Gels are giant single molecules that consist of a very large number (∼Avogadro's number) of cross-linked nanometer-size polymer chains. Unlike most low-molecular-weight compounds, the extensively crosslinked gel networks typically do not exhibit a well-defined structure. In a previous study, we disproved this preconception and demonstrated that by applying suitable percolation conditions during the gelation process, a highly homogeneous gel with an ordered structure can be synthesized. In the present study, we further demonstrate that by tuning the percolation conditions, stable polymer-rich or polymer-poor nanodefects can be selectively introduced in the gel network; the controlled addition of such nanodefects has not been achieved before. The successful introduction of nanodefects was confirmed using laser speckle tests, and their structures and dynamics were evaluated in Fourier space using static and dynamic scattering measurements. While the addition of polymer-rich defects had a relatively little effect on the elastic modulus of gels, the addition of pores significantly lowered the elastic modulus, suggesting that substantial topological defects were introduced simultaneously when the packing ratio was low. The controlled addition of such nanodefects may potentially modulate the structural, mechanical, optical, and mass transportation properties of the gels effectively, and thus serve as a new design strategy for gel materials.

The dynamics of thermoresponsive and biocompatible gels was investigated using dynamic light scattering. The gels were copolymerized by two types of ethylene glycol methacrylates having short and long hydrophilic side chains consisting of ethylene oxide units. Their dynamics was decomposed to fast and slow modes. The critical temperature was determined from the fast mode (cooperative diffusion) and was controlled lineally by the copolymerization ratio of the two monomers, which is one of the advantages in the gel system. The formation mechanism of hydrophobic domains in the gels was investigated from the slow mode. The hydrophobic domains grew in the gels by rising temperature, and they were copolymerization ratio dependent. The domain formation was suppressed as the copolymerization ratio of the longer side chain was increased. The results of this study should lead to new design strategy for the bioapplications, including drug delivery systems that require a retention of their smart functions.

Dynamically crosslinked gels are appealing materials for applications that require timedependent mechanical responses. DNA duplexes are ideal crosslinkers for building such gels because of their excellent sequence addressability and flexible tunability in bond energy.However, the mechanical responses of most DNA gels are complicated and unpredictable despite the high potential of DNA. Here, we demonstrate a DNA gel with a highly homogeneous gel network and well-predictable mechanical behaviors by using a pair of starpolymer-DNA precursors with presimulated DNA sequences showing the two-state transition.The melting curve analysis of the DNA gels reveals the good correspondence between the thermodynamic potentials of the DNA crosslinkers and the presimulated values by DNA calculators. Stress-relaxation tests and dissociation kinetics measurements show that the macroscopic relaxation time of the DNA gels is approximately equal to the lifetime of the DNA crosslinkers over four orders of magnitude from 0.1-2,000 sec. Furthermore, a series of durability tests find the DNA gels are hysteresis-less and self-healable after the applications of repeated temperature and mechanical stimuli. These results demonstrate the great potential of star-polymer-DNA precursors for building gels with predictable and tunable viscoelastic properties, suitable for applications such as stress-response extracellular matrices, injectable solids, and soft robotics.

Partially deuterated poly(ethylene glycol) (PEG) networks were fabricated by cross-linking a protonated four-arm PEG prepolymer (4hPEG) with a deuterated linear PEG (2dPEG) via a cross-end-coupling reaction. The structure of the resulting partially deuterated 4h-2dPEG networks and the corresponding non-cross-linked polymer solutions, i.e., a mixture of 4hPEG and 2dPEG homopolymers, was investigated in D 2 O solvent by small-angle neutron scattering (SANS) in a contrast-matched condition. The observed SANS profiles were analyzed with an extended random phase approximation (RPA) model, where the RPA theory was expanded for three-component multiarm star block copolymer systems, e.g., 4hPEG, 2dPEG, and solvent. A correlation-hole peak was exclusively observed in 4h-2dPEG gels but not in solutions of the homopolymer mixture of 4hPEG and 2dPEG. The extended RPA theory well reproduced the scattering profiles of both gels and polymer solutions. The Flory interaction parameter between PEG and water and the segment length of PEG were in good agreement with literature values. In addition, the behavior of the theoretical scattering function of the partially labeled multiarm star block copolymer was examined with respect to the fraction of polymer volume, the fraction of protonated chains in the star block copolymer, and the Flory interaction parameter between polymer chains and solvent molecules.

Topology transformations of polymer architectures via dynamic covalent chemistry have attracted considerable attention in recent decades, as they change the primary structure of the polymer architecture and thus the polymer properties. However, topology transformations to produce cyclic topologies remain challenging. In this study, various cyclic polymers were synthesized based on the dynamic behavior of the bis(2,2,6,6tetramethylpiperidin-1-yl)disulfide (BiTEMPS) linkage. Linear and 4-branched polymers were transformed into cyclic topologies, namely, cyclic polymers and figure-eight-shaped polymers, using the following sequence: (1) A thiol−ene reaction was used to introduce BiTEMPS units into the terminal structures of the polymers to be cyclized; (2) an entropy-driven transformation to give the desired cyclic topology through the exchange reaction of the BiTEMPS units was induced by dilution and heating; (3) acyclic impurities with reactive groups were selectively removed via a thiol−ene click reaction with polystyrene particles that contain dangling thiol groups on their surface and subsequent simple filtration. The radicals generated by BiTEMPS upon heating are highly tolerant toward a variety of chemical species, including oxygen and olefins, and exhibit high reactivity in exchange reactions, making them applicable for various skeletons. The simplicity and substrate versatility of this procedure are demonstrated via the highly efficient gram-scale synthesis of cyclic and figure-eightshaped polymers. Moreover, we describe the topology transformation of the obtained cyclic and/or figure-eight-shaped polymers into cross-linked polymers with controlled physical properties using their cyclic or figure-eight topology and the dynamic nature of the BiTEMPS units in their structures.