• Columns. Two SDV (styrene–divinylbenzene copolymer) Olexis columns with
particle diameters of 13 μm and column sizes of 300 Â 8 mm i.d. (Polymer
Laboratories, Church Stretton, England).
• Mobile phase. TCB for both AF4 and SEC, distilled prior to use.
• Detectors. IR detector (IR4, Polymer Char, Valencia, Spain) and a HT-MALLS
detector (Heleos 2, Wyatt Technology, Santa Barbara, USA). The specific RI
increments used in TCB at 145
C were—0.091 for PE and 0.097 for PP.
• Separation temperature. 145
C for both AF4 and SEC.
• Sample preparation. Stirring and shaking of polymers during sample preparation
was avoided to reduce shear degradation. Samples were prepared by dissolving
the polymers in distilled TCB (2 mg/mL). Various parameters of sample preparation, namely the amount of stabilizer, the dissolution temperature and time
were varied. The absence of oxygen in the vials was ensured by the addition of
argon for some samples before dissolution. The remaining oxygen from the
solvent for all the measurements was removed with a degasser. For the samples
to which argon was added, the sample solvent was also flushed before passing
the degasser. Butylated hydroxytoluene (1 mg/mL) was added to the sample
solvent and the mobile phase for stabilization. An aluminium block was used to
heat the sample solutions in the vials. The temperature control during the sample
dissolution was ensured by a sensor inside the aluminium block. PTFE-silicone
septa work well at high temperature and were used to close the vials.
• Injection volume. 200 μL.
4.3.2.4 Measurement and Evaluation
A high dissolution temperature of 160
C had to be used for the samples to ensure
complete dissolution of samples containing significant amounts of UHM material
(consideration of the latter being the focus of this study). As a result, an initial
thermal degradation has to be accepted, especially for the PP samples. A sample of
UHM PE was dissolved under various conditions (see Table 4.2) and separated with
HT-AF4 and HT-SEC, resulting in MMDs as shown in Fig. 4.13.
Comparison of the MMDs and the average molar masses shows that the absence
of both antioxidant BHT and argon in the sample solution leads to a very pronounced decrease in molar masses. A few hours difference in the dissolving time of
the samples without BHT or argon leads to a very strong shift of the MMDs towards
lower values. The addition of argon alone or argon and BHT stabilizer slows down
the degradation process, but does not stop it completely.
There are large differences in the molar mass averages obtained from HT-SEC
and HT-AF4. In general, molar masses obtained from HT-AF4 are much higher
than the molar masses obtained from HT-SEC. The low and incorrect MMDs
obtained by SEC are attributed to shear degradation of the polymer samples during
SEC separation in the column packing and the inlet frits [9–11, 33, 34, 37–
39]. Furthermore, the thermo-oxidative degradation of the macromolecules during
the dissolution step superimposes the intensive shear degradation in SEC. In
comparison to SEC, polymer molecules are less exposed to shear degradation in
AF4 due to the absence of a stationary phase and, therefore, higher MMDs are
4.3 Analysis of Polyolefins by Asymmetric Flow FFF
163
particle diameters of 13 μm and column sizes of 300 Â 8 mm i.d. (Polymer
Laboratories, Church Stretton, England).
• Mobile phase. TCB for both AF4 and SEC, distilled prior to use.
• Detectors. IR detector (IR4, Polymer Char, Valencia, Spain) and a HT-MALLS
detector (Heleos 2, Wyatt Technology, Santa Barbara, USA). The specific RI
increments used in TCB at 145
C were—0.091 for PE and 0.097 for PP.
• Separation temperature. 145
C for both AF4 and SEC.
• Sample preparation. Stirring and shaking of polymers during sample preparation
was avoided to reduce shear degradation. Samples were prepared by dissolving
the polymers in distilled TCB (2 mg/mL). Various parameters of sample preparation, namely the amount of stabilizer, the dissolution temperature and time
were varied. The absence of oxygen in the vials was ensured by the addition of
argon for some samples before dissolution. The remaining oxygen from the
solvent for all the measurements was removed with a degasser. For the samples
to which argon was added, the sample solvent was also flushed before passing
the degasser. Butylated hydroxytoluene (1 mg/mL) was added to the sample
solvent and the mobile phase for stabilization. An aluminium block was used to
heat the sample solutions in the vials. The temperature control during the sample
dissolution was ensured by a sensor inside the aluminium block. PTFE-silicone
septa work well at high temperature and were used to close the vials.
• Injection volume. 200 μL.
4.3.2.4 Measurement and Evaluation
A high dissolution temperature of 160
C had to be used for the samples to ensure
complete dissolution of samples containing significant amounts of UHM material
(consideration of the latter being the focus of this study). As a result, an initial
thermal degradation has to be accepted, especially for the PP samples. A sample of
UHM PE was dissolved under various conditions (see Table 4.2) and separated with
HT-AF4 and HT-SEC, resulting in MMDs as shown in Fig. 4.13.
Comparison of the MMDs and the average molar masses shows that the absence
of both antioxidant BHT and argon in the sample solution leads to a very pronounced decrease in molar masses. A few hours difference in the dissolving time of
the samples without BHT or argon leads to a very strong shift of the MMDs towards
lower values. The addition of argon alone or argon and BHT stabilizer slows down
the degradation process, but does not stop it completely.
There are large differences in the molar mass averages obtained from HT-SEC
and HT-AF4. In general, molar masses obtained from HT-AF4 are much higher
than the molar masses obtained from HT-SEC. The low and incorrect MMDs
obtained by SEC are attributed to shear degradation of the polymer samples during
SEC separation in the column packing and the inlet frits [9–11, 33, 34, 37–
39]. Furthermore, the thermo-oxidative degradation of the macromolecules during
the dissolution step superimposes the intensive shear degradation in SEC. In
comparison to SEC, polymer molecules are less exposed to shear degradation in
AF4 due to the absence of a stationary phase and, therefore, higher MMDs are
4.3 Analysis of Polyolefins by Asymmetric Flow FFF
163