GPC with Branched Polymers

The final step in establishing the branching structure is to relate g to the branching structure. This requires some knowledge of the way the polymer was synthesized. For a number of known structures, theoretical equations have been derived. Equations 2.17 to 2.26 are examples, and Small [7] has listed sources for many others. The GPC-MALLS technique is clearly a very valuable tool in the study of branched polymers. On the one hand, it can be used to verify polymerization models, and on the other, it produces data that can be used to predict rheological properties using the models presented in Chapters 9 and 11. 

The long-chain branched metallocene polyethylenes made with the constrained geometry catalysts described in Chapter 3 pose their own special problem, because the level of branching, 0.01 to 0.1 LCB per 1,000 carbon atoms, is too low to be detected by means of GPC-MALLS. For these materials the factor j8 in Eq. 2.104 has been established to be 0.5 by use of C-13 NMR [57], and this makes it possible to use GPC-LALLS-DV to determine the distribution of radii of gyration and the number of branches per 1000 carbon atoms. Then using the results 

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