(32a) Invited Talk: Phase Behavior of Model Tapered Diblock Copolymers

Motivated by the significant recent interest in using block copolymers to create solvent-free, mechanically robust battery electrolytes, we use theory and simulation to study the microphase separated structures of model copolymers. Besides being safe and nonflammable, certain salt-doped microphase separated polymer electrolytes could save battery weight because they have a high enough shear modulus to be used in conjunction with a solid lithium electrode. This is because they are composed of AB diblock copolymers, where A is soft and facilitates ion transport while B provides mechanical strength. Depending on the strength of interactions and fraction of A, the system phase separates into various ordered phases. It would be advantages for transport and mechanical properties if regions of A and B were each continuous in all three dimensions. Unfortunately, bicontinuous phases such as the double gyroid are difficult to access at high molecular weights (desirable for good mechanical properties).

Recent experimental and theoretical work showed that linear tapered copolymers, composed of blocks of pure A and B separated by a middle “tapered” block with mixed composition that linearly varies from pure A to pure B (or from B to A for an inverse taper), can form the gyroid phase at high molecular weight. In such systems, taper length is a new adjustable parameter; we aim to guide experiments in using this parameter to control interfacial and phase behavior, with special interest in creating the gyroid phase. First, phase diagrams were created from self-consistent field theory (SCFT). The phase diagrams of normal tapered systems are similar to those of diblocks, but with a lower order-disorder transition temperature and a larger gyroid region. For more detailed information, we employ fluids density functional theory (fDFT) and molecular dynamics (MD) simulations together. These capture monomer scale packing effects and are implemented using very similar models. By comparing with the fDFT results, we can ensure the appropriate equilibrium state is formed in the MD simulations. Density profiles from SCFT, fDFT, and MD are in close agreement; tapers widen the interfacial region and large tapers decrease the maximum purity of the microphases.