Continuity development in polymer blends of very low interfacial tension AbstractPhase continuity development and co-continuous morphologies are highly influenced by the nature of the interface in immiscible polymer blends. Blends of ethylene-propylene-diene terpolymer (EPDM) and polypropylene (PP) possess an interfacial tension of about 0.3 mN/m and provide an interesting model system to study the detailed morphology development in a very low interfacial tension binary system. A variety of blends with viscosity ratios of 0.2-5.0 and shear stresses of 11.7-231.4 kPa were considered. Using a variety of sophisticated morphology protocols it is shown that at low blend compositions, the dispersed phase actually exists as stable fibers of extremely small diameter of 50-200 nm and the continuity develops by fiber-fiber coalescence. An analysis using break-up times from Tomotika theory also supports the notion of highly stable dispersed fiber formation. These results challenge the current view of the dispersed phase as small spherical droplets. It is shown, under these conditions, that a seven-fold variation in the viscosity ratio has virtually no influence on % continuity or morphology, while a large change in the matrix shear stress from 11.7 to 90.9 kPa has an important effect on pore diameter. Both sides of the continuity diagram are studied and highly symmetrical continuity behavior is observed with composition. In fact a single master continuity curve is observed for these blends varying in viscosity ratio from 0.7-5.0 and with shear stresses from 11.7-90.9 kPa. Although the glass transition temperatures indicate that these materials are completely immiscible after melt mixing and cooling, it is shown that the blends demonstrate the morphological features of a partially miscible system. These results support a concept that the blend was partially miscible during melt blending, at which time the gross morphological features of the blend were developed, but becomes fully phase separated upon cooling. It appears that the quenching of the EPDM/PP blend from the melt is rapid enough to preserve the imprint of that partial miscibility on the gross blend morphology. q
This work studies morphology development in blends of polyamide (PA) with poly(isobutyleneco-p-methylstyrene) (IMSM) and brominated poly(isobutylene-co-p-methylstyrene) (BIMSM). The presence of pendent benzylic bromine in BIMSM facilitates the rapid in situ formation of BIMSM-PA graft copolymer. IMSM represents the nonreactive reference for this blend system. In this study several important anomalies have been observed for the reactive BIMSM/PA system as compared to classical interfacially modified blend systems. These anomalies are as follows: as much as a 37-fold reduction in volume average diameter for the reactive system as compared to the nonreactive one; high phase size distribution (d v /d n ), at all blend compositions, with the fine droplets being in the 50-80 nm range scale; extensive droplet in droplet formation for BIMSM in PA in a BIMSM matrix; very high extents of reaction, i.e., 46 wt % of the total blend material reacts over a short time of mixing; and an emulsification study which demonstrates a linear drop in particle size and requires a very high concentration of copolymer (20 IMSM/80 BIMSM) to reach a plateau value. This is well beyond the amount of copolymer needed to saturate the interface even though static interfacial tension studies show that only 5% BIMSM in IMSM completely suppresses capillary breakup. These results are explained by a novel mechanism of reactive morphology development termed here as "interfacial erosion". The mechanism considers the formation of a very high viscosity graft copolymer right at the interface during dynamic mixing, resulting from the mutual contact of the BIMSM and PA molecules. The viscosity mismatch between the formed graft copolymer and the other constituents of the blend lead to the subsequent erosion of interphase material during dynamic mixing to form fine, nanometer-sized micelles in the bulk. The removal of the copolymer from the interface exposes nonreacted material and primes the interfacial region for further copolymer formation. In this fashion, most of the BIMSM can be made to react, and the resulting blend is a nanoscale dispersion with a number-average diameter of 50-80 nm and a volume average diameter of 300 nm. This work raises important considerations concerning the use of graft copolymers, in general, in polymer blend systems.
Effect of dynamic vulcanization on co-continuous morphology This study examines the effect of dynamic vulcanization on the co-continuous morphology in ethylene-propylene-diene terpolymer (EPDM)/polypropylene (PP) blends using a technique of morphology investigation involving focused ion beam (FIB) etching of the sample surface, followed by topological investigation of the sample surface using tapping mode atomic force microscopy (TMAFM). The FIB ion etching rates of EPDM and PP are distinctly different, and these differences create a significant topological contrast between the phases when subsequently analyzed by atomic force microscopy. This approach allows for the high-resolution observation of dispersed EPDM, and phases as small as 100 nm, are clearly identified. Since it is shown that the etching rates of noncrosslinked and crosslinked EPDM are similar, it was necessary to selectively remove the noncrosslinked EPDM phase by solvent dissolution. This combination of techniques then allows for the clear distinction of polypropylene, as well as noncrosslinked and crosslinked EPDM phases in the blend. The high-resolution micrographs together with the continuity data, surprisingly, indicate that a noncrosslinked co-continuous EPDM phase (␣-network) transits to a finer network of crosslinked EPDM (-network) after dynamic crosslinking. It is suggested that this
This work studies continuity development and cocontinuity in high viscosity ratio EPDM/PP blends. A very low interfacial tension (0.3 mN/m) between the blend components together with high viscosity ratios (11 and 17) result in a variety of unusual morphological features, including isolated nanometer diameter fibers, very large particles, partially coalesced particles, and numerous particles interconnected by fibers. This unique combination of morphologies leads the blend to a novel and stable cocontinuous structure of partially coalesced particles and particles interconnected by fibers. Compared with low to medium viscosity ratio EPDM/PP blends, these cocontinuous networks demonstrate early percolation thresholds, rapid continuity development, and attain cocontinuity at lower compositions of minor phase. The slow surface erosion of the high viscosity EPDM phase during melt blending is shown to be responsible for the generation of these unusual morphological structures. Typically the timescale for erosion phenomena are so small that they have defied study in the mixing environment itself and typical blend morphology studies almost always examine the final steady‐state morphology obtained after several minutes of mixing. The combination of very low interfacial tension and very high viscosity ratios of these EPDM/PP systems provide a unique opportunity to examine erosion phenomena persisting over longer time scales during melt mixing. We propose a new concentration‐dependant erosion mechanism that is based on particle collision–coalescence–separation dynamics. The proposed conceptual mechanism is shown to dramatically accelerate the erosion process and maintain cocontinuity over prolonged periods of mixing. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 1919–1929, 2006
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