LOCATION AND MOBILITY OF FUNCTIONAL-GROUPS AT SURFACE OF OXIDIZED, LOW-DENSITY POLYETHYLENE FILM
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Samples of low-density polyethylene film modified by introduction of surface carbonyl hydrazide groups (PE-CONHNH2) react with 4-thiocyanato-2,2,6,6-tetramethylpiperidin-l-oxyl and 5-dimethylamino-l-naphthalenesulfonyl chloride to yield spin-labeled (PE-CONHNHCSNH-TMPO) and fluorescence-labeled (PE-CONHNH-DANS) films, respectively. Polyethylene acid chloride (PE-COCl[SOCl2]), prepared by treatment of PE-CO2H with thionyl chloride, reacts with 4-amino-2,2,6,6-tetramethylpiperidin-l-oxyl in high yield. Films prepared by this method (PE-CONH-TMPO[TMPO]) are contaminated with material not covalently bonded to the surface via an amide linkage, and, apparently, generated by reaction of thionyl chloride (or an impurity in the thionyl chloride) dissolved in the polyethylene with the primary amine. Extensive washing of the surface of films prepared by reaction of amines with polyethylene containing thionyl chloride using common organic solvents removes little of the contaminating material; extraction of the film with hot 2-propanol removes most but not all of it. Several experiments demonstrate that groups attached to these PE-CONHNH2 films via carbonyl hydrazide moieties react with aqueous reagents which are insoluble in polyethylene, and establish that the labeling groups are located predominantly at the polyethylene film surface. Immersion of nitroxide-labeled film in 0.1 M aqueous sodium ascorbate solution (pH 6.8) for 5 h reduces the intensity of the ESR signal of the film by >85%; treatment of the film with 3.0 N aqueous HCl reduces its ESR signal intensity by 70-95%. The fluorescence emission maximum of PE-CONHNH-DANS falls at 468 nm when the film is dry; in contact with water, it shifts to 510 nm. Aqueous 1.0 N HCl quenches the fluorescence almost completely. Both observations are compatible with localization of the dansyl groups at the polyethylene-water interface. Treatment of PE-CONHNH2 with cold, aqueous nitrous acid rapidly generates the corresponding acyl azide (PE-CON3). This material, in turn, is rapidly hydrolyzed by aqueous 1.0 N HCl. In contrast to PE-CONHNHCSNH-TMPO, the nitroxyl groups of PE-H[TMPO] (prepared by reaction of polyethylene containing dissolved thionyl chloride, PE-H[SOCl2], with H2N-TMPO) are not reduced by 0.1 M aqueous sodium ascorbate or 3 N aqueous HC1. The effects of temperature and organic solvents upon the mobility of nitroxyl radicals bonded to surface-functionalized polyethylene indicate that the nitroxyl groups experience little restriction to free rotation when the films are in contact with organic liquids. Rotation of the nitroxyl groups of dry film is more hindered. 1977, American Chemical Society. All rights reserved.
