A new perspective of the glass transition of polymer single-chain nanoglobules

The glass transition behavior of polyacrylamide (PAL) was investigated in the bulk entangled state, the single-chain isolated state, and the partially entangled state. It was observed that the glass transition temperature increases as the degree of entanglement decreases. Single-chain nanoglobules of PAL were prepared by spray drying and the partially entangled PAL sample was prepared by freeze-drying from solutions of different concentrations. The glass transition temperature (T-g) was measured by using differential scanning calorimetry (DSC). The enthalpy relaxation peak of the single-chain nanoglobules was found to be sharper and higher than that of the bulk PAL, whereas the partially entangled PAL exhibited two enthalpy relaxation peaks. The cohesional entanglement theory is adopted in the discussion. The DSC results of this study appear to be consistent with the concept of cohesional entanglement. It was also found that, at the same annealing condition, the single-chain PAL globules have much higher T-g than that of the bulk PAL. The single chains, after spray-drying, are in the form of loose chain coils; upon thermal treatment, the chain coil becomes more compact. The bulk polymer also shows the same trend, but due to the entanglement in the bulk polymer, the increase in T, is smaller compared to the T-g increase for single-chain globules.