Multiphase polymers impact strengths

X HE IMPORTANCE AND UTILITY of multiphase copolymer systems have been well documented in the literature (1-4), with emphasis on their unique combination of properties and their potential material applications. Or-ganosiloxane block polymers are a particularly interesting type of multiphase copolymer system because of the unusual characteristics of poly siloxanes, such as their stability to heat and UV radiation, low glass transition temperature, high gas permeability, and low surface energy (i, 2, 5). The incorporation of polysiloxanes into various engineering polymers offers an opportunity for many improvements, such as lower temperatures for the ductile-to-brittle transitions and improved impact strength. 

This new theory of the non-equilibrium thermodynamics of multiphase polymer systems offers a better explanation of the conductivity breakthrough in polymer blends than the percolation theory, and the mesoscopic metal concept explains conductivity on the molecular level better than the exciton model based on semiconductors. It can also be used to explain other complex phenomena, such as the improvement in the impact strength of polymers due to dispersion of rubber particles, the increase in the viscosity of filled systems, or the formation of gels in colloids or microemulsions. It is thus possible to draw valuable conclusions and make forecasts for the industrial application of such systems. 

The commercial value of some polymers depends on their morphology. Thus, high-impact polystyrene (HIPS), which has an impact strength 5-10 times that of the neat polystyrene, is a multiphase material in which polybutadiene rubbery domains are distributed within the polystyrene matrix (Figure 1.4). Each of these domains contains polystyrene... 

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... 

Multiphase polymers are commonly toughened plastics which contain a soft, elastomeric or rubbery component in a hard glassy matrix or in a thermoplastic matrix. An example of the typical brittle fracture morphology of an unmodified thermoplastic is shown by SEM of nylon (Fig. 5.45A). Addition of an elastomeric phase modifies the brittle fracture behavior of the matrix, as shown in a fracture surface of a modified nylon (Fig. 5.45B). The modification depends on the composition and deformation mechanism of the material [204, 215], but normally it increases the fracture toughness and strength from that of the unmodified matrix resin. Impact strength, as measured for instance by an Izod impact testing apparatus, is affected by the dispersed phase... 

Processing plays a major role in the nature of the dispersed phase in multiphase polymers. Changes in the shear forces and the temperature provide different structures. In the case of PS modified with polyisoprene, TEM studies showed that smaller particles, broken down in size by melt shearing, resulted in lowered impact strength and increased tensile strength [183]. Particle dimensions have also been shown to be affected by the viscosity of the molten polymer and the concentration modifier. Heikens et al. [184] investigated copolymer modified PS and... 

Figure6.9 Synergistic toughness behavior of PEEK/PEi biends (tensiie impact strength) (reproduced from Harris, . E.and Robeson, L.M., in ContemporaryTopics in Polymer Science, Vol.6 Multiphase Macromolecular Systems, Cuibertson, B. M. (Ed.) (1989) Pienum Press, New York, p. 519 with kind permission of Springer Science and Business Media)...

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