intention of modifying the polyethylene density. In the case of polypropylene, the
addition of ethylene extended the product range to copolymers.
Characterization techniques being used in the 1950s and 1960s, such as NMR,
infrared spectroscopy, X-ray diffraction, microscopy etc., could only measure the
average values; this was also the case for the molar mass measurement. In some
cases, there was a need to separate the amorphous and crystalline fractions by
solvent extraction methods because no other means to measure the distributions
were available.
We had to wait for quite some years for the development of new separation
principles like gel permeation chromatography (GPC) in the late 1960s and
temperature rising elution fractionation (TREF) in the late 1970s to better define
the polyolefins microstructure by its distributions (molar mass and composition
distributions). It took a significant time and effort to fully develop these separation
techniques. Separation first means dissolution, which with the good chemical
resistance of polyolefins demands a high temperature with special solvents; the
second challenge is good detection, which with the lack of chemical functionality
could only be done by refractive index and later on with more sensitive infrared
detectors.
The development of single-site catalysts in the 1980s together with new multireactor processes and new comonomers opened the route for the design of
new resins with improved performance for different applications. New polyolefin
copolymers may have a complex microstructure and, besides molar mass and
composition distribution, it is necessary to characterize the bivariate distribution
(interdependence of molar mass and composition) and, on occasions, the level of
long chain branching and stereoregularity.
Spectroscopic techniques have improved significantly in the last 50 years,
especially in sensitivity, and they are of great value for investigating new structures
or understanding the intramolecular inhomogeneity of polyolefins. However, more
effort has been demanded in separation science, and the new developments to deal
with the analysis of these complex structures will be the subject of this chapter.
2 Polyolefin Microstructure
Polyolefins have the simplest chemistry of all synthetic polymers, just carbon and
hydrogen atoms, but can have complex microstructures. Besides the molar mass
distribution, there exists a wide range of significant features in the polyolefin
molecular architecture such as:
• The presence of short chain branches, by the addition of one or various
comonomers, which could result in intermolecular homogeneous (single-site
catalyst) or heterogeneous (multiple-site catalysts) incorporation
• The presence of long chain branches, which even in small quantities have a
significant influence on rheological properties
206
B. Monrabal
addition of ethylene extended the product range to copolymers.
Characterization techniques being used in the 1950s and 1960s, such as NMR,
infrared spectroscopy, X-ray diffraction, microscopy etc., could only measure the
average values; this was also the case for the molar mass measurement. In some
cases, there was a need to separate the amorphous and crystalline fractions by
solvent extraction methods because no other means to measure the distributions
were available.
We had to wait for quite some years for the development of new separation
principles like gel permeation chromatography (GPC) in the late 1960s and
temperature rising elution fractionation (TREF) in the late 1970s to better define
the polyolefins microstructure by its distributions (molar mass and composition
distributions). It took a significant time and effort to fully develop these separation
techniques. Separation first means dissolution, which with the good chemical
resistance of polyolefins demands a high temperature with special solvents; the
second challenge is good detection, which with the lack of chemical functionality
could only be done by refractive index and later on with more sensitive infrared
detectors.
The development of single-site catalysts in the 1980s together with new multireactor processes and new comonomers opened the route for the design of
new resins with improved performance for different applications. New polyolefin
copolymers may have a complex microstructure and, besides molar mass and
composition distribution, it is necessary to characterize the bivariate distribution
(interdependence of molar mass and composition) and, on occasions, the level of
long chain branching and stereoregularity.
Spectroscopic techniques have improved significantly in the last 50 years,
especially in sensitivity, and they are of great value for investigating new structures
or understanding the intramolecular inhomogeneity of polyolefins. However, more
effort has been demanded in separation science, and the new developments to deal
with the analysis of these complex structures will be the subject of this chapter.
2 Polyolefin Microstructure
Polyolefins have the simplest chemistry of all synthetic polymers, just carbon and
hydrogen atoms, but can have complex microstructures. Besides the molar mass
distribution, there exists a wide range of significant features in the polyolefin
molecular architecture such as:
• The presence of short chain branches, by the addition of one or various
comonomers, which could result in intermolecular homogeneous (single-site
catalyst) or heterogeneous (multiple-site catalysts) incorporation
• The presence of long chain branches, which even in small quantities have a
significant influence on rheological properties
206
B. Monrabal