When dealing with block copolymers, the calorimetric methods may provide
some additional information not accessible by TREF, where the dominating separation mechanism is according to the most crystalline part of the block copolymer.
4.1.2 Temperature Rising Elution Fractionation
The most comprehensive analytical approach to characterizing the CCD in recent
years has been TREF, implemented in the polyolefin characterization world by
Wild et al. in the late 1970s [74, 75], which led to an understanding of the LLDPE
structure in relation to the multiple sites in Ziegler-type catalyst.
The initial dissolution fractionation of polyethylene according to composition
by increasing temperature was first described by Desreux and Spiegels [76] in
1950 using an extraction technique with a single solvent at increasing temperatures.
This was used with success by Hawkins and Smith [77] and Shirayama et al. [78],
who first named the technique temperature rising elution fractionation, but it was
the work of Wild et al. [74, 75] in the late 1970s with the development of analytical
TREF that established the technique as a standard in the polyolefin industry.
Various reviews on TREF have been published by Wild [79], Glo ¨ckner [68],
Monrabal [80], Fonseca and Harrison [81], Soares and Hamielec [82, 83], and
recently by Monrabal in a comprehensive review chapter in the Encyclopedia of
Analytical Chemistry [84].
The TREF analytical process resembles a liquid chromatography separation
with a column, an eluent, and a detector or collecting device depending on whether
an analytical or preparative approach is intended. TREF needs to be performed
at the high temperatures (up to 160
C) demanded by the polyolefin dissolution
step. In TREF, the separation requires two temperature cycles (crystallization and
dissolution) as shown schematically in Fig. 13. Once the polymer sample is
dissolved in a proper solvent at high temperature, the solution is introduced into
a column containing a non-active support; this is followed by a crystallization
step at a slow cooling rate during which polymer fractionation will occur, as the
temperature drops, by deposition (crystallization) of layers of decreasing crystallinity or increasing branch content on the column support particles; fractionation
takes place within this cycle that is usually carried out at a low crystallization rate.
At this stage, the polymer is already segregated into crystal aggregates of
different composition on the inert support particles inside the column although all
of them are still mixed together (there is not yet a physical separation of fractions).
The TREF technique still requires a second temperature cycle to quantify or
collect those fractions of different crystallinity. This is achieved by washing the
column with new solvent while the temperature is being increased. The eluent
dissolves fractions of increasing crystallinity, or decreasing branch content, as
temperature rises. These fractions are monitored with an IR detector (analytical
TREF) to generate the CCD curve, or collected (preparative TREF) to perform
further analysis. The name temperature rising elution fractionation comes from
this second temperature cycle.
Polyolefin Characterization: Recent Advances in Separation Techniques
221
some additional information not accessible by TREF, where the dominating separation mechanism is according to the most crystalline part of the block copolymer.
4.1.2 Temperature Rising Elution Fractionation
The most comprehensive analytical approach to characterizing the CCD in recent
years has been TREF, implemented in the polyolefin characterization world by
Wild et al. in the late 1970s [74, 75], which led to an understanding of the LLDPE
structure in relation to the multiple sites in Ziegler-type catalyst.
The initial dissolution fractionation of polyethylene according to composition
by increasing temperature was first described by Desreux and Spiegels [76] in
1950 using an extraction technique with a single solvent at increasing temperatures.
This was used with success by Hawkins and Smith [77] and Shirayama et al. [78],
who first named the technique temperature rising elution fractionation, but it was
the work of Wild et al. [74, 75] in the late 1970s with the development of analytical
TREF that established the technique as a standard in the polyolefin industry.
Various reviews on TREF have been published by Wild [79], Glo ¨ckner [68],
Monrabal [80], Fonseca and Harrison [81], Soares and Hamielec [82, 83], and
recently by Monrabal in a comprehensive review chapter in the Encyclopedia of
Analytical Chemistry [84].
The TREF analytical process resembles a liquid chromatography separation
with a column, an eluent, and a detector or collecting device depending on whether
an analytical or preparative approach is intended. TREF needs to be performed
at the high temperatures (up to 160
C) demanded by the polyolefin dissolution
step. In TREF, the separation requires two temperature cycles (crystallization and
dissolution) as shown schematically in Fig. 13. Once the polymer sample is
dissolved in a proper solvent at high temperature, the solution is introduced into
a column containing a non-active support; this is followed by a crystallization
step at a slow cooling rate during which polymer fractionation will occur, as the
temperature drops, by deposition (crystallization) of layers of decreasing crystallinity or increasing branch content on the column support particles; fractionation
takes place within this cycle that is usually carried out at a low crystallization rate.
At this stage, the polymer is already segregated into crystal aggregates of
different composition on the inert support particles inside the column although all
of them are still mixed together (there is not yet a physical separation of fractions).
The TREF technique still requires a second temperature cycle to quantify or
collect those fractions of different crystallinity. This is achieved by washing the
column with new solvent while the temperature is being increased. The eluent
dissolves fractions of increasing crystallinity, or decreasing branch content, as
temperature rises. These fractions are monitored with an IR detector (analytical
TREF) to generate the CCD curve, or collected (preparative TREF) to perform
further analysis. The name temperature rising elution fractionation comes from
this second temperature cycle.
Polyolefin Characterization: Recent Advances in Separation Techniques
221