Networks assembled by reversible association of telechelic polymers constitute a common class of soft materials. Various mechanisms of chain migration in associative networks have been proposed, yet there remains little quantitative experimental data to discriminate among them. Proposed mechanisms for chain migration include multichain aggregate diffusion as well as singlechain mechanisms such as "walking" and "hopping", wherein diffusion is achieved by either partial ("walking") or complete ("hopping") disengagement of the associated chain segments. Here we provide evidence that hopping can dominate the effective diffusion of chains in associative networks due to a strong entropic penalty for bridge formation imposed by local network structure; chains become conformationally restricted upon association with two or more spatially separated binding sites. This restriction decreases the effective binding strength of chains with multiple associative domains, thereby increasing the probability that a chain will hop. For telechelic chains this manifests as binding asymmetry, wherein the first association is effectively stronger than the second. We derive a simple thermodynamic model that predicts the fraction of chains that are free to hop as a function of tunable molecular and network properties. A large set of self-diffusivity measurements on a series of model associative polymers finds good agreement with this model.
Coiled-coil domains can direct the assembly of protein block copolymers into physically crosslinked, viscoelastic hydrogels. Here we describe the use of fluorescence recovery after photobleaching (FRAP) to probe chain mobility in reversible hydrogels assembled from engineered proteins bearing terminal coiled-coil domains. We show that chain mobility can be related to the underlying dynamics of the coiled-coil domains by application of a 3-state "hopping" model of chain migration. We further show that genetic programming allows the effective mobility of network chains to be varied 500-fold through modest changes in protein sequence. Destabilization of the coiled-coil domains by site-directed mutagenesis increases the effective diffusivity of probe chains. Conversely, probe mobility is reduced by expanding the hydrophobic surface area of the coiled-coil domains through introduction of the bulky leucine surrogate homoisoleucine. Predictions from the 3-state model imply asymmetric sequential binding of the terminal domains. Brownian Dynamics simulations suggest that binding asymmetry is a general feature of reversible gels, arising from a loss in entropy as chains transition to a conformationally restricted bridged state.
We report a new catalytic method for alcohol sulfamoylation that deploys electron-deficient aryl sulfamates as activated group transfer reagents. The reaction utilizes the simple organic base N-methylimidazole, proceeds under mild conditions, and provides intrinsic selectivity for 1° over 2° alcohols (up to >40:1 for certain nucleosides). The requisite aryl sulfamate donors are stable crystalline solids that can be readily prepared on a large scale. Mechanistic considerations support the intermediacy of HNSO2 “aza-sulfene” in the transfer reaction.
To study the effect of the addition of calcium chloride on portland cements of widely varying composition, and on portland-cement concretes, a carefully controlled laboratory investigation was conducted. Such measurements as heats of hydration, setting times, consistencies and the strengths at various ages under several curing temperatures were made. The addition of calcium chloride appears to increase somewhat the heat contributed at the end of 24 hours by dicalcium silicate and tetracalcium aluminoferrite and to decrease the heat contributed by tricalcium aluminate. Calcium chloride has but little effect on the heat contributed by tricalcium silicate. Calcium chloride increases the rate at which the heat is evolved from all cements. The strength contributed by the two silicate compounds in cement was increased and the strength contributed by tricalcium aluminate was decreased by the addition of calcium chloride. Calcium chloride decreased the time of set of 11 commercial cements and increased the flow and the strength of the resulting concrete. The physical properties of the experimental cements were similarly affected by the addition of calcium chloride. The optimum amounts to be added for different types of cement and at various curing temperatures are indicated.
Viscoelastic forces can affect the dynamics of pattern formation during phase separation in polymeric materials. We programmed an artificial protein hydrogel to undergo viscoelastic phase separation above a critical temperature. Highly dynamic phase patterns that coarsened under the influence of viscoelastic
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