Temperature Scan DMA Measurements of Differently Vulcanized SBR Samples
Purpose
Temperature dependent DMA measurements of differently vulcanized SBR.
Sample
The samples are described in Section 4.2.2.
Conditions
Measuring cell: DMA/SDTA861e with shear sample holder
Sample preparation: Cylinders of 5-mm diameter were punched out from a 1.2-mm thick film and mounted in the shear sample holder with 10% predeformation.
DMA measurement: The measurements were performed at 10 Hz and a heating rate of 2 K/min in the temperature range –60 °C to 100 °C. Maximum force amplitude 10 N, maximum displacement amplitude 10 Pm, offset control zero.
Interpretation
As already known from the DSC measurements, the glass transition temperature shifts to higher temperatures with increasing degree of vulcanization. This behavior can also be observed in the DMA curves. From about 30 °C onward, after the glass transition has occurred, the course of the storage modulus curve is different depending on the degree of vulcanization. The weakly vulcanized sample SBR1 shows a decrease with increasing temperature. This is due to viscous flow. If the network density is so large that flow cannot occur, the storage modulus increases almost linearly with temperature. This is in fact the theoretically expected behavior of a rubbery elastic network, for which the relationship between the shear modulus and the temperature, T, can be described by the equation G=ϕρRZT. Here G is the equilibrium shear modulus, R the gas constant, ρ the density and ϕ an empirical parameter. Z characterizes the average molar mass between the cross-linking points (Me). In an ideal network, Z = 1/Me and ϕ= 1. With greater cross-linking density, therefore, both the modulus in the rubbery plateau and G should increase with temperature. This is confirmed by the behavior of the heavily cross-linked samples, SBR3 and SBR4. If the sulfur content is doubled, one would expect twice the concentration of network sites, giving rise to a larger modulus. At 60 °C the value of the storage modulus of SBR4 is 1.1 MPa, which is about double the value of SBR3 (0.58 MPa). Cross-linking causes the molecular inhomogeneity of the material to increase. Since the concentration of cross-linking points is relatively low, the cross-linking points are distributed statistically and not homogeneously. This inhomogeneity leads to a broadening of the glass transition. This means that with an increasing degree of vulcanization, the glass transition should become broader. This effect can be clearly seen in the G" peak.
Evaluation
In the case of an ideal network, the cross-linking density, k, can be estimated from the equation 
If the density, ρ, is assumed to be 1 g/cm3 , for SBR3 a cross-linking density of 1.07*10-4 mol/g is obtained. With SBR4, the cross-linking density, N, is 2.03104 mol/g. The ratio of the two values is 1.90 and agrees with the ratio of the sulfur contents of 1.87. In order to investigate the effect of cross-linking on the glass transition, the width at half height of the G" peak was evaluated (in linear presentation). In the following diagram, the width at half height is plotted against the sulfur content. It can be seen that the width at half height increases with increasing sulfur content.
The relationship between the width of the G" peak and the sulfur content can be used to estimate the sulfur content of an unknown elastomer.
Conclusions
Temperature-dependent mechanical measurements provide additional information about the elastomer network besides the relationship between the glass transition temperature and the degree of cross-linking. The width of the GƎ peak and the storage modulus in the rubbery plateau can be used to estimate the sulfur content and the cross-linking density. The storage modulus allows information to be obtained about the structure of the elastomer network.
Temperature Scan DMA Measurements of Differently Vulcanized SBR Samples | Thermal Analysis Application No. HB446 | Application published in METTLER TOLEDO TA Application Handbook Elastomers, Volume 2