Spectroscopic Technique Reveals Motion of Single Molecules Within a Medium
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matrix. Fracture tests were performed using double-cantilever-beam specimens. Environmental scanning electron micrographs showed a thin polymer film on the fracture surface and illustrated the rupture process of an embedded microcapsule. Infrared spectroscopy supplied chemical information confirming that the film is the polymerized healing agent. Virgin and healed load-displacement curves from fracture tests assessed the crack-healing efficiency of the composite sample, which in this case was up to 75% recovery of the virgin fracture toughness. “Self-healing composites possess great potential for solving some of the most limiting problems of polymeric structural materials: microcracking and hidden damage,” said the researchers. They said that the material cannot only “recover structural function,” but also “increase the reliability and service life of thermosetting polymers used in a wide variety of applications ranging from microelectronics to aerospace.” YUE HU
Increased Lateral Electrostriction Realized in Ferroelectric LiquidCrystalline Elastomer Films The strain due to the external electric field of electrostrictive materials forms the basis of such micromachines as transducers, actuators, pumps, and medical microrobots. Of the reported electrostrictive materials—including piezoelectric crystals, grafted polyglutamates, carbon nanotubes, and polycopolymers—the best performance exhibited in these materials is 4% strain at an electric field 150 MVm -1 . Ferroelectric liquid-crystalline elastomer (FLCE) films, however, have shown the same constriction of 4%, but require only a 100× lower electric field of 1.5 MVm -1. F. Kremer, whose group studied the films at the Institute for Experimental Physics at Leipzig University, said the principle of FLCE lies in the fact that the orientation of liquid crystal (LC) molecules is influenced by an applied electric field, and that the ferroelectric LC molecules can respond to the external electric field at a very high speed. As reported in the March 22 issue of Nature, the paraelectric SA* phase LC material organizes itself in a layered liquid-crystalline structure. In each layer, the long axes of the rodlike LC molecules are aligned perpendicular to the layer plane. Such LC polymers can be synthesized by means of attaching the LC molecules to a polysiloxane backbone perpendicularly with a flexible alkyl chain, and cross-linking of the polymer LC. In this way, the layered structure can be preserved under mechanical stress. When a lateral electric field is applied, the LC molecules will tilt MRS BULLETIN/JUNE 2001
at an angle proportional to the external electric field, thus thinning the layer and the overall thickness of the film. Mathematical analysis shows that the constriction of FLCE films is proportional to the square of the lateral electric field, and that the frequency of the effect is twice that of the applied alternating electric field, which is characteristic of electrostriction. The electrostriction of a freely suspended FLCE film was measured at continuously in
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