This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: A type of transformation best known for hardening steel and enabling shape-memory alloys may also occur in a much softer class of materials, according to new research from Rice University. Researchers in the departments of chemistry and chemical and biomolecular engineering and at the Smalley-Curl Institute at Rice have shown that an organic semiconductor can transform from a liquid crystal into a solid crystal through a rapid, highly coordinated process resembling a martensitic transformation—a phenomenon traditionally associated with changes between two solid crystalline structures.
The findings, published in the Proceedings of the National Academy of Sciences, expand scientists' understanding of how materials reorganize during phase transitions and could eventually offer a new way to produce highly ordered organic semiconductor crystals for technologies such as flexible electronics and transistors. "Martensitic transformations are usually thought of as exclusively solid-state processes," said Kushal Bagchi, assistant professor and the Norman Hackerman-Welch Young Investigator in the Department of Chemistry at Rice and corresponding author of the study. "What we found is that a transition from something with fluidity into a solid can still exhibit the essential characteristics of a martensitic transformation." Martensitic transformations are unusual because atoms or molecules move together in a coordinated way rather than slowly rearranging one by one.
The process can happen quickly while allowing the new material to retain some structural "memory" of the phase it came from. The Rice team studied HAT6, an organic semiconductor made of disk-shaped molecules that form columns in its liquid-crystalline state. They aligned those columns inside microscopic channels, then rapidly cooled the material into a solid crystal.
Instead of losing its original organization during crystallization, much of the molecular alignment remained. At rapid cooling rates, the crystal retained roughly 70% of the alignment present in the liquid crystal. When the material cooled more slowly, however, the molecules had more time to rearrange and progressively lost that orientation.
"We had three qualitative indicators," Bagchi said. "The transition appeared only when we cooled very fast, the daughter phase remembered the structure of the parent phase, and the transition was largely reversible. Once we saw all three, we became confident that this was qualitatively similar to a martensitic transformation." "What was exciting to us was seeing how much of the molecular organization in the liquid crystal could survive as the material became a solid," added Matteo Pasquali, the A.J.
Hartsook Professor of Chemical and Biomolecular Engineering and an author of the study. "It suggests that we may be able to use the structure that already exists in a liquid crystal as a kind of blueprint for building highly ordered crystalline materials." The researchers found that the unusual transition occurred only within a narrow cooling range: The material had to cool quickly enough to prevent conventional crystallization but not so quickly that it became a glass instead of a crystal. The transformation was also remarkably fast.
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