Thermal degradation behavior of gamma-irradiated acetyloxy end-capped poly(oxymethylene)

The radiation effect on the thermal stability of poly(oxymethylene) diaceta te (POMDA) in film has been investigated on irradiation up to 1000 kGy in a ir with a Co-60 gamma-ray source. The gel permeation chromatography (GPC) i ndicated that the molecular weight of POMDA was lowered in the polydisperse index (M-w/M-n) approaching the ultimate value of 2 as the radiation dose increased. As observed in the differential scanning calorimetry (DSC), the melting temperature of irradiated POMDA decreased progressively with the lo wering of the molecular weight, and the thermally less stable components th an the original POMDA were produced in increased yields with increasing rad iation dose. According to the thermogravimetry (TG), about 55% of higher-do se irradiated POMDA maintained thermal stability comparable or even superio r to that of the original POMDA before irradiation, while about 45% had les s stability to thermal degradation. The formation of hydroxyl (G(-OH) = 0.5 ), formyloxy (G(-OCHO) = 0.6) and methoxy (G(-OCH3) = 0.4) end-groups was i dentified by the measurement of infrared (IR) dichroism of uniaxially orien ted POMDA film after irradiation. The radiation chemical modification of ch ain-end groups is accounted for by a radical chain reaction mechanism invol ving random scission of POMDA main-chain as an initiation step. It was conc luded that a POM chain possessing at least one hydroxyl end-group is conclu ded to be thermally less stable than POMDA, while another POM family withou t bearing any hydroxyl end-group may have thermal stability comparable or s uperior to the original POMDA. (C) 1999 Elsevier Science Ltd. All rights re served.