due to the intensive stabilization is indicated by Fig. 4.15a. A slight shift of the
entire distribution curve in the direction of low molar masses manifests degradation. The strong degradation of the same sample without BHT and argon is shown in
Fig. 4.15b as indicated by the decrease of the high molar mass region in favour of a
shoulder which is formed in the low molar mass region. In addition the whole molar
mass distribution is shifted towards lower molar masses.
The average molar masses of branched polymers as given in Table 4.3 stay high
in comparison to the linear materials (see Table 4.2) despite the changes in the
molar mass distribution curves even for long dissolving times and without stabilization. The tertiary C-H bonds, that have low dissociation energy and result in more
stable radicals, are the preferred sites for the start of the thermo-oxidative degradation. More tertiary C-H bonds are present in LDPE than in linear materials because
free radical polymerization induces the formation of long- and short-chain branches
at the main chain and at the side chains [40, 41]. Accordingly, the probability of
chain scission or radical formation during thermal treatment at the various side
chains that contain additional branching points is rather high. The lower molar mass
degradation for the branched samples seems to contradict, therefore, the degradation mechanism. However, one has to keep in mind that side chains in branched
samples are significantly shorter than the main backbone of a linear or slightly
branched polymer. Even a significant number of chain scissions resulting in the
abstraction of a number of side chains will not lead to a significant decrease in
average molar mass. The amount of low molar mass material will increase at the
expense of the highest molar mass fractions as depicted in Fig. 4.15. It can,
therefore, be assumed that the impact of degradation on the entire molar mass
distribution of LDPE will not be as pronounced as for a comparable linear or
slightly branched PE.
In conclusion, the effects of the dissolution process, as well as the separation
conditions and methods on the obtained molar masses are, impressively
demonstrated. All samples undergo shear degradation in SEC, but this has been
successfully eliminated in HT-AF4. The molar mass dependence of shear degradation is also shown, which prevents the possible use of correction factors for the
molar mass analysis by SEC.
Most problems in SEC are caused by the stationary phase, which is not used in
HT-AF4. Without the effects of the stationary phase, HT-AF4 gives a clear picture
on the thermo-oxidative degradation of polyolefins that is a result of sample
preparation and treatment. The results have shown that the thermal degradation is
equally molar mass dependent. Degradation can be significantly reduced by stabilization using BHT and/or argon gas. However, even stabilized samples show a
decrease in molar mass when long dissolution times are used.
4.3 Analysis of Polyolefins by Asymmetric Flow FFF
171
entire distribution curve in the direction of low molar masses manifests degradation. The strong degradation of the same sample without BHT and argon is shown in
Fig. 4.15b as indicated by the decrease of the high molar mass region in favour of a
shoulder which is formed in the low molar mass region. In addition the whole molar
mass distribution is shifted towards lower molar masses.
The average molar masses of branched polymers as given in Table 4.3 stay high
in comparison to the linear materials (see Table 4.2) despite the changes in the
molar mass distribution curves even for long dissolving times and without stabilization. The tertiary C-H bonds, that have low dissociation energy and result in more
stable radicals, are the preferred sites for the start of the thermo-oxidative degradation. More tertiary C-H bonds are present in LDPE than in linear materials because
free radical polymerization induces the formation of long- and short-chain branches
at the main chain and at the side chains [40, 41]. Accordingly, the probability of
chain scission or radical formation during thermal treatment at the various side
chains that contain additional branching points is rather high. The lower molar mass
degradation for the branched samples seems to contradict, therefore, the degradation mechanism. However, one has to keep in mind that side chains in branched
samples are significantly shorter than the main backbone of a linear or slightly
branched polymer. Even a significant number of chain scissions resulting in the
abstraction of a number of side chains will not lead to a significant decrease in
average molar mass. The amount of low molar mass material will increase at the
expense of the highest molar mass fractions as depicted in Fig. 4.15. It can,
therefore, be assumed that the impact of degradation on the entire molar mass
distribution of LDPE will not be as pronounced as for a comparable linear or
slightly branched PE.
In conclusion, the effects of the dissolution process, as well as the separation
conditions and methods on the obtained molar masses are, impressively
demonstrated. All samples undergo shear degradation in SEC, but this has been
successfully eliminated in HT-AF4. The molar mass dependence of shear degradation is also shown, which prevents the possible use of correction factors for the
molar mass analysis by SEC.
Most problems in SEC are caused by the stationary phase, which is not used in
HT-AF4. Without the effects of the stationary phase, HT-AF4 gives a clear picture
on the thermo-oxidative degradation of polyolefins that is a result of sample
preparation and treatment. The results have shown that the thermal degradation is
equally molar mass dependent. Degradation can be significantly reduced by stabilization using BHT and/or argon gas. However, even stabilized samples show a
decrease in molar mass when long dissolution times are used.
4.3 Analysis of Polyolefins by Asymmetric Flow FFF
171