Block Copolymers
The usual block copolymers are composed of two types of monomers that do not give miscible homopolymers.
An example is a block copolymer of polystyrene and poly(ethylene oxide).
The two homopolymers would macroscopically separate from one another.
But when the monomers are present in the same chain, they can only achieve a microphase separation (Bates, F. S.; Fredrickson, G. H. "Block Copolymer Thermodynamics. Theory and Experiment" Annu. Rev. Phys. Chem. 1990, 41, 525-557).
Diblock Copolymers
The typical organized structures formed spontaneously by diblock copolymers in their melts depend on the ratio of the sizes of the two blocks and the energy of the interaction between the two types of segments.
- If one block is extremely large, and the other block is extremely small, the system is not easily ordered.
- If both blocks are of the same size, the system forms lamellae upon cooling of the melt below a particular temperature, the order-disorder transition temperature (ODT).
- In between these two extremes of ratios of the sizes, the systems can organize into
- Spheres of the minor component in a continuous matrix of the major component
- Cylinders of the minor component in a continuous matrix of the major component
- A variety of other interesting structures that are not completely understood.
In dilute solution in a selective solvent (good solvent for one block, poor solvent for the other block), the diblock copolymers can form micelles.
An approximately spherical core composed of the insoluble blocks from several chains is surrounded by a corona of their soluble blocks.
The chains in the corona are swollen by their favorable interaction with the medium.
If the medium is extremely hostile to the insoluble block, the micelles may not be able to come to equilibrium, due to the very slow rate at which they exchange chains with one another.
Symmetric Triblock Copolymers
In a selective solvent, the behavior of a triblock copolymer depends on whether the medium is a good solvent for the two end blocks or the single middle block, i. e., whether it is ABA or BAB.
Triblock copolymers can form micelles, but the ease with which they do so depends on whether the sequences is ABA or BAB.
Micelle formation is more difficult when the medium is a good solvent for the middle block than when it is a good solvent for the end blocks.
Formation of an independent micelle is difficult in the former case because the chain in a micelle must either
- leave one of its end blocks dangling free in the hostile medium, or
- sacrifice conformational entropy of its middle block in order to place both end blocks in the core of the same micelle.
At higher concentration (where the micelles are separated, on average, by distances much shorter than the length of the middle block), another structure becomes available to the system where the middle block is in a good solvent.
Now the triblock copolymer can place one end block in the core of one micelle, and the other end block in the core of another micelle, with the soluble middle block forming a bridge between the two micelles.
Continuation of this process with other chains, and other micelles, can eventually result in the formation of a three-dimensional thermoreversible network of cross-linked micelles that permeates throughout the system.
The network, or gel, is thermoreversible because the cross-links are maintained by noncovalent interactions, not by covalent bonds.
A change in the interaction between the insoluble block and the medium (such as a change in solvent or temperature) so that it becomes less hostile to the end blocks may result in disruption of the network.
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July 1, 1999
Wayne L. Mattice: wlm@polymer.uakron.edu