3.3. TEM
TEM-images of TPV and SEBS/PP/oil blends are shown in
Fig. 5(a) and (b). The dark phase represents the rubber phase
and the white phase is PP. This technique proves to be the best
for visualizing the morphology of these blends. The image on
the left clearly shows elongated EPDM-domains dispersed in
the PP-matrix. The same seems to happen for the SEBS-phase
in the image shown on the right. A higher magnification image
of the SEBS-phase shows distinct polystyrene domains
(Fig. 5(c)) more or less clustered together, and phase separated
from the ethylene–butylene matrix. The PP phase is more
evenly distributed within the SEBS-phase as compared to the
EPDM-phase in Fig. 5(a), which hints in the direction of cocontinuous
morphology. The contrast between the rubber and
the PP-phase is very high as compared to the one seen with
LVSEM. The thickness of ultrathin sections (w50 nm),
staining time (w30 min) and the concentration of the staining
agent (1% solution of RuO4) were found to be very crucial for
obtaining good images.
The reason why TEM-images could reveal the finest
morphological details as compared to LVSEM can be
explained by the fact that the image in TEM involves detecting
the electrons transmitted through a sample, while SEM
involves detecting the number of secondary electrons scattered
by the sample. The voltage used in TEM is about 200 keV
while with low voltage SEM it is around 1 keV. This means
that the signal to noise ratio is much higher in TEM as
compared to LVSEM. The amount of secondary electrons
produced in LVSEM is too low to be useful in creating highresolution
images. Detecting transmitted electrons in TEM
demands a very thin section: w50 nm. This also reduces
sample charging at high acceleration voltages, because the
charge on the sample can easily be grounded.
3.4. Electron tomography
Representative tomographic images of co-continuous TPVand
SEBS/PP/oil-blends are shown in Fig. 6(a) and (b),
respectively. These images only show the oultline of the
EPDM-phase in Fig. 6(a) and for the SEBS-phase in Fig. 6(b).
The tomographic images shown in Fig. 6(a) show
interesting details of TPV morphology that cannot be observed
with conventional TEM. TEM images of TPV samples, having
a high loading of EPDM-phase often lead to confusion about
the dispersed nature of the EPDM-phase. In those blends, the
interparticle distances between the EPDM domains are rather
small. Since the image in TEM is a superposition of all EPDM
domains present along the electron beam trajectory, in the 2Dimage
overlapping domains with small interparticle distances
appear to touch each other. This gives the impression of a cocontinuous
EPDM structure. Tilting the sample under TEM
during a tomography experiment provides a different side of
view, which increases the accuracy of observation and resolves
any conflicts about overlapping domains. An example of this
effect is given in Fig. 7. The EPDM-domains that appear to
touch each other in Fig. 7(a) look separated on tilting, Fig. 7(b).
The resulting tomographic model obtained by backprojecting
these 2D-TEM images, as shown in Fig. 6(a), conclusively
proves the dispersed nature of the EPDM-phase, even in blends
containing high amounts of the EPDM-phase.
A TEM image of a 28:33:39 wt% SEBS/PP/oil blend
composition was shown in Fig. 5(b). In the image the