The relationship between the energy release rate (or adhesion energy) and the crack velocity has been measured for a variety of cross-linked elastomers based on carboxylated poly(n-butyl acrylate). Methylation of the carboxyl groups decreased adhesion energies to the glassy polymer substrates used in these experiments, and neutralization of the carboxyl groups increased the adhesion energy to these substrates. The primary effect of methylation or neutralization was to change the velocity dependence of the adhesion energy. These changes were consistent with changes in the measured relaxation times of un-cross-linked carboxylated, methylated and neutralized polymers. Effects on the critical energy release rate required for measurable crack motions were consistent with a contact charging mechanism, involving transfer of a proton for the carboxylated elastomers, and transfer of a sodium ion for the neutralized polymers.