  The influence of structural constraints on the relaxation dynamics of three
polyurea networks with varying degree of crosslinking, has been studied by
means of a thorough analysis of broadband dielectric spectroscopy measurements.
Two different relaxation processes are observed, namely, a fast process
involving the soft poly(propylene oxide) chains, and a slower and much broader
process associated to the immediate surroundings of the hard crosslinkers.
Microphase separation in soft and hard domains characterizes the systems in the
presence of hydrogen bondings. In this case, different confinement conditions
are explored by varying the soft chain length; overall, the so called
'adsorption' effects dominate. With respect to both cooperativity and
rearrangement energy threshold in fast relaxation, it is found that the
enhancement of configurational constraints is similar to cooling, but only on
qualitative grounds. An upper bound of the hard domain's interface thickness,
in which the slow relaxation is believed to take place, is estimated from the
analysis of the fast relaxation in the system characterized by the highest
degree of confinement, taking into account the results of the structural
analysis. Dropping the hydrogen bonding mechanism, phase separation does not
occur anymore and the configurational constraints at the ends of the soft
chains are reduced, leaving just those imposed by the rigid crosslinkers. This
leads to a significant increase in cooperativity, on approaching the glass
transition, and to a complex behavior that is thoroughly discussed in
comparison with those observed in the micro-segregated systems.
