The effect of stereoregularity upon the adsorption behavior of high molecular weight poly(isopropyl acrylate)
The adsorption of high molecular weight atactic, syndiotactic, and isotactic poly(isopropyl acrylate) (PIPA) on finely divided silica has been studied at 24°C. Trichloroethylene was used as the solvent. The PIPA isotherms are typical for polymers and differences in stereoregularity cannot be detected from the isotherm data. Infrared-bound fraction determinations show that all adsorbed PIPA samples have a high fraction of their segments attached directly to silica. However, surface coverage and length of agitation affect bound fraction values, p. Below surface saturation, atactic and syndiotactic PIPA have values of p = 0.36−0.52 while for isotactic PIPA p = 0.47−0.58. In these cases, prolonged agitation causes an increases in values of p. At surface saturation, values of p for atactic and syndiotactic PIPA decrease to 0.34–0.39, and length of agitation has little effect on p. It was difficult to obtain reliable values of p for isotactic PIPA at surface saturation due to dispersion instability. Silica was always readily removed from dispersions that contained isotactic PIPA. In contrast, a substantial fraction of silica could not be removed from dispersions that contained excess atactic and syndiotactic PIPA, even after prolonged centrifugation. Many of these observations are consistent with a bridging model for flocculation of colloidal dispersions by polymers.