Putting terahertz radiation in context

I read with excitement Mitch Jacoby’s article “Terahertz Radiation Probes Polymers” (C&EN, Aug. 15/22, page 30). The development of the terahertz radiation probe must be taken in context with other analytical and morphological techniques such as transmission electron microscopy when studying the crystallization of polymers such as poly(3-hydroxybutyrate), or PHB.

As can be seen in the photomicrographs on this page, the crystallization of PHB from solution undergoes significant ultrastructural changes as a function of time. The ultrastructural study of PHB pictured below was undertaken by P. H. Geil and me as a function of time over 36 hours.

As can be seen in figure 1, PHB initially forms as fibrous micelles that are 4 nm thick as measured by the shadow distance. The micelles then undergo a form of epitaxial crystallization. As can be seen in figure 2, the epitaxial fibers form into epitaxial lathlike and needle crystals on the order of 6 nm thick. Lastly, as can be seen in figure 3, the crystallites of PHB form a number of ultrastructures greater than 6 nm thick that are similar to those presented in Jacoby’s article.

The bottom line of this letter is for one to note that ultrastructural studies of PHB must be undertaken as a function of the polymer’s molecular weight, the concentration of polymer and solvent, the evaporation environment, time, and the available analytical techniques.

Sumner Barenberg
Boston