Radiation-induced structural changes in poly(propylene-ran-ethylene) film: Effect of antioxidant 2,2,6,6-tetramethylpiperidine derivatives

Abstract

The isotactic random copolymer of propylene and 2.3 wt% ethylene in films with and without bis(2,2,6,6-tetramethyl-4-piperidinyl)sebacate (LS-770) or bis( N-chloro-2,2,6,6-tetramethyl-4-piperidinyl)-sebacate (LS-770-Cl) was γ-irridiated up to 20 Mrad in air at room temperature. Progressive oxidation proceeded to form hydroperoxide and carbonyl groups, which were depressed by the antioxidizing action of the additives. The apparent antioxidizing activity of LS-770 was higher than of LS-770-Cl. From differential scanning calorimetry, to evaluate the melting temperature and heat of fusion, main-chain scission and subsequent recrystallization were suggested to occur in the interlamellar amorphous regions. In association with recrystallization, the helical regularity in the isotactic sequence of the propylene units increased with increasing radiation dose, as evaluated from the sequence-sensitive i.r. absorption bands. X-Ray diffraction analysis indicated a transformation of disordered crystallines to the closest packed monoclinic structure during the irradiation. A density rise was also observed, which was probably due to the combined effects of the recrystallization and the oxygen uptake. These physical-structural changes were effectively depressed by the additives, for which the LS-770-Cl was more active than the LS-770.