Block Copolymers: An Effective Tool for Fundamental and Applied Chemical Engineering

Block copolymer (BCP) self-assembly has garnered significant attention for several decades because it can yield ordered structures in a wide range of morphologies with potential or practical applications in many fields. A diblock copolymer is a polymer consisting of two distinct monomers. The monomers are arranged such that there are distinct chains of each monomer, and the chains are covalently linked together to form a single copolymer chain. In BCPs, the enthalpic contributions to free energy are often significant enough overcome entropy, resulting in the formation of microdomains of each type of monomer. This self-assembly is useful for fundamental studies of molecular conformation and for engineering materials wherein the useful properties of two chemically distinct chains are incorporated into a single molecule. In this work we will demonstrate the extraordinary effectiveness block copolymers to both types of studies./nIn this dissertation work, we used peptoid diblock copolymers to identify a motif common to all bulk phase crystalline peptoid polymers. Poly N-substituted glycine materials (peptoids) have the capacity for prolific diversity due to their large library of monomers, synthetic sequence control, and monodispersity. These properties make peptoids an ideal material for the study of the relationship between chemical structure and supramolecular structure. In order to probe this relationship, we synthesized and analyzed a series of crystalline peptoid copolymers, systematically varying peptoid side-chain length (S) and main-chain length (N). In all peptoids, we found that the three unit cell dimensions - a, b, and c - are simple linear functions of S and N. These relationships indicate that the molecules adopt extended, planar conformations. This new structural motif can be used to design broad classes of assemblies which have specific unit cell sizes, functional group densities, or aqueous monolayer thicknesses, based upon a specific backbone conformation and packing preference. Furthermore, these materials ordered well enough in water to achieve the first 2 Å level transmission electron microscopy of a synthetic polymer./nIn the second study, we used block copolymers to impart two properties in a material effective for the pervaporation – a separation consisting of permeation and evaporation – of aqueous volatile organic compounds (VOCs). We performed this separation using a microphase separated polystyrene-block-polydimethylsiloxane-block-polystyrene (SDS) copolymer membrane. The PDMS domains are rubbery and have good permeation properties for volatile organic compounds (VOCs). The PS domains are glassy and provide the membrane with structural integrity. We find that using SDS block copolymer membranes is effective for the removal of inhibitors from lignocellulosic dilute-acid hydrolysate. Furthermore, the pervaporation-treated hydrolysates are suitable for ethanol fermentation with Saccharomyces cerevisiae . These results indicate that block copolymer-based pervaporation is a viable approach for hydrolysate detoxification in an industrial bioethanol production process./nTaken together, these studies demonstrate that block copolymers are an effective tool with which to implement both fundamental molecular engineering studies and chemical engineering design.

Advisor: Balsara, Nitash P