Chemistry Reference
In-Depth Information
techniques, however, require significant
amounts of solvents, time and labour.
Within the large variety of liquid chro-
matographic techniques, only size exclusion
chromatography (SEC) has been used so
far for the analysis of polyolefins. High
temperature SEC enables a more or less
correct determination of molar mass distri-
butions of olefin copolymers, but compo-
nents with different chemical compositions
may co-elute. Even by coupling SEC with
FTIR spectroscopy, only average chemical
compositions corresponding to each elution
volume, but not CCDs, can be deter-
mined.
[23-25]
High performance liquid chromatogra-
phy (HPLC) is an important tool for fast
separation of complex polymers with regard
to chemical composition.
[26,27]
Different
separation mechanisms such as adsorption-
desorption or precipitation-redissolution
are used.
[28,29]
Typical concentration detec-
tors like differential refractive index (DRI)
and evaporative light scattering detectors
(ELSD) do not provide information on
the chemical composition of the separated
species. When HPLC is coupled to FTIR,
however, information on the chemical com-
position of the chromatographic fractions
can be obtained.
[30-32]
Unfortunately, up to now, HPLC techni-
ques for the separation of polymers have
been used only at
EGMBE is used as the mobile phase.
With a gradient of EGMBE and TCB,
the separation of polypropylene and poly-
ethylene can be achieved, where poly-
propylene elutes in the SEC mode and
polyethylene elutes with the solvent gra-
dient, respectively.
[33]
In the present paper, the separation of
EP copolymers by high temperature gra-
dient HPLC according to chemical compo-
sition is reported. For the first time the
coupling of gradient HPLC with FTIR
spectroscopy at temperatures that are
suitable for the characterization of poly-
olefins is described.
Experimental Part
High-Temperature Chromatograph
PL XT-220
A prototype high-temperature gradient
HPLC system PL XT-220 (Polymer Labor-
atories, Varian Inc, Church Stretton, Eng-
land) was used.
[34]
The stationary phase
was silica gel Nucleosil 500, column size
25
0.46 cm I.D., average particle diameter
5
m (Macherey Nagel, D ยจ ren, Germany).
For dissolution and injection of the samples
a robotic sample handling system PL-XTR
(Polymer Laboratories) was used. The
temperature of the auto sampler with the
sample block and the injection needle,
the injection port and the transfer line
between the auto sampler and the column
compartment was set to 140
m
temperatures below
100
C. For polyolefins, however, tempera-
tures between 130-160
8
C are necessary for
keeping the polymer samples in solution. In
our previous work,
[33]
a gradient HPLC
system was developed that enables to
separate blends of olefin homopolymers
(PE and PP) according to their chemical
composition at high temperatures. Polar
silica gel, as the stationary phase, and
a mobile phase comprising ethylene gly-
col monobutylether (EGMBE) and 1,2,4-
trichlorobenzene (TCB) were used. The
separation was based on the fact that
EGMBE is a non-solvent for linear poly-
ethylene (above
20 kg/mol) and a sol-
vent for isotactic polypropylene. Thus,
polyethylene precipitates on the column
while polypropylene is eluted when pure
8
C. The mobile
phase flow rate was 1 mL/min and 50
m
L
of sample solutions were injected. The
polymers were dissolved in TCB at a
concentration of 1-1.2 mg/mL at a tem-
perature of 150
8
C. The column outlet was
connected either to an evaporative light
scattering detector (ELSD, model PL-ELS
1000, Polymer Laboratories) or to a LC-
Transform FTIR Interface (Series 300, Lab
Connections, Carrboro, USA). The ELSD
was operated at a nebulisation temperature
of 160
8
C, an evaporation temperature of
270
8
C and an air flow of 1.5 mL/min.
The LC-Transform was operated at a stage
temperature of 164
8
C and a temperature
8
