Branching, Chain Flexibility, and Molecular Weight

As Tg marks the onset of molecular motion, a number of factors that affect rotation about links (necessary for movement of polymer chains) will also influence the Tg of a polymer. These include (a) chain flexibility, (b) molecular structure (steric effects), (c) molecular weight, and (d) branching and crosslinking. 

In this section, we present the rheological behavior of linear flexible homopolymers (i.e., without side-chain branching). We first present methods for obtaining temperature-independent plots for shear viscosity ( ) and first normal stress difference (N ) in steady-state shear flow and for dynamic moduli (G" and G") in oscillatory shear flow. We then discuss the effect of molecular weight and molecular weight distribution on the rheological behavior of linear flexible homopolymers. 

The conformation plot is the scaling law between macromolecule size and molecular weight (i.e., the chain length) and it is very useful in studying the stiffness (flexible, stiff, or compact conformation) of polymers. Also branching, degree of functionalization, or derivatization could be studied using the conformation plot of the polymer. Therefore, it is very useful in the molecular characterization of polymers. 

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