Unveiling the effects of molecular topology on the flow property of entangled polymers under gelation

As the applications for crosslinked polymers continue to expand, understanding the evolution of their microstructures becomes imperative to produce materials with the desired physical and mechanical properties for the application. Here, on the basis of ring opening polymerization, statistical copolymers are synthesized with the distribution of crosslinkable sites of NHS (n-hydroxysuccinimide) ester along the linear norbornene backbone. Through the controlled NHS ester amine reactions, side-linking model networks with tunable crosslinking density are achieved by varying the stochiometric ratio of mono-amines to diamines. These model networks are crosslinked in dilute solutions but characterized in neat with the corresponding architecture and rheological properties investigated by our recently developed "reaction-topology-viscoelastic" model. Compared to typical network of end-linking chemistry, a much higher fraction of loop-like structures is found for the above network structure. For a serial of side-linking model networks crosslinked under different concentrations, a nearly unchanged linear rheology indicates these high-loop-containing molecules behaves similarly in entangled regime as those of elastic network strands over the frequency window of interest. A further analogy to radical induced crosslinking of polyolefin melts on the characteristics of network defects as well as the rheology reveals two different gelation mechanisms indicating the great importance of molecular topology and the associated synthesis conditions on the properties of thermoset and gels.