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Reactive processing

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Mechanically blending polymers with different characteristics can give new materials with some of the properties of both parents. This is a standard method for generating value-added plastics such as rubber-toughened adhesives and high impact polystyrene. The disadvantage with simple mechanical blending, however, is that the intimacy of the blend is limited.

A number of techniques have been developed to improve the stability of mirco-phase morphologies, and so increase the intimacy of the blend, including:

• Reaction induced phase separation in which polymerisation of an initially compatible mixture results in phase separation.
• The addition of copolymers to stabilise interfaces.
• Chemical coupling of reactive groups on each polymer leading to the formation of copolymers or grafts at the interface between phases ('reactive blending').

To date the ability to use these techniques in practice has been limited, as only an empirical understanding of the processes is available. The IRC is now conducting a broad range of experimental and theoretical modelling studies aimed at quantifying the interaction between flow, reactivity and structure leading to the development of stable micro-phase structures. These include:

• The evolution of bubbles and diffusion of gasses during processing, important in the formation of foams.
• Localised effects on composition and degree of reaction during processing.
• Process-induced phase separation and shear banding.

Whilst in the past the importance of blends has been in their mechanical properties, increased control over the process offers new product possibilities. Materials with sub-micron to nano-scale structures, have been demonstrated to have valuable optical and electronic properties. The ability to blend polymers at these scale lengths potentially offers access to new 'smart' and 'reactive' products.