TEM Analysis of Polyethylene-EPDM Blends
New Method for TEM Staining of Saturated Olefin Rubbers
- Fig. 1: TEM images of D1, D2, D3 and D4 samples. Composition: D1=EPDM2/VLDPE2 (30:70); D2=EPDM1/VLDPE2 (30:70); D3=EPDM2/VLDPE1 (30:70); D4=EPDM1/VLDPE1 (30:70). Scale bar 500 nm.
- Fig. 2: TEM images of D5 and D6 samples. Composition: D5=EPDM2/VLDPE1 (50:50); D6=EPDM1/VLDPE1 (50:50). Scale bar 500 nm.
- Fig. 3: TEM detail of the morphology of EPDM2 in D3 blend. Composition: 30wt% of EPDM2 and 70wt% of VLDPE1. Scale bar 200 nm.
The development of an innovative staining method for TEM analysis was performed in order to study blends of Very Low Density Polyethylene (VLDPE) and Ethylene – Propylene - Diene Rubber (EPDM). The blends were prepared by melt mixing different PE and EPDM samples and using different EPDM concentrations. In particular the distribution of the elastomeric component in PE matrix and the morphology of the rubber were investigated.
Image contrast in Transmission Electron Microscopy (TEM) is the result of variations in electron density between the phases or the structures of the specimen under test. Most polymers are composed of low atomic number elements, and thus they exhibit little variation in electron density. This variation is often not sufficient to provide an acceptable contrast in the image . The primary methods that have proved useful in contrast enhancement are staining, generally by the addition of heavy atoms to specific structures. Staining involves the chemical or physical incorporation of electron dense atoms into one or more specific components of the polymer, in order to increase the electron density and thus enhance contrast. Moreover, this technique has the advantage of hardening the sample, a feature that facilitates the sectioning with ultramicrotome of soft polymeric materials or of materials which contain soft components or organic substances. Staining agents depend on the type of polymer and of the properties investigated.
M. Kakugo, Sadatoshi H. and M. Yokoyama were among the first to successfully implement selective contrast of saturated olefin rubbers in polyolefins for TEM analysis, applying it specifically to binary blends of polypropylene (PP) / Ethylene - Propylene Rubber (EPR) and ternary PP/EPR/PE (Polyethylene) . They implemented a technique that involves two steps: the use of a staining agent (OsO4) and of an adjuvant (1,7 - octadiene).
In detail, the following steps are performed: trimming of the sample, immersion in 1,7 - octadiene for 3 hours at room temperature, drying for 1,5 minutes in the atmosphere, staining in aqueous solution at 1% of OsO4 for 3h at 60°C and finally sectioning with the cryoultramicrotome at -40°C.
Pretreatment with the 1,7 - octadiene, which preferentially penetrates the amorphous phase of the rubber, is decisive and allows osmium tetroxide to bind to itself thanks to the presence of two carbon - carbon double bonds .
The method developed by Kakugo et al. has been suitably modified and applied to the study of the morphology of blends of Very Low Density Polyethylene (VLDPE) and Ethylene - Propylene - Diene Monomer Rubber (EPDM). The blends were prepared by melt mixing of two different PE and EPDM and using different EPDM concentrations (wt%).
Materials & Methods
VLDPE/EPDM blends with different rubber content were prepared at Basic Chemicals & Plastics Research Centre of Versalis SpA in Mantova using a twin screw extruder. Two VLDPE matrices with melt flow rate of 7.5 and 3 g/10 min (190°C/2.16 kg - ISO 1133) and with molecular weights of 70 KDa and 85 KDa, named VLDPE1 and VLDPE2 respectively, were used. The selected EPDM polymers, named EPDM1 and EPDM2, have the same polypropylene content (28wt%) but two different molecular weights (110 KDa vs 140 KDa) and Mooney viscosities (30 MU vs 44 MU - ML 1+4, 100°C). They were produced by suspension polymerization using Ziegler – Natta catalysis. In particular, six types of blends were produced, named from D1 to D6: D1=EPDM2/VLDPE2 (30:70); D2=EPDM1/VLDPE2 (30:70); D3=EPDM2/VLDPE1 (30:70); D4=EPDM1/VLDPE1 (30:70); D5=EPDM2/VLDPE1 (50:50); D6=EPDM1/VLDPE1 (50:50).
The method of Kakugo et al. has been modified by changing the adjuvant: the 1,7 octadiene was substituted with the 1 - octene, which is a linear alkene with the double bond in the alpha position. Moreover, the immersion/drying times were varied and the concentration of the aqueous solution of OsO4 was increased from 1% to 4%. The immersion procedure was replaced with an exposure to vapors.
TEM analyses at different magnifications were carried out applying the innovative staining. EPDM phases can be easily identified, their morphology is clearly distinguished and the relevant particle size can be measured. It is also possible to analyze their dispersion quality in the two matrices, identifying the best combination of VLDPE/EPDM. In figure 1 are reported the TEM images of all the samples obtained mixing 30wt% of rubber and 70wt% of polyethylene: both EPDM1 and EPDM2 seems to be more dispersed in VLDPE1. In general, samples made with EPDM 1 appear to have a finer dispersion of the rubber in VLDPE. In particular, the D4 sample seems to be the best combination of the two materials.
EPDM 1 is more finely dispersed even in the case of blends containing 50wt% of EPDM. This feature appears quite clear observing figure 2, where the morphology of D5 and D6 are reported. Furthermore, in these last two samples the morphology of the rubber is more clearly distinguished: it has irregular shaped particles that tend to agglomerate. A detail of EPDM2 morphology is represented in figure 3.
The staining method specially developed was particularly useful for the analysis of VLDPE/EPDM blends and can be applied to saturated and amorphous olefinic rubbers. TEM micrographs show that the staining is successful: the rubber particles are clearly visible and distinguishable.
Alessia Priante1 and Massimo Vighi1
1Versalis SpA – Eni, Basic Chemicals & Plastics Research Centre, Mantova, Italy
 Linda C. Sawyer, David T. Grubb, Gregory F. Meyers: Polymer Microscopy, Springer-Verlag New York (2008) doi: 10.1007/978-0-387-72628-1
 Masahiro Kakugo, Hajime Sadatoshi, Masakazu Yokoyama: Transmission Electron Microscopy of Saturated Rubber Modified Polymer Systems, Journal of Polymer Science Part C: Polymer Letters, 24 (4): 171–175 (1986) doi: 10.1002/pol.1986.140240405