Strain-Dependent Kinetics in the Cis-to-Trans Isomerization of Azobenzene in Bulk Elastomers

The cis-to-trans isomerization of azobenzene is accelerated in a bulk PDMS elastomer under uniaxial tension. The kinetics are cleanly described by a single-exponential first-order process (k = 2.7 X 10(-5) s(-1)) in the absence of tension but become multiexponential under constant strains of 40-90%. The complex kinetics can be reasonably modeled as a two-component process. The majority (similar to 92%) process is slower and occurs with a rate constant that is similar to that of the unstrained system (k = 2.3-2.7 x 10(-5)s(-1)) whereas the rate constant of the minority (similar to 8%) process increases from k = 10.1 X 10(-5) s(-1) at 40% strain to k = 21.3 X 10(-5) s(-1) at 90% strain. Simple models of expected force-rate relationships suggest that the average force of tension per strand in the minority component ranges from 28 to 44 pN across strains of 40-90%.