Handbook of Size Exclusion Chromatography and Related ...

Natural and Synthetic Rubbe177
variation between different samples of NR depending on both the origin of trees
and processing methods. Also, samples of NR havean additional complication as
aresult of the oxidation and gelation that take place in the bulk state or even in
solution.
In1972,Subramaniam(8)reportedacomprehensivestudyonthemolecular
weight distribution of selected samples of NR by SEC. Solutions of NR were
prepared on fresh latex obtained from six clones of Hevea brasiliensis. Figure 1
showstheSECchromatogramofthepurifiednaturalrubbersample.Itcanbeseen
in this curve that NR has avery broad molecular weight distribution with a
distinctive bimodal curve. Comparing this to that obtained on a sample of
synthetic polyisoprene (IR), Natsyn 400, the bimodality can be seen more clearly.
Usingtheuniversalcalibrationmethod(63),heshowedthattheintegralmolecular
weight distribution curves for six clones of NR ranges from 10 4 to10 7 .However,
the average molecular weights derived from SEC curves are too low compared
with values obtained conventionally.He pointed out that this error was aresult of
not considering chain branching. In other words, by this method chain branching
wasnotcompletelydetected.FurtherstudyisneededtouseSEC/LALLSorother
relevant methods.
Subramaniam also described difficulty with the practice of SEC for NR.
This difficulty was the partial blockage of the columns experienced with some
rubber samples. This is caused by the gel “plug” in the NR solution. When
plugged, the plugged gel parts are usually removed by opening the column.
Another remedy to this problem was to clean the plugged gel by injecting a3%
(vol/vol)solutionofxylylmercaptan,whichhadnoeffectontheefficiencyofthe
columns in fractionating polymers.
The degradation of NR during milling has been studied by SEC. A
representative graph shows that the molecular weight distribution is narrowed as
therubber ismilled for anincreasinglylonger time underfixed milling conditions
(Fig.3)(66).Thepeakofthedistributioncurveshiftstolowerandlowermolecular
weights with increased milling time. The molecular weight distribution curve
becomes much narrower than the original curve.
A comparison of the milling down rate of different diene rubbers was
measured easily by SEC. From the SEC analysis results for different diene rubbers
under fixed milling conditions, the milling down rate is in the order
NR . IR . SBR ’ BR (67).
4.2 SEC of Polyorganophosphazene Rubber Z
A typical example of the application of SEC for difficult samples is seen in
the molecular weight analysis of polyorganophosphazene rubber. The
molecular characterization of Z by SEC has been studied extensively in
recent years. Nevertheless, no satisfactory results were obtained until the work
© 2004 by Marcel Dekker, Inc.