Modification of Polymer Blend Morphology Using Electric Fields

Transcription

1 Modification of Polymer Blend Morphology Using Electric Fields G. VENUGOPAL, S. KRAUSE and G. E. WNEK, Department of Chemistry, Rensselaer Polytechnic Institute, Troy, NY INTRODUCTION Droplets of a liquid suspended in another liquid of different dielectric constant may deform and burst in an electric field,' or one may observe "pearl chain formation," in which the liquid drops form chains in the direction of the field? Moriya et al? have observed some of these effects in a blend of polystyrene (PS) and polyethylene oxide (PEO) with the PEO as inclusions in a matrix of PS. Alignment of these minor phases in the matrix gives blends with unique morphologies which should result in unique properties like anisotropic dielectric constants, electrical conductivities, and mechanical properties. In this article we report preliminary experimental work on the morphologies of PEO-PS blend films that have been made by solvent casting in the presence of a DC electric field. EXPERIMENTAL Two samples of PEO and PS were used: PEOl (Aldrich), weight average molecular weight, M, = 8000, PE02 (Aldrich), M, = 1 x lo5, PSI (Gulf) M, = 2.5 X lo5, and PS2 (Aldrich) M, = 2.5 x lo5. Cyclohexanone for solvent casting was purified by drying with anhydrous magnesium sulfate and subsequent vacuum distillation. In all experiments, the total polymer concentration was 4% (weight/volume) and the PEO : PS ratio was 10 : 90 (by weight). The electrodes for the electric field experiments were made by evaporating copper onto glass microscope slides. The evaporation was done using a CVC high vacuum evaporator. A mask made of 3M Scotch Tape was used to coat selectively. A Hewlett-Packard 6516A DC power supply was used to generate electric fields up to 10.5 kv/cm. Since the electrodes were extremely thin, the solution always flowed over them. The resulting polymer films were characterized by differential scanning calorimetry (DSC) and optical microscopy. A Perkin-Elmer DSC-4 was used to make the DSC measurements at a heating rate of 10"C/min. The morphologies of the blends were characterized using a Leitz- Wetzlar optical microscope. RESULTS AND DISCUSSION The upper DSC curve in Fig. 1 is for PSI homopolymer after cooling from 220 C to 20 C at a rate of lo"c/min, and shows a glass transition (T,) at 104 C. The lower curve corresponds to a 10% PEO1-PS1 blend that was cast outside a field. Prior to acquiring the data, this sample was heated in a vacuum oven at 150 C for 24 hours. The endotherm near 65 C is for the melting of the crystalline PEO. The T, of the PS Journal of Polymer Science: Part C: Polymer Letters, Vol. 27, (1989) John Wiley & Sons, Inc. CCC /89/ $04.00

2 498 VENUGOPAL, KRAUSE, AND WNEK 1 ae LL w I c 0 z W Fig TMPERAfllRE Kl DSC thennograms of PS1 (top) and 10% PEO1-PSI blend (bottom). remains unchanged indicating that the two polymers are immiscible in their amorphous state. The morphologies of 10% PEO-PS blend films made outside the field are shown in Fig. 2a (PEO1-PS2) and 2b (PE02-PS2). An increase in the molecular weight yielded larger PEO phases under our conditions. The phase size in Fig. 2a is less than 1 micrometer, the smallest that could be observed by this method; the phase size in Fig. 2b is about 3 micrometers. On casting a PEO1-PS2 film in a 3kV/cm electric field, a significant increase in the phase size was observed (Fig. 3a). The arrows on the micrographs indicate the direction of the field. A few of the PEO phases appear deformed in the field direction. The phenomenon of deformation may occur when the force on the droplet due to the electric field overcomes the interfacial tension. Both the increase in phase size and the deformation of the phases became more obvious in the PE02-PS2 blend (Fig. 3b), the PE02 phases forming short, thick columns oriented in the direction of the field. The observation that the larger phases are more easily deformed agrees with the theory of O'Konski and Tha~her,~ who studied the effects of electric fields on aerosol particles. The formation of larger phases in the field suggests the existence of a fusion phenomenon. Such processes are known to occur with biological cells! At higher fields, 10.5 kv/cm, in the PEO1-PS2 blends, we observed pearl chaining (Fig. 4a). This process, which occurs due to an effect called "mutual seemed to occur to a lesser extent in the PE02-PS2 blend made in the same field strength. In this blend the PEO phase formed column-like structures (Fig. 4b) which appeared to be elongated versions of the shorter columns that were formed at lower fields. The wave-like shape of the columns may be attributed to a "rippling" motion of the solution that was produced when the electric field was on. This motion increases with field strength and presumably arises due to the inhomogeneity of the field at the edges of the electrodes and on the electrodes. Some of the column structures in the

3 POLYMER BLEND MORPHOLOGY 499 Fig. 2. field. (a) (b) Optical Micrographs of (a) PEO1-PS2 and (b) PE02-PS2 blend films cast outside a (a) (6) Fig. 3. Optical Micrographs of (a) PEO1-PS2 and (b) PE02-PS2 blend films cast in a 3 kv/cm field. Arrows indicate the direction of the field.

4 500 VENUGOPAL, KRAUSE, AND WNEK (4 (b) Fig. 4. Optical Micrographs of (a) PEO1-PS2 and (b) PE02-PS2 blend films cast in a 10.5 kv/cm field. Arrows indicate the direction of the field. micrograph seem to be at the point of breaking up and some others already have, a process similar to the necking of liquid cylinders? This effect was also noticed by Moriya et. al.,3 who were studying the effects of much larger electric fields on solvent free PEO-PS blends at 420K, above the Tg of PS. Garton and Krasucki predicted that a droplet will not deform indefinitely, but will break up when the ratio of the major to the minor axis exceeds a critical value. Both phenomena, deformation and breakup, depend on the viscosities of the matrix and the droplet? If the droplet viscosity is much higher than the matrix viscosity, the deformation process takes a longer time. Furthermore, in such systems at higher fields, fine threads are drawn from either end of the deformed droplet. The following summarizes the observations of our study so far: (1) Blends made in low fields showed an increase in the size of the minor phases, when compared to those made outside the field, suggesting that the electric field induces a coalescence or fusion process. Compared to the PEO1-PS2 blends the high molecular weight PEO blends (PE02-PS2) showed a significant deformation of the minor phases. (2) At high fields the morphology of the PEO1-PS2 blends showed pearl chain formation, while the PE02-PS2 blends were characterized by a column-like morphology. The pearl chain morphology in the PE02-PS blends probably occurs because of the breakup of some of the columns. We thank Prof. C. I. Chung for the use of his DSC equipment, and Prof. E. B. Nauman for the optical microscope. This work was supported in part by DARPA and the RPI Science Initiatives Program. References 1. C. G. Garton and 2. Krasucki, Roc. R. SOC. London A, (1964).

5 POLYMER BLEND MORPHOLOGY H. P. Schwan and L. D. Sher, J. Electrochem. SOC., 116, 22c (1969). 3. S. Moriya, K. Adachi, and T. Kotaka, Pob. Commun., 26, 235 (1985). 4. C. T. O Konski and H. C. Thacher Jr., J. Chem. Phys., 55, 955 (1953). 5. U. Zimmerman, Biochim. Biophys. Acta., 694, 227 (1982). 6. H. A. Pohl, Dielectrophoresis, Cambridge University Press, Cambridge, 1978, p A. W. Adamson, Physical Chemistry of Surfaces, John Wiley and Sons, New York, 1982, p S. Moriya, K. Adachi, and T. Kotaka, Langnuir, 2, 155 and 161 (1986). Received May 16, 1989 Accepted June 12, 1989