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 × 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 (∼92%) process is slower and occurs with a rate constant that is similar to that of the unstrained system ( k = 2.3–2.7 × 10 –5 s –1 ), whereas the rate constant of the minority (∼8%) process increases from k = 10.1 × 10 –5 s –1 at 40% strain to k = 21.3 × 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%.