Relationship between Segregation Strength and Permeability of Ethanol/Water Mixtures through Block Copolymer Membranes

A series of poly(styrene-b-dimethylsiloxane-b-styrene) (SDS) triblock copolymers with molecular weights ranging from 55 to 150 kg/mol and polydimethylsiloxane (PDMS) volume fractions ranging from 0.59 to 0.83 were used to fabricate membranes for ethanol/water separation by pervaporation. The rigid polystyrene (PS) microphase provides the membrane with structural integrity, while the rubbery PDMS microphase provides nanoscale channels for ethanol transport. We use a simple model to study the effect of morphology and PDMS volume fraction on permeabilitites of ethanol and water through the block copolymer membranes. We defined normalized permeabilities of ethanol and water to account for differences in morphology and PDMS volume fraction. We found that the normalized ethanol permeability in SDS copolymers was independent of the total polymer molecular weight. This is qualitatively different from what was previously reported for poly(styrene-b-butadiene-b-styrene) (SBS) membranes, where the normalized ethanol permeability was found to be a sensitive function of total molecular weight [J. Membr. Sci. 2011, 373, 112]. We demonstrate that this is due to differences in the Flory-Huggins interaction parameter (χ) for the two systems. When χN is less than 100 (N is the number of segments per chain), the two microphases are weakly segregated, and the presence of glassy PS segments in the transporting microphase impedes ethanol transport. When χN exceeds 100, the two microphases are strongly segregated and the glassy PS segments do not mix with the transporting phase. We compare these results with normalized ionic conductivity data previously reported for mixtures of a lithium salt and polystyrene-b-poly(ethylene oxide) (SEO). Evidence suggests that the product χN governs the transport of widely different species such as ethanol and lithium salts through block copolymer membranes. Surprisingly, the normalized permeability of water is independent of total molecular weight for both SDS and SBS block copolymers. © 2013 American Chemical Society

Synthesis of well-defined polyethylene-polydimethylsiloxane-polyethylene triblock copolymers by diimide-based hydrogenation of polybutadiene blocks

Polyethylene, PE, is a crystalline solid with a relatively high melting temperature, and it exhibits excellent solvent resistance at room temperature. In contrast, polydimethylsiloxane, PDMS, is a rubbery polymer with an ultralow glass transition temperature and poor solvent resistance. PE-PDMS block copolymers have the potential to synergistically combine these disparate properties. In spite of this potential, synthesis of PE-PDMS block copolymers has not been widely explored. We report a facile route for the synthesis of well-defined polyethylene-b-polydimethylsiloxane-b-polyethylene (EDE) triblock copolymers. Poly(1,4-butadiene)-b-polydimethylsiloxane-b-poly(1,4-butadiene) (BDB) copolymer precursors were synthesized by anionic polymerization, followed by diimide-based hydrogenation. Under the standard hydrogenation conditions established by the work of Hahn, the siloxane bond undergoes scission resulting into significant degradation of the PDMS block. Our main accomplishment is the discovery of reaction conditions that avoid PDMS degradation. We used mechanistic insight into arrive at the optimal hydrogenation conditions, and we established the efficacy of our approach by successfully synthesizing a wide variety of block copolymers with total molecular weights ranging from 124 to 340 kg/mol and PDMS volume fractions ranging from 0.22 to 0.77. © 2014 American Chemical Society

Poly(vinylidene fluoride) membranes with hyperbranched antifouling and antibacterial polymer brushes

10.1021/ie302762wIndustrial and Engineering Chemistry Research514915962-15973IECR