ELSEVIER zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA Po/pw Dqrudation und Srahiliry 54 zyxwvutsrqponmlkjihgfed (lYY6) XY-YX 0 lYY6 Elsevier Science Limited Printed m Northern Ireland. All rights reserved PII: SO141-3910(96)00106-l 014l-3Y10/Y6/$15.00 zyxwvutsrqponm Characterisation of polypropylene peroxides; their thermo-oxidative stability and reactivity towards dimethylsulfide A. Kron; B. Stenberg”* & T. Reitbergef’ zyxwvutsrqponmlkjihgfedcbaZYXWVUT “Department of Polymer Technology, Royal Institute of Technology, S-100 44 Srockhotm, Sw eden ‘Department of Chemistry, Royal Institute of Technologyy, S-100 44 Stockholm, Sw eden (Received 27 March 1996: accepted 10 April 1996) Exposure to dimethylsultide (DMS) has previously been presented as a tool to distinguish between different types of polypropylene (PP) peroxides. The reasons for the differences in peroxide reactivity towards DMS are investigated for PP peroxides. This was accomplished using iodometry, infrared spectros- copy, measurements of total emitted chemiluminescence, chemiluminescence imaging and measurements of glass transition and melting temperatures. On the basis of our results, we propose that DMS reacts preferentially with peroxides which are adjacent to each other, rather than with isolated peroxides.’ The higher reactivity of adjacent peroxides towards DMS is believed to be due to a catalytic action of protic species. An increase in glass transition temperature during oxidation was observed, which indicates an increasingly restricted segmental mobility. Furthermore, changes in concentra- tion of DMS-resistant peroxides during oxidation were found to correlate with the changes in glass transition and melting temperatures. Consequently, we propose that the segmental mobility in the polymer affects the reactivity of the isolated peroxides towards DMS. 0 1996 Elsevier Science Limited 1 INTRODUCTION Polyolefins are susceptible to oxidative degrada- tion, leading to deterioration of their physical properties. Incorporation of oxidation products and scissions in the polymer backbone reduces the fracture toughness of the polyolefins. Polypropylene (PP) is particularly vulnerable to main chain scission since the backbone contains tertiary carbons. The oxidative degradation of polyolefins has been extensively studied,le3 which has led to the development of more efficient stabilising systems and to materials of better long-term performance. Oxidative degradation of polymers is a sequence of radical reactions which leads to the incorporation of oxygen into the polymer. The reaction mechanism is often described by the Basic Autocatalytic Scheme (BAS).” The BAS has been used for kinetic evaluations of polymer * To whom correspondence should be addressed. 89 oxidation and a significant development of the scheme has recently been reported by Gillen et al.’ The formation and decomposition of peroxides are key reactions in the oxidation process. The peroxides are primary oxidation-products which decompose to form new radicals. It is desirable to know the identities and reactivites of the different types of peroxides formed in order to understand and predict the oxidation process. However, it is not trivial to distinguish between different types of peroxides or to estimate their respective concentrations. Techniques for meas- uring hydroperoxides have been reviewed by Carlsson et aLh and more recently by Scheirs et aL7 The oxidation of polymers is accompanied by the emission of low levels of light. Several research groups have independently shown that measurement of such chemiluminescence (CL) is a valid and very sensitive technique for