Rheological Behavior of Copolymers

An important class of commercial polymers is that of copolymers of ethylene and alpha-olefins, which are commonly referred to as linear low density polyethylenes (LLDPE). The use of a copolymer introduces short-chain side branches onto the polyethylene backbone, and the effect of these short-chain branches on rheological properties depends very much on the method of polymerization. If a heterogeneous, Ziegler catalyst is used, the side-chains tend to be distributed in blocks rather than randomly along the backbone, and Wardhaugh and Williams [74] point out that this can lead to microphase separation in the melt, which could have an important effect on rheological behavior. 

Fetters et al. [60 ] proposed that their empirical equations (Eq. 5.25a and b above) for calculating the plateau modulus of a poly(a-olefin), given only the average molecular weight per backbone bond, could be used to estimate the plateau moduli of ethylene/a-olefin copolymers. 

Arnett and Thomas [78] used hydrogenated polybutadiene to study the effect of ethyl branches on the zero-shear viscosity. The data did not obey either of the standard expressions for the effect of temperature, but data at the highest temperatures were fitted to the Arrhenius expression (Eq. 4.68), and log(E3) was found to be linear in the number of ethyl branches per total carbon atoms, n. They correlated all of their data by use of Eq. 5.41. 

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