Dynamics of Telechelic Ionomers with Distribution of Number of Ionic Stickers at Chain Ends

Strong and Tough Nanostructured Hydrogels and Organogels Prepared by Polymerization-Induced Self-Assembly

In nature, the hierarchical structure of biological tissues endows them with outstanding mechanics and elaborated functions. However, it remains a great challenge to construct biomimetic hydrogels with well-defined nanostructures and good mechanical properties. Herein, polymerization-induced self-assembly (PISA) is for the first time exploited as a general strategy for nanostructured hydrogels and organogels with tailored nanodomains and outstanding mechanical properties. As a proof-of-concept, PISA of BAB triblock copolymer is used to fabricate hydrogels with precisely regulated spherical nanodomains. These nanostructured hydrogels are strong, tough, stretchable, and recoverable, with mechanical properties correlating to their nanostructure. The outstanding mechanical properties are ascribed to the unique network architecture, where the entanglements of the hydrophilic chains act as slip links that transmit the tension to the micellar crosslinkers, while the micellar crosslinkers dissipate the energy via reversible deformation and irreversible detachment of the constituting polymers. The general feasibility of the PISA strategy toward nanostructured gels is confirmed by the successful fabrication of nanostructured hydrogels, alcogels, poly(ethylene glycol) gels, and ionogels with various PISA formulations. This work has provided a general platform for the design and fabrication of biomimetic hydrogels and organogels with tailorable nanostructures and mechanics and will inspire the design of functional nanostructured gels.

Effect of sticker clustering on the dynamics of associative networks

Recent experimental and theoretical work has shown that sticker clustering can be used to enhance properties such as toughness and creep resistance of polymer networks. While it is clear that the changes in properties are related to a change in network topology, the mechanistic relationship is still not well understood. In this work, the effect of sticker clustering was investigated by comparing the dynamics of random copolymers with those where the stickers are clustered at the ends of the chain in the unentangled regime using both linear mechanics and diffusion measurements. Copolymers of N,N-dimethyl acrylamide (DMA) and pendant histidine groups were synthesized using reversible addition-fragmentation chain transfer (RAFT) polymerization. The clustered polymers were synthesized using a bifunctional RAFT agent, such that the midblock consisted of PDMA and the two end blocks were random copolymers of DMA and the histidine-functionalized monomer. Upon addition of Ni ions, transient metal-coordinate crosslinks are formed as histidine-Ni complexes. Combined studies of rheology, neutron scattering and self-diffusion measurements using forced Rayleigh scattering revealed changes to the network topology and stress relaxation modes. The network topology is proposed to consist of aggregates of the histidine-Ni complexes bridged by the non-associative midblock. Therefore, stress relaxation requires the cooperative dissociation of multiple bonds, resulting in increased relaxation times. The increased relaxation times, however, were accompanied by faster diffusion. This is attributed to the presence of defects such as elastically inactive chain loops. This study demonstrates that the effects of cooperative sticker dissociation can be observed even in the presence of a significant fraction of loop defects which are known to alter the nonlinear properties of conventional telechelic polymers.

Brittle-to-Ductile Transition of Sulfonated Polystyrene Ionomers

This study examines the brittle-to-ductile transition of sulfonated polystyrene ionomers (SPS) with different counterions. The polystyrene precursor was unentangled and had two ionic groups per chain on average. Thus, its terminal relaxation time was comparable to the lifetime of the associating ionic groups. Three types of ionomer samples were used to tune the association lifetime: (1) fully neutralized SPS with different alkali-metal counterions, (2) fully neutralized SPS with mixed sodium and cesium counterions, and (3) partially neutralized SPS with sodium or cesium counterions. For all three systems, the brittle-to-ductile transition could be represented by a diagram of two Weissenberg numbers, Wi and Wi_R, defined with respect to the terminal and Rouse relaxation times, respectively. A flowable region existed at sufficiently low Wi, independent of Wi_R. At higher Wi, a brittle-to-ductile transition of the ionomer melt occurred above a critical value of Wi_R. To achieve ductility during the application of rapid elongational flow, the Rouse-type motions should be sufficiently slow relative to the rate of ion-dissociation, so that the strain-induced breakup of the ionic cross-links would not cause very strong chain retraction that may further lead to the macroscopic fracture.

Physical Bond Breaking in Associating Copolymer Liquids

We combine ideas from polymer and glassy liquid physics to construct a new model for the bond-breaking time scale of attractive sticker groups in associating copolymer liquids that form transient networks. The activated event is argued to be a two-step process, involving first the release of the nonsticker dynamic caging constraints that defines the primary alpha relaxation, followed by attractive stickers surmounting an association free-energy barrier subject to a local frictional resistance which can be strongly affected by relaxation-diffusion decoupling. The ideas embedded in the model produce a consistent and good description of the bond-breaking time scale for diverse polymer chemistries and architectures as a function of temperature and fraction of sticky groups. Chemically sensible values for association free energies are deduced. In strong contrast, the existing phenomenological models are shown to incur qualitative failures.

Synthesis and hierarchical self-assembly of luminescent platinum(ii)-containing telechelic metallopolymers

Luminescent telechelic metallopolymers functionalized with platinum(ii) complexes can self-assemble into flowerlike micelles, and the resulting flowers can further form vesicle-like architectures in solution.

Using Coupling Motion of Connecting Ions in Designing Telechelic Ionomers

Conventional telechelic ionomers have one ion fixed at each end, enabling the chains to form a physical network. Here we report a type of telechelic ionomers with a distribution of the number of ions at the chain ends, which endows them with very rich rheological properties. We synthesized the ionomer samples via a two-step polymerization. Namely, we synthesized a precursor chain first and then polymerized a few ion-containing monomers at its two ends. An average number of ion-containing monomers per chain end, m, varies from 0 to 3.0. Linear viscoelasticity of these samples can be well explained through considering the Poisson distribution of m, and the hierarchical relaxation of the chains ends according to the number of connecting ions that exhibit the coupling motion.

A Critical Approach to Polymer Dynamics in Supramolecular Polymers

Over the past few years, the concurrent (1) development of polymer synthesis and (2) introduction of new mathematical models for polymer dynamics have evolved the classical framework for polymer dynamics once established by Doi-Edwards/de Gennes. Although the analysis of supramolecular polymer dynamics based on linear rheology has improved a lot recently, there are a large number of insecurities behind the conclusions, which originate from the complexity of these novel systems. The interdependent effect of supramolecular entities (stickers) and chain dynamics can be overwhelming depending on the type and location of stickers as well as the architecture and chemistry of polymers. This Perspective illustrates these parameters and strives to determine what is still missing and has to be improved in the future works.