OSTWALD RIPENING IN IMMISCIBLE POLYOLEFIN BLENDS

The coarsening in the quiescent melt of the phase-segregated particles of a polymer blend, composed of a narrow molecular weight fraction of an unbranched high-density polyethylene (HDPE) and a highly branched (100 ethyl branches/1000 C atoms) hydrogenated polybutadiene (HPB) was studied. The system was effectively binary, due to the narrow molecul ar weight and composition distributions of each component. The system was composed of 90 wt % of the HDPE and 10 wt % of the HPB and it form ed a two-phase system in the melt at 177 degrees C. The blend was prec ipitated from xylene solution in order to obtain an initially intimate ly mixed system. This was the third study in a series of studies of th e coarsening of phase-segregated particles in polymer blends. This stu dy was unique in that the system studied was binary in this case while the previous systems were multicomponent. Since the present system wa s binary, exact thermodynamic calculations of the phase state of this system could be applied with a high level of confidence. The droplet p hase particles, which were mainly composed of the HPB, were observed t o coarsen on storage in the melt for times of from 5 s to 1 h. At the shortest storage time of 5 s the particles had an average radius of ab out 0.05 mu m and coarsened to about 0.2 mu m after 1 h storage in the melt state. Particle dimensions were measured by scanning electron mi croscopy of n-heptane-etched and gold-coated sections. It was found th at the volume of the particles increased linearly with time and that t he rate constant of coarsening was K-exp = 1.23 X 10(-18) cm(3)/s and this agreed fairly well with the rate constant calculated from Ostwald ripening theory of K-ce = 0.86 X 10(-18) cm(3)/s. In contrast the rat e constant for direct particle diffusion and coalescence was K-c = 3.6 X 10(-20) cm(3)/s. Since this was two orders of magnitude smaller tha n the rate constant for Ostwald ripening, it was concluded that, altho ugh the linear increase of volume with time was also consistent with t he particle diffusion and coalescence mechanism, this was not a signif icant contributor to the coarsening mechanism. The major cause for the insignificance of the particle diffusion and coalescence mechanism wa s the high melt viscosity of the matrix polymers. The application of t he Ostwald ripening theory to this system could be made with a high le vel of confidence because it was binary. It was found that the phase c oncentration of the droplet phase apparently underwent a rapid increas e during the first 1-2 min of storage in the melt, indicating that the system did not reach phase equilibrium (i.e., did not completely phas e-segregate) for about 1-2 min. This further indicated that the long-t ime coarsening regime was not entered until after this length of time. The particle size distributions remained approximately self-similar o ver the period of coarsening, as predicted by Ostwald ripening theory. (C) 1995 John Wiley & Sons, Inc.