Crystallization-Based Fractionation
Techniques
2
The vast majority of commercial polyolefins are semi-crystalline materials.
Depending on the chemical composition and tacticity, their melting temperatures
range from ambient to temperatures exceeding 160
C. Polyolefins form various
crystalline structures that can be investigated with microscopic, spectroscopic and
scattering techniques.
When a semi-crystalline polyolefin is dissolved in a solvent at high temperature
(usually above the melting temperature), followed by a continuous or stepwise
decrease of the solution temperature, the polyolefin starts to form crystals that will
precipitate out of the solution. The crystallization temperature, the shape and the
amount of crystals depend on the molecular structure of the polyolefin, mainly its
chemical composition, its tacticity and the degree of branching. Highly crystalline
materials will crystallize out of the solution at higher temperatures than materials
with lower crystallinity.
Flory–Huggins statistical thermodynamic treatment accounts for melting point
depression due to the presence of a diluent in a crystallizing system. The diluent can
be a solvent or a comonomer. In either case, the crystallization temperature
decreases with increasing diluent concentration. Therefore, for copolymers that
do not have long chain branches, the separation by crystallizability can be regarded
as a separation according to chemical composition. A precondition is that the
concentration of the diluent is low and it does not enter into the crystal lattice of
the crystallizing polymer. For copolymers where the non-crystallizing comonomer
is the diluent, a linear dependence of the melting or crystallization temperature on
the amount of comonomer incorporated is observed. Such linear dependencies have
been seen in temperature rising elution fractionation (TREF), differential scanning
calorimetry (DSC) and crystallization analysis fractionation (CRYSTAF)
experiments [1–4].
The potential of crystallization behaviour of semi-crystalline polymers as an
analytical tool was recognized by Desreux and Spiegels [5] in the early history of
polyolefin fractionation. TREF, CRYSTAF and crystallization elution fractionation
(CEF) are the main techniques that are used today in this category. The differences
# Springer International Publishing Switzerland 2014
H. Pasch, M.I. Malik, Advanced Separation Techniques for Polyolefins, Springer
Laboratory, DOI 10.1007/978-3-319-08632-3_2
11
Techniques
2
The vast majority of commercial polyolefins are semi-crystalline materials.
Depending on the chemical composition and tacticity, their melting temperatures
range from ambient to temperatures exceeding 160
C. Polyolefins form various
crystalline structures that can be investigated with microscopic, spectroscopic and
scattering techniques.
When a semi-crystalline polyolefin is dissolved in a solvent at high temperature
(usually above the melting temperature), followed by a continuous or stepwise
decrease of the solution temperature, the polyolefin starts to form crystals that will
precipitate out of the solution. The crystallization temperature, the shape and the
amount of crystals depend on the molecular structure of the polyolefin, mainly its
chemical composition, its tacticity and the degree of branching. Highly crystalline
materials will crystallize out of the solution at higher temperatures than materials
with lower crystallinity.
Flory–Huggins statistical thermodynamic treatment accounts for melting point
depression due to the presence of a diluent in a crystallizing system. The diluent can
be a solvent or a comonomer. In either case, the crystallization temperature
decreases with increasing diluent concentration. Therefore, for copolymers that
do not have long chain branches, the separation by crystallizability can be regarded
as a separation according to chemical composition. A precondition is that the
concentration of the diluent is low and it does not enter into the crystal lattice of
the crystallizing polymer. For copolymers where the non-crystallizing comonomer
is the diluent, a linear dependence of the melting or crystallization temperature on
the amount of comonomer incorporated is observed. Such linear dependencies have
been seen in temperature rising elution fractionation (TREF), differential scanning
calorimetry (DSC) and crystallization analysis fractionation (CRYSTAF)
experiments [1–4].
The potential of crystallization behaviour of semi-crystalline polymers as an
analytical tool was recognized by Desreux and Spiegels [5] in the early history of
polyolefin fractionation. TREF, CRYSTAF and crystallization elution fractionation
(CEF) are the main techniques that are used today in this category. The differences
# Springer International Publishing Switzerland 2014
H. Pasch, M.I. Malik, Advanced Separation Techniques for Polyolefins, Springer
Laboratory, DOI 10.1007/978-3-319-08632-3_2
11