Block Copolymers in Bulk

Block copolymers are composed of long sequences (“blocks”) of the same monomer unit, covalently bound to sequences of unlike type. The blocks can be connected in a variety of ways; schematics of AB diblock and ABA triblock structures are shown below, but structures such as ABC triblocks, (AB)n multiblocks, (AB)n starblocks, and even (ABC)n starblocks have been examined in our laboratory.

Dibock and Triblock Copolymers

Copolymer self-assembly

Despite this diversity of macromolecular architectures, all block copolymers share a common physics:  translational entropy drives the blocks to mix, while the enthalpic repulsion between the chemically different A and B units drives the blocks to segregate.  As per the Second Law of Thermodynamics, the entropic contribution is weighted by temperature, so for a suitably designed polymer, it is possible for the blocks to intermix at sufficiently high temperature, generating the “disordered” structure shown above at right, while at lower temperature, the blocks to spontaneously self-assemble (“order”, as shown above at left) into one of a diversity of mesophases, with the size scale governed by the polymer’s radius of gyration (order tens of nanometers). In the mesophases, dissimilar blocks exist in distinct “microdomains” which are highly enriched in blocks of the same type, sometimes to the point of being essentially pure. The covalent bonds linking the dissimilar blocks are thus localized to the vicinity of the microdomain interfaces. While the cartoon above illustrates the case where the A and B blocks are of comparable lengths, the block ratio is easily varied during polymer synthesis to alter the mesophase structure. Amongst the known equilibrium mesophases for diblock copolymers are spheres, cylinders, gyroid, and lamellae, as shown in the phase diagram below:

Copolymer Phase Diagram
Schematic diblock copolymer phase diagram: fA = volume fraction of A block, χ = Flory interaction parameter, N = diblock degree of polymerization. Equilibrium mesophases include spheres, cylinders, gyroid, and lamellae, as well as the disordered (homogeneous) state at small interblock segregation strength (χN). Diagram adapted from Matsen and Bates, Macromolecules, 29, 1091 (1996).

In the ordered state, glassy microdomains serve to anchor rubbery segments of the polymer, permitting these materials to be used (at a level exceeding $2 billion/year worldwide) as melt-processible adhesives and rubbers (“thermoplastic elastomers”). Since block copolymers require no vulcanization, they are simpler to process than conventional rubbers and are even recyclable.

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