Ultrasonic Attenuation Measurements in Poly-l-Glutamic Acid Solutions

Ultrasonic attenuation measurements of poly‐l‐glutamic acid in 0.2M NaCl(aqueous)—dioxane (2:1) have been carried out over a frequency range from 6 to 175 Mc/sec using a pulse‐echo method. Attenuation in excess of the pure solvent was found over the entire pH range investigated (5.4–9.0). A distribution of relaxation times is observed at all pH values. This distribution approaches a single relaxation time as the pH is raised. The amplitude parameter characterizing the attenuation and the average relaxation time does not vary markedly with pH. The amplitude parameter is approximately proportional to the polymer concentration, in contrast to the average relaxation time which is not strongly dependent on the polymer concentration. The process primarily responsible for the observed chemical relaxation is probably the interaction between solvent and polymer. The minimal volume change associated with the observed process has been calculated and is approximately ±3 cc/mole of monomer. Although the ultrasonic attenuation clearly depends on whether the polymer molecule is in the form of a helix or random coil, the transition from helix to coil is not primarily responsible for the observed excess absorption. By combining these results with other data, bounds for the average relaxation time characteristic of the helix—coil transition can be estimated as 5×10⁻⁸<$\mathrm{\tau ̄}$ <10⁻⁵ sec.