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Incoming links: Hydrodynamic volume
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.
Hydroxide attacks a PLA ester carbonyl, moving the electrons onto oxygen. Collapse of the tetrahedral intermediate expels an alkoxide; proton transfer produces an alcohol and a carboxylate. The carboxylate stabilizes the hydrolysis products and makes this saponification effectively irreversible under the basic conditions. Repetition cleaves the PLA block, including its terminal ester linkage to the junction alcohol, while the carbon–carbon polystyrene backbone remains intact.
The principal surviving polymer is a polystyrene chain bearing the junction end and its nitroxide-derived distal end. In gel permeation chromatography, the intact block copolymer normally elutes earlier than this smaller surviving polystyrene coil. Lactate and short hydrolysis fragments elute later still, provided the column and detector resolve such small solutes. The polymer peak therefore shifts to larger elution volume; its height need not remain constant under a mass-sensitive detector. Elution reflects hydrodynamic volume, so calibration against polystyrene standards does not supply an exact absolute mass for the original mixed block.