Nanoscale Multifunctional Tubular Networks
Block polymers can self-assemble into an ever-increasing variety of nanoscale-sized periodic patterns. The simplest block polymer is an A/B diblock – a linear chain comprised of covalently linked A monomers followed by a similar a run of covalently linked B monomers with a single covalent junction joining the two blocks.
At high temperature or when mixed into a solvent mutually good for both blocks, the system is homogeneous, but upon cooling or solvent evaporation, the A monomers segregate from the B’s and vice versa, leading to nearly pure domains of A and B that are periodically juxtaposed since the junction bonds belong to both blocks and must reside on the interface between the A and B domains. This forces microphase separation with the length scale of the domains set by the sizes of the respective A and B runs of diblock.
So far these simple AB diblocks have formed a number of interesting patterns – that vary with the volume fractions of the blocks. At low minority block content, there are various packings of spheres of the minority block, followed by hexagonally packed cylinders at higher minority volume fraction and then most interestingly, 3D tubular networks of various symmetries leading to a 1D periodic alternating A layer- B layer structure which occupies the large middle portion of the phase diagram. The basic considerations that govern what equilibrium shapes the A and B blocks choose to take as function of their size and volume fraction (and even chain architecture, e.g. branched molecules in addition to linear and for 3 (or more) blocks) are now relatively well understood for the non-tubular structures. However, the growing set of microdomain network structures constitute a bit of a theoretical puzzle. Of the various microdomain patterns, the 3D networks are the most complex and arguably the most interesting from both the fundamental and applied points of view.
There are 4 patterns: two cubic ones – the double gyroid (shown in the figure with red and blue networks) and the double diamond, and two lower symmetry orthorhombic structures, the O70 and O52. . All are characterized by continuity of each type of domain in all 3 directions of space. The gyroid and orthorhombic networks are comprised of nodes where 3 short cylinder-like struts meet, while the double diamond arrangement consists of tetrahedral symmetry nodes where 4 tubular units meet. The bicontinuous nature of these networks is key to how such structures can display outstanding multifunctionality. Some important physical property combinations are unique to networks such as outstanding strength, stiffness, yet excellent conductivity of charge and mass, arising for example from the mechanical properties of the A phase and the transport behavior of the B phase.
Also, the 3D symmetries give rise to unusual wave propagation phenomena with band gaps due to the scattering of both optical and sound waves. Since the bicontinuous nanoscale tubular network structures are self-supporting even when a constituent is removed, additional functionality can be readily accessed by etching away one component (since it is continuous, the entire component can be removed). Such a nanoporous material can then be used as a template for infiltration leading to completely unprecedented properties and performance.
These findings are described in the article entitled Nanoscale 3D ordered polymer networks, published in the journal Science China Chemistry. This work was led by Edwin L. Thomas from Rice University.