Poly molecular weight between entanglements

Creton et al. [32] used a series ofblock copolymers with different molecular weights to compatibilize blends of PS and poly(2-vinylpyridine) (PVP). Only those copolymers whose PVP block polymerization degree was above 200 were found to be effective in preventing failure at the interface. This critical value corresponded roughly to the molecular weight between entanglements (Me), suggesting therefore... 

Another example that demonstrates the influence of chain entanglements is found in the melt crystallization kinetics of pure poly(dimethyl siloxane) (Mn = 740 000).(50b) It is found that in this case the derived Avrami equation with n = 3 can account for 95% of the transformation. This relatively rare event can be explained by the fact the molecular weight between entanglements of this polymer is 12 000 as compared, for example, with 830 for linear polyethylene.(50c) Thus, the number of entanglements per chain is relatively low as compared to other polymers. It will also be shown in Chapter 13 that a derived Avrami expression explains more than 90% of the crystallization of linear polyethylene from dilute solution. 

The polymer concentration (c) dependence of the zero-shear-rate viscosity (r]o) for aqueous poly(acrylamide) solutions of various viscosity-average molecular weights does not seem to follow the entanglement model, wherein polymer chain interpenetration would dominate the viscometric behavior, and a master curve should result when rjo is plotted against cM,. Instead, the data can be better described using a suspension model, wherein c[r]] correlates the r]o data on a master plot. A concise presentation of the relationship between r]Q, Af, and c for poly(acrylamide) in water at 25°C has been made (12). 

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