep test, valid frequency sweep tests can be performed. Filled mbber compounds however hardly exhibit a linear viscoelastic response when submitted to harmonic strains and the current practice consists in testing such materials at the lowest permitted strain for satisfactory reproducibility an approach that obviously provides apparent material properties, at best. From a fundamental point of view, for instance in terms of material sciences, such measurements have a limited meaning because theoretical relationships that relate material structure to properties have so far been established only in the linear viscoelastic domain. Nevertheless, experience proves that apparent test results can be well reproducible and related to a number of other viscoelastic effects, including certain processing phenomena. 

The four variables in dynamic oscillatory tests are strain amplitude (or stress amplitude in the case of controlled stress dynamic rheometers), frequency, temperature and time (Gunasekaran and Ak, 2002). Dynamic oscillatory tests can thus take the form of a strain (or stress) amplitude sweep (frequency and temperature held constant), a frequency sweep (strain or stress amplitude and temperature held constant), a temperature sweep (strain or stress amplitude and frequency held constant), or a time sweep (strain or stress amplitude, temperature and frequency held constant). A strain or stress amplitude sweep is normally carried out first to determine the limit of linear viscoelastic behavior. In processing data from both static and dynamic tests it is always necessary to check that measurements were made in the linear region. This is done by calculating viscoelastic properties from the experimental data and determining whether or not they are independent of the magnitude of applied stresses and strains. 

The generation of frequency sweeps under computer control, described in Section II, 3 (p. 13) is one small aspect of a general tendency towards control of analytical instrumentation by means of digital computers, through appropriate interfacing devices. Several commercial and individually built systems have appeared in which all of the instrumental parameters for frequency sweeps or pulse-excitation (see Section IV, p. 45) are selected by teletype or numerical keyboard input to the computer, which then acquires the spectral data automatically and performs any further processing required. Automatic analysis of spectral peak positions and areas by a computer, and printing of the numerical results on a teletype or... 

The method for probe tuning on older spectrometers that are unable to produce the frequency sweep display is to place a directional coupler between the transmitter/receiver and the probe and to apply rf as a series of very rapid pulses. The directional coupler provides some form of display, usually a simple meter, which represents the total power being reflected back from the probe. The aim is to minimise this response by the tuning and matching process so that the maximum power is able to enter the sample. Unfortunately with this process, unlike the method described above, there is no display showing errors in tune and match separately, and there is no indication of the direction in which changes need be made, one simply has an indication of the overall response of the system. This method is clearly the inferior of the two, but may be the only option available. 

Figure 31 show s the tan 5 response in an RPA frequency sweep on two SBR 1006 rubbers with the same Mooney viscosity. The RPA distinguishes polymers by their processing properties even though they may have the same Mooney viscosity. (Typically rubber lots can have the same Mooney viscosity but still process quite differently.) The RPA gives the viscoelastic profiles for raw rubbers, which relates directly to processing differences [121],... 

Microcontroller and Computer Systems - The Arduino Nano board (from Atmel) is used as the interface between AD5933 (I2C) and the conputer (USB). The embedded software in the microcontroller executes special commands to configure operating parameters of the AD5933 and data read are sent to the computer. The software allows the setting of some parameters of the AD5933, such as frequency sweep number of collecting data, time between data collection, Vout amplitude and resistor calibration value. Impedance data are processed, converted in terms of R, Xc and PA values, which are stored in a text format. 

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