4.2
Application of Field-Flow Fractionation to Polyolefins
Similar to column-based chromatography, in FFF the sample under investigation
must be dissolved in a suitable solvent and injected into the FFF system. Since most
polyolefins are semi-crystalline materials, they dissolve only at high temperatures
and, therefore, must be dissolved in high boiling point solvents. The rules that apply
to HT-SEC regarding solvent preparation and the type of solvents also apply to
HT-FFF; typical solvents are 1,2,4-trichlorobenzene (TCB), ortho-dichlorobenzene
(ODCB), decaline, and similar. Working temperatures must be between 120 and
150
C in order to ensure that the sample is completely dissolved.
The severe experimental conditions that must be used for HT-FFF limit the
application of the various sub-techniques to AF4. ThFFF, which is based on a
temperature gradient in the FFF channel, is not widely used for polyolefin analysis.
One of the problems relates to the fact that typical temperature differences between
the ‘hot’ and the ‘cold’ plate are around 50
C. If the ‘cold’ plate must have a
minimum temperature of 130–150
C to keep the sample in solution, the hot plate
should have a temperature of 180–200
C. Considering the low thermo-oxidative
stability of most polyolefins, these are not favourable experimental conditions.
AF4 for medium (MT) and high (HT) temperatures (MT-AF4 and HT-AF4) was
developed and commercialized by Postnova Analytics (Landsberg, Germany) only
a few years ago. HT-AF4 has been specifically developed for the separation and
characterization of high molar mass complex polyolefins that are difficult to analyse
by HT-SEC. Various detectors, such as infrared (IR), refractive index (RI), multiangle light scattering (MALS) and dynamic light scattering (DLS), were applied in
different experimental set-ups [27].
HT-AF4 is a specific variation of flow field-flow fractionation, in which separation is achieved by a cross-flow perpendicular to the solvent flow, as shown in
Fig. 4.3. Through the empty channel, a constant solvent flow passes and forms a
parabolic velocity profile. The macromolecules are held back by the cross-flow
through a semipermeable membrane and, consequently, they are pushed against the
membrane. From the accumulation membrane, the macromolecules move back into
the channel by diffusion. The diffusion ability of the macromolecules is dependent
upon their molecular size, i.e. small molecules diffuse faster than large molecules.
This results in a distribution of the macromolecules with respect to their size in the
channel. The macromolecules situated near the centre of the profile will be small in
size. The maximum velocity of the flow profile will be in the centre and it decreases
as it approaches the walls. Therefore, small molecules will elute earlier and larger
molecules will elute later, opposite to the elution order in SEC.
A stainless steel channel and a flexible ceramic accumulation wall membrane are
utilized in HT-AF4. Measurements with chlorinated organic solvents like TCB at
temperatures up to 220
C are possible because of the use of the above-mentioned
materials. A Mylar spacer is cut to form the trapezoid channel with a thickness of
250–350 μm.
4.2 Application of Field-Flow Fractionation to Polyolefins
151
Application of Field-Flow Fractionation to Polyolefins
Similar to column-based chromatography, in FFF the sample under investigation
must be dissolved in a suitable solvent and injected into the FFF system. Since most
polyolefins are semi-crystalline materials, they dissolve only at high temperatures
and, therefore, must be dissolved in high boiling point solvents. The rules that apply
to HT-SEC regarding solvent preparation and the type of solvents also apply to
HT-FFF; typical solvents are 1,2,4-trichlorobenzene (TCB), ortho-dichlorobenzene
(ODCB), decaline, and similar. Working temperatures must be between 120 and
150
C in order to ensure that the sample is completely dissolved.
The severe experimental conditions that must be used for HT-FFF limit the
application of the various sub-techniques to AF4. ThFFF, which is based on a
temperature gradient in the FFF channel, is not widely used for polyolefin analysis.
One of the problems relates to the fact that typical temperature differences between
the ‘hot’ and the ‘cold’ plate are around 50
C. If the ‘cold’ plate must have a
minimum temperature of 130–150
C to keep the sample in solution, the hot plate
should have a temperature of 180–200
C. Considering the low thermo-oxidative
stability of most polyolefins, these are not favourable experimental conditions.
AF4 for medium (MT) and high (HT) temperatures (MT-AF4 and HT-AF4) was
developed and commercialized by Postnova Analytics (Landsberg, Germany) only
a few years ago. HT-AF4 has been specifically developed for the separation and
characterization of high molar mass complex polyolefins that are difficult to analyse
by HT-SEC. Various detectors, such as infrared (IR), refractive index (RI), multiangle light scattering (MALS) and dynamic light scattering (DLS), were applied in
different experimental set-ups [27].
HT-AF4 is a specific variation of flow field-flow fractionation, in which separation is achieved by a cross-flow perpendicular to the solvent flow, as shown in
Fig. 4.3. Through the empty channel, a constant solvent flow passes and forms a
parabolic velocity profile. The macromolecules are held back by the cross-flow
through a semipermeable membrane and, consequently, they are pushed against the
membrane. From the accumulation membrane, the macromolecules move back into
the channel by diffusion. The diffusion ability of the macromolecules is dependent
upon their molecular size, i.e. small molecules diffuse faster than large molecules.
This results in a distribution of the macromolecules with respect to their size in the
channel. The macromolecules situated near the centre of the profile will be small in
size. The maximum velocity of the flow profile will be in the centre and it decreases
as it approaches the walls. Therefore, small molecules will elute earlier and larger
molecules will elute later, opposite to the elution order in SEC.
A stainless steel channel and a flexible ceramic accumulation wall membrane are
utilized in HT-AF4. Measurements with chlorinated organic solvents like TCB at
temperatures up to 220
C are possible because of the use of the above-mentioned
materials. A Mylar spacer is cut to form the trapezoid channel with a thickness of
250–350 μm.
4.2 Application of Field-Flow Fractionation to Polyolefins
151