Tale of Two Tails: Molecular Exchange Kinetics of Telechelic Polymer Micelles

Telechelic polymers contain two chain ends that are able to promote self-assembly into “flowerlike” or interconnected micellar structures. Here, we investigate the molecular exchange kinetics of such micelles using time-resolved small-angle neutron scattering. We show that the activation energies of monofunctional and telechelic chain exchange are identical. This demonstrates that the two chain ends are not simultaneously released in a single event. Instead, the results show that, contrary to regular micelles, the kinetics occurs in a multistep process involving a collision-induced single-molecule exchange mechanism where the exchange rate is directly proportional to the polymer concentration. We show that this novel mechanism can be quantitatively explained by a simple kinetic model.

Figure 1

(Top) Illustration of the kinetic zero average contrast technique to monitor chain exchange in diblock and triblock mixed micelles. (Bottom) Corresponding TR-SANS data of ${\mathrm{C}}_{28}\text{−}\mathrm{PEO}5/{\mathrm{C}}_{28}\text{−}\mathrm{PEO}10\text{−}{\mathrm{C}}_{28}$ micelles obtained at 37 °C and 1.0 vol% polymer volume fraction after mixing equal amounts of the deuterated and proteated reservoirs.

Figure 2

Relaxation functions $R(t)$ of C22F50 at selected temperatures. Black lines represent fits according to Eq. (5). (Inset) Comparison of $R(t)$ of C22F50 and C22F00 (pure monofunctional) at 14 °C. The data were obtained using a stopped-flow mixing device.

Figure 3

Arrhenius representation of the rate constants of ${\mathrm{C}}_{28}$ and ${\mathrm{C}}_{22}$ monofunctional (mon) and telechelic (tel) polymers, obtained via a simultaneous fit of all temperatures. For comparison, previous data of the pure monofunctional samples (F00) are included [29]. The gray shades are contour plots of differential scanning calorimetry traces, indicating the melting transitions of the micellar cores.

Figure 4

Relaxation curves of C28F40 measured at $T=37°\mathrm{C}$ and three different polymer volume fractions. (Inset) Respective rate constants obtained by fitting Eq. (5). The fast process is concentration independent, while the slow process exhibits a $k\sim \varphi$ dependence.

Figure 5

Visualization of the exchange mechanism of telechelic chains in flowerlike micelles and clusters of micelles via a sequence of consecutive equilibrium steps involving collision-induced exchange. Diffusion of free telechelic chains as an important pathway is excluded.