Addition of Block Copolymers

The addition of block copolymers to a binary polymer mixture can aid compatibilization of the components of the blend by reduction of the interfacial energy between the two phase separated domains. Copolymers can depress the build-up of the interfacial energy between the phases and/or make a favourable contribution to the entropy changes on mixing at the homogeneous/hetero-geneous phase separation temperature with the result that phase separation is delayed. 

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Reduction in interfadal tension of a homopolymer Mend by addition of block copolymer... 

The reduction in interfacial tension in homopolymer blends effected by addition of block copolymer is due to the segregation of copolymer to the interface. The... 

Two semi-empirical relations between the interfacial tension coefficient and compatibilizer concentration were derived. The first was obtained assuming an analogy between addition of block copolymer to a polymer blend and titration of an emulsion with surfactant [Utracki and Shi, 1992] ... 

The effects of minor proportions of PVC and PS in the polyolefins are quite dramatic. As little as 5% of PVC or PS in LDPE reduces the impact strength (toughness) of the latter by about 65%. This results from their presence as separate phases in the polyolefin matrix which leads to rapid crack propagation on impact. The effect of PP is very much less 10% of PP in LDPE reduced the energy absorbing capacity (toughness) of the matrix by only 1 % and 20% of PP reduced it by only 5%. The addition of block copolymers which act as compatibilisers or more correctly solid-phase dispersants (SPDs) for a second incompatible phase reduces the size of the heterogeneous domains and improves impact resistance. However, a considerable concentration ( 20%) of SPD is required, which unacceptably increases the cost in most cases. 

Block copolymers of styrene and butadiene are extensively used in blends as well as neat products. Most of the blends are made with polystyrene resins, where addition of block copolymer improves the toughness of otherwise brittle polystyrene. Such materials find widespread use in food packaging, from yogurt cups to polystyrene film used for food packaging. Some volume of copolymer is also used as an impact modifier in the manufacture of specialty grades HIPS. 

The effects of compatibilization both by reaction and addition of block copolymers are demonstrated in Fig. 5.10. Either type of compatibilization suppresses coalescence, resulting in lower dispersed phase sizes compared to the non-compatibilized blend. The reactive compatibilization is clearly more potent than addition of block copolymer and evidently suppresses coalescence completely over the concentration of dispersed phase investigated. Curiously, in this blend neither compatibilization scheme substantially reduces the dispersed phase domain size in the low concentration limit. The reason for this is not clear, although it may be due to the presence of an interphase with different rheological properties than either of the main components. Figure 5.11 shows a more... 

First, unassociated chains A combine to form a dimer A2 in a stage with equilibrium constant K2 which is followed by further stages involving successive additions of block copolymer chains to the growing micelle. Formation of an AT-mer involves multiple additions. 

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