Temperature dependence on free volume in cured natural rubber and styrene-butadiene rubber blends

(Received 19 December 2010; revised manuscript received 7 April 2011; published 27 May 2011)A systematic study on the evolution of free volume as a function of the temperature in vulcanized at 433 Knatural rubber (NR) and styrene butadiene rubber (SBR) in 25-75, 50-50, 75-25 NR-SBR (percent content ofpure NR and SBR, respectively) blends was studied by positron annihilation lifetime spectroscopy. All sampleswerepreparedwithsulfurandTBBS(n-t-butyl-2-benzothiazolesulfenamide)asaccelerator.Theglasstransitiontemperaturesofthesamplesstudiedweredeterminedbydifferentialscanningcalorimetry(DSC)andfromlifetimedata. In general, a sigmoidal-like complex behavior of the long-lived lifetime component, linked to the nanoholefree volume, as a function of the temperature was found. For SBR, the slope of the ortho-positronium lifetimeagainst temperature curves could be well-fitted using a linear function. For blends and also for NR, two differentlinear functions were necessary. This last behavior is explained in terms of the supercooled process involving areconfigurationoftheelastomericchains.Inthecaseofblends,thestateofcureofNRandSBRineachNR-SBRsample was also taken into account in the discussion of the results obtained. Besides, thermal expansion coeffi-cients of the free volumes in the transition and glassy region of all compounds were estimated. The differencesobserved in the values of this parameter are discussed by taking into account the morphology and formulation ofeachblend,thecrosslinkdensities,andtheroleoftheinterphasesformedbetweenbothNRandSBRelastomers.DOI: 10.1103/PhysRevE.83.051805 PACS number(s): 61