The Extreme of Disentanglement: 100-fold Viscosity Reduction in Model Single-Chain Nanoparticles

The melt rheology of single-chain nanoparticles (SCNPs), which are synthesized by intrachain cross-linking of linear precursors, remains an unresolved issue. In this study, we synthesized linear precursors with different molecular weights (MW) and varying mole percentages of cross-linkers (CrF). The cross-linker, 4-vinylbenzocyclobutene, undergoes thermal-induced coupling. The resulting SCNPs in the melt state exhibited four distinct types of linear viscoelastic behavior: 1) reduced entanglement, 2) viscous-dominant liquids, 3) viscoelastic liquid with an elastic plateau and terminal relaxation, and 4) viscoelastic solid, a persistent elastic plateau with unmeasurable long terminal relaxation times. We observed a nonmonotonic change in viscoelastic behavior as the cross-linking increases. At CrF equal to 4%, Type 1 SCNP is still polymer-like, but the degree of entanglement is reduced. When CrF increased to 8%, the elastic plateau completely disappeared, and the viscous-dominant behavior is similar to the Zimm or Rouse model. Complete disentanglement is achieved even for high MW. In Type 2, the relaxation times and zero-shear viscosities show weaker dependence (a power of 1) on MW compared to linear polymers (a power of 3.4). Thus, the relaxation times and viscosities of such SCNPs can be 100 times smaller than their linear counterparts of the same MW. At even higher CrF, the storage modulus would surpass the loss modulus again if the MW is sufficiently high. This elasticity is due to a mechanism other than entanglements but is analogous to the colloidal domain. Relaxation times or viscosities increase divergently and exceed the experimental measurability in Type 4. The boundary between Types 3 and 4 might be considered as the boundary between polymers and colloids. Using SCNPs as a model system for detailed rheological characterization, we demonstrate the dynamics of the transition from linear polymers to particles. Type 2 SCNPs could be regarded as the extreme of disentanglement, achieved by an appropriate amount of intrachain cross-linking. Such a significant viscosity reduction might support the development of more energy-efficient polymers.