Domain Growth and Wetting in Polymer Mixtures
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DOMAIN GROWTH AND WETTING IN POLYMER MIXTURES ANDREW W. CUMMING'ý and PIERRE WILTZIUS" 4University of Florida, Department of Physics, Gainesville, FL 32611 d2AT&T Bell Laboratories, Murray Hill, NJ 07974 Abstract: We present the results of phase-separation experiments performed on the binary polymer blend polyisoprene-poly(ethylene-propylene). Elastic light scattering and optical microscopy were used to follow the phase-separation after quenches into the coexistence region of the phase diagram. For critical quenches, we observed the well known bicontinuous infinite cluster morphology normally associated with spinodal decomposition, but with two distinct growth modes. In the bulk, we observed L(t) - t 13 consistent with diffusion driven dynamics, whereas near the sample walls, there obtained a novel fast growth with L(t) - t 3/2, inconsistent with either diffusion or interface driven dynamics. We attribute this large exponent to wetting effects, but still lack any theoretical understanding of the phenomenon. Experimental D :eta: It has been well established theoretically1 and experimentally 2 that in the nonlinear pattern coarsening regime of spinodal decomposition that the pattern length scale grows with time as L(t) - t 113 in the case that material transport is by diffusion alone, and L(t) ~ t in the case that bulk hydrodynamic flows driven by surface tension are allowed. By contrast, we have observed power-law growth with a new exponent L(t) - t 3/2, which on the basis of a dynamical scaling argument, we have shown to arise from a two-dimensional scattering structure. We conjecture that this phenomenon (the Fast Mode) is associated with some long-range van der Waals mediated, or wetting effect. Recently, the role played by wetting in binary systems has received much theoretical and experimental attention, 3 , 4, 5 although a derivation for our observation oft 3/2 behavior has yet eluded theorists. The experiment.consisted of allowing the sample to equilibrate in the disordered state, approximately 0.1'C above the phase-separation threshold, at critical composition. A quench was then effected to a final temperature below the phase-separation threshold by between approximately 0.10C and 1.0'C. After the quench, determinations of the structure were made by measuring the 6 scattering with a charge-coupled device (CCD) camera, as described elsewhere. At the high temperature equilibration point, the temperature was regulated by the Lakeshore temperature controller, reading a thermistor and supplying a Minco heater. The quench was effected by simultaneously turning off the heater and routing water of pre-regulated temperature through passages in the sample holder. This water was pumped from a Lauda circulator/regulator, resulting in characteristic quench times on the order of 15 to 20 seconds. Mat. Res. Soc. Symp. Proc. Vol. 248. @1992Materials Research Society
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We observed the well known ring associated with the bicontinuous in
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