Gamma irradiation of a polyisobutylene rubber: effects on the molecular weight and the subsequent thermal stability

• Published in: Polymer degradation and stability Vol. 62; no. 2; pp. 211 - 223
• Main Authors: Lehrle, R.S., Pattenden, C.S.
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Ltd 01-01-1998; Elsevier Science
• Subjects: Applied sciences; Exact sciences and technology; Organic polymers; Physicochemistry of polymers; Properties and characterization; Solution and gel properties; Isobutene polymer; Pyrolysis; Chemical solution; Radiation dose; Gamma radiation; Radiation effect; Kinetic parameter; Experimental study; Property processing relationship; Molecular weight property relation; Thermal stability; Thermal degradation
• ISSN: 0141-3910; 1873-2321
• DOI: 10.1016/S0141-3910(97)00278-4

Polyisobutylene (PIB) rubber as a 1% solution in methylcyclohexane has been exposed to gamma radiation in a
60Co source. The samples received doses of 8.23 and 75.7
kGy. The molecular weight of the polymer in solution was measured by solution viscosity before and after irradiation, and the results showed a dramatic reduction in molecular weight as the dose increased. The thermal stability of the non-irradiated sample and the highly irradiated sample have been studied by pyrolysis-gas chromatography-mass spectrometry (py-g.c.-m.s.) using a thermocouple-controlled resistive filament pyrolyser. The measured rate constants for thermal degradation showed that the irradiated sample had slightly greater thermal stability than the non-irradiated sample. It is proposed that this observation is consistent with the following interpretations: (i) the reduced molecular weight of the irradiated sample results in a reduction of the melt viscosity during the subsequent degradation, and this facilitates the bimolecular termination of degradation chains, and (ii) the reduced molecular weight of the irradiated sample is less than the average kinetic chain length for the non-irradiated sample, hence for a given scission, less product is evolved and the stability is increased. Another interpretation is feasible if it is assumed that the original polymer contained some ‘weak links’. If these were broken first by the irradiation there would be no depropagation at the low ambient irradiation temperature. Potential initiation sites would thereby be lost before the temperature was elevated during subsequent pyrolysis.