Multiphase polymers elastomers with

In general, there is a paucity of information on the relationship between polymer structure and degradation kinetics. This becomes especially critical in multiphase polymers like heterophasic copolymers, thermoplastic elastomers and blends. This review should stimulate research in this important area, which could ultimately lead to polymers with better photo-thermal and radiation resistance as well as more effective stabilizers. 

Multiphase or multicomponent polymers can clearly be more complex structurally than single phase materials, for there is the distribution of the various phases to describe as well as their internal structure. Most polymer blends, block and graft copolymers and interpenetrating networks are multiphase systems. A major commercial set of multiphase polymer systems are the toughened, high impact or impact modified polymers. These are combinations of polymers with dispersed elastomer (rubber) particles in a continuous matrix. Most commonly the matrix is a glassy amorphous thermoplastic, but it can also be crystalline or a thermoset. The impact modified materials may be blends, block or graft copolymers or even all of these at once. 

As may be guessed from the names for these systems, the rubber particles are added to improve the mechanical properties of the matrix material, particularly to improve their low impact strength. The size of the rubber particles, their distribution, composition and compatability with the matrix all influence the mechanical properties of the final engineering resin. T) ical multiphase polymers which include elastomers... 

Major factors that affect the size and distribution of dispersed phases in multiphase polymers are elastomer content, compatibility, processing and viscosity. If the elastomer is present as the smaller volume fraction, it will most likely be the dispersed phase. As the volume fraction increases, the size of the dispersed phase can be larger and there is more likelihood of subinclusions of the matrix polymer in the elastomer [217, 218]. These subinclusions appear to enhance impact properties. The shape of the dispersed phase often changes with differences... 

Walters and Keyte [82] first observed dispersed particles in blends of rubber polymers by phase contrast optical microscopy. Marsh et al. [83] studied elastomer blends by both optical phase contrast and TEM. Electron microscopy was applied to study blends of natural rubber, styrene-butadiene rubber (SBR), cis-polybutadiene (PB) and chlorobutyl rubber [84]. It became obvious that both hardening of the rubber and staining were necessary for producing sections with contrast for TEM. Today, the most common methods of observing multiphase polymers are by phase contrast OM of thin sections, TEM of stained ultrathin sections and SEM of etched or fractured surfaces. 

Multiphase polymer particles prepared by emulsion polymerizations find a number of important commercial applications such as elastomers, coatings, adhesives, and impact resistant thermoplastics. Latex products, which exhibit nonuniform particle morphology, are produced when two or more monomers react with one another such that separate polymer phases form during emulsion polymerization. The incompatibility of different polymers or the sequence and location of the formation of polymers can result in separate polymer phases. 

Representative SEM images (Fig. 4.41) show a range of different multiphase polymers in notched Izod impact fractured specimens. A polymer with large, nonuniform, dispersed phase particles, not well adhered to the matrix, is shown in Fig. 4.41A. A much finer dispersed phase is shown in Fig. 4.41B with both particles and holes from particle pullouts. Smaller particles are not as obvious in Fig. 4.41 C, although the dispersed phase accounts for 15% of the specimen. Finally, the SEM image in Fig. 4.41 D does not reveal the elastomer, so the size and distribution of the dispersed phase must be provided by some other microscopy technique. 

The commercial success of ABS led to the development of many other multiphase materials. ABS consists of a rubbery phase dispersed in a glassy, or plastics, phase. Unfortunately the rubber used (polybutadiene) is not particularly light resistant, as it is prone to oxidation, and so ABS plastics are not naturally weather resistant. If the polybutadiene is replaced with a rubber that contains no main chain, double bonds (i.e the unsaturation is removed) then a more heat and light resistant material results. When an acrylic ester rubber (or elastomer) is used then, the material is called an acrylate-styrene-acrylonitrile polymer. 

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