Otte et al. [7, 21, 22, 31] conducted several detailed studies on the analysis of
UHM polyolefins following the work of Mes et al. [6]. The problems associated
with SEC of UHM samples, namely the shear degradation of high molar mass
structures and the anomalous late co-elution effects, can be successfully avoided in
HT-AF4. This allows accurate molar mass analysis of samples up to R g > 1,000 nm.
The curvature that existed in the conformation plot based on SEC measurements is
absent in similar AF4 plots. This curvature results in erroneous branching and molar
mass calculations. Accordingly, molar mass averages calculated from HT-AF4 are
significantly higher than those obtained from HT-SEC [7]. In addition, HT-AF4
allows the visualization of the thermo-oxidative degradation of polyolefins in
solution [31]. Improper sample treatment, including sample preparation for
HT-SEC, can be evaluated by this approach.
To summarize, the application of AF4 for the characterization of polyolefins
enables wider insight into molecular properties, which are apparently more complex than has been found to date by traditional separation methods.
4.3.1 Characterization of Branched Ultrahigh Molar Mass
Polyethylene by AF4 [7]
Polymers with weight average molar masses (M w ) > 500 kg/mol are typically
considered as UHM polymers. The mechanical stability and specific weight of
UHM PEs are superior compared to traditional materials. Therefore, UHM PE is
used for special applications such as ultrastrong fibers, implants, or toothed wheels.
Knowledge of the MMD and the chain structure of these materials is extremely
important for the development of new products from UHMPE. The mechanical
properties of the final products, the morphology and the rheological behaviour of
the melt are influenced by these parameters [32].
SEC that is typically used for the molar mass analysis of polymers has its
limitations regarding the measurements of UHM polymers. Such samples may
undergo shear degradation during the chromatographic separation process. If the
samples are not strictly linear, then co-elution of branched macromolecules with
smaller linear macromolecules prevents an accurate analysis of the chain structure.
4.3.1.1 Aim
In the present study, UHM polyolefins shall be analysed regarding their molar
masses. HT-SEC and HT-AF4 shall be used for fractionation; the detection shall be
conducted by multiangle laser light scattering (MALLS). The molar mass and
molecular size information obtained by both methods shall be compared and the
effect of branching shall be discussed.
4.3.1.2 Materials
• Polymers. LDPE samples CSTR-LDPE 1 and 2 synthesized by free radical
polymerization in a continuous stirred tank reactor (CSTR) using different
amounts of chain transfer agent propionic aldehyde; technical PE samples PE
4.3 Analysis of Polyolefins by Asymmetric Flow FFF
155
UHM polyolefins following the work of Mes et al. [6]. The problems associated
with SEC of UHM samples, namely the shear degradation of high molar mass
structures and the anomalous late co-elution effects, can be successfully avoided in
HT-AF4. This allows accurate molar mass analysis of samples up to R g > 1,000 nm.
The curvature that existed in the conformation plot based on SEC measurements is
absent in similar AF4 plots. This curvature results in erroneous branching and molar
mass calculations. Accordingly, molar mass averages calculated from HT-AF4 are
significantly higher than those obtained from HT-SEC [7]. In addition, HT-AF4
allows the visualization of the thermo-oxidative degradation of polyolefins in
solution [31]. Improper sample treatment, including sample preparation for
HT-SEC, can be evaluated by this approach.
To summarize, the application of AF4 for the characterization of polyolefins
enables wider insight into molecular properties, which are apparently more complex than has been found to date by traditional separation methods.
4.3.1 Characterization of Branched Ultrahigh Molar Mass
Polyethylene by AF4 [7]
Polymers with weight average molar masses (M w ) > 500 kg/mol are typically
considered as UHM polymers. The mechanical stability and specific weight of
UHM PEs are superior compared to traditional materials. Therefore, UHM PE is
used for special applications such as ultrastrong fibers, implants, or toothed wheels.
Knowledge of the MMD and the chain structure of these materials is extremely
important for the development of new products from UHMPE. The mechanical
properties of the final products, the morphology and the rheological behaviour of
the melt are influenced by these parameters [32].
SEC that is typically used for the molar mass analysis of polymers has its
limitations regarding the measurements of UHM polymers. Such samples may
undergo shear degradation during the chromatographic separation process. If the
samples are not strictly linear, then co-elution of branched macromolecules with
smaller linear macromolecules prevents an accurate analysis of the chain structure.
4.3.1.1 Aim
In the present study, UHM polyolefins shall be analysed regarding their molar
masses. HT-SEC and HT-AF4 shall be used for fractionation; the detection shall be
conducted by multiangle laser light scattering (MALLS). The molar mass and
molecular size information obtained by both methods shall be compared and the
effect of branching shall be discussed.
4.3.1.2 Materials
• Polymers. LDPE samples CSTR-LDPE 1 and 2 synthesized by free radical
polymerization in a continuous stirred tank reactor (CSTR) using different
amounts of chain transfer agent propionic aldehyde; technical PE samples PE
4.3 Analysis of Polyolefins by Asymmetric Flow FFF
155