The vulcanization of carboxylated nitrile rubber (XNBR) with zinc peroxide,
which produces ionic crosslinks, has been studied in relation to vulcaniza
tion time. Vulcanized compounds present two transitions, corresponding to t
he glass transition of the polymer at low temperatures and the ionic transi
tion resulting from the formation of ionic aggregates. Both transitions are
displaced to higher temperatures with increasing crosslink density. The io
nic associations which give rise to high values of mechanical properties di
sappear on exposure of the vulcanized compounds to saturated ammonia vapour
. This treatment produces a decreased crosslink density resulting in the di
sappearance of the ionic transition. When the action of ammonia is terminat
ed by immersion in solvent followed by drying, the original crosslink densi
ty is recovered and the ionic transition reappears, although at higher temp
eratures. However, with increasing crosslink density, the difference betwee
n the temperatures at which both transitions take place diminishes. All the
se factors can be interpreted as reflecting the generation of a new and mor
e compatible arrangement of the newly-appearing ionic clusters. (C) 1999 So
ciety of Chemical Industry.