Long Chain Branching Distribution

Because PVAc is a widely used thermoplastic, developments in its characterization have tracked advances in SEC technology as a whole. This chapter outlines procedures and conditions for characterizing the molecular weight and long-chain branching distributions in PVAc. Several studies were discussed, indicating that the principle of universal calibration applies to PVAc in THF. 

In this chapter, the analysis of LCB in macromolecules using SEC with multiple detection is described. While the qualitative information obtained from this type of analysis is mentioned, particular attention is paid to the requirements necessary for accurate, quantitative determination of LCB, of the long-chain branching distribution (LCBD), and of the fractal dimension (dt) of macromolecules. 

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One chain-end is typically unsaturated due to chain transfer and termination mechanisms. Mol wts can range from several hundred to several million. There is no long-chain branching unless special synthesis methods ate employed. The mol wt distribution is commonly the most probable,... 

Variation in the molecular weight distribution (which may in part depend on the long chain branching). 

In addition, subsequent chain transfer reactions may occur on side chains and the larger the resulting polymer, the more likely will it be to be attacked. These features tend to cause a wide molecular weight distribution for these materials and it is sometimes difficult to check whether an effect is due inherently to a wide molecular weight distribution or simply due to long chain branching. 

It is well known that LCB has a pronounced effect on the flow behavior of polymers under shear and extensional flow. Increasing LCB will increase elasticity and the shear rate sensitivity of the melt viscosity ( ). Environmental stress cracking and low-temperature brittleness can be strongly influenced by the LCB. Thus, the ability to measure long chain branching and its molecular weight distribution is critical in order to tailor product performance. 

The method outlined above for characterizing branched polymers will hereafter be referred to as the molecular weight and branching distribution (MWBD) method. In the following sections, its application to the long chain branching in polyvinyl acetate and high pressure low density polyethylene will be demonstrated. 

The MWBD method, when coupled with high speed SEC techni-gues, is more rapid for long chain branching measurements than NMR. In addition, the branching distribution information that it provides, once epsilon has been determined, can not be obtained by other branching characterization methods unless the polymer is fractionated. 

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