Separation of polymers by molecular topology using monolithic columns

Long chain branching (LCB) is most often incorporated into polymeric materials to enhance rheology and processing. Presently, the characterization of LCB in polymers is limited to the determination of the number of long chain branches per molecule. The ability to characterize other branching parameters such as the molecular weight of the branch and the shape of the branched molecule (star, comb, "T", "H", etc.)would have considerable value in material design. A new fractionation method, molecular topology fractionation (MTF), is described herein for the separation of polymer molecules based on LCB topology. This first successful practice of MTF uses monolithic columns to create a tortuous path having macropores or flow-through channels of the order of polymer molecular dimensions as the separation medium. Because long chain branched molecules are retained more than linear molecules of the same size, the method can be used to separate branched from linear chains. Performed in a thermodynamically good solvent, the key parameters influencing the onset of MTF include flow rate, macropore dimensions, and molecular weight of the polymer to be separated. The onset of the MTF mechanism is characterized by a flow rate-dependent reversal in the elution order of narrow molecular weight polystyrene standards, denoting a distinct change in retention mechanism. Below the reversal point, retention consistent with a hydrodynamic chromatography mechanism is active. Above the reversal point, a new mechanism yields increased retention with increasing molecular weight.