Entanglements and glass transition in polymer blends

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Everybody is surrounded by polymeric materials - modern life would not be possible without them. Typical polymer materials consists of two or more different polymers. As a blend has many degrees of freedom to play with, this, in principle, allows the material designer to tweak the material properties to his needs. Therefore a knowledge of the physics behind these materials is highly desired. Both mechanical and flow properties of typical polymeric melts are dominated by two phenomena: entanglements and glassy effects. The theory of entanglements attempts to describe the fact that long polymer chains can not pass through each other. This fact seriously reduces chain mobility in polymeric liquids and leads to extremely long relaxation times (in the order of seconds or even days as compared with picoseconds for simple liquids).The transition from a liquid to an amorphous solid is known as the glass transition. Approaching the glass transition also has a drastic effect on the relaxation times. The relaxation time of a statistical segment, the basic unit of a chain, can increase by orders of magnitude by only a small temperature change. As with entanglements this can also lead to extremely long relaxation times. The effect of the glass transition is very important even far above the transition: the monomer mobility in the melt, which is one of the most crucial parameters in all theories, depends critically on the distance from the glass transition. Thus, in order to understand the behaviour of molten polymers, it is essential to understand both entanglements and glass transition phenomena.The tube theory describing the linear rheology of linear monodisperse polymers is well-established but this system is highly idealized and therefore of limited use in industrial applications. There the processes are strongly non-linear, and include mixtures of different molecular weights and more importantly blends of different polymers. It is this situation of mixtures of two different polymers that is much less understood theoretically. Both components are susceptible in a different way to the glass transition and to entanglements effects. The aim of the project is to develop new theoretical understanding of the linear rheology of such a polymer blend. This is carried out by using massive state-of-the-art computer simulations to study the simplest model for a polymer blends covering a wide range of chain lengths, chain diameter and densities, and developing smart analysing tools relevant for supporting the development of the theory. After linear regime is understood, we then plan to investigate the effect of flow on glass transition and entanglements in polymer blends.