Chemistry Reference
In-Depth Information
Equation (1):
polymer solution concentration; as tem-
perature goes down the most crystalline
fractions, composed of molecules with zero
or very few branches (highly crystalline)
will precipitate first, resulting in a steep
decrease in the solution concentration on
the cumulative plot. This is followed by
precipitation of fractions of increasing
branch content (or less crystallinity) as
temperature continues to decrease; the last
data point, corresponding to the lowest
temperature of the crystallization cycle,
represents the fraction which has not
crystallized (mainly highly branched or
amorphous material) and remains in solu-
tion at the lowest temperature. The first
derivative of this curve corresponds to the
CCD, very similar in shape to the one
obtained in TREF with the only difference
of a temperature shift, as equilibrium is not
reached, and CRYSTAF is measured in the
crystallization while TREF is measured in
the dissolution (melting).
With this approach, the CCD can be
analyzed relatively fast in a single crystal-
lization cycle without physical separation of
the fractions. The term crystallization
analysis fractionation stands for this pro-
cess. Reviews of the CRYSTAF technique
have been done by Soares and Hamielec,
[7]
Anantawaraskul et al.
[8]
and Monrabal.
[9]
2
R
ð
T
m
Þ
T
m
T
m
mc
(1)
D
H
u
where T
m
is the equilibrium melting tem-
perature of the polymer-diluent mixture,
0
m
the melting temperature of the pure poly-
mer, and
T
D
H
u
the heat of fusion per polymer
repeating unit. Equation (1) assumes that
D
H
u
is constant in the crystallization tem-
perature range and that the presence
of solvent, when crystallizing in solution,
plays as an additional temperature shift
factor.
Analysis of the CCD by TREF is today a
common practice in the polyolefin industry
and the long analysis time of the first
homemade instruments
[1,3]
(three or four
days per sample) has been reduced sig-
nificantly down to a few hours; still, there is
an interest in further reducing TREF
analysis time. Reviews of the TREF techni-
que have been done byWild,
[4]
Gl
¨
ckner,
[5]
Fonseca and Harrison,
[6]
Soares and
Hamielec,
[7]
Anantawaraskul, Soares and
Wodd-Adams,
[8]
and Monrabal.
[9]
Crystallization Analysis Fractionation
Crystallization analysis fractionation (CRYS-
TAF) was developed by Monrabal
[10,11]
in
1991 as a process to speed up the analysis of
the CCD, and it shares with TREF the same
fundamentals on separation according to
crystallizability, but the whole fractionation
process is carried out during crystallization.
In CRYSTAF the analysis is carried out
in stirred crystallization vessels with no
support, by monitoring the polymer solu-
tion concentration, through the crystal-
lization process, while decreasing tempera-
ture. Aliquots of the solution are filtered
(through an internal filter inside the vessel)
and analyzed by a concentration detector.
In fact, the whole process is similar to a
classical stepwise fractionation by precipita-
tion with the exception that, in this approach,
no attention is paid to the precipitate but to
the polymer that remains in solution.
The first data points, taken at tempera-
tures above any crystallization, provide a
constant concentration equal to the initial
Dynamic Crystallization
In the previous sections, TREF and CRYS-
TAF methods have been reviewed, and it
has been discussed how both techniques
share the same principles of fractionation
on the basis of crystallizability through a
slow cooling of a polymer solution. TREF is
carried out in a packed column and
demands two full temperature cycles,
crystallization and elution, to achieve the
analysis of the composition distribution. In
CRYSTAF the analysis is performed in a
single step, the crystallization cycle.
In Figure 1.a, the analysis of a blend of
three different components by TREF is
represented in three steps: 1) Sample
loading into the column, 2) Crystallization
cycle where the components are being
crystallized in the same location where
