Chemistry Reference
In-Depth Information
1,0
1,0
70
EP 2
EP 1
50
60
0,8
0,8
40
50
0,6
0,6
30
40
30
20
0,4
0,4
20
10
0,2
0,2
10
0
0
0,0
0,0
0
2
4
6
8
10
12
0
2
4
6
8
10
Elution Volume (mL)
Elution Volume (mL)
60
1,0
35
1,0
EP 3
EP 4
30
50
0,8
0,8
25
40
0,6
20
0,6
30
15
0,4
0,4
20
10
10
5
0,2
0,2
0
0
0,0
0,0
0
2
4
6
8
10
0
2
4
6
8
10
Elution Volume (mL)
Elution Volume (mL)
Figure 7.
Gram-Schmidt plots of the HT-gradient HPLC separation of EP copolymers 1-4, full line Gram-Schmidt plots,
&
relative amount of CH
3
groups (peak ratio CH
3
/CH
2
), chromatographic conditions see Figure 3.
with the sample solvent peak. From the fact
that the peak area ratio is less than for pure
PP (0.75 in Figure 6) it can be concluded
that the early eluting peaks do not contain
PP but EP copolymer with low ethylene
contents. This is in a good agreement with
the CRYSTAF measurements, where no
isotactic PP and no PE were detected. With
increasing PE content the EP copolymer
becomes less and less soluble in EGMBE
and elutes with the solvent gradient accord-
ing to the solvent strength. The EP copoly-
mers start to elute close to the starting point
of the gradient between 7.1 and 7.5 mL.
Apparently, there is a solubility threshold
at a certain copolymer composition. Accor-
dingly, rather similar CH
3
/CH
2
ratios are
obtained for the end of the first peak and
the beginning of the second peak. For the
EP copolymer fractions with the lowest
propylene content, peak area ratios of
0.25 to 0.35 are measured. Compared to
the blend separation in Figure 5 a shift to
lower elution volumes is observed for the
late eluting peaks. This additionally indi-
cates that the peaks are due to EP copoly-
mer and not PE homopolymer. Finally, the
decreasing peak area ratio with increasing
elution volume within the late eluting peaks
clearly confirms the separation with regard
to chemical composition. Due to the steep
gradient it is not surprising that the peak
positions of the EP copolymers do not vary
much. For a better separation of the
fractions A more shallow gradient has to
be chosen.
As mentioned earlier, the higher boiling
point of n-decanol compared to EGMBE
results in a certain spreading of n-decanol
on the Germanium disc. The elution peak
appearing at an elution volume of 4 mL
corresponds to the solvent peak in Figure 3.
This peak contains small amounts of EP
copolymer with medium ethylene content
as can be seen in Figure 7.
As indicated in Figure 7, the analyzed
EP copolymers exhibit broad distributions
with chemical compositions ranging from
0.7 to 0.3 (relative amount of CH
3
/CH
2
).
The differences between the samples can be
