Structure and rheology of dual-associative protein hydrogels under nonlinear shear flow.

Dual-associative protein di- and triblock copolymers composed of sticker-decorated midblocks and micelle-forming elastin-like polypeptide (ELP) endblocks form shear-thinning, thermoresponsively reinforceable hydrogels that are potentially useful as injectable materials for a variety of applications. Here, the combination of rheological and in situ scattering measurements under shear on these dual-associative gels is employed in order to better understand how block architecture plays a role in controlling microscopic structural rearrangement and the resulting macroscopic mechanical responses. These gels, which form a disordered sphere phase due to endblock aggregation under quiescent conditions with the midblock domains physically crosslinked by protein associations, exhibit both viscoelastic and thixotropic signatures with relative magnitudes dependent upon gel concentration and block architecture. In situ SAXS measurements during flow indicate that these thixotropic responses correspond to the development of ordered domains following start-up of shear. For both architectures, the rate of alignment increases with increasing concentration. However, the rate of domain formation when increasing the temperature from 35 to 50 °C depends on the interplay between thermoresponsive toughening of the endblocks and softening of the coiled-coil domains such that rate of rearrangement decreases in the triblock while it increases in the diblock. Following a step-down in shear flow, structural rearrangement within the samples results in a thixotropic stress response. Upon cessation of flow, gel recovery is characterized by a concentration-dependent restoration of the micellar network over time, with two timescales observed that correspond to two different length scales of network relaxation.

Mechanical response of transient telechelic networks with many-part stickers.

A central question in soft matter is understanding how several individual, weak bonds act together to produce collective interactions. Here, gel-forming telechelic polymers with multiple stickers at each chain end are studied through Brownian dynamics simulations to understand how collective interaction of the bonds affects mechanical response of the gels. These polymers are modeled as finitely extensible dumbbells using an explicit tau-leap algorithm and the binding energy of these associations was kept constant regardless of the number of stickers. The addition of multiple bonds to the associating ends of telechelic polymers increases or decreases the network relaxation time depending on the relative kinetics of association but increases both shear stress and extensional viscosity. The relationship between the rate of association and the Rouse time of dangling chains results in two different regimes for the equilibrium stress relaxation of associating physical networks. In case I, a dissociated dangling chain is able to fully relax before re-associating to the network, resulting in two characteristic relaxation times and a non-monotonic terminal relaxation time with increasing number of bonds per polymer endgroup. In case II, the dissociated dangling chain is only able to relax a fraction of the way before it re-attaches to the network, and increasing the number of bonds per endgroup monotonically increases the terminal relaxation time. In flow, increasing the number of stickers increases the steady-state shear and extensional viscosities even though the overall bond kinetics and equilibrium constant remain unchanged. Increased dissipation in the simulations is primarily due to higher average chain extension with increasing bond number. These results indicate that toughness and dissipation in physically associating networks can both be increased by breaking single, strong bonds into smaller components.

Classical Challenges in the Physical Chemistry of Polymer Networks and the Design of New Materials.

Polymer networks are widely used from commodity to biomedical materials. The space-spanning, net-like structure gives polymer networks their advantageous mechanical and dynamic properties, the most essential factor that governs their responses to external electrical, thermal, and chemical stimuli. Despite the ubiquity of applications and a century of active research on these materials, the way that chemistry and processing interact to yield the final structure and the material properties of polymer networks is not fully understood, which leads to a number of classical challenges in the physical chemistry of gels. Fundamentally, it is not yet possible to quantitatively predict the mechanical response of a polymer network based on its chemical design, limiting our ability to understand and characterize the nanostructure of gels and rationally design new materials. In this Account, we summarize our recent theoretical and experimental approaches to study the physical chemistry of polymer networks. First, our understanding of the impact of molecular defects on topology and elasticity of polymer networks is discussed. By systematically incorporating the effects of different orders of loop structure, we develop a kinetic graph theory and real elastic network theory that bridge the chemical design, the network topology, and the mechanical properties of the gel. These theories show good agreement with the recent experimental data without any fitting parameters. Next, associative polymer gel dynamics is discussed, focusing on our evolving understanding of the effect of transient bonds on the mechanical response. Using forced Rayleigh scattering (FRS), we are able to probe diffusivity across a wide range of length and time scales in gels. A superdiffusive region is observed in different associative network systems, which can be captured by a two-state kinetic model. Further, the effects of the architecture and chemistry of polymer chains on gel nanostructure are studied. By incorporating shear-thinning coiled-coil protein motifs into the midblock of a micelle-forming block copolymer, we are able to responsively adjust the gel toughness through controlling the nanostructure. Finally, we review the development of novel application-oriented materials that emerge from our enhanced understanding of gel physical chemistry, including injectable gel hemostats designed to treat internal wounds and engineered nucleoporin-like polypeptide (NLP) hydrogels that act as biologically selective filters. We believe that the fundamental physical chemistry questions articulated in this Account will provide inspiration to fully understand the design of polymer networks, a group of mysterious yet critically important materials.

Celebrating Soft Matter's 10th Anniversary: chain configuration and rate-dependent mechanical properties in transient networks.

Numerical solution of a coupled set of Smoluchowski convection-diffusion equations of associating polymers modelled as finitely extensible dumbbells enables computation of time-dependent end-to-end distributions for bridged, dangling, and looped chains in three dimensions as a function of associating end-group kinetics. Non-monotonic flow curves which can lead to flow instabilities during shear flow result at low equilibrium constant and high association rate from two complementary phenomena: a decrease in the fraction of elastically active chains with increasing shear rate and non-monotonic extension in the population of elastically active chains. Chain tumbling leads to reformation of bridges, resulting in an increased fraction of bridged chains at high Deborah number and significant reduction in the average bridge chain extension. In the start-up of steady shear, force-activated chain dissociation and chain tumbling cause both stress overshoot and stress ringing behaviour prior to reaching steady state stress values. During stress relaxation following steady shear, chain kinetics and extension mediate both the number of relaxations and the length of time required for system relaxation. While at low association rate relaxation is limited by the relaxation of dangling chains and the rate of dangling chain formation, at high association rate coupling of dangling and bridged chains leads to simultaneous relaxation of all chains due to a dynamic equilibrium between dangling and bridged states.

Low-power upconversion in dye-doped polymer nanoparticles.

Examples of nanoscale low-power upconverting systems are rapidly increasing because of their potential application in numerous areas such as bioimaging or drug delivery. The fabrication of dye-doped cross-linked rubbery nanoparticles that exhibit upconversion even at relatively low power densities is reported here. The nanoparticles were prepared by surfactant-free emulsion polymerization of n-butylacrylate with divinylbenzene as a cross-linker, followed by dyeing of the resulting particles with a two-chromophore system composed of a palladium porphyrin sensitizer, and diphenylanthracene. Blue emission (≈440 nm) of these systems was observed upon excitation at 532 nm. In addition to their optical properties, the particles were characterized by electron microscopy and dynamic light scattering.