Effect of polymer molecular weight distribution on solute sequestration in two-phase partitioning bioreactors

Polymeric solids are effective absorbents in two-phase partitioning bioreactors (TPPBs) when they provide adequate absorptive capacity for the target solute, as well as the physical properties required by solid-liquid TPPB operations. This study demonstrates the influence of molecular weight distribution (MWD) on solute uptake, as measured by solute partition coefficient (PC), and mechanical strength, as measured by the polymer's complex modulus (G∗). Experimental PC data for n-octanol absorption from aqueous solution by poly(dimethyl siloxane) (PDMS) demonstrate a decline in absorptive capacity with increasing number average molecular weight (Mn), in agreement with Flory-Huggins solution theory predictions. Importantly, MWD is shown to have no effect on solute uptake, with both unimodal and bimodal distributions generating the same PC at a given Mn. This is in contrast to G∗, whose MWD sensitivity is exploited to formulate bimodal mixtures of poly(isobutylene) (PIB) that provide high n-octanol PC values as well as satisfactory material strength. This bimodal MWD strategy to TPPB absorbent design is extended to miscible solutions of high MW PIB and cyclohexylbenzene, which are shown to generate superior n-octanol PC at a given G∗.