• Stereoregularity differences in the case of polypropylene
• The recent appearance of block copolymers
Very often, with the goal of optimizing final product performance, the industrial
polyolefin products are a complex combination of resins with some of the features
listed above. It is not surprising that various separation techniques may be required
to cover a full characterization task.
In the following sections, the microstructure features of the most important
polyolefins are described.
2.1 Polyethylene Microstructure
The chemical structure of a linear polyethylene homopolymer is solely defined by
the molar mass distribution (MMD) of the resin. This important distribution,
together with the additives incorporated and the final morphology achieved in the
processing, defines the polymer performance in a given application.
In the resin manufacturing process, and due to the difficulties in obtaining
fast MMD data, homopolymer resins are controlled by a parameter related to the
average molar mass (M), the melt index, and on occasions by additional rheology
measurements that reflect the broadness of the distribution; however, when full
characterization of a linear high-density polyethylene (HDPE) homopolymer product
is required, the whole MMD must be measured.
To expand the application range of polyethylene produced with Ziegler-type
[2, 3] or chromium oxide catalyst processes [4, 5], comonomers such as propylene,
butene, hexene, or octene are incorporated into the linear chain and become
short chain branches that reduce the crystallizability of the polymer, extending
the density range from 0.96 g/mL of the HDPE homopolymer through medium
density resins (0.94 g/mL) down to the linear low-density polyethylene (LLDPE)
resin types (0.91 g/mL) and below to the elastomers region. The density value
of a given polyethylene resin correlates to the average comonomer mole
percentage incorporated; however, when dealing with multiple-site catalyst
systems (Ziegler-type), the intermolecular incorporation of comonomer is not
uniform and, in those cases, there is further need to know the chemical composition
distribution (CCD) and the molar mass–composition interdependence.
A scheme of the extended chain molecular population and branching in an
LLDPE resin is shown in Fig. 1a, where it is seen that the larger the molecule
the lower chances of comonomer incorporation (lower branch content). Most
interesting in LLDPE is the bimodality of the CCD, sometimes referred to as
the short-chain branching distribution (SCBD), as shown in Fig. 1b, due to the
population discontinuity observed between (1) the fraction of linear molecules,
practically excluding the comonomer incorporation in certain catalyst sites, and
(2) the remaining fractions with increasing amounts of comonomer incorporated.
Catalyst sites, where the bulkier and less reactive comonomer can be incorporated,
Polyolefin Characterization: Recent Advances in Separation Techniques
207
• The recent appearance of block copolymers
Very often, with the goal of optimizing final product performance, the industrial
polyolefin products are a complex combination of resins with some of the features
listed above. It is not surprising that various separation techniques may be required
to cover a full characterization task.
In the following sections, the microstructure features of the most important
polyolefins are described.
2.1 Polyethylene Microstructure
The chemical structure of a linear polyethylene homopolymer is solely defined by
the molar mass distribution (MMD) of the resin. This important distribution,
together with the additives incorporated and the final morphology achieved in the
processing, defines the polymer performance in a given application.
In the resin manufacturing process, and due to the difficulties in obtaining
fast MMD data, homopolymer resins are controlled by a parameter related to the
average molar mass (M), the melt index, and on occasions by additional rheology
measurements that reflect the broadness of the distribution; however, when full
characterization of a linear high-density polyethylene (HDPE) homopolymer product
is required, the whole MMD must be measured.
To expand the application range of polyethylene produced with Ziegler-type
[2, 3] or chromium oxide catalyst processes [4, 5], comonomers such as propylene,
butene, hexene, or octene are incorporated into the linear chain and become
short chain branches that reduce the crystallizability of the polymer, extending
the density range from 0.96 g/mL of the HDPE homopolymer through medium
density resins (0.94 g/mL) down to the linear low-density polyethylene (LLDPE)
resin types (0.91 g/mL) and below to the elastomers region. The density value
of a given polyethylene resin correlates to the average comonomer mole
percentage incorporated; however, when dealing with multiple-site catalyst
systems (Ziegler-type), the intermolecular incorporation of comonomer is not
uniform and, in those cases, there is further need to know the chemical composition
distribution (CCD) and the molar mass–composition interdependence.
A scheme of the extended chain molecular population and branching in an
LLDPE resin is shown in Fig. 1a, where it is seen that the larger the molecule
the lower chances of comonomer incorporation (lower branch content). Most
interesting in LLDPE is the bimodality of the CCD, sometimes referred to as
the short-chain branching distribution (SCBD), as shown in Fig. 1b, due to the
population discontinuity observed between (1) the fraction of linear molecules,
practically excluding the comonomer incorporation in certain catalyst sites, and
(2) the remaining fractions with increasing amounts of comonomer incorporated.
Catalyst sites, where the bulkier and less reactive comonomer can be incorporated,
Polyolefin Characterization: Recent Advances in Separation Techniques
207