Waters Alliance System:
Gradient Analysis of Polymer Blends
Introduction: Gel permeation chromatography (GPC) is a chromatographic
technique that provides valuable information regarding the molecular weight
distribution of a polymer. If the polymer is a true homopolymer, (not a
copolymer or blend of polymers), the calculated molecular weight distribution
will be accurate for that sample. However, GPC analysis provides no
information about the actual composition of the material. Chemists must rely
on other methods to determine the composition of the copolymer or polymer
blend. This Performance PerSPECtive discusses use of a novel reversedphase, gradient HPLC technique for polymer composition analysis.
GPC Analysis of a Polymer Blend: Manufacturers frequently blend two or
more polymers in order to obtain specific physical properties for a final
product. For example, an elastomer such as polybutadiene or polyisoprene
may be blended with a more brittle polymer to impart flexibility to the finished
material making it less susceptible to stress cracking. Many times polymers
are blended to provide chemical resistance as when polyacrylonitrile is added
to polystyrene. Chemists frequently need to determine both the molecular
weight distribution as well as the percent composition of the individual
components contained in blended polymer systems. Figure 1 illustrates
overlays of several GPC chromatograms from the analyses of polystyrene,
styrene-butadiene rubber (SBR), styrene-acrylonitrile (SAN), and a blend
containing all three polymers. Molecular weight distribution information from
these GPC analyses provide no information on composition.
Figure 1: Overlay of GPC chromatograms of polystyrene,
SBR, SAN, and an equal blend of the three polymers
58
(2)Styragel ®HR5E, (1)HR2
Columns
Waters Alliance System
Waters 2414 RI Detector
1.0 mL/min, 100 µL injection
56
54
52
50
Styrene - Butadiene Rubber
(50% Styrene)
48
46
mV
44
42
Polystyrene
40
38
36
Styrene - Acrylonitrile
SBR/PS/SAN
Blend
34
32
(25% Acrylonitrile)
30
28
13
14
15
16
17
18
19
20
Minutes
21
22
23
24
25
26
27
Gradient Analysis of Polymers: Reversed-phase, gradient HPLC methods can
help scientists obtain composition information on polymers. Compared to GPC,
this HPLC separation mechanism relies primarily upon a precipitation /
redissolution process. The polymer blend is dissolved in a suitable solvent such
as tetrahydrofuran (THF) and injected onto the head of a reversed-phase C8 or
C18 column that has been equilibrated with 100% acetonitrile (ACN). The
polymer blend precipitates as soon as it is exposed to the ACN at the head of the
column. A gradient program, from 100% acetonitrile to 100% THF, is then run.
During the solvent change, the three polymers in the blend begin to re-dissolve
at different rates. In many cases, the column solely serves as a matrix to
accommodate the separation and does not affect the separation. In other
instances, the column actually plays a major role in the separation of the polymer
blend, with selective adsorption being an important separation mechanism.
Compound identification is made by comparing retention times between the
eluted peaks from the polymer blend with the retention times obtained from the
analysis of known polymer standards (with the same structure) under the same
gradient conditions. Typically, a detector that is not affected by the gradient (UV,
evaporative light scattering detector - ELSD) is required. In the example below,
an ELSD was used.
Figure 2 shows the gradient HPLC separation of the same polymer blend that
was separated by GPC in Figure 1. Note the baseline resolution of all three
components. Using standards with known composition and concentration, both
qualitative and quantitative information on the three components contained in the
blend are readily obtained.
Figure 2: Reversed-Phase gradient HPLC separation of
the tri-polymer blend of Polystyrene, SBR, and SAN
100
Polystyrene
90
Styrene-Butadiene Rubber
(50% Styrene)
SymmetryShield™ C8 Column (3.9 mm x150mm)
100% ACN to 100% THF over 20 minutes
Evaporative Light Scattering Detection
80
70
60
% THF
50
40
Styrene-Acrylonitrile
(25% Acrylonitrile)
30
20
10
0
0
2
4
6
Minutes
8
10
12
14
16
Summary: GPC analysis provides molecular weight distribution information on a
polymer blend, but reverse-phase gradient HPLC analysis can be used to
separate polymers in a blend based on composition.
Waters, Alliance, Styragel, and Symmetry Shield are registered trademarks of Waters Corporation.
Copyright 2001 Waters Corporation
Printed in the U.S.A.
WPP227
720000395EN