Longest relaxation time of linear polybutadiene chains trappedin rubber networks

Abstract

We have used dynamic-mechanical measurements to probe the long-timedynamics of linear, almost monodisperse polybutadiene chains in twomedia: a) their own melts and b) as trapped in our recentlyprepared elastomeric networks. The results are compared with theclassical reptation-in-a-tube model of de Gennes and Doi andEdwards. Within experimental accuracy, the obtained experimentalscaling of the longest relaxation time τL relative to thechain mass M (up to 9·105 g/mol) is the same in bothmedia: τM ∼ Mx with x = 3.33 ± 0.10 as against x = 3predicted by the "pure” reptation-in-a-tube model. However, thenetwork data are well described in the range of molar masses 3.4·104 − 5.5·105 g/mol, after correcting thereptation model for chain-end effects (in the form of contour lengthfluctuations). The τL for the chains trapped in a networkwere found to be 1.9–3.2 times longer than the τL of therespective melts. We present data on ten polybutadienes confined intwo kinds of networks and covering the range of weight average molarmasses 3.4·104–1.5·106 g/mol.