Disorder-Driven Polymers Prefer Crystalline Order

Author: Phuongmai Truong
Institution:  UC Berkeley

Surprisingly, not only atoms prefer to form crystals. For a system of polymer-like chains of particles, the best way to maximize entropy is letting them form crystal structures as well. Manuel Laso and his team at the Technical University of Madrid reported the finding in the 24th July Physical Review Letter.

Nature favors an increase in entropy, a measurement of disorder, in any given system. That explains why air molecules spread throughout an entire room, or broken pieces of glass cannot naturally stick together to form a window. It is therefore expected that a group of spherical, uncharged, non-interactive particles, when packed densely together, would tend to form a random structure instead of an orderly one. Early studies have found that such spheres naturally arrange themselves in a crystalline pattern, because the crystal structure gives more room among spheres for them to "rattle" around. However, a random structure would lead to a much more rigid, compact formation.

Manuel Laso and his team at the Technical University of Madrid recently published an even more surprising result: a long chain of connected beads can unravel itself from a tangled bundle to become a crystal, much like unconnected spheres. The computer program simulates an initial random state of the chain, the chain can then freely move without resistance to bending or twisting, and there is no force among the beads. Therefore, the system is completely entropy-driven. It is a highly idealized model, nonetheless promising. Researchers believe that further development will explain the formation of protein, which is biological polymer. Although proteins are not in crystal form, their three-dimensional structure possesses interactions analogous to those of crystallizing polymers. "Good fundamental work like this does advance the field", said Thomas Truskett of the University of Texas at Austin, "even if we don't know how to apply it right away."

-Source: American Physical Society - http://focus.aps.org/story/v24/st5