First observation of two-stage collapsing kinetics of a single synthetic polymer chain

Using a narrowly distributed poly($N$-isoporpylacrylamide) (PNIPAM) chain with a degree of polymerization ($N$) of 3100, randomly labeled with pyrene, we have, for the first time, observed the two-stage kinetics of the coil-to-globule transition. Two characteristic relaxation times, ${\ensuremath{\tau}}_{\mathrm{fast}}$ for the crumpling of a random coil ($\ensuremath{\sim}12\text{ }\text{ }\mathrm{ms}$) and ${\ensuremath{\tau}}_{\mathrm{slow}}$ for the collapsing of the crumpled chain to a compact globule ($\ensuremath{\sim}270\text{ }\text{ }\mathrm{ms}$), were measured. To our knowledge, this is the first experimental evidence supporting the two-stage collapsing kinetics of single synthetic polymer chain previously proposed by de Gennes, Dawson, and Grosberg.