3.2.3.5 Measurement and Evaluation
Using Hypercarb as the stationary phase and the 1-decanol-TCB solvent gradient,
the propylene copolymers were separated. As can be seen in Fig. 3.27, the
copolymers with the shorter 1-olefins (1-butene, 1-hexene, 1-octene) did not adsorb
on the stationary phase but eluted in the SEC mode before the start of the solvent
gradient. For the 1-octene copolymers a small portion of material was detected that
eluted later with the solvent gradient. These are probably the copolymer molecules
with the highest comonomer content.
A different behaviour is obtained for copolymers with longer branches; see
Fig. 3.28 for the propylene copolymers with 1-tetradecene and 1-octadecene. The
elution volume increases with increasing 1-alkene content indicating that the longer
alkyl branches adsorb on the stationary phase. This is in excellent agreement with
the behaviour of ethylene copolymers where the longer ethylene segments interact
with the stationary phase. It can be concluded that, although the iPP polymer
backbone does not adsorb, substituents (branches) with a minimum number of
ethylene units (>6) promote adsorption.
As has been shown earlier, the retention of PP is stereospecific, i.e. iPP is not
retained, while sPP adsorbs on the Hypercarb stationary phase. When short alkyl
branches are introduced into the sPP structure, the interactions with the graphite
surface are disturbed and adsorption decreases. This is shown in Fig. 3.29 for
propylene-1-pentene copolymers.
A similar trend is found for ethylene-1-hexene copolymers; see Fig. 3.30. The
C4 branches in these copolymers are too short to interact with the stationary phase.
Adsorption takes place only based on the long ethylene sequences. Accordingly,
retention decreases with increasing comonomer content.
In the present application, ELS detection is used. It is known that peak intensity
in ELS detection is influenced by a number of factors, including the composition of
the analyte and the mobile phase. As can be seen in Fig. 3.30, the peak intensity
decreases with increasing comonomer content although similar sample amounts
were injected. At the same time samples of different compositions elute at different
elution volumes and, hence, get exposed to different solvent compositions. This
situation must be taken into account when the ELSD signal intensity is converted
into concentration. Typically, an ELSD calibration must be conducted.
In conclusion, the retention behaviour as a function of copolymer type and
composition is summarized in Fig. 3.31. The fact that copolymers containing the
same backbone but different comonomers (branches) behave differently may open
the way to characterize polyolefins regarding their branching microstructure.
3.3
Temperature Gradient Interaction Chromatography
Solvent gradient interaction chromatography (SGIC) is a very powerful technique
for the chemical composition separation of polyolefins, but it does have a few
limitations: one is the limited number of detectors that can be used. When using a
solvent gradient, typical concentration detectors such as RI and IR cannot be used.
3.3 Temperature Gradient Interaction Chromatography
109
Using Hypercarb as the stationary phase and the 1-decanol-TCB solvent gradient,
the propylene copolymers were separated. As can be seen in Fig. 3.27, the
copolymers with the shorter 1-olefins (1-butene, 1-hexene, 1-octene) did not adsorb
on the stationary phase but eluted in the SEC mode before the start of the solvent
gradient. For the 1-octene copolymers a small portion of material was detected that
eluted later with the solvent gradient. These are probably the copolymer molecules
with the highest comonomer content.
A different behaviour is obtained for copolymers with longer branches; see
Fig. 3.28 for the propylene copolymers with 1-tetradecene and 1-octadecene. The
elution volume increases with increasing 1-alkene content indicating that the longer
alkyl branches adsorb on the stationary phase. This is in excellent agreement with
the behaviour of ethylene copolymers where the longer ethylene segments interact
with the stationary phase. It can be concluded that, although the iPP polymer
backbone does not adsorb, substituents (branches) with a minimum number of
ethylene units (>6) promote adsorption.
As has been shown earlier, the retention of PP is stereospecific, i.e. iPP is not
retained, while sPP adsorbs on the Hypercarb stationary phase. When short alkyl
branches are introduced into the sPP structure, the interactions with the graphite
surface are disturbed and adsorption decreases. This is shown in Fig. 3.29 for
propylene-1-pentene copolymers.
A similar trend is found for ethylene-1-hexene copolymers; see Fig. 3.30. The
C4 branches in these copolymers are too short to interact with the stationary phase.
Adsorption takes place only based on the long ethylene sequences. Accordingly,
retention decreases with increasing comonomer content.
In the present application, ELS detection is used. It is known that peak intensity
in ELS detection is influenced by a number of factors, including the composition of
the analyte and the mobile phase. As can be seen in Fig. 3.30, the peak intensity
decreases with increasing comonomer content although similar sample amounts
were injected. At the same time samples of different compositions elute at different
elution volumes and, hence, get exposed to different solvent compositions. This
situation must be taken into account when the ELSD signal intensity is converted
into concentration. Typically, an ELSD calibration must be conducted.
In conclusion, the retention behaviour as a function of copolymer type and
composition is summarized in Fig. 3.31. The fact that copolymers containing the
same backbone but different comonomers (branches) behave differently may open
the way to characterize polyolefins regarding their branching microstructure.
3.3
Temperature Gradient Interaction Chromatography
Solvent gradient interaction chromatography (SGIC) is a very powerful technique
for the chemical composition separation of polyolefins, but it does have a few
limitations: one is the limited number of detectors that can be used. When using a
solvent gradient, typical concentration detectors such as RI and IR cannot be used.
3.3 Temperature Gradient Interaction Chromatography
109