significant attention because of the simplicity of the isocratic system and easy
detection, as will be discussed in Sect. 4.2.2.
Both solvent gradient and thermal gradient systems have become new tools
for characterizing copolymers in short analysis times and for extending the range
of polymers to be analyzed towards the elastomers region that could not be
characterized by crystallization techniques.
4.2.1 Solvent Gradient Interaction Chromatography
The use of a solvent/non-solvent approach to separating PE and PP in a preparative
mode was shown by Lehtinen et al. [106] using ethylene glycol monobutyl ether
(EGMBE) as a non-solvent. Macko et al. [107] were the first to implement this
approach in analytical HPLC, using EGMBE as a mobile phase in an isocratic
mode but depositing the polymer in the column with TCB; a separation of PE
and PP was obtained but without full recovery of the PE resin. Heinz et al. [108],
from the same group at DKI, used a solvent gradient approach (EGMBE-TCB) to
achieve a separation of PE and PP (for PE of molecular weight higher than 50,000
g/mol) with full recovery of PE. A similar approach was used by Albrecht et al.
[109] to separate EP copolymers and ethylene-vinyl acetate (EVA) resins [110],
and by Dolle et al. [111] to characterize an LLDPE resin.
In all cases, an ELSD was the only possible detection system because of the
solvent gradient. Pasch et al. [112] reported the separation of EVA and ethylenemethyl acrylate (EMA), and also combined the solvent gradient separation with
collection of germanium disks for FTIR measurement.
A significant breakthrough came with the separation of polyolefins by adsorption on a carbon-based column (Hypercarb); Macko and Pasch [113] obtained a
separation of isotactic, syndiotactic, and atactic polypropylene together with linear
polyethylene using a gradient of decanol-TCB in a very short analysis time, as
shown in Fig. 30.
Using the same Hypercarb column and eluents, Macko et al. have shown a
separation of ethylene copolymers by the level of comonomer incorporation
[114, 115]. Similar results were obtained by Miller et al. [116] on the same
Hypercarb column. The presence of branches in the ethylene copolymers reduces
the adsorption potential on the atomic level flat surface of graphite and a linear
correlation is obtained between the comonomer mole percentage incorporated and
the elution volume, as shown in Fig. 31 for various types of copolymers.
Solvent gradient interaction chromatography (SGIC) can be used to analyze
copolymers in the whole range of 0–100% of comonomer incorporation, which was
not possible with crystallization techniques.
The combination of SGIC with SEC in a second dimension (SGIC2D) was
shown by Roy et al. [117] using a gradient of decanol or EGMBE and TCB on a
Hypercarb column; a second dimension with the standard GPC columns and
isocratic TCB solvent was used with IR detection. Besides the convenience and
linearity of the IR detector, the molar mass–composition interdependence could be
Polyolefin Characterization: Recent Advances in Separation Techniques
237
detection, as will be discussed in Sect. 4.2.2.
Both solvent gradient and thermal gradient systems have become new tools
for characterizing copolymers in short analysis times and for extending the range
of polymers to be analyzed towards the elastomers region that could not be
characterized by crystallization techniques.
4.2.1 Solvent Gradient Interaction Chromatography
The use of a solvent/non-solvent approach to separating PE and PP in a preparative
mode was shown by Lehtinen et al. [106] using ethylene glycol monobutyl ether
(EGMBE) as a non-solvent. Macko et al. [107] were the first to implement this
approach in analytical HPLC, using EGMBE as a mobile phase in an isocratic
mode but depositing the polymer in the column with TCB; a separation of PE
and PP was obtained but without full recovery of the PE resin. Heinz et al. [108],
from the same group at DKI, used a solvent gradient approach (EGMBE-TCB) to
achieve a separation of PE and PP (for PE of molecular weight higher than 50,000
g/mol) with full recovery of PE. A similar approach was used by Albrecht et al.
[109] to separate EP copolymers and ethylene-vinyl acetate (EVA) resins [110],
and by Dolle et al. [111] to characterize an LLDPE resin.
In all cases, an ELSD was the only possible detection system because of the
solvent gradient. Pasch et al. [112] reported the separation of EVA and ethylenemethyl acrylate (EMA), and also combined the solvent gradient separation with
collection of germanium disks for FTIR measurement.
A significant breakthrough came with the separation of polyolefins by adsorption on a carbon-based column (Hypercarb); Macko and Pasch [113] obtained a
separation of isotactic, syndiotactic, and atactic polypropylene together with linear
polyethylene using a gradient of decanol-TCB in a very short analysis time, as
shown in Fig. 30.
Using the same Hypercarb column and eluents, Macko et al. have shown a
separation of ethylene copolymers by the level of comonomer incorporation
[114, 115]. Similar results were obtained by Miller et al. [116] on the same
Hypercarb column. The presence of branches in the ethylene copolymers reduces
the adsorption potential on the atomic level flat surface of graphite and a linear
correlation is obtained between the comonomer mole percentage incorporated and
the elution volume, as shown in Fig. 31 for various types of copolymers.
Solvent gradient interaction chromatography (SGIC) can be used to analyze
copolymers in the whole range of 0–100% of comonomer incorporation, which was
not possible with crystallization techniques.
The combination of SGIC with SEC in a second dimension (SGIC2D) was
shown by Roy et al. [117] using a gradient of decanol or EGMBE and TCB on a
Hypercarb column; a second dimension with the standard GPC columns and
isocratic TCB solvent was used with IR detection. Besides the convenience and
linearity of the IR detector, the molar mass–composition interdependence could be
Polyolefin Characterization: Recent Advances in Separation Techniques
237