Molecular Weight-Sensitive Separation of a Bimodal Polymer Mixture Using Nanoscale Porous Materials

We present a novel scheme for the fractionation of polydisperse polymers in solution. The molecular weight-sensitive separation utilizes nanoscale porous materials. The entropy of a polymer chain that diffuses within the pore is significantly reduced compared with a chain in the free solution. This effect is used in the scheme, in which a semidilute solution of a polymer with a bimodal molecular weight distribution (for illustration) is allowed to equilibrate with the porous material saturated with pure solvent. The high osmotic pressure of the solution and the sharp dependence of the entropy reduction on molecular weight drive the low molar mass fraction preferentially into the pore channels. The high molar mass fraction remains outside. After equilibrium is reached, the external solution is thus enriched in the high molar mass fraction and can be extracted with high yield and in high purity. Concomitantly the solution in the interior of the pores retains the low molar mass fraction also with high purity. When the porous material is allowed to subsequently contact the pure solvent, we can recover the low molar mass fraction from the external solution after the second equilibrium. We estimate the efficiency of this fractionation scheme and propose protocols to be followed for an optimal separation.