Single Molecules Probing the Freezing of Polymer Melts: A Molecular Dynamics Study for Various Molecule-Chain Linkages

We present molecular dynamics simulations of coarse-grained model systems of a glass-forming polymer matrix containing fluorescent probe molecules. These probe molecules are either dispersed in the matrix or covalently attached to the center or the end of a dilute fraction of the polymer chains. We show that in all cases the translational and rotational relaxation of the probe molecules is a faithful sensor for the glass transition of the matrix as determined from a mode-coupling analysis or Vogel-Fulcher analysis of their α-relaxation behavior. Matrix and dumbbell related relaxation processes show a clear violation of the Stokes-Einstein-Debye laws. In accordance with recent experimental results, the long time behavior of single molecule spectroscopy observables like the linear dichroism is not susceptible to distinguish between center-attached and end-attached fluorophores. However we show that it is different from the behavior of dispersed fluorophores. We also show that the difference between the two attachment forms does show up in the caging regime of the relaxation functions and that this difference increases upon supercooling the melt toward its glass transition.