2.2
Crystallization Analysis Fractionation
Besides all the merits of TREF, one of the important shortcomings is the very long
time that is required for the slow crystallization and elution steps. This makes a
TREF experiment a very time-consuming procedure that requires from a few hours
to a few days, depending on the experimental protocol. CRYSTAF was developed
by Monrabal in 1999 to overcome this problem and to speed up the CCD analysis of
olefin copolymers [8, 101]. CRYSTAF is based on the same principles of separation
by crystallizability from dilute solutions but, instead of two steps—crystallization
and elution, it makes use of only one step—crystallization. This crystallization
takes place in a stirred vessel with no support. The polyolefin sample is dissolved at
high temperature, followed by a slow decrease of the temperature of the solution.
Depending on the composition of the sample, fractions of different crystallizability
(chemical composition) precipitate out of solution at different temperatures.
The crystallization process is continuously monitored as a function of temperature using a suitable detector, typically a dual wavelength IR detector. Aliquots of
the polymer solution are analysed by the detector after filtration through the internal
filter in the vessel. The detector reading is assumed to provide relative concentration information. Consequently, a profile of polymer concentration in the solution
as a function of temperature is obtained; it is termed as a cumulative CRYSTAF
profile. As the temperature of the solution is decreased, an increasing fraction of
polymer in solution crystallizes out and, accordingly, polymer concentration in
solution decreases. Similar to TREF, a correlation between polymer concentration
in solution at a given temperature and chemical composition is developed through a
calibration curve. Copolymer standards with narrow CCDs are used to create the
calibration curve for particular experimental conditions (cooling rate, comonomer
type, solvent, etc.). The typical means of obtaining polyolefins with narrow CCDs
are P-TREF fractionation or direct synthesis using single-site catalysts.
Polymer Char (Valencia, Spain) is the only supplier of CRYSTAF instrumentation; the schematic diagram of their commercial version is illustrated in Fig. 2.25.
The instrument is equipped with five stainless steel crystallization vessels with
stirrers and a temperature programmable oven for parallel analysis. A nitrogen line,
a waste line and a sampling line with an inline filter are provided to all five vessels.
A dual wavelength online IR detector is connected to the sampling line. The
detector is also heated to 150
C and polymer concentration in solution as a function
of temperature is measured. A good solvent for the polymers such as TCB is used
for dissolution of the sample, while keeping the concentration of the polymer in
solution between 0.1 mg/mL and 1.0 mg/mL. Interchain interactions and
co-crystallization could occur if higher concentrations are used. On the other
hand, low concentrations can lead to poor signal-to-noise ratio (SNR). Experience
and literature suggest that the most suitable stirring rate during dissolution and
stabilization is 200 rpm. The stirring rate should be reduced to 100 rpm during
crystallization. The Co-crystallization should be avoided by cooling at a very slow
rate. A typical cooling rate is kept at 0.1–0.2
C/min during the crystallization step.
2.2 Crystallization Analysis Fractionation
47
Crystallization Analysis Fractionation
Besides all the merits of TREF, one of the important shortcomings is the very long
time that is required for the slow crystallization and elution steps. This makes a
TREF experiment a very time-consuming procedure that requires from a few hours
to a few days, depending on the experimental protocol. CRYSTAF was developed
by Monrabal in 1999 to overcome this problem and to speed up the CCD analysis of
olefin copolymers [8, 101]. CRYSTAF is based on the same principles of separation
by crystallizability from dilute solutions but, instead of two steps—crystallization
and elution, it makes use of only one step—crystallization. This crystallization
takes place in a stirred vessel with no support. The polyolefin sample is dissolved at
high temperature, followed by a slow decrease of the temperature of the solution.
Depending on the composition of the sample, fractions of different crystallizability
(chemical composition) precipitate out of solution at different temperatures.
The crystallization process is continuously monitored as a function of temperature using a suitable detector, typically a dual wavelength IR detector. Aliquots of
the polymer solution are analysed by the detector after filtration through the internal
filter in the vessel. The detector reading is assumed to provide relative concentration information. Consequently, a profile of polymer concentration in the solution
as a function of temperature is obtained; it is termed as a cumulative CRYSTAF
profile. As the temperature of the solution is decreased, an increasing fraction of
polymer in solution crystallizes out and, accordingly, polymer concentration in
solution decreases. Similar to TREF, a correlation between polymer concentration
in solution at a given temperature and chemical composition is developed through a
calibration curve. Copolymer standards with narrow CCDs are used to create the
calibration curve for particular experimental conditions (cooling rate, comonomer
type, solvent, etc.). The typical means of obtaining polyolefins with narrow CCDs
are P-TREF fractionation or direct synthesis using single-site catalysts.
Polymer Char (Valencia, Spain) is the only supplier of CRYSTAF instrumentation; the schematic diagram of their commercial version is illustrated in Fig. 2.25.
The instrument is equipped with five stainless steel crystallization vessels with
stirrers and a temperature programmable oven for parallel analysis. A nitrogen line,
a waste line and a sampling line with an inline filter are provided to all five vessels.
A dual wavelength online IR detector is connected to the sampling line. The
detector is also heated to 150
C and polymer concentration in solution as a function
of temperature is measured. A good solvent for the polymers such as TCB is used
for dissolution of the sample, while keeping the concentration of the polymer in
solution between 0.1 mg/mL and 1.0 mg/mL. Interchain interactions and
co-crystallization could occur if higher concentrations are used. On the other
hand, low concentrations can lead to poor signal-to-noise ratio (SNR). Experience
and literature suggest that the most suitable stirring rate during dissolution and
stabilization is 200 rpm. The stirring rate should be reduced to 100 rpm during
crystallization. The Co-crystallization should be avoided by cooling at a very slow
rate. A typical cooling rate is kept at 0.1–0.2
C/min during the crystallization step.
2.2 Crystallization Analysis Fractionation
47