Langfristige konformationsänderung von makromolekülen in lösung, 1. Polyacrylamide

The viscosity of aqueous polyacrylamide solutions does not remain constant. During a period of many weeks it decreases by as much as 30%. Simultaneously a loss of flocculation activity occurs. This unusual behaviour — also called ageing or solution instability — is examined. Viscosity and light scattering investigations were made. The viscosity was also determined by addition of substances which influence hydrogen bonds. It could be demonstrated, that in high molecular weight polyacrylamide hydrogen bonds are effective. Light scattering measurements result in curved angular dependence of scattered light, which must be regarded as real and specific for the high molecular weight polyacrylamides in water. The Zimm plot for polyacrylamide in ethylene glycol was undistorted. The refractive index increment decreases with time during the investigations. From the Zimm plots it follows, that the radius of gyration decreases with time. The molecular weight however remains constant. Based on the experimental results a mechanism is assumed with a long-term conformational change of the macromolecules. The orientated segments — stabilized by hydrogen bonds — are slowly destroyed by occupation of “active sites” by water molecules. Chain degradation as well as disentanglement could be excluded. A molecular interpretation is given.