POLYMER LETTERS VOL. 8, PP. 855-859 (1970)

MICROSTRUCTURE OF PARTICLE-FORM POLYETHYLENE

When polyethylene is synthesized below its dissolution temperature, the polymer is formed as small particles or granules. This letter describes the morphology of particle-form polyethylene as revealed by scanning electron microscopy.

The polymerization of ethylene over catalysts of chromium oxide sup- ported on silica or silica-alumina leads to essentially linear polyethylene. On a commercial basis, there are two principal modes of operation which utilize this catalyst system (1-4).

In the solution process, monomer and a diluent such as cyclohexane are brought into contact with catalyst at pressures in the range400 to 500 psi and at temperatures between 120 and 160°C. Under these condi- tions, the polymer dissolves in the hydrocarbon diluent and is subsequently separated by precipitation after removal of the catalyst and unreacted monomer. The morphology of polymer produced by the solution process depends upon the conditions of precipitation and is usually altered further by pelletization.

In the particle-form (PF) or slurry process, the reaction temperature is kept low, typically 90 to llO"C, and consequently the polymer is not dissolved in the paraffinic diluent. Reaction pressures are typically 400 to 450 psi and catalyst concentrations range from 0.004 to 0.03 weight per cent. Polyethylene forms on the catalyst particles and crystallizes during polymerization. The macromolecules are neither melted nor dissolved in this process. Thus granules of particle-form polymer retain their is-poly- merized morphology.

Granules of PF polyethylene range in size from 4 mm in diameter down to a fine powder. An analysis of molecular weight by GPC for one of the samples used in this investigation showed: M, = 21,000, M, = 31 1,000, M, = 1,210,000, M,l = 2,060,000, and very little material of molecular weight below 1500.

Granules of PF polyethylene from commercial sources were prepared for examination in the scanning electron microscope by fracturing them in liquid nitrogen. The interior surfaces exposed in this manner must be rendered conductive to avoid build-up of electrostatic charge in the micro- scope. This is accomplished by evaporating a light coating of gold onto the polymer sample. Experiments with uncoated and unfractured specimens show no visible introduction of artifacts from the sample treatment in- dicated above.

form polyethylene specimen. Numerous complex structures are present, Figure 1 shows a general view of the interior morphology of a particle-

855 0 1970 by John Wiey & Sons, Inc.

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Fig. 1 . Particle-form polyethylene morphology.

Fig. 2. Helix in PF polyethylene.

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Fig. 3. Type I and Type I1 structures.

Fig. 4. Type I1 and Type I11 structures.

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showing a variety of states of aggregation. In approximate order, according to increasing size these are:

Type I: Type 11: Platelets Type 111: Type IV: Helices.

Thin filaments (sometimes present in bundles)

Nodules, worms, and polyps

The morphologies listed above occur in varying proportions in different PF samples. Type I1 platelets can be seen as overgrowth on some Type I11 material while other Type I11 structures have surfaces which appear smooth at the resolution presently attainable. As shown in Figure 4, there are some instances in which platelets develop as oriented overgrowths on parent platelets. The presence of helices is most striking. A typical helix, such as that shown in Figure 2, has measured dimensions of 4.8 microns length, 2.5 microns outside diameter, and consists of turns 0.7 micron thick. In- dividual turns are inclined at an angle of about 15 degrees to the helix axis. The “contour length” of such a helix is approximately 50 microns.

Several pioneering studies have been made of the morphology of nascent polymers synthesized with Ziegler-Natta catalysts. Both fibrous and nodular structures have been found in polyethylene (5-9,11,12) and in other poly- mers (5,6,10,12). In their syntheses of polystyrene and poly (4-methylpen- tene-1) on Ziegler-Natta catalysts, Blais and Manley observed spiral structures protruding from polymer globules (6). Further elucidation of the structure of the observed spirals would be of interest for comparison to the helical morphology observed in the present study. In polyethylene, the helices shown in Figures 1 and 2 are unique insofar as is known. In a material with as complex a morphology as particle-form polyethylene, it is likely that further details of the microstructure remain to be discovered.

Use of the scanning electron microscope was made possible through Engis Equipment Co. and Kent Cambridge Scientific, Inc., Morton Grove, Illinois. Particle-form polyethylene was provided by Celanese Research Company, Chemplex Ccmpany, and Phillips Petroleum Company. This research was supported in part by the Advanced Research Projects Agency through the Northwestern University Materials Research Center.

References

(1) J. P. Hogan and R. L. Banks, U. S. Patent 2,825,721 (1958). (2) J. P. Hogan and R. W. Myerholtz, in Kirk-Othmer Encyclopedia of

Chemical Technology, 2nd Ed., Vol. 14, Wiley, New York, N. Y., pp. 251- 254, (1967).

(3) H. N. Friedlander, in Crystalline Olefm Polymers, R. A. V. Raff and K. W. Do&, Eds., Part I, Interscience, New York, N.Y. ch. 6 (1965).

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(4) M. Sittig, Polyolefii Resin Processes, Noyes Development Corpora-

(5) P. Blais and R. St. John Manley, Science, 153, 539 (1966). (6) P. Blais and R. St. John Manley, J. Polym. Sci., A - l , 6 , 291 (1968). (7) P. Mackie, M. N. Berger, B. M. Grieveson, and D. Lawson, J. Polym.

(8) H. Chanzy, A. Day, and R. H. Marchessault, Polymer, S, 567 (1967). (9) H. Chanzy and R. H. Marchessault, Macromolecules, 2, 108 (1969). (10) J. Y. Cuttman and J. E. Guillet, Macromolecules, 1, 461 (1968). (1 1) P. Ingram and A. Schindler, Makromol. Chem., 111 ,267 (1968). (12) L. A. M. Rodriguez and H. V. Van Looy, J. Polym. Sci., A - l , s ,

tion, Park Ridge, N. J., 1967.

Sci., B, 5, 493 (1967).

1971 (1966).

Theodore Davidson

Depts. of Materials Science & Chemical Eng. Northwestern University Evanston, Illinois 60201

Received August 20, 1970