Trapping Polymer Entanglements via Prolonged Initiation

Abstract Entanglements in polymer networks can be either trapped or transient, and their ratio is crucial to the mechanical properties of soft materials. Traditionally, entanglement formation has been mostly linked to chain length─polymers entangle when their lengths exceed the entanglement molecular weight─without independent control of the ratio between the two. Here, we demonstrate that the initiation rate significantly affects the fraction of trapped entanglements and the resulting mechanical properties. We hypothesize that more monomers grow in the presence of existing polymers as the initiation rate decreases, forming more trapped entanglements. To demonstrate this, we synthesize UV-curable, highly entangled polyacrylamide hydrogels in which the initiation rate varies with UV intensity. Upon swelling, transient entanglements can detangle, whereas trapped entanglements cannot, by which we characterize the fraction of entanglements. We observe that the swelling ratio of the same precursor decreases significantly as UV intensity decreases, indicating a higher fraction of trapped entanglements. Additionally, such hydrogels with many trapped entanglements exhibit superior fracture resistance due to their swell-resistance. Our work offers a kinetic approach to network topology design, expanding the material property space of polymer networks.