Inhomogeneous Polymers with Reversible Bonding

Institute for Multiscale Materials Studies Presents: Inhomogeneous Polymers with Reversible Bonding

Speaker: Won Bo Lee, University of California Santa Barbara

Thermoreversible, supramolecular self-assembly in multi-block copolymer melts is studied within the framework of self-consistent field theory. This approach is adapted to study three systems which are supramolecular diblock copolymer melts, supramolecular triblock copolymer melts and lipid/water mixtures. Firstly, the supramolecular diblock copolymer melts are composed of two homopolymers of different species can reversibly bond at terminal binding sites to form a diblock copolymer. Unlike the analogous model for a three component blend of A and B homopolymers with an irreversibly bonded AB diblock copolymer, N is turned out to be another independent parameter in addition to xN. Two mean-field phase diagrams are presented. For symmetric systems with equal volume fractions and chain lengths of the two homopolymers, we predict re-entrant behavior upon cooling in narrow parameter ranges for both disordered and lamellar phases. In the case of the lamellar phase, we find re-entrant behavior in which the intermediate phase is either disordered or macrophase separated. Secondly, the supramolecular triblock system is composed of two chemically distinct, but reactive homopolymer species: a linear A-homopolymer with a single reactive group at one of the ends, and a linear B-homopolymer with reactive groups at both ends. Reversible bonding occurs between the functional groups from different polymer species so that the reacting system can contain A, B, AB and ABA (co)polymer species whose overall volume fractions are controlled by the segmental incompatibility, bonding strength and homopolymer chain lengths. Architectural variations of these copolymers, arising from the differing lengths of A and B homopolymers, have a dramatic effect on not only the micro-phase separation but also on the extent of reversible bonding. Two characteristic phase diagrams are constructed to illustrate this behavior. Lastly, we present a coarse-grained model in order to describe the unusual sequence of mesophases observed in aqueous solutions of non-ionic lipids, such as monoolein. The lipid molecules are modeled as a rigid head and a flexible Gaussian tail, and water is treated explicitly. A key component of the model is thermally reversible hydrogen bonding between the lipid head and water resulting in changes in both head volume and the interactions of the hydrated head with its surroundings. Phase diagrams obtained from unit-cell, self-consistent field simulations capture the qualitative thermotropic and lyotropic phase behavior of the monoolein/water system [Hyde et. al., Z. Kristallogr. 168, 213 (1984); Qiu and Caffrey, Biomaterials 21, 223 (2000)].