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SURFACE TENSION OF POLYMER LIQUIDS yielded spectra which exhibited no isosbestic points in the ultraviolet region; presumably the protonation products existed as a mixture of geometric isomers. Even unprotonated cyanines may form such isomers, l4 as was evident from the visible absorption spectra of However, certain carbocyanines at low temperature. the simple cyanines studied here gave no indication for
the presence of isomeric species in an alcohol glass a t liquid nitrogen temperature. (14) G. S. Levinson, W. T. Simpson, and W. Curtis, J . Am. Chem. Soc., 79, 4314 (1957). (15) G. Scheibe and 0. Worz, Angew. Chem.,78,304 (1966);W. West, 8. Pearce, and F. Grum, J . Phga. Chem., 71, 1316 (1967).
Surface Tension of Polymer Liquids by Ryong-Joon Roe Electrochemicala Department, E . I . du Pont de Nemoura and Co., Inc., Experimental Station, Wilmington, Delaware 19898 (Received October 10, 1967)
The surface tensions of homopolymers above their melting points were measured by the pendant-drop method. The polymers studied are, in the order of decreasing surface tension, poly(ethy1ene oxide), linear polyethylene, branched polyethylene, polyisobutylene, atactic polypropylene, and polydimethylsiloxane. The Macleod exponent p , which is usually around 4 for simple liquids, has been found to be smaller, generally between 3 and 3.5, reflecting the restrictions imposed on the conformations of polymer molecules a t the surface.
Surface tension of molten polymer liquids has reThis is due ceived attention only in recent in part to the experimental difficulties in performing measurements on highly viscous liquids a t elevated temperatures. Interest has been stimulated more recently (i) by the development of the concept of the critical surface tension of wettingg as a measure of surface energy of polymeric solids, and also (ii) by the recognition that it is possible to correlateloJ1 the surface properties of polymer liquids with those of low molecular weight homologs. I n this work we have adopted the pendant-drop method, originally proposed by Andreas, Hauser, and Tucker.12 A further refinement of the technique, described elsewhere,13is tested and utilized here. The static nature of the method allows evaluation of the equilibrium surface tension uninfluenced by high viscosities of these polymeric liquids. A preliminary account of the result was presented earlier4 and it was demonstrated that the concept of parachor can be applied to polymers. I n this paper the details are given. Experimental Section ( A ) The optical train, used for taking enlarged photographs of the drops, is similar to those described by others12J4but with some further modifications. A collimated light beam from a mercury arc lamp was filtered through Wratten Filters No. 77 and 75 for
isolation of 546-mp line before its entrance into the pendant-drop cell. A Unitron microscope objective of a magnification 2X was placed in front of a Bausch and Lomb Type H photomicrographic camera with a Polaroid Land camera back. A variable aperture was placed, as a telecentric stop, at the rear focal plane of the objective lens, as recommended by Andreas, et a1.12 When the aperture was made small, however, (1) H. Tarkow, J . Polyn. Sci., 28, 35 (1958). (2) N. Ogata, Bull. Chew SOC.Jap., 3 3 , 212 (1960). (3)F. J. Hybart and T. R. White, J . Appl. Polym. Sci., 3 , 118 (1960). (4)R. J. Roe, J . Phys. Chem., 69,2809 (1965). (5) (a) H.Schonhorn and L. H. Sharpe, J . Polgm. Sci., Part A , 3 , 569 (1965); (b) H. Schonhorn and L. H. Sharpe, Polyn. Lett., 3 , 235 (1965). (6) R. H. Dettre and R. E. Johnson, Jr., J . Colloid Sci., 2 1 , 367 (1966). (7) T. Sakai, Polyn., 6, 659 (1965). (8) H.Schonhorn, F. W. Ryan, and L. H. Sharpe, J . Polyn. Sci., Part A-8, 4, 538 (1966). (9) W. A. Zisman, Advances in Chemistry Series, No. 43,American Chemical Society, Washington, D. C., 1964,p 1. (10) H. W.Starkweather, Jr., SPE Trans., 5 , 5 (1965). (11) R. H.Dettre and R. E. Johnson, Jr., J. Phya. Chem., 71, 1529 (1967). (12) J. M.Andreas, E. A. Hauser, and W. B. Tucker, ibid., 42, 1001 (1938). (13) R. J. Roe, V. L. Bacchetta, and P. iM.G. Wong, ibid., 71,4190 (1967). (14) H.L. Frisch and S. AI-Madfai, J . Amer. Chem. SOC.,80, 3561 (1958). Volume 72,Number 6 June 1068
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the photographic image became less sharp due to a diffraction effect, and its utility appeared to be doubtful, except when the drop was surrounded by a liquid of nearly the same refractive index, as in interfacialtension studies. A small plumb bob suspended with a fine nylon monofilament was placed inside the Polaroid camera, just in front of the film plane. A sharp shadow of the nylon filament thus imprinted on every photograph served as the reference for the vertical direction. The positions of the objective lens and the camera film plane were fixed on an optical bench for a series of measurements. Focusing of the drop image was accomplished by moving the pendant-drop cell back and forth. This kept constant magnification for all the photographs taken in a series. At the beginning and end of the series, a reticle with a calibrated grid pattern was photographed to determine the magnification factor. Vibration of the drop was a source of difficulty whenever the liquid was fairly fluid at higher temperatures. The difficulty was overcome by placing the whole optical bench on a sturdy steel frame mounted in a ground level laboratory. No attempt was made to use a photoflash for illumination, since the sharper photograph thus obtained does not necessarily represent an equilibrium drop shape. (23) Figure 1 illustrates the construction of the pendant-drop cell. A spectrophotometric absorption cell of silica (10 mm square) (A) is fitted snugly in the cavity inside an aluminum heating block (B). The light beam passes through the horizontal hole fitted with circular quartz disks (C) at both ends. A thermocouple is inserted in the hole (D) drilled close to the inner cavity. Six long, thin cartridge heaters (E) are placed symmetrically around the cavity. I n addition, there are four hollow channels running vertically inside the block (not shown), through which a cold air stream can be blown for rapid cooling. The tube F is used to introduce argon gas under pressure to keep the atmosphere around the drop oxygen free. A sheet-metal housing (G) with quartz windows (H) surrounds the block to insulate it against drafts. The whole assembly is supported on an optical bench with the rod J. Conductance of heat to the rod is minimized by use of Transite (I
