curves to examine the distribution of ethylene and propylene crystallinity across the
molar mass profiles; see Fig. 2.16.
The low elution volume component of the 60
C fraction shows a very low level
of propylene isotacticity or crystallinity. The higher elution volume component
(corresponding to lower molar mass) shows higher ratios of 998 cm
À1 /972 cm
À1 ,
indicating the presence of PP homopolymer. Crystalline ethylene sequences are
present only in the lower elution volume component, which can be identified as the
semi-crystalline EPC component of this fraction. The absence of the 730 cm
À1 and
720 cm
À1 absorption bands resulted in the discontinuation of this profile within the
higher elution volume component of the distribution.
In the case of the 80
C and 90
C fractions, the ratio of the 998 cm
À1 /972 cm
À1
absorption bands increases across the bimodal distribution towards higher elution
volumes where PP homopolymer is located. Ethylene crystallinity is only seen in
the lower elution volume component. This is the region where EPC elutes. Therefore, crystalline ethylene and propylene segments are found in the EPC phase and
highly crystalline isotactic PP is found in the lower molar mass PP phase. The
crystalline ethylene segments of the EPC are represented by lower temperature melt
endotherms in the DSC heating curve. The higher temperature melt endotherm in
the DSC heating curve is due to melting of propylene segments of EPC and PP
homopolymer. A uniform propylene concentration is detected in the 100
C fraction
across the Gram–Schmidt curve. The higher elution volume end of the PP homopolymer component of preceding fractions and this fraction show similar values for
the 998 cm
À1 /972 cm
À1 ratio. There is only a slight variation at the lower elution
volume shoulder, where EPC elutes, as indicated by the CH 3 /CH 2 ratio. Crystalline
ethylene segments are also detected only in the low elution volume shoulder of the
Gram–Schmidt plot. The SEC-FTIR results for ethylene and propylene crystallinity
agree well with DSC results on the thermal behaviour of the fractions.
2.1.3 Analysis of Thermo-oxidatively Degraded
Polypropylene [83]
Polyolefins are susceptible to degradation which takes place throughout the life
cycle of the material. Degradation occurs during polymerization, processing, application and recycling. It influences the polymer properties, thereby limiting the
lifetime of the materials and leading to economic losses [84, 85]. To reduce the
degradation of a particular material, the sources of degradation and the degradation
pathways must be understood. This is a strong motivation to search for new
analytical methods to analyse and monitor the degradation of polyolefins [86]. A
particular aspect is the increasing importance of polymer recycling with the aim not
to downgrade the material.
One can distinguish between photo-oxidative and thermo-oxidative degradation.
Polyolefins can also be attacked by strong acids [87]. The generally accepted free
radical oxidation model of polyolefins involves radical initiation, propagation and
termination reactions [88, 89]. Following an initiation reaction, which usually
2.1 Temperature Rising Elution Fractionation
35
molar mass profiles; see Fig. 2.16.
The low elution volume component of the 60
C fraction shows a very low level
of propylene isotacticity or crystallinity. The higher elution volume component
(corresponding to lower molar mass) shows higher ratios of 998 cm
À1 /972 cm
À1 ,
indicating the presence of PP homopolymer. Crystalline ethylene sequences are
present only in the lower elution volume component, which can be identified as the
semi-crystalline EPC component of this fraction. The absence of the 730 cm
À1 and
720 cm
À1 absorption bands resulted in the discontinuation of this profile within the
higher elution volume component of the distribution.
In the case of the 80
C and 90
C fractions, the ratio of the 998 cm
À1 /972 cm
À1
absorption bands increases across the bimodal distribution towards higher elution
volumes where PP homopolymer is located. Ethylene crystallinity is only seen in
the lower elution volume component. This is the region where EPC elutes. Therefore, crystalline ethylene and propylene segments are found in the EPC phase and
highly crystalline isotactic PP is found in the lower molar mass PP phase. The
crystalline ethylene segments of the EPC are represented by lower temperature melt
endotherms in the DSC heating curve. The higher temperature melt endotherm in
the DSC heating curve is due to melting of propylene segments of EPC and PP
homopolymer. A uniform propylene concentration is detected in the 100
C fraction
across the Gram–Schmidt curve. The higher elution volume end of the PP homopolymer component of preceding fractions and this fraction show similar values for
the 998 cm
À1 /972 cm
À1 ratio. There is only a slight variation at the lower elution
volume shoulder, where EPC elutes, as indicated by the CH 3 /CH 2 ratio. Crystalline
ethylene segments are also detected only in the low elution volume shoulder of the
Gram–Schmidt plot. The SEC-FTIR results for ethylene and propylene crystallinity
agree well with DSC results on the thermal behaviour of the fractions.
2.1.3 Analysis of Thermo-oxidatively Degraded
Polypropylene [83]
Polyolefins are susceptible to degradation which takes place throughout the life
cycle of the material. Degradation occurs during polymerization, processing, application and recycling. It influences the polymer properties, thereby limiting the
lifetime of the materials and leading to economic losses [84, 85]. To reduce the
degradation of a particular material, the sources of degradation and the degradation
pathways must be understood. This is a strong motivation to search for new
analytical methods to analyse and monitor the degradation of polyolefins [86]. A
particular aspect is the increasing importance of polymer recycling with the aim not
to downgrade the material.
One can distinguish between photo-oxidative and thermo-oxidative degradation.
Polyolefins can also be attacked by strong acids [87]. The generally accepted free
radical oxidation model of polyolefins involves radical initiation, propagation and
termination reactions [88, 89]. Following an initiation reaction, which usually
2.1 Temperature Rising Elution Fractionation
35