Wiesner Group - Triblock Copolymers

Triblock terpolymers are a natural extension of our work with diblock copolymers. Tri-block terpolymers may be thought of simply as a diblock copolymer (AB) upon which a third block (C) is grown.

This tremendous volume of experimental space is why there are so many fascinating phases in ter-polymer systems as shown at right. Many of these phases are core-shell analogs of well known diblock phases in addition to many brand new phases. The rich phase space available within triblock terpolymer systems opens the doors to many new and exciting applications of mesomorphic materials.

Triblock copolymers are of interest because they have more experimental parameters and many more possible morphologies than diblock copolymers. For a given triblock copolymer system there are three different Flory-Huggins interaction parameters chi-AB, chi-BC and chi-AC.

Furthermore, for each particular polymer there are three more independent parameters: the volume fraction of the A block (f A), the volume fraction of the B block (f B), and the total length of the chain (N). All together, there are six parameters that determine of given triblock terpolymers equilibrium structure.

We have explored the phase space of this ABC polymer as shown in the ternary phase diagram at right.

Characteristic triblock copolymer morphologies were observed including spherical, tubular and lamellar morphologies as shown below.

In 2004, we reported the first triblock terpolymer ever to have a poly(ethylene oxide) (PEO) middle block.

References:

G. Toombes, S.Mahajan, M. Weyland, A. Jain, P. Du, M. Kamperman, S.Gruner, D.Muller, U. Wiesner. "Self-Assembly of Four-Layer Woodpile Structure from Zigzag ABC Copolymer/Aluminosilicate Concertinas" Macromolecules (2008)

S. Mahajan, B-K. Cho, J. Allgaier, L. Fetters, G. Coates, U. Wiesner. "Synthesis of amphiphilic ABC triblock copolymers with PEO as the middle block" Macromolecular Rapid Communications, 25(22), 2004, 1889-1894.

Triblock morphologies figure adapted from:

F. Bates, G. Fredrickson. Block copolymers - designer soft materials. Physics Today, 52(2), 1999, 32-38

PEO is commonly used in block copolymers because of its versatility, but integrating it into the B block of an ABC triblock copolymer introduced synthetic challenges. Most well-defined block copolymers are created using anionic polymerization. However, PEO is usually an end block in anionic polymerization since it it difficult to grow subsequent polymer blocks onto PEO due to its nucleophilicity.

To overcome this challenge, we functionalized the the end of PEO block with a bromine containing molecule and continued the chain growth with atom transfer radical polymerization.

The resulting polymer (shown above) is poly(ethylene-alt-
propylene)-block-poly(ethylene oxide)-block-poly(hexyl methacrylate).

Analogous to our work with diblock copolymers, this system was extended to structure direct silica based materials and was found to create a variety of interesting morphologies including the hexagonal morphology shown below.

Wiesner Research Group - Cornell University