Determining the polymer domain structure of TPE blends by microscopy techniques.

The effective development of multi-phase polymer blends requires microscopy techniques capable of showing the morphologies which result from various methods of blending and compatibilizing the base resins. Photomicrographs also make it possible to relate physical properties to the size, shape and distribution of the polymer domains, to the distribution of other compounding ingredients within those domains and to the stability of the morphology during subsequent forming operations. When new blends are brought to the production stage, microscopy is needed to determine whether the desired morphologies are being achieved in large scale commercial processes.

This article will discuss some of the cryogenic ultramicrotoming, transmission electron microscopy (TEM), scanning electron microscopy (SEM) and light microscopy (LM) techniques that are useful when the components include olefinic polymers that are characteristically low in chemical reactivity and often difficult to distinguish form each other in blends.

The polymer blends discussed here are all thermoplastic elastomers produced by crosslinking one or more thermoset elastomers under conditions of high shear in a melt mix with a thermoplastic olefin resin such as polypropylene. Commercial products of this type are referred to generally as thermoplastic vulcanizates (TPVs) or dynamically vulcanized alloys (DVAs) (refs. 1 and 2).

Experimental

The polymers studied were dynamically vulcanized halogenated butyls, ethylene propylene rubber terpolymers of varied unsaturation and ethylene content, chloroprene rubber and polypropylene thermoplastic.

The solvent extrusion (etching) technique which is very useful with rubber-thermoplastic blends generally cannot be used with DVAs because the rubber, which constitutes the major phase, is crosslinked and therefore insoluble. If the rubber could be extracted, the initial swelling would distort the thin thermoplastic phase causing it to collapse when the rubber was removed (refs. 3 and 4). Due to their elastic properties all DVAs had to be cryo-microtomed at temperatures equal to or below -130[degrees]C. The TEM and LM sections, [is less than or equal to]100 nm in thickness, were collected on carbon film coated copper grids (15 nm in thickness) and on LM glass slides. The same sample blocks from which the LM and TEM sections were cut were also used from the SEM study.

To obtain polymer domain contrast in the TEM or SEM, both [OsO.sub.4] and [RuO.sub.4] staining were used. In the case of chlorobutyl and bromobutyl, the polymer degradation (chain scissioning) which occurs under electron bombardment, was also exploited for obtaining contrast. The [OsO.sub.4] stain produces good contrast only with unsaturated polymers (refs. 4-6). However RuO4 also stains or diffuses into amorphous regions in semicrystalline polymers, at the same time reacting with functional groups on polymers (refs. 4 and 7).

The staining was carried out with the vapors of 0.5 wt% aqueous solution of the respective staining agents. The samples were suspended above |1 ml of the staining agent in a closed glass jar of |50-60 ml in volume for 20-30 minutes. These concentrations and staining times were sufficient to obtain good contrast in both the TEM and SEM.

The TEM images were obtained with 100 KV electrons and the samples were protected by a liquid nitrogen cooled anticontaminator. The SEM images were obtained with an efficient backscattered electron detector (10-15 KV primary electrons). This allowed good differentiation between polymer phases because the stained polymer domains backscattered more primary electrons than the unstained ones. The SEM images formed by secondary electrons emitted from the surface of samples are undesirable since useful information about polymer morphology, i.e. domain location and size, is suppressed. Backscattered electrons are much more specific, when samples are stained, in imaging polymer morphology (ref. 8).

To obtain sufficient contrast in the light microscope the polymer domain sizes must be greater than 1 [micro]m. There must also be enough difference in refractive index between the polymer phases to use phase contrast microscopy (refs. 9 and 10). A 1/20 wavelength shift seems to be sufficient.

Results and discussion

The first figure shows a DVA made with polypropylene (PP) and ethylene propylene diene elastomer (EPDM) as part of a mixing study. This is a blend of 83 weight percent compounded rubber and 17 weight percent resin. The TEM images were made using [RuO.sub.4] stained thin section (50-100 nm) samples. The "dark-gray" EPDM particles appear to have a wide particle size distribution (0.2-2 [micro]m) and the black particles are a filler in this compound. A large number of the rubber particles appear to be aggregated, however most likely there is a thin layer of PP which is coating the particles not visible in these images.

Next is a chlorobutyl (CIIR)/PP made with 81 weight percent compounded CIIR. Sample preparation and TEM imaging procedures were the same as in figure 1. The rubber particle size is larger in this blend than in the PP/EPDM DVA (1-7 [micro]m); again, the black particles represent a filler which appears to be located within the CIIR phase (figure 2).

When bromobutyl (BIIR) was mixed and cured in the same way as CIIR rubber, the BIIR particles appeared to be much larger than the CIIR particles and the BIIR particles tended to be highly elongated (figure 3).

To see which staining agent gives the most useful information on the DVA morphology an experiment was carried out using a BIIR/EPDM/PP blend. Various blends of this type have been proposed (refs. 11 and 12). It appears that the [RuO.sub.4] stain gives the best contract and information on the rubber phase particle size. However, the [OsO.sub.4] stain gives better information than the [RuO.sub.4] stain on the EPDM phase morphology, and if a filler is present, its location can be seen more clearly with this stain (see figures 4 and 5). The lighter (white) areas represent BIIR which has scissioned under electron bombardment and evaporated in the TEM's high vacuum.

The light microscope, using phase contrast optics with a polarizer to increase contrast, was used to determine chloroprene (CR) and chlorobutyl (CIIR) rubber domain sizes in a CR/CIIR/PP DVA 13 (figure 6). The microtomed sections were 0.15 microns thick. The CR particles (darker phase) have sufficient contrast that their size and distribution can easily be determined; the CIIR phase is much more difficult to see and separate from the PP background. This technique proved to be useful in support of mixing studies directed toward improving physical properties by reducing the size of the chloroprene particles.

When olefinic DVAs are combined or adhered with other olefinic compounds during fabrication, as in co-extrusion, the most useful instrument for observing the interface between them is the SEM. Figure 7 demonstrates this in a construction where two different DVAs both based on polypropylene and halogenated butyl rubber are joined. One is a hard DVA (b -left bottom) and the other is a soft one (a- right top). The sample was microtomed and RuO4 stained and a Robinson electron backscatter detector was used to enhance contrast between the phases. The harder compound contains less rubber, i.e. fewer discrete rubber particles (light particles, produced by backscattered electrons). The interface between the two compounds is diffuse indicating interdiffusion of the PP resins which for the continuous phases.

Summary and conclusions

Electron and light microscope techniques were successfully used to determine the morphology of DVAs (TPVs); the combination of [OsO.sub.4] and [RuO.sub.4] staining helped to locate the polymer domains in triblends (PP/CIIR or BIIR/EPDM) in the electron microscope and the differences in refractive index between the different polymer phases (PP/CR/CIIR) were used to form sufficient contrast in the phase contrast light microscope. The interface between DVA compounds, containing different EPDM concentrations, was located with the SEM using a sensitive backscattered electron detector.

References

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