Molecular weight between entanglements

The properties of styrenic block copolymers are dependent on many factors besides the polymerization process. The styrene end block is typically atactic. Atactic polystyrene has a molecular weight between entanglements (Me) of about 18,000 g/mol. The typical end block molecular weight of styrenic block copolymers is less than Mg. Thus the softening point of these polymers is less than that of pure polystyrene. In fact many of the raw materials in hot melts are in the oligomeric region, where properties still depend on molecular weight (see Fig. 1). 

Recently, Tong et al. [195] have shown that in methyl methacrylate-b-alkyl acrylate-b-methyl methacrylate (MAM) and SIS-type triblock copolymers, the ultimate tensile strength is inversely proportional to the molecular weight between the chain entanglements in the middle soft block at comparable proportion of the outer block. 

Swelling data and stress-strain results in SBS have shown that Me is approximately the molecular weight between entanglements. Choudhury and Bhowmick [201] have shown in the case of different NR-based TPEs the following relation ... 

Tong J.D. and Jerome R., Dependence of the ultimate tensile strength of thermoplastic elastomers of the triblock type on the molecular weight between chain entanglements of the central block. Macromolecules, 33, 1479, 2000. 

Comments on Calculated Data. In several studies (13,18,19), G ax has been found to equal, or possibly be somewhat less than, the plateau modulus, G j, of a high molecular weight polymer whose chemical composition is the same as that of the network chains. Although G j for amorphous PPO has not been reported, it can be estimated from Zc, the number of chain atoms per molecule above which the viscosity increases approximately with the 3.4 power of Z. This quantity has been reported (25,26) to be about 400. As the chain atoms between entanglements is commonly about Zc/2, it follows that the molecular weight between entanglement loci is about 3900, and thus G j [ = (p/Me)RT] is about 0.65 MPa at 30°C. 

Diluents increase the molecular weight between entanglements according to the equation... 

Table 7.2. Values for the molecular weight between entanglements Me from the equilibrium properties of crosslinked networks... 

Molecular weight between entanglements in undiluted polymer. Characteristic molecular weight from t]0 vs M behavior of undiluted polymers. 

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