The fitting function is based on that used for the static measurements. The quantum mechanical treatment of this experiment gives the efficiency factor in the kernel function (Equation ) that is reduced to ϵ = 0.78. 4 Results and discussion The DMTA experiments give a first indication of the bulk changes occurring in the weathered samples. The storage and loss modulus (G' and G", respectively) obtained from temperature ramps of NR and SBR samples subjected to various durations of weathering are shown in Figure . Below Tg, NR reveals a reduction in modulus with the exposure duration. With an increase in the temperature, the 72 and 168 h weathered specimens demonstrate identically marginal retention of the G', indicating hardening compared to the unaged specimen. Thus, a marginal increase in the Tg from the unaged sample is also observable. Distinct changes occur around 500 h of weathering, where stiffening of the sample due to weathering leads to a significant increase in modulus. In addition, a broad second transition around 60 °C can also be seen, which probably is an indication of a second glass transition corresponding to over cross-linked regions of the specimen. With further weathering, stiffness drops below that of the 504 h weathered sample. At higher temperatures, the specimen undergoes softening, well below the plateau modulus of 0 h specimen, due to softening of the hardened regions and the possible presence of decomposed chains. In the case of SBR, the rubber continuously stiffens in both temperature regions due to weathering-induced hardening. The additional transition appearing upon 500 h of weathering seems to be due to differently aged regions withing the sample volume. One may expect that the surface is more aged than the core, thus creating regions of different stiffness. This difference seems to be lost after further weathering. The additional transition observed in the previous weathering interval probably is masked due to the stiffening of the overall sample. The relative stiffening in SBR is higher than NR and remains a decade stiffer even at 100 °C.