collected copolymer fractions was determined using the new cryoprobe. Therefore,
the practical applicability and excellent improvement in detectability of polyolefins
in NMR were demonstrated.
3.2
Solvent Gradient Interaction Chromatography
TREF, CRYSTAF and crystallization elution fractionation (CEF) are excellent
methods for the fractionation of polyolefins according to chemical composition.
As has been pointed out earlier, these techniques relate only to the crystallizable
part of the sample while the non-crystallizable (amorphous) part is obtained as a
bulk fraction. Another problem associated with the crystallization-based fractionation techniques is that they are quite time-consuming. CEF overcomes the long
analysis times but is still based on crystallization.
An excellent and fast separation of complex polymers can be achieved by high
performance liquid chromatography (HPLC). HPLC separation is based on chemical composition rather than crystallizability, contrary to crystallization-based
techniques discussed in Part 2. Different mechanisms that are operative in HPLC
separations include adsorption-desorption and precipitation-redissolution. Precipitation and adsorption processes are usually combined in gradient HPLC. Previously, standard methods for the separation of polymers with respect to chemical
composition, including solvent gradient chromatography or liquid chromatography
at critical conditions (LCCC), were limited to ambient or slightly elevated
temperatures [58, 59]. The maximum operating temperature for such separations
was 80
C. Polyolefins dissolve above their melting points; hence, this temperature
is not sufficient for their dissolution. Frequently, the minimum required temperature
for dissolution of polyolefins is 120
C. Therefore, it was a real challenge to develop
high-temperature (>120
C) HPLC methods for polyolefin fractionation with
regard to chemical composition.
In 2003, first attempts to establish interactive chromatographic methods for
polyolefins were reported by Macko et al. [60, 61]. They used an isocratic system
for the separation of PE-PP blends. The elution behaviours of PE and PP differed
significantly. PP eluted first in SEC mode, while PE eluted later irrespective of its
molar mass under limiting conditions. The method, however, had several
limitations, including limited resolution and poor solubility of the samples. The
studies showed that a major challenge in the development of HPLC methods is the
solubility of polyolefins. Therefore, the solubility of polyolefins in different
solvents was studied by using cloud point titrations [62]. In the process of method
development of interaction chromatography, zeolites were tested as selective stationary phases [63–66]. On specific zeolites, PE can be adsorbed from some polar
nonsolvents as well as from good solvents, such as decalin or TCB (typical solvents
for SEC of polyolefins). Depending on the nature of the column packing, full or
partial adsorption of PE and PP was found [67, 68]. By using tetrachloroethane or
trichloropropane as mobile phases on silica gel and other macroporous sorbents,
3.2 Solvent Gradient Interaction Chromatography
91
the practical applicability and excellent improvement in detectability of polyolefins
in NMR were demonstrated.
3.2
Solvent Gradient Interaction Chromatography
TREF, CRYSTAF and crystallization elution fractionation (CEF) are excellent
methods for the fractionation of polyolefins according to chemical composition.
As has been pointed out earlier, these techniques relate only to the crystallizable
part of the sample while the non-crystallizable (amorphous) part is obtained as a
bulk fraction. Another problem associated with the crystallization-based fractionation techniques is that they are quite time-consuming. CEF overcomes the long
analysis times but is still based on crystallization.
An excellent and fast separation of complex polymers can be achieved by high
performance liquid chromatography (HPLC). HPLC separation is based on chemical composition rather than crystallizability, contrary to crystallization-based
techniques discussed in Part 2. Different mechanisms that are operative in HPLC
separations include adsorption-desorption and precipitation-redissolution. Precipitation and adsorption processes are usually combined in gradient HPLC. Previously, standard methods for the separation of polymers with respect to chemical
composition, including solvent gradient chromatography or liquid chromatography
at critical conditions (LCCC), were limited to ambient or slightly elevated
temperatures [58, 59]. The maximum operating temperature for such separations
was 80
C. Polyolefins dissolve above their melting points; hence, this temperature
is not sufficient for their dissolution. Frequently, the minimum required temperature
for dissolution of polyolefins is 120
C. Therefore, it was a real challenge to develop
high-temperature (>120
C) HPLC methods for polyolefin fractionation with
regard to chemical composition.
In 2003, first attempts to establish interactive chromatographic methods for
polyolefins were reported by Macko et al. [60, 61]. They used an isocratic system
for the separation of PE-PP blends. The elution behaviours of PE and PP differed
significantly. PP eluted first in SEC mode, while PE eluted later irrespective of its
molar mass under limiting conditions. The method, however, had several
limitations, including limited resolution and poor solubility of the samples. The
studies showed that a major challenge in the development of HPLC methods is the
solubility of polyolefins. Therefore, the solubility of polyolefins in different
solvents was studied by using cloud point titrations [62]. In the process of method
development of interaction chromatography, zeolites were tested as selective stationary phases [63–66]. On specific zeolites, PE can be adsorbed from some polar
nonsolvents as well as from good solvents, such as decalin or TCB (typical solvents
for SEC of polyolefins). Depending on the nature of the column packing, full or
partial adsorption of PE and PP was found [67, 68]. By using tetrachloroethane or
trichloropropane as mobile phases on silica gel and other macroporous sorbents,
3.2 Solvent Gradient Interaction Chromatography
91