Enthalpy relaxation near the glass transition of polystyrenes with controlled interchain proximity

Interchain coupling of polymers was investigated by detecting the segmental proximity using H-1 solid-state NMR under fast magic angle spinning. The NMR experiment based on the dipole-dipole interaction provided information about intersegment distances on length scales of several angstroms, over which the van der Waals attraction force occurs. The NMR and DSC were combined to get insight into the dependence of the enthalpy relaxation by sub-T-g annealing on the local segmental motion. Parameters of the TNM model were used to quantitatively discuss the glassy-state structural relaxation behavior near the glass transition of the prepared samples. Our experimental results demonstrated that if the intersegment distance was controlled to be larger than 0.5 nm, the enthalpy relaxation time spectrum through the glass transition region became narrower and the stretch exponent beta for the TNM model was larger. When the polystyrene (PS) sample was a cast film or powder prepared from a concentrated solution, the chains were detected to be in close proximity, and thus their beta value became smaller. It is interesting that the PS sample from a 10 wt % solution in DOP exhibited nearly the same relaxation behavior as the sample prepared from a 0.1 wt % solution in benzene. H-1 solid-state NMR detection indicated that these two samples showed a similar level of intersegmental proximities, although they had different entanglement concentrations. On the basis of our experimental results, we argued that the segmental proximity restricted the local segmental motions on short length scales and thus influenced the glassy-state relaxation behavior.