Past exam of the chemistry course of the year 4 of the natural sciences course of the University of Cambridge / 2022 / Paper 1 / 1 / d

Gel permeation chromatography separates by solution hydrodynamic volume, rather than molar mass alone. At equal molar mass, a more extended chain accesses fewer pores and elutes earlier. The relatively rigid, conjugated poly(3-hexylthiophene) is the natural first-eluting candidate, compared with the more flexible chains, provided all samples are molecularly dissolved.

This is a structural prediction, not an unconditional ordering: solvent quality, polymer aggregation and column interactions affect retention, and the hydrogen-bonding polyamide may not dissolve adequately in tetrahydrofuran. An insoluble sample cannot be assigned an ordinary solution gel permeation chromatography elution time.

A differential refractive index detector can detect all four polymers when their refractive-index contrasts are nonzero. Light scattering with an appropriate concentration detector can additionally provide molar mass. A single ultraviolet detector is unsuitable as the universal detector because polypropylene lacks the strong chromophores present in the aromatic and conjugated products.