Handbook of Size Exclusion Chromatography and Related ...

weight distribution. This type of analysis is often performed by SEC systems
having more than two detectors.
When one of the constituents A or B of a copolymer has ultraviolet (UV)
absorption and the other does not, a UV-RI dual-detector system can be used for the
detection of the chemical heterogeneity of the copolymer. As with molecular weight
distribution, 1:1 eluant–eluant composition against the retention volume Ve is
calculated from the two chromatograms, and a compositional variation is plotted as a
function of molecular weight. However, the response factors of the two components
in the two detectors must be calibrated first. This method has been applied to the
determination of chemical heterogeneity for styrene–butadiene copolymers (14,59).
SBR is one of the most widely used synthetic rubbers. In the earliest stage of
introduction of SEC for SBR, the molecular weights and molecular weight
distribution were only included in the analysis by RI detection. However, by using a
UVabsorption detector, additional comonomer styrene UV maxima can be obtained
separately. If a UV photodiode array detector is used, various low-molecular-weight
additives that have different UV maxima can be detected at one time (14).
Other detection methods, such a turbidometric titration (19) and Fourier
transform infrared spectrometry (35), have been used for compositional detection
in copolymer rubbers.
Recently, rubbers have been modified by blending or by chemical reaction to
suit specific needs for the product. In these cases, the compositional analysis is
very important. The same SEC analysis is used as an effective companion method.
For the SEC of rubber blends, it is crucial that SEC equipped with two or
three properly selected detectors, instead of the conventional single RI detector, be
used (65).
3.6 Preparative SEC for Rubbers
From the beginning, preparative SEC was applied to the preparation of narrow
molecular weight samples of a specified rubber polymer. Nevertheless, the literature
survey shows that only a few studies have been reported. The reason, as Chaturcedi
and Patel (43) describe, is that the preparative SEC method is tedious and time
consuming compared with the conventional preferred precipitation method.
Fractionation of trans-1,4-polyisoprene by preparative SEC was reported by
Chaturcedi and others (43). However, they obtained only three fractions that could
be measured by further viscometry.
4 TYPICAL APPLICATIONS OF SEC RUBBERS
4.1 SEC for NR and IR
Although the molecular weight distribution of NR has been studied extensively,
different results have been reported. The reason appears to be that there is a
© 2004 by Marcel Dekker, Inc.