Simultaneous Determination of Surface Tension and Density of Polymer Melts Using Axisymmetric Drop Shape Analysis 1 M. Wulf,* S. Michel,* K. Grundke,* ,2 O. I. del Rio,† D. Y. Kwok,† and A. W. Neumann† *Institute of Polymer Research, Hohe Str. 6, 01069 Dresden, Germany; and †Department of Mechanical and Industrial Engineering, University of Toronto, Toronto, Canada E-mail: grundke@argos.ipfdd.de Received July 8, 1998; accepted October 19, 1998 By employing a new strategy, we show that axisymmetric drop shape analysis (ADSA) can be used to determine simultaneously the surface tension and the density of polymer melts from sessile drops at elevated temperatures. To achieve this, two developments were necessary. First, the ADSA algorithm had to be modified to replace the density by the mass of the drop as an input parameter. Since ADSA also yields the volume, the density became output rather than input. Second, a closed high-temperature chamber whose temperature could be precisely controlled and a sample holder that allowed the formation of highly axisymmetric sessile drops at elevated temperatures had to be developed. Fora typical polymeric material (polystyrene), it is demonstrated that measure- ments with sessile drops yield essentially the same surface tension values and temperature coefficients as measurements with pen- dant drops. The densities determined with ADSA are comparable to independent PVT results. © 1999 Academic Press Key Words: surface tension; polymer melt; density; drop shape analysis; pendant drop; sessile drop. 1. INTRODUCTION The surface (interfacial) tension of polymer melts is an important thermodynamic parameter that plays a key role in many technological processes such as wetting, coating, poly- mer blending, and the reinforcement of polymers with fibers. However, the high viscosity and the limited thermal stability of polymer melts as well as the high temperatures cause several difficulties in the experimental determination of their surface tension. Therefore, exact experimental data are often not avail- able. Regarding the measuring technique, considerable effort has been made to develop and to modify drop shape methods and the Wilhelmy balance technique for the determination of sur- face (interfacial) tensions of polymer melts (1–7). In the Wilhelmy experiment a solid surface is immersed into the liquid. The measured quantity is the increase in weight due to the wetting of the surface. In the case of polymer melts, thin fibers were used as solid probes. This has the advantage that no correction for buoyancy has to be made and therefore the knowledge of the melt density at elevated temperatures is not necessary (5). A drawback of the Wilhelmy technique is that the surface tension is not measured directly. Since the mea- sured quantity equals the so-called wetting tension  l cos , complete wetting of the fiber by the polymer melt (contact angle  = 0°) is required to obtain the surface tension  l (4, 5). Drop shape methods are based on the idea that the shape of a sessile or pendant drop is determined by a combination of surface tension and gravity effects. When gravitational and surface tension effects are comparable, it is possible, in prin- ciple, to determine the surface (interfacial) tension from the measurement of the shape of a drop. Compared with the Wilhelmy technique two major advan- tages of drop shape methods should be stressed. First, the surface tension is obtained directly. No requirements like com- plete wetting are needed. Second, they can be used to study both liquid–vapor and liquid–liquid interfacial tensions of polymer melts. But, since gravity is involved, one has to know the density of the polymers at elevated temperatures; this has to be evaluated separately by time-consuming methods of dilatometry (8, 9). In the past, the precision of drop shape methods was strongly dependent on the measurement of some selected critical points of the drop shape that had to be interpreted using different sets of tables (10 –12). By means of this classical calculation pro- cedure accurate and consistent results are difficult to obtain. Computer based developments started with the work of Maze and Burnet (13, 14). Recent developments utilize video digital image processing techniques to extract the entire experimental drop profile with subsequent numerical procedures to calculate the surface (interfacial) tension based on the Laplace equation of capillarity (1, 2, 15–19). The surface tension measurements of polymer melts using drop shape methods are typically based on pendant drop ex- periments, both in the older, e.g. (20), as well as in the recent literature, e.g. (21, 22). There are, in essence, two reasons for 1 This paper is dedicated to the memory of Professor Hans-Jo ¨rg Jacobasch, who died on April 15, 1998. 2 To whom correspondence should be addressed. Journal of Colloid and Interface Science 210, 172–181 (1999) Article ID jcis.1998.5942, available online at http://www.idealibrary.com on 172 0021-9797/99 $30.00 Copyright © 1999 by Academic Press All rights of reproduction in any form reserved.