Another feature of TREF and CRYSTAF is that only the crystallizing parts are
fractionated while the non-crystallizing amorphous parts are not.
More recently, crystallization elution fractionation (CEF), as a refinement of the
basic TREF technique, has been pioneered by Monrabal. CEF allows rapid analysis
and good separation of a variety of polyolefins [20]. Another new technique based
on turbidity fractionation analysis, solution crystallization analysis by laser light
scattering (SCALLS), has been used for CCD analysis of polyolefins [21]. This
technique yields similar results to CRYSTAF but in shorter periods of time, uses a
comparatively low amount of solvent, and has a greater sensitivity in some cases.
However, all these techniques are limited to crystallizable samples.
Differential scanning calorimetry (DSC) is a thermal analysis method suitable
for studying the melting and crystallization behaviour; it is related to the chemical
structure of the polymer chain (chemical composition). DSC has been used as an
alternative tool for the qualitative analysis of CCD.
13 C-NMR spectroscopy is the
method of choice for the analysis of polyolefin microstructure, based on the
analysis of comonomer sequences and tactic units. Like all other bulk spectroscopic
methods, it provides only the average chemical composition. It is difficult to
determine the exact CCD due to the relatively low concentration of individual
components in complex polyolefins; thus, a preparative fractionation is required as
the first step.
Another well established spectroscopic method, FTIR spectroscopy, provides
information regarding comonomer composition, polymer chain configuration,
branching and crystallinity. A combined method of chromatography and IR spectroscopy may be employed to map the distribution of monomers in copolymer
samples [20]. The coupling of high temperature SEC (HT-SEC) to multiple
detectors allows detailed and fast molecular characterization. Several method
combinations have been applied to determine the copolymer composition and
comonomer distribution along the MMD. TREF combined with SEC and FTIR,
SEC with TREF and CRYSTAF and SEC with triple detectors (TriSEC) have been
reported to perform such type of analysis [22–26].
It is well established that fractionation and subsequent analysis of the separated
fractions is an essential approach to study the heterogeneity in multicomponent
systems. Preparative fractionation followed by subsequent analysis of the fractions
by SEC and NMR or SEC-FTIR can provide a detailed picture of the chemical
composition as a function of molar mass [27]. The selection of the analytical
method does not depend only on factors such as accuracy, labour and time demands
but also on the versatility and practicability of the approach. Preparative TREF
fractionation followed by SEC-FTIR is capable of analysing even very heterogeneous samples with low branching in a rapid and satisfactory manner [28–30]. However, only the average chemical composition per molar mass fraction can be
obtained by this approach; due to the heterogeneity of the chemical composition
within each molar mass fraction, the CCD cannot be obtained. The main drawback
of this approach is that preparative TREF involves time consuming operations such
as separation, filtration and drying of the fractions.
6
1 Introduction
fractionated while the non-crystallizing amorphous parts are not.
More recently, crystallization elution fractionation (CEF), as a refinement of the
basic TREF technique, has been pioneered by Monrabal. CEF allows rapid analysis
and good separation of a variety of polyolefins [20]. Another new technique based
on turbidity fractionation analysis, solution crystallization analysis by laser light
scattering (SCALLS), has been used for CCD analysis of polyolefins [21]. This
technique yields similar results to CRYSTAF but in shorter periods of time, uses a
comparatively low amount of solvent, and has a greater sensitivity in some cases.
However, all these techniques are limited to crystallizable samples.
Differential scanning calorimetry (DSC) is a thermal analysis method suitable
for studying the melting and crystallization behaviour; it is related to the chemical
structure of the polymer chain (chemical composition). DSC has been used as an
alternative tool for the qualitative analysis of CCD.
13 C-NMR spectroscopy is the
method of choice for the analysis of polyolefin microstructure, based on the
analysis of comonomer sequences and tactic units. Like all other bulk spectroscopic
methods, it provides only the average chemical composition. It is difficult to
determine the exact CCD due to the relatively low concentration of individual
components in complex polyolefins; thus, a preparative fractionation is required as
the first step.
Another well established spectroscopic method, FTIR spectroscopy, provides
information regarding comonomer composition, polymer chain configuration,
branching and crystallinity. A combined method of chromatography and IR spectroscopy may be employed to map the distribution of monomers in copolymer
samples [20]. The coupling of high temperature SEC (HT-SEC) to multiple
detectors allows detailed and fast molecular characterization. Several method
combinations have been applied to determine the copolymer composition and
comonomer distribution along the MMD. TREF combined with SEC and FTIR,
SEC with TREF and CRYSTAF and SEC with triple detectors (TriSEC) have been
reported to perform such type of analysis [22–26].
It is well established that fractionation and subsequent analysis of the separated
fractions is an essential approach to study the heterogeneity in multicomponent
systems. Preparative fractionation followed by subsequent analysis of the fractions
by SEC and NMR or SEC-FTIR can provide a detailed picture of the chemical
composition as a function of molar mass [27]. The selection of the analytical
method does not depend only on factors such as accuracy, labour and time demands
but also on the versatility and practicability of the approach. Preparative TREF
fractionation followed by SEC-FTIR is capable of analysing even very heterogeneous samples with low branching in a rapid and satisfactory manner [28–30]. However, only the average chemical composition per molar mass fraction can be
obtained by this approach; due to the heterogeneity of the chemical composition
within each molar mass fraction, the CCD cannot be obtained. The main drawback
of this approach is that preparative TREF involves time consuming operations such
as separation, filtration and drying of the fractions.
6
1 Introduction