Similar to the 60
C fraction, the HPLC results for the 3V 80
C fraction
(Fig. 3.50e) show a complex distribution of different EP copolymers. In comparison, the chromatogram of the 80
C fraction of 3VA (Fig. 3.50f) shows mainly iPP
(that elutes before the start of the gradient) and a very late eluting component that
may be assigned to PE. This comparison shows clearly that there are significant
compositional differences between the two samples that are mainly due to the
different ethylene contents. These results again confirm the capability of
HT-HPLC as the only suitable method to distinguish the CCD present in such
complex TREF fractions, which are indistinguishable by SEC (MMD) and DSC
(melting behaviour). The chromatograms of the higher TREF fractions indicate
mainly the presence of iPP eluting in two peaks, depending on molar mass and
isotacticity.
More detailed information on the chemical composition of the fractions obtained
by HT-HPLC is accessible by combining this fractionation with FTIR spectroscopy. The experimental background of such measurements has been outlined
earlier and shall not be discussed again here. As an example, the analysis of the
bulk sample 3VA is shown in Fig. 3.51. Hence, with the help of the corresponding
FTIR spectra, it is proved that the early eluting fractions are PP while the late
eluting fractions are EP copolymers and PE.
Figures 3.52 and 3.53 present the chemical composition (propylene and ethylene
contents) and the crystallinity distributions of the TREF fractions as obtained by
HT-HPLC-FTIR. The ratio of the peak areas CH 3 /CH 2 decreased with increasing
elution volume, indicating an increase in ethylene content. The results are in
agreement with the separation mechanism, which assumes strong retention of
ethylene sequences and no retention of linear propylene sequences. At an elution
volume of approximately 6 mL, PE homopolymers with low branching elute (lower
CH 3 /CH 2 value for late eluting fractions). Propylene sequences with higher crystallinity were found in early eluting fractions where crystalline PE is not present.
Crystalline PE is present only in the late eluting fractions. This again confirms the
proposed adsorption-desorption mechanism on the Hypercarb stationary phase
according to the E/P content and sequence lengths in the sample. According to
the TREF separation mechanism, the higher temperature TREF fractions are mainly
composed of highly crystalline iPP (not discussed here).
In order to further confirm these results, individual IR spectra at peak maximum
for each component were analysed. These spectra proved that the component
eluting at 2.0 mL is iPP homopolymer, and the component eluting at 4.2 mL is
EP copolymer or a propylene-rich or branched copolymer. The individual spectrum
for the component eluting at 5.5 mL was identical to that of PE homopolymer. In
the HT-HPLC-FTIR results, it was observed that the elution volume of the first
component in each fraction is nearly identical. The elution volume of the second
component in the 80
C and 90
C fractions decreased compared to that of the
similar component in the 30
C and 60
C fractions, and the elution volume of the
late eluting fraction increased from the 30
C to the 90
C fraction. The first
component of all the fractions is iPP homopolymer, which co-crystallized with
other components during the TREF crystallization step, either due to differences in
3.4 Two-Dimensional Liquid Chromatography
137
C fraction, the HPLC results for the 3V 80
C fraction
(Fig. 3.50e) show a complex distribution of different EP copolymers. In comparison, the chromatogram of the 80
C fraction of 3VA (Fig. 3.50f) shows mainly iPP
(that elutes before the start of the gradient) and a very late eluting component that
may be assigned to PE. This comparison shows clearly that there are significant
compositional differences between the two samples that are mainly due to the
different ethylene contents. These results again confirm the capability of
HT-HPLC as the only suitable method to distinguish the CCD present in such
complex TREF fractions, which are indistinguishable by SEC (MMD) and DSC
(melting behaviour). The chromatograms of the higher TREF fractions indicate
mainly the presence of iPP eluting in two peaks, depending on molar mass and
isotacticity.
More detailed information on the chemical composition of the fractions obtained
by HT-HPLC is accessible by combining this fractionation with FTIR spectroscopy. The experimental background of such measurements has been outlined
earlier and shall not be discussed again here. As an example, the analysis of the
bulk sample 3VA is shown in Fig. 3.51. Hence, with the help of the corresponding
FTIR spectra, it is proved that the early eluting fractions are PP while the late
eluting fractions are EP copolymers and PE.
Figures 3.52 and 3.53 present the chemical composition (propylene and ethylene
contents) and the crystallinity distributions of the TREF fractions as obtained by
HT-HPLC-FTIR. The ratio of the peak areas CH 3 /CH 2 decreased with increasing
elution volume, indicating an increase in ethylene content. The results are in
agreement with the separation mechanism, which assumes strong retention of
ethylene sequences and no retention of linear propylene sequences. At an elution
volume of approximately 6 mL, PE homopolymers with low branching elute (lower
CH 3 /CH 2 value for late eluting fractions). Propylene sequences with higher crystallinity were found in early eluting fractions where crystalline PE is not present.
Crystalline PE is present only in the late eluting fractions. This again confirms the
proposed adsorption-desorption mechanism on the Hypercarb stationary phase
according to the E/P content and sequence lengths in the sample. According to
the TREF separation mechanism, the higher temperature TREF fractions are mainly
composed of highly crystalline iPP (not discussed here).
In order to further confirm these results, individual IR spectra at peak maximum
for each component were analysed. These spectra proved that the component
eluting at 2.0 mL is iPP homopolymer, and the component eluting at 4.2 mL is
EP copolymer or a propylene-rich or branched copolymer. The individual spectrum
for the component eluting at 5.5 mL was identical to that of PE homopolymer. In
the HT-HPLC-FTIR results, it was observed that the elution volume of the first
component in each fraction is nearly identical. The elution volume of the second
component in the 80
C and 90
C fractions decreased compared to that of the
similar component in the 30
C and 60
C fractions, and the elution volume of the
late eluting fraction increased from the 30
C to the 90
C fraction. The first
component of all the fractions is iPP homopolymer, which co-crystallized with
other components during the TREF crystallization step, either due to differences in
3.4 Two-Dimensional Liquid Chromatography
137