Polymer Blending

We are exploring pressure induced miscibility and phase separation as a technique to produce novel polymer-polymer blends, in particular blends of polymers that are mutually incompatible. In the preceding section on miscibility we have already commented on the solution blending of polyethylene and isotactic polypropylene. A more recent approach that we have been evaluating is based on impregnating a host polymer that swells but does not dissolve in a hi h pressure fluid with a solution of a second polymer that dissolves in the same fluid2122 The system is then subjected to a pressure quench upon which the dissolved polymer is in-situ precipitated inside the host polymer. The entrapment is further enhanced with rapid solidification ofthe host polymer which accompanies the escape of the dissolved fluid from the polymer. Poly(dimethylsiloxane) and polyethylene present a unique polymer pair to explore this type of blending. This is because PDMS, as we have shown in the previous sections dissolves in supercritical carbon dioxide whereas polyethylene does not. High molecular weight PDMS and PE are also known to be highly incompatible.22 In a recent study, pellets of the polyethylene (M, = 121,000) sample discussed in the previous sections were impregnated with a 0.1 wt % solution of PDMS (M, = 94,000), also discussed in the previous sections, in supercritical carbon dioxide. The impregnation was carried out in a view cell loaded with the polymer pellets, PDMS and CO2 at 130 OC and 57 MPa. The temperature of impregnation was chosen to be close to the crystalline melting temperature of polyethylene so that it can be swollen with carbon dioxide. At these conditions, the translucent appearance of the initially opaque polymer pellet provides a visual verification of the penetration of carbon dioxide into the host polymer matrix. The pressure conditions were chosen to insure complete miscibility for PDMS in carbon dioxide. After equilibration at these conditions, the pressure was reduced rapidly to atmospheric pressures by venting the carbon dioxide from the cell. The depressurization was carried out either at the initial impregnation temperature of 130 oC, or at lower temperature of 50 oC. The pellets were then removed, freeze-fractured in liquid nitrogen, carbon-coated and characterized by Energy Dispersive X-Ray Spectroscopy (EDX) for silicon atom distribution across the cross-section. The microstructural changes were also examined by electron microscopic investigations. Figure 12 shows the silicon count across the cross-section of the pellets. Even though near the outer edges of the pellet a decrease in the silicon count is noted, an average net silicon count of about 5000 is observed throughout the pellet. This overall picture is similar in the pellets depressurized at 130 oC or at 50 oC. Closer analysis of the distribution in the first 100 micron range from the outer edges give indications that silicon count shows an initial decrease but then passes through a localized maximum. This phase localization and unique gradient formation becomes more distinctly observable and highly dependent on the temperature of depressurization when the host polymer is a glassy polymer like polystyrene instead of semicrystalline polymer like polyethylene.22 This localization has been attributed to the vitrification /crystallization - enhanced entrapment of PDMS while it is carried away from the inner regions of the pellet towards the outer surface during decompression.

Figure 12. Silicon distribution in polyethylene blended with PDMS by impregnation with 0.1 % solution of PDMS in carbon dioxide at 57 MPa and 130 OCand depressurization at 130 OC(left) and 50 °C (right).

A consequence of pressure-induced phase-separation in polymers swollen with a fluid like supercritical carbon dioxide is the foaming and porous structure formation. Figure 13 shows the microstructure of the polyethylene pellet before and after the impregnation (at 130 oC) with 0.1 % PDMS solution and depressurization (at 50 oC) treatment at a magnification of about X 10,000. While the initial polymer is nonporous, during the process a nano-porous polyethylene has been produced. Fine-tuning of these processes to achieve such unique microstructures and compositional distributions is a new challenge. It requires a balanced understanding of the thermodynamics (miscibility and phase separation), kinetics (time scale of phase growth, diffusional rates) and other physicochemical properties of the fluids and the materials such as the viscosity, glass transition temperature, interfacial tension, melting and crystallization temperatures and their compositional dependence at high pressures.

Figure 13. Electron micrographs of polyethylene before (left) and after swelling in 0.1 % solution of PDMS in carbon dioxide at 57 MPa and 130 OC followed by depressurization at 50 OC (right).

References

1. E. Kiran, Polymer miscibility and kinetics of pressure-induced phase separation in near-critical and supercritical fluids, Chapter 6 in Supercritical Fluids. Fundamentals and Applications, E. Kiran, P. G. Debenedetti and C. J. Peters, Eds., Kluwer Academic Publishers, Dordrecht, The Netherlands (2000).

2. W. Zhuang and E. Kiran, Kinetics of pressure-induced phase separation (PIPS) from polymer solutions by time-resolved light scattering. Polyethylene + n-pentane, Polymer, 39,2903-2915 (1998).

3. T. Hashimoto, Structure formation in polymer mixtures by spinodal decomposition, in Current Topics in Polymer Science, Vol 11, R. M. Ottenbrite and L. A. Utracki, Eds., Hanser, New York (1987).

4. L. A. Utracki, Thermodynamics and kinetics of phase separation in Interpenetrating Polymer Networks, D. Klemper and L. H. Sperling, Eds., ACS Advances in Chemistry Series, Vol. 239, ACS, Washington, D.C., (1994); pp. 77123

5. E. Kiran, Polymer formation, modification and processing in or with supercritical fluids, in Supercritical Fluids. Fundamentals for Applications, E. Kiran and J. M. H. Levelt Sengers, Eds. Kluwer Academic Publishers, Dordrecht, The Netherlands (1994); pp. 541-588.

6. E. Kiran and W. Zhuang, Miscibility and phase separation of polymers in near- and supercritical fluids, in Supercritical Fluids- Extraction and Pollution Prevention, M. A. Abraham and A. K. Sunol, Eds, ACS Symp. Ser. No 670, ACS, Washington, D. C., (1997); pp. 2-36.

7. B. Folie and M. Radosz, Phase equilibria in high-pressure polyethylene technology, Ind. Eng. Chem. Res., 34, 1501-1516 (1995).

8. A. D. Shine, Polymers in supercritical fluids, Chapter 18 in Physical Properties of Polymers Handbook, J. E. Mark, Ed., American Institute of Physics, New York (1996).

9. C. F. Kirby and M. A. McHugh, Phase behavior of polymers in supercritical fluid solvents, Chem. Rev., 99, 565-602 (1999).

10. Z. Bayraktar and E. Kiran, Miscibility, phase separation and volumetric properties of solutions of poly(dimethylsiloxane) in supercritical carbon dioxide, J. Appl. Polym. Sci., 75, 1397-1403 (2000).

1 1. H.-G. Elias, An Introduction to Polymer Science, VCH, New York (1997), p.23 1.

12. W. Zhuang, Miscibility, demixing and kinetics of phase separation of polymers in near and supercritical fluids by light scattering, Ph. D. Thesis, University of Maine, Orono, Maine, USA (Thesis Advisor: E. Kiran), 1995.

13. E. Kiran and H. Pohler, Alternative solvents for cellulose derivatives: Miscibility and density of cellulosic polymers in carbon dioxide + acetone and carbon dioxide + ethanol binary fluid mixtures, J. Supercrit. Fluids, 13, 135-147 (1998).

14. E. Kiran, Y. Xiong, and W. Zhuang, Effect of polydispersity on the demixing pressures of polyethylene in near- and supercritical alkanes, J. Supercrit. Fluids, 7, 283-287 (1994).

15. H. Pohler and E. Kiran, Miscibility and phase separation of polymers in supercritical fluids: Polymer-polymer-solvent and polymer-polymer-solvent ternary mixtures, presented at the AIChE Annual Meeting, Chicago, Illinois, November 10-15, 1996.

16. Y. Xiong and E. Kiran, High-pressure light scattering apparatus to study pressure-induced phase separation in polymer solutions, Rev. Sci. Instrum., 69, 1463-1471 (1998).

17. Y. Xiong and E. Kiran, Kinetics of pressure-induced phase separation (PIPS) in polystyrene + methylcyclohexane solutions at high pressure, Polymer, 41, 37593777 (2000).

18. K. Liu and E. Kiran, Kinetics of pressure-induced phase separation (PIPS) in solutions of poly(dimethylsiloxane) in supercritical carbon dioxide: Crossover from nucleation and growth to spinodal decomposition. J. Supercrit. Fluids, 16, 59-79 (1999).

19. K. Liu and E. Kiran, Pressure-induced phase separation (PIPS) in polymer solutions: Kinetics of phase separation and crossover from nucleation and growth to spinodal decomposition in solutions of polyethylene in n-pentane, Proc. 5th Int. Symp. Supercritical Fluids, ISSF 2000, April 9-1 1,2000, Atlanta, Georgia.

20. T. Hashimoto, Self assembly of polymer blends at phase transition- morphology control by pinning of domain growth, in Progress in Pacific Polymer Science, Vol 2., Y. Imanishi, Ed., Springer, Berlin (1992), pp.175-187.

21. Z. Bayraktar and E. Kiran, Microblending of polymers with swelling and impregnation from supercritical fluid solutions. An investigation of the polymer distribution using energy dispersive X-ray spectroscopy, Presented at the AIChE Annual Meeting, Dallas, Texas, October 31-November 5, 1999.

22. Z. Bayraktar and E. Kiran, Polymer blending by pressure-induced swelling. Impregnation and phase separation in supercritical fluids. Gradient blending and phase localization of poly(dimethylsiloxane) in polystyrene and polyethylene, submitted for publication in Macromolecules.

THE COMPATIBILIZATION OF POLYMER BLENDS WITH LINEAR COPOLYMERS: COMPARISON BETWEEN SIMULATION AND EXPERIMENT

Chemistry Department The University of Tennessee Knoxville, TN 37996-1600

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