Impact of Nanostructure on Mechanical Properties of Norbornene-based Block Copolymers under Simulated Operating Conditions for Biobutanol Membranes

The structure and mechanical properties of a novel block copolymer (BCP) system with T_g's for both segments exceeding 300°C, poly(butylnorbornene)-block-poly(hydroxyhexafluoroisopropyl norbornene) (BuNB-b-HFANB), are investigated as a function of processing conditions used for solvent vapor annealing (SVA). Solvent selection impacts long-range order markedly, but unexpectedly vertical orientation of cylinders are preferred over a wide range of solubility parameters, as determined by atomic force microscopy and grazing incidence small-angle X-ray scattering. The mechanical properties (elastic modulus, fracture strength, and onset fracture strain) are dependent upon the long-range order induced during SVA and determined using the combination of surface wrinkling and cracking. The modulus and fracture strength of the films increase from 1.44 GPa and 12.1 MPa to 1.77 GPa and 17.5 MPa, respectively, whereas the onset fracture strain decreases from 1.6% to approximately 0.6% as the ordering is improved. The polarity difference in the segments of the BCP is attractive for membrane separations, especially butanol-water. For biobutanol recovery, the titers are typically <3 wt % butanol; exposure of the BCP membrane to aqueous 1 wt % butanol decreases the elastic modulus to approximately 0.90 GPa, irrespective of the morphology, despite the high T_g of both segments and limited swelling (5.0 wt %). Correspondingly, the onset fracture strain of these swollen films is estimated to increase significantly to 6-7%. These results indicate that operating conditions impact the mechanical performance of BCP membranes more than their morphology despite the high T_g of the neat copolymer. Wrinkling and cracking provide a facile route to test the mechanical properties of membranes under simulated operando conditions. (Figure Presented).