Interplay between structure and relaxation in polyurea networks: the point of view from a novel method of cooperativity analysis of dielectric response

TL;DR

This study investigates how structural constraints influence relaxation dynamics in polyurea networks using broadband dielectric spectroscopy and introduces a novel cooperativity analysis method to understand the interplay between structure and relaxation.

Contribution

It presents a new method for analyzing cooperativity in dielectric response and applies it to polyurea networks, revealing how hydrogen bonding and phase separation affect relaxation processes.

Findings

Two relaxation processes identified: fast soft chain relaxation and slow hard domain relaxation.

Hydrogen bonding and phase separation influence the confinement and cooperativity of relaxation.

Removing hydrogen bonds reduces phase separation and increases cooperativity near the glass transition.

Abstract

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.