Mechanical Spectra of an Unfilled Elastomer Blend
Purpose
To construct a master curve for an SBR/NR blend.
Sample
The measurements were performed using the elastomer blend described in Section 4.5.5. Temperature-dependent DMA measurements of an unfilled SBR/NR elastomer.
Composition:
Blend of 40% NR and 60% L-SBR, vulcanized with 2 phr sulfur
Conditions
Measuring cell: DMA/SDTA861e with shear sample holder
Sample preparation: Cylinder of 6-mm diameter punched out from a 1.6-mm thick film and mounted in the shear sample holder with 10% predeformation.
DMA measurement: The sample was measured isothermally at different frequencies beginning at –50 °C in steps of 10 K up to 140 °C. At each step the frequency range used was between 0.03 Hz and 1000 Hz. The displacement amplitude was 10 μm.
The measured curves were used to construct a master curve whereby G' and G" were shifted simultaneously to achieve the best degree of overlap for both curves. The reference temperature of the master curve is –10 °C.
Interpretation
The glass transition of NR is measured in the master curve at the highest frequencies. It was possible to construct an optimum master curve in this region. The same applies to the frequency range in which the glass transition of the SBR occurs, i.e. at frequencies below 102 Hz. Between the glass transitions, it is not possible to obtain a reasonable overlap of the measured curve segments. The reason for this is that the measured curves contain information from both relaxation processes, but these however exhibit different temperature dependencies. The distance between the relaxation ranges changes with measurement temperature and a perfect master curve cannot be obtained. A master curve can also be constructed in the rubbery plateau below 101 Hz. The flow relaxation can be seen in the GƎ curve at about 3106 Hz. Both this effect and the value of the storage modulus of about 0.4 MPa are indicators for the cross-linking density. The curve is typical for a sulfur content of about 2 phr (see Section 4.2.2.Master curves of differently vulcanized SBR samples).
Conclusions
With polymer blends, it is not possible to construct a continuous master curve because of the different temperature-dependencies of the relaxation ranges. The curve obtained, nevertheless, describes the frequency–dependent mechanical behavior of the material to a good approximation. Important information can therefore also be gained from these curves about the properties and structure of materials. Despite the limitations discussed concerning the overlapping of relaxation ranges, the master curves obtained are often more useful than temperature-dependent measurements. It is essential to perform measurements over a wide frequency range in order to improve the quality of the curves, especially in cases where the construction of master curves is difficult.
Mechanical Spectra of an Unfilled Elastomer Blend | Thermal Analysis Handbook No.HB477 | Application published in METTLER TOLEDO TA Application Handbook Elastomers Volume 2