RESTRICTION OF SPIN-PROBE MOTIONS IN POLYMER COMPOSITES DUE TO CHEMICAL FACTORS

Hyperfine splitting constants of the nitroxyl radical, with and withou t hydrogen bonds to the surrounding molecules, have been calculated us ing the UHF method on a 6-31G base. In polyethylene filled with silic a, hydrogen bonds are formed between nitroxyl radicals and -OH groups of the filler. The formation of hydrogen bonds leads to a change in th e A(ZZ) value from 3.33 mT for an isolated nitroxyl radical to 3.83 mT for a radical with a hydrogen bond. The relevant values as measured e xperimentally are 3.4 and 4.0 mT, respectively. The same procedure was used to calculate the theoretical A,, value for a nitroxyl radical in teracting with polyamide via a hydrogen bond. The value was found to b e 3.63 mT (experimental value = 3.6 mT). Hydrogen bond formation resul ts in a restricted motion of the nitroxyl radical in a polymeric mediu m.