Anomalous molecular-weight dependence of aqueous polypropylene glycol in swelling of amphiphilic norbornene copolymers containing hexafluoroisopropanol

Polymer membranes offer a low-cost path to separate bio-products. Amphiphilic polyol, commonly used as surfactant in fermentation process to suppress foaming, can severely swell membrane to a degraded performance. Here we systematically investigate the role of hydrophobicity and molecular mass (M_n) of the polyol using quartz crystal microbalance with dissipation (QCM-D) to quantify the swelling and rheological properties. We examined M_n-dependent sorption behavior of polyethylene glycol (PEG) and polypropylene glycol (PPG) at 10 ppm in aqueous solution for a series of norbornene copolymers containing approximately 50 mol% hexafluoroisopropanol (HFA) and 50 mol% alkyl (methyl to decyl). The alkyl chain length did not dramatically influence the swelling. We attribute this to the swelling driven by hydrogen bonding between hydroxy groups on the polyol and HFA group. The swelling by PEG in solution is nearly invariant with M_n and is significantly less than the swelling by the higher M_n PPG. A maximum in swelling is observed at M_n=3kg/mol for the PPG with the swelling dramatically decreasing for higher and lower M_n PPG. This strong M_n-dependence of the swelling may provide a potential route to mitigate these effects through selection of the size of polyols.