Branched polymers characterized by comprehensive two-dimensional separations with fully orthogonal mechanisms: Molecular-topology fractionation × size-exclusion chromatography

• Molecular-topology fractionation was demonstrated on monoliths with various pore sizes.
• Flow rate affects selectivity in MTF; separations can be tuned for different pore sizes.
• Conditions exist where hydrodynamic effects and reptation effects on retention cancel out.
• Critical separations (no effect of molecular size) may reveal the degree of branching.
• Range of separations based solely on branching may be extended using flow-rate programming in MTF.

Polymer separations under non-conventional conditions have been explored to obtain a separation of long-chain branched polymers from linear polymers with identical hydrodynamic size. In separation media with flow-through channels of the same order as the size of the analyte molecules in solution, the separation and the elution order of polymers are strongly affected by the flow rate. At low flow rates, the largest polymers are eluted last. At high flow rates, they are eluted first. By tuning the channel size and flow rate, conditions can be found where separation becomes independent of molar mass or size of linear polymers. Long-chain branched polymers did experience lower migration rates under these conditions and can be separated from linear polymers. This type of separation is referred to as molecular-topology fractionation (MTF) at critical conditions. Separation by comprehensive two-dimensional molecular-topology fractionation and size-exclusion chromatography (MTF × SEC) was used to study the retention characteristics of MTF. Branching selectivity was demonstrated for three- and four-arm “star” polystyrenes of 3–5 × 106 g/mol molar mass. Baseline separation could be obtained between linear polymer, Y-shaped molecules, and X-shaped molecules in a single experiment at constant flow rate. For randomly branched polymers, the branching selectivity inevitably results in an envelope of peaks, because it is not possible to fully resolve the huge numbers of different branched and linear polymers of varying molar mass. It was concluded that MTF involves partial deformation of polymer coils in solution. The increased coil density and resistance to deformation can explain the different retention behavior of branched molecules.