Rubbery dispersed phase separation

Finally, a different and quite complementary form of crazing occurs on a much larger microstructural scale in diblock copolymers composed of phase-separated blocks of compliant rubbery components such as PB in spherical morphologies or randomly dispersed spheres or wavy rods in a majority component of a stiff polymer such as PS. In such hetero-polymers, under stress the compliant component can be made to cavitate, which triggers plastic expansion of the remaining carcass of the stiff polymer to form a very regular kind of cellular craze matter over substantial parts of the volume and can result in a very tough response. 

Toughened adhesives contain a dispersed, physically separate, though chemically attached, resilient rubbery phase. The toughened concept (see Section 1.2) - in the modern sense - has so far only been successfully applied to two adhesive families - anaerobics and epoxies described in Section 5.1.2 and 5.1.5 respectively. It has also led to the creation of an entirely new species of adhesive - the toughened acrylic - which is discussed in Section 5.1.12.2 below. 

PP homopolymer is copolymerised with ethylene. In block copolymers, the ethylene content is much higher than the random copolymers. The copolymerised part of the material is rubbery and forms a separate dispersed phase within the PP matrix. As a result, block copolymerised PP is much tougher than homopolymerised PP and can withstand higher impact even at low temperatures but at the expense of transparency and softening point. The main applications of the block copolymerised PP are similar to those of elastomer-modified PP but where the impact property requirement is not that critical. 

It was pointed out in Section 2.16.9 that anionic living polymerisation can be used to prepare ABA tri>block copolymers suitable for use as thermoplastic elastomers. In such copolymers the A blocks are normally of a homopolymer which is glassy and the B block is of a rubbery homopolymer (e.g. a polydiene such as polybutadiene or polyisoprene). The characteristic properties of these materials stems from the fact that two polymers which contain repeat units of a different chemical type tend to be incompatible on the molecular level. Thus the block copolymers phase separate into domains which are rich in one or the other type of repeat unit. In the case of the polystyrene-polydiene-polystyrene types of tri-block copolymers used for thermoplastic elastomers (with about 25% by weight polystyrene blocks), the structure is phase-separated at ambient temperature into approximately spherical polystyrene-rich domains which are dispersed in a matrix of the polydiene chains. This type of structure is shown schematically in Fig. 4.36 where it can be seen that the polystyrene blocks are anchored in the spherical domains. At ambient temperature the polystyrene is below its Tg whereas the polydiene is above its Tg. Hence the material consists of a rubbery matrix containing a rigid dispersed phase. 

Polymer alloys often exhibit microphase separation. The heterogeneous morphologies are determined not only by the composition of the system but by the processing conditions as well. The microstructure influences the properties of polymeric alloys [40]. For example, the addition of a second phase of dispersed rubbery particles into the polymer matrix results in a great enhancement of toughness [41]. 

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