Crystalline Transitions and Mechanical Resonance Dispersion in Vinyl and Ethyl Stearate

Abstract

Dynamic shear compliance measurements on vinyl and ethyl stearate yield results depending critically on the particular crystalline modification in which these materials exist at the time of measurement. When first crystallized from the melt the materials are in a metastable state and only a few very small resonance dispersions are found in the range 50 to 5000 cps. At later times many large resonances occur; the largest is in the vicinity of 300 cps for both vinyl and ethyl stearate. While vinyl stearate is in its metastable state (about 60 days) a reversible transition takes place at 24.85±15°C such that above this temperature there are no resonances from 50 to 600 cps, but below the transition point the large resonance near 300 cps always appears. A break in the slope of density-temperature curves for vinyl stearate occurs between 25 and 24°C indicating that a transition occurs in the vinyl stearate while in its β form (with the molecules tilted with respect to the planes formed by the terminal groups). X-ray powder diffraction photographs show no changes in the long spacing between planes so the transition must involve alterations in cross-sectional packing of the chains or a change from a monoclinic to triclinic unit cell rather than a variation in angle of tilt. While the details of the crystalline modification at 24–25°C are not as yet sufficiently clear to define a specific mechanism for the resonance phenomena, the results do prove conclusively that the dispersions are intimately connected with the crystalline state of the material. Eventually a kind of mechanical spectroscopy may be evolved yielding information on the actual crystalline state of solids in much the same way that electromagnetic spectra now aid in determinations of molecular structures.