Random Polybutadiene Rubber Networks with Unattached Chains

Abstract

The preparation and characterization of randomly cross-linked polybutadiene (PB) networks containing unattached PB chains are presented. These were used as model systems for the experimental study of the dynamics of linear chains trapped in networks, which is the basic model of the reptation-in-a-tube model of Doi and Edwards. The preparation consists of the conversion at random of a small fraction of the PB chains' double bonds into epoxide groups and in the subsequent selective use of these as cross-linking centers in the presence of unmodified PB chains; these last remain chemically unattached to the network. The selectivity of the cross-linking reaction was examined by swelling−extraction of networks containing known fractions of relatively short “monodisperse” unattached chains. The polymeric phase composition of the networks with unattached chains (nuc) was tested by glass transition temperature measurements. Dynamic−mechanical measurements were shown to be sensitive to the polymer-phase composition of nuc, as well. The dynamics of the linear PB chains in the melts and in the networks was studied by dynamic−mechanical measurements. The experimental results for the longest relaxation time τL were compared with the predictions of the reptation-in-a-tube model with and without chain-end effects. Within experimental accuracy, the obtained scaling of τL relative to the chain mass M (up to 9 × 105 g/mol) is the same in both media: τL ∼ Mx with x = 3.35 (±0.10 as against x = ±3 from the pure reptation-in-a-tube model; correcting for chain-end effects in the form of contour length fluctuation, results in a good description of the experimental data for the τLs of unattached chains. τL for the chains trapped in networks were found to be 1.9−3.2 times longer than the τL of the respective melts. Data on 14 PB prepared by anionic polymerization and covering the range of weight average molar masses 1.0 × 104 to 1.5 × 106 g/mol are presented.