SPI-Chem™ Ruthenium Tetroxide Staining Kit

Applications for multiphase polymer systems

Ruthenium tetroxide is generally believed to be a "stronger" oxidizing agent than osmium tetroxide, itself an extremely strong oxidizing agent. Indeed it is so strong that it is almost too strong for the routine staining of a normal polybutadiene rubber modifier. In the case of an EPDM modified polymer, however, the stronger ruthenium tetroxide produces a far more desirable kind of result than osmium tetroxide. And the staining can be done far more quickly as well.

Ruthenium tetroxide will also stain a number of polymers for which osmium tetroxide would have no reaction whatsoever. For example, ruthenium tetroxide will stain polypropylene, and for certain other polymers, ruthenium tetroxide will oxidize aromatic rings.

Although we expect that osmium tetroxide will remain the main staining agent of choice for polymers, there are a growing number of instances where ruthenium is preferred, not only because osmium lacks the necessary reactivity, but also because in some polymers, ruthenium tetroxide has superior penetrating characteristics.

When comparing ruthenium vs. osmium tetroxide staining, some researchers have failed to recognize the need to slow down (or reduce the staining time) the reaction when using ruthenium. If the reaction proceeds too fast, the fine discrimination between a finely divided phase can be lost.

Perhaps the most widely ruthenium tetroxide stained polymer systems commercially today are those that are "acrylic modified". For example, where as both osmium tetroxide and ruthenium tetroxide will stain the typical ABS (acrylonitrile-butadiene-styrene) system, for the system acrylonitrile-styrene-acrylate, there is absolutely no staining with osmium tetroxide but strong staining with rutheniu tetroxide. The reaction is generally perceived to be the result of a cross-linking mechanism on the ester groups of the acrylate phase.

Some other polymer systems that can be uniquely stained with the ruthenium tetroxide system (but not osmium tetroxide) are the following:

polystyrene-polymethyl methacrylate (PS-PMMA)
ABS/nylon 11
high density polyethylene (HDPE) is stained slightly
isotactic and atactic polypropylene are slightly to moderately stained
poly(butylene terephthalate) (PBT) and polycarbonate (PC): PC is stained but PBT is not
core shell morphology of acrylic graft polybutadiene seed latex suspensions
polyvinyl alcohol
polyethers
bisphenol A based epoxies

Publications:
For the staining of polymers, one excellent paper on the use of ruthenium tetroxide for the staining of polymers is by G. M. Brown and J. H. Butler, New method for the characterization of domain morphology of polymer blends using ruthenium tetroxide staining and low voltage scanning electron microscopy (LVSEM)", Polymer 38 (15), 3937 (1997).

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