Using a fluorescent molecular rotor probe enables accurate determination of glass transition temperatures in polymers: Experiment and atomistic simulation

In this presentation, we show that a fluorescent molecular rotor consisting of a julolidine headgroup and a farnesyl tail, farnesyl-(2-carboxy-2-cyanovinyl)-julolidine (FCVJ), can be used as a probe for experimental determination of T_g for bulk and nanoconfined polymers. Experiments show that FCVJ has a higher sensitivity to changes in free volume and is thus able to detect the glass transition more reliably than other types of molecular rotors previously studied. Atomistic MD simulations show that the difference in conformational flexibility between FCVJ’s head and tail groups produces anomalous kinetic energy distributions between these groups below T_g. The T_g values determined for a low-molecular weight polystyrene by experiment and simulation, both using FCVJ as a probe for detection of glass transition, agreed with each other and also with the value obtained by DSC. These results also establish that the T_g of a polymer can be accurately determined by atomistic MD simulation—by tracing the motions of the probe molecule’s head and tail groups (rather than tracing the motions of polymer segments themselves), which enables to circumvent the well-documented dilemma of T_g overestimation that occurs due to a fast cooling rate employed in atomistic MD simulation.