Thermo-oxidative degradation is known to be a major problem in HT-SEC
experiments of high molar mass polyolefins. Depending on the solvent used, the
sample concentration, the dissolution temperature and time, chain scission can take
place to a larger or lesser extent. Therefore, suitable protocols for SEC sample
preparation are of utmost importance for the validity of the analytical results.
4.3.2.1 Aim
Thermo-oxidative degradation and shear stress in parts of the HT-SEC system are
the main reasons for the decrease in PE molar masses [14, 33–36]. In the present
application, the influence of the dissolution procedure on the molar mass of UHM PE
shall be investigated. HT-SEC measurements shall be compared to results of HT-AF4
analyses, where shear stress or unwanted interaction of the macromolecules with the
stationary phase (such as in SEC) are avoided. The absence of shear stress and
co-elution in AF4 makes it possible to track the influence of the thermo-oxidative
degradation process on the molar mass distribution without disturbing secondary
effects that often significantly influence the SEC results.
4.3.2.2 Materials
• Polymers. HDPE, LDPE, linear and branched PP. HDPE and PP were technical
products (Basell, Frankfurt, Germany). The UHM PE AK1 was obtained from
the University of Freiburg, Germany. The LDPE samples were synthesized by a
free radical polymerization process.
4.3.2.3 Equipment
• AF4 instrument. AF2000 (Postnova Analytics, Landsberg/Lech, Germany)
which was specifically configured to be used at high temperature above
130
C. For this aim the AF4 pump system was connected to a PL GPC-220
chromatograph (Polymer Laboratories, Church Stretton, England). The HT-AF4
channel was placed inside the column oven. The AF4 channel was connected to
three pumps from Postnova Analytics and to an additional pump management
system which ensured a constant flow rate to the detectors during the entire
separation. The HT channel had a spacer of 350 μm thickness; it was clamped
between two plates made of stainless steel. For HT work, a ceramic membrane
with a cut-off of approx. 50 kg/mol PE in TCB was used for separation. The flow
rate at the detector was 0.5 mL/min.
• HT-SEC. AF2000 (Postnova Analytics, Landsberg/Lech, Germany) was
configured for HT-SEC work. The columns were installed inside the PL-220
chromatograph next to the AF4 channel. Three 6-port HT valves (Valco
Instruments, Waterbury, USA) were used to control the injection and switching
between two separation systems. The detectors were connected to the outlet of
the channel and the columns. A temperature of 145
C in TCB was used for all
separations. Argon was flushed through the solvent to remove any remaining
oxygen. The flow rate was set at 0.5 mL/min.
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4 Field-Flow Fractionation
experiments of high molar mass polyolefins. Depending on the solvent used, the
sample concentration, the dissolution temperature and time, chain scission can take
place to a larger or lesser extent. Therefore, suitable protocols for SEC sample
preparation are of utmost importance for the validity of the analytical results.
4.3.2.1 Aim
Thermo-oxidative degradation and shear stress in parts of the HT-SEC system are
the main reasons for the decrease in PE molar masses [14, 33–36]. In the present
application, the influence of the dissolution procedure on the molar mass of UHM PE
shall be investigated. HT-SEC measurements shall be compared to results of HT-AF4
analyses, where shear stress or unwanted interaction of the macromolecules with the
stationary phase (such as in SEC) are avoided. The absence of shear stress and
co-elution in AF4 makes it possible to track the influence of the thermo-oxidative
degradation process on the molar mass distribution without disturbing secondary
effects that often significantly influence the SEC results.
4.3.2.2 Materials
• Polymers. HDPE, LDPE, linear and branched PP. HDPE and PP were technical
products (Basell, Frankfurt, Germany). The UHM PE AK1 was obtained from
the University of Freiburg, Germany. The LDPE samples were synthesized by a
free radical polymerization process.
4.3.2.3 Equipment
• AF4 instrument. AF2000 (Postnova Analytics, Landsberg/Lech, Germany)
which was specifically configured to be used at high temperature above
130
C. For this aim the AF4 pump system was connected to a PL GPC-220
chromatograph (Polymer Laboratories, Church Stretton, England). The HT-AF4
channel was placed inside the column oven. The AF4 channel was connected to
three pumps from Postnova Analytics and to an additional pump management
system which ensured a constant flow rate to the detectors during the entire
separation. The HT channel had a spacer of 350 μm thickness; it was clamped
between two plates made of stainless steel. For HT work, a ceramic membrane
with a cut-off of approx. 50 kg/mol PE in TCB was used for separation. The flow
rate at the detector was 0.5 mL/min.
• HT-SEC. AF2000 (Postnova Analytics, Landsberg/Lech, Germany) was
configured for HT-SEC work. The columns were installed inside the PL-220
chromatograph next to the AF4 channel. Three 6-port HT valves (Valco
Instruments, Waterbury, USA) were used to control the injection and switching
between two separation systems. The detectors were connected to the outlet of
the channel and the columns. A temperature of 145
C in TCB was used for all
separations. Argon was flushed through the solvent to remove any remaining
oxygen. The flow rate was set at 0.5 mL/min.
162
4 Field-Flow Fractionation