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Small-angle neutron scattering experiments revealed elongation-induced demixing in polymer blends. Such concentration fluctuations can enhance or reduce the local segmental friction and thereby affect the rheological behavior. 

The linear viscoelastic response, LVR, of a hydrated polyelectrolyte complex coacervate, PEC, was evaluated over a range of frequencies, temperatures, and salt concentrations. The PEC was a nearly-stoichiometric blend of a quaternary ammonium poly([3-(methacrylamido)propyl]trimethylammonium chloride), PMAPTAC, and poly(2-acrylamido-2-methyl-1-propanesulfonic acid sodium salt), PAMPS, an aliphatic sulfonate, selected because they remain fully charged over the conditions of use. Narrow molecular weight distribution polyelectrolytes were prepared using fractionation techniques. A partially deuterated version of PMAPTAC was incorporated to determine the coil radius of gyration, Rg, within PECs using small angle neutron scattering. Chain dimensions were determined to be Gaussian with a Kuhn length of 2.37 nm, which remained constant from 25 to 65 0C. The LVR for a series of matched molecular weight PECs, mostly above the entanglement threshold, exhibited crossovers of modulus versus frequency classically attributed to the reptation time, relaxation between entanglements, and the relaxation of a Kuhn length of units (the “monomer” time). The scaling for zero shear viscosity, η0, versus chain length N, was η0 ~ N3.1, in agreement with “sticky reptation” theory. The lifetime and activation energy, Ep, of a pair between polyanion and polycation repeat units, Pol+Pol-, were determined from diffusion coefficients of salt ions within the PEC. The activation energy for LVR of salt-free PECs was 2Ep, showing that the key mechanism limiting the dynamics of undoped PECs is pair exchange. An FTIR technique was used to distinguish whether SCN- acts as a counterion or a co-ion within PECs. Doping of PECs with NaSCN breaks Pol+Pol- pairing efficiently, which decreases effective crosslinking and decreases viscosity. An equation was derived that quantitatively predicts this effect. 

Creating tissue and organ equivalents with intricate architectures and multiscale functional feature sizes is the first step toward the reconstruction of transplantable human tissues and organs. Existing embedded ink writing approaches are limited by achievable feature sizes ranging from hundreds of microns to tens of millimeters, which hinders their ability to accurately duplicate structures found in various human tissues and organs. In this study, a multiscale embedded printing (MSEP) strategy is developed, in which a stimuli-responsive yield-stress fluid is applied to facilitate the printing process. A dynamic layer height control method is developed to print the cornea with a smooth surface on the order of microns, which can effectively overcome the layered morphology in conventional extrusion-based three-dimensional bioprinting methods. Since the support bath is sensitive to temperature change, it can be easily removed after printing by tuning the ambient temperature, which facilitates the fabrication of human eyeballs with optic nerves and aortic heart valves with overhanging leaflets on the order of a few millimeters. The thermosensitivity of the support bath also enables the reconstruction of the full-scale human heart on the order of tens of centimeters by on-demand adding support bath materials during printing. The proposed MSEP demonstrates broader printable functional feature sizes ranging from microns to centimeters, providing a viable and reliable technical solution for tissue and organ printing in the future. 

Interactions between molecules in the synovial fluid and the cartilage surface may play a vital role in the formation of adsorbed films that contribute to the low friction of cartilage boundary lubrication. Osteoarthritis (OA) is the most common degenerative joint disease. Previous studies have shown that in OA-diseased joints, hyaluronan (HA) not only breaks down resulting in a much lower molecular weight (MW), but also its concentration is reduced ten times. Here, we have investigated the structural changes of lipid-HA complexes as a function of HA concentration and MW to simulate the physiologically relevant conditions that exist in healthy and diseased joints. Small angle neutron scattering and dynamic light scattering were used to determine the structure of HA-lipid vesicles in bulk solution, while a combination of atomic force microscopy and quartz crystal microbalance was applied to study their assembly on a gold surface. We infer a significant influence of both MW and HA concentrations on the structure of HA-lipid complexes in bulk and assembled on a gold surface. Our results suggest that low MW HA cannot form an amorphous layer on the gold surface, which is expected to negatively impact the mechanical integrity and longevity of the boundary layer and could contribute to the increased wear of the cartilage that has been reported in joints diseased with OA. 

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.