  Vitrimers are polymer networks that can undergo bond exchange reactions. They
dynamically rearrange their structures while maintaining their overall
integrity, thus resulting in unique properties such as self-healing,
reprocessability, shape memory and adaptability. Here, we show that the
introduction of dynamic bonds directly impacts the polymer density. For a
limiting case, where the dynamic bonds are the same size as the polymer chain
bonds, simulations and theory show an enhancement in the density, because these
bonds induce an increased cohesive force in the liquid, which is entropic in
origin. The crosslinks are well mixed in the bulk but are depleted from the air
and polymer interface. These findings implicate density as a key variable in
polymers with dynamic crosslinkers, one that can be used to facilely tune their
properties.
