Designing a Block Copolymer Membrane for Selective Transport of Lactic Acid from Aqueous Mixtures

We report the design and synthesis of a triblock copolymer-based membrane for enabling selective transport of lactic acid from aqueous solutions. This is relevant to the production of polylactic acid, one of the few biodegradable and biobased polymers with sufficient mechanical strength for practical applications. The end blocks are positively charged with negatively charged lactate counterions. The middle block is polybutadiene (PBD). Due to microphase separation, the charged blocks form channels for transporting lactic acid. The mechanical integrity of the membrane is controlled by cross-linking the PBD block. Transport of lactic acid and water across the membrane was studied by placing the membrane between two chambers, a feed chamber containing aqueous lactic acid solutions, and a receiving chamber containing pure water. The lactic acid concentration in the receiving chamber was monitored as a function of time using conductivity, HPLC, and NMR. The corresponding flux of water from the receiving chamber to the feed chamber was measured using an NMR-based approach. The lactic acid and water permeabilities through our membrane were (1.12 ± 0.05) × 10^–8 and (8.58 ± 0.75) × 10^–9 cm² s^–1. To our knowledge, there are no reports of lactic acid permeabilities through any membrane in the literature. The separation factor of our membrane, α_LA/water, 1.305 ± 0.123, is comparable to that of membranes used for selective transport of ethanol, despite the fact that lactic acid is a much larger molecule than ethanol. Selective transport of lactic acid in our membrane is governed mainly by differences in solubility; lactic acid is 18 times more soluble in the membrane than water.