microstructure became more complex and parameters like chemical composition
distribution (CCD) and tacticity became the focus of polyolefin research. Initially,
bulk analysis techniques like Fourier transform infrared spectroscopy (FTIR),
nuclear magnetic resonance (NMR), microscopy and crystal analysis were used.
Later, further advancements led to fractionation techniques such as temperature
rising elution fractionation (TREF) and size exclusion chromatography (SEC).
One of the major technical advantages of polyolefins is that they are stable
against solvent attack and are insoluble in most of the typical organic solvents. This
is, however, a disadvantage for solution based analytical methods. The prerequisite
of most polymer fractionation methods is proper solubility of all the components of
the sample to obtain representative dilute solutions. The majority of technically
important polyolefins are semi-crystalline materials with melting points above
100
C. Typically, polyolefin materials must be heated above their melting
temperatures to achieve complete solubility; hence, high boiling solvents are
required in polyolefin analysis. Polyolefin fractionation is usually carried out in
1,2,4-trichlorobenzene (TCB), 1,2-dichlorobenzene, decaline and in some cases
cyclohexane at temperatures between 130 and 160
C [5, 6]. Typically, stabilizers
and antioxidants are added to the solvent to prevent degradation.
Polyolefins generally exhibit multiple distributions of molecular parameters,
e.g. low density polyethylene (LDPE) and linear low density polyethylene
(LLDPE) have long chain branching and CCD, respectively, along with the
MMD. Polyolefin materials can be either copolymers or blends of homopolymers.
From this point of view, polyolefins are not different from other synthetic polymers,
and similar analytical approaches can be used. These approaches, together with
specific separation and analysis methods, will be presented in the chapters that
follow and typical applications will be discussed.
1.1
Molecular Heterogeneity of Polyolefins
Classical polyolefins contain only carbon and hydrogen, making them the simplest
of all synthetic polymers. However, similar to all other synthetic polymers,
polyolefins exhibit distributions in molar mass, chemical composition, molecular
topology and (sometimes) functional groups. The constitution, configuration and
conformation of macromolecules are characterized by their chemical structures.
The sequences of repeat units (alternating, random or block in the case of
copolymers) must be described in addition to their type and quantity. Several
constitutions of macromolecules (head-to-tail vs. head-to-head arrangements, linear vs. branched molecules) can exist in macromolecules with similar chemical
compositions. In addition, repeat units can exist in different steric patterns (isotactic, syndiotactic and atactic sequences in the polymer chain) despite having similar
constitutions, which is referred to as configurational isomerism. Variations in the
polymerization procedure and the composition of the monomer feed can lead to
different types of heterogeneities in the products. High density polyethylene
(HDPE) is a mostly linear homopolymer that is distributed only regarding molar
2
1 Introduction
distribution (CCD) and tacticity became the focus of polyolefin research. Initially,
bulk analysis techniques like Fourier transform infrared spectroscopy (FTIR),
nuclear magnetic resonance (NMR), microscopy and crystal analysis were used.
Later, further advancements led to fractionation techniques such as temperature
rising elution fractionation (TREF) and size exclusion chromatography (SEC).
One of the major technical advantages of polyolefins is that they are stable
against solvent attack and are insoluble in most of the typical organic solvents. This
is, however, a disadvantage for solution based analytical methods. The prerequisite
of most polymer fractionation methods is proper solubility of all the components of
the sample to obtain representative dilute solutions. The majority of technically
important polyolefins are semi-crystalline materials with melting points above
100
C. Typically, polyolefin materials must be heated above their melting
temperatures to achieve complete solubility; hence, high boiling solvents are
required in polyolefin analysis. Polyolefin fractionation is usually carried out in
1,2,4-trichlorobenzene (TCB), 1,2-dichlorobenzene, decaline and in some cases
cyclohexane at temperatures between 130 and 160
C [5, 6]. Typically, stabilizers
and antioxidants are added to the solvent to prevent degradation.
Polyolefins generally exhibit multiple distributions of molecular parameters,
e.g. low density polyethylene (LDPE) and linear low density polyethylene
(LLDPE) have long chain branching and CCD, respectively, along with the
MMD. Polyolefin materials can be either copolymers or blends of homopolymers.
From this point of view, polyolefins are not different from other synthetic polymers,
and similar analytical approaches can be used. These approaches, together with
specific separation and analysis methods, will be presented in the chapters that
follow and typical applications will be discussed.
1.1
Molecular Heterogeneity of Polyolefins
Classical polyolefins contain only carbon and hydrogen, making them the simplest
of all synthetic polymers. However, similar to all other synthetic polymers,
polyolefins exhibit distributions in molar mass, chemical composition, molecular
topology and (sometimes) functional groups. The constitution, configuration and
conformation of macromolecules are characterized by their chemical structures.
The sequences of repeat units (alternating, random or block in the case of
copolymers) must be described in addition to their type and quantity. Several
constitutions of macromolecules (head-to-tail vs. head-to-head arrangements, linear vs. branched molecules) can exist in macromolecules with similar chemical
compositions. In addition, repeat units can exist in different steric patterns (isotactic, syndiotactic and atactic sequences in the polymer chain) despite having similar
constitutions, which is referred to as configurational isomerism. Variations in the
polymerization procedure and the composition of the monomer feed can lead to
different types of heterogeneities in the products. High density polyethylene
(HDPE) is a mostly linear homopolymer that is distributed only regarding molar
2
1 Introduction