The Mark–Houwink plot – Differentiation of Branching and Composition
Using OMNISEC, we will show how to separate the effects of a structural difference induced by a compositional change to a polymer (e.g. substitution) and the structural change induced by polymer chain branching.
The Mark–Houwink plot – Differentiation of Branching and Composition
Using OMNISEC, we will show how to separate the effects of a structural difference induced by a compositional change to a polymer (e.g. substitution) and the structural change induced by polymer chain branching.
Introduction
The Mark–Houwink plot is a powerful tool for investigating polymer structure in solution as it clearly reveals the structure-molecular weight relationship with high sensitivity. It is generated by plotting the molecular weight (MW) against the intrinsic viscosity (IV) on a log-log graph. The molecular weight, of course, indicates the length of the polymer chains (or degree of polymerization) but on its own cannot give any indication of structure. The intrinsic viscosity (expressed in dL/g) is a measurement of the molecular density of the polymer chains in solution. The tighter the chains fold or coil in solution, the higher the density and the lower the intrinsic viscosity. This measurement is independent of the molecular weight, so two different structures having the same molecular weight can have different intrinsic viscosities – for example a linear (unbranched) polymer and a branched polymer of the same molecular weight will have different intrinsic viscosities. Furthermore, if the polymer changes structure across its molecular weight distribution (e.g. becomes more substituted), the intrinsic viscosity changes will be easily detected. This is what makes the Mark-Houwink plot so useful and powerful.
