Understanding the interactions of polyols with hexafluoroisopropanol containing polynorbornene biobutanol membranes using QCM-D

Polymer membranes offer a low-cost path to separate bio-products. However, interactions of the membrane with components in the fermentation broth can alter its performance. Here we describe how ppm levels of polyol surfactant designed to inhibit foaming of the broth can dramatically swell and plasticizing the polynorbornene copolymers in the bio-butanol separation. in-situ quartz crystal microbalance with dissipation (QCM-D) enable the quantification of both the swelling and rheological properties. We examined the molecular-weight-dependent sorption behavior of 10ppm of polyethylene glycol (PEG) and polypropylene glycol (PPG) for a series of copolymer membranes that all contain 50 mol% hexafluoroisopropanol norbornene and 50 mol% alkyl (methyl to decyl) norbornene. Changing the alkyl side length only modestly impacts the swelling behavior, which is consistent with hydrogen bonding between hydroxy groups driving the sorption. It shows that PPG, not PEG, is the primary cause for the enhanced swelling. Increasing molecular weight of the PPG leads to increased swelling (entropic effect) but also slower the sorption rate (size effect). These results illustrate that details about components added to the broth can dramatically affect the membrane properties.