Polymer, branched Molecular weight

MALLS detectors can provide information on the branching ratio of a polymer and molecular weight. 

In a later study, Roovers and Graessley [39] investigated the role that molecular weight between branches plays in the rheology of polystyrene combs. Two backbone polymers with molecular weights of 275000 and 860 000 g/mol... 

By convention, the term branched implies that the polymer molecules are discrete, which is to say that their sizes can be measured by at least some of the usual analytical methods described in Chapter 3. A network polymer is an interconnected branch polymer. The molecular weight of such polymers is infinite, in the sense that it is too high to be measured by standard techniques. If the average functionality of a mixture of monomers is greater than 2, reaction to sufficiently high conversion yields network structures (p. 174). 

Molecular weight distributions in commercial polymers are characterized by ratios of about 3 for substances like polystyrene in which transfer to polymer does not appear to be important. Where long branches can be formed by chain transfer to polymer, the molecular weight distribution will be even broader and M /M ratios of 50 and more are observed in some polyethylenes made by free-radical syntheses. 

Problem 3.12 Suppose you have monodispersed star-branched melts of some polymer with molecular weights 20,000, 40,000, and 100,000. You measure the longest relaxation times of the two lower-molecular-weight samples and find Ti = 1 sec and 10 sec, respectively, for M =... 

Solid line. Theoretical random scission L =. 72 Dashed line and A. Experimental for branched polymer DYNF molecular weight > 9100, 3.7 CHs per 100 CH, (16, 27) Vertical scale is to he multiplied by 2... 

The use of the GPC technique spread very quickly but, very soon, it became obvious that this method was imperfect. Every polymer has its own calibration curve which differs from one polymer to another. Moreover, for copolymers or branched polymers, the molecular weight calibration curve does not work correctly. 

Brookhart and co-workers [79-81] introduced catalysts based largely on chelating, nitrogen-based ligands that are active for the homopolymerization of ethylene and the copolymerization of ethylene with 1-olefins and polar comonomers (31). Ni, Co, Fe or Pd are used as late transition metals. The diimine ligands have big substituents to prevent 6-hydride elimination. Ni(II) or Pd(II) complexes form cations by combination with MAO and polymerize ethylene to highly branched polymers with molecular weights up to one million. The activities reach TON... 

The recommended heat capacity data are currently being analyzed in terms of chemical structure, structure of the polymers in the glassy, crystalline and molten states. The data are further being analyzed to study the effect of branching, molecular weight and tacticity on the heat capacity of polymers. Comprehensive tabulation of heat capacities of various structural units are being prepared and will be available in the near future. 

The concept of a polymer chain implies two ends per chain. However, because of the nature of the process that is used to form the polymer, the chain may contain one or more branch points resulting in multiple ends per chain. These ends can have a marked influence on polymer performance. Branching, molecular weight, and molecular weight distribution also have been shown to affect processability. The optimum macrostructure often represents a compromise between processing and ultimate performance. 

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