Heterogeneous Local Dynamics in Mussel-Inspired Elastomers

Abstract

Materials with increased mechanical strength and toughness bearing mussel-inspired iron–catechol complexes and network architectures exhibit several dynamic features, e.g., high and broad glass-“transition” temperature, not explored so far. By combining differential scanning calorimetry and dielectric spectroscopy, the latter as a function of temperature and pressure, we have explored the increase in the glass temperature, T g, and the concomitant increase in the breath of T g in bioinspired networks bearing one (covalent) or two (covalent and coordination) types of cross-links. Cross-linked networks experience heterogeneous segmental dynamics that are responsible for the broad T g range observed in the thermal measurements. The two distinct dynamics reflect the relaxation of segments in the vicinity versus more distant cross-linked units. The various topologies are shown to have different fragilities and tend to form stronger glasses with increasing network topology. In addition, the ionic conductivity is influenced by the segmental dynamics and the increased T g in the networks. These features (reduced specific heat step, Δc p, at T g and “strong” dynamic behavior) are also found in permanently cross-linked polymers with dynamic network topology known as vitrimers. The similarities in the two network systems are discussed.

Competing Effects of Molecular Additives and Cross-Link Density on the Segmental Dynamics and Mechanical Properties of Cross-Linked Polymers

“…Xia and co-workers have recently shown that increasing the strength of the cohesive interaction can lead to a significant increase in T g while at the same time leading to a significant decrease in the fragility of glass formation in the thermoset material. This competing effect has been observed recently in polymer network materials with cross-links intended to model charged physical cross-links found in mussels . These results collectively reveal another apparently general behavior of GF materials, changes in T g and fragility are related to factors altering the cohesive interaction strength of the material.…”

“…This competing effect has been observed recently in polymer network materials with cross-links intended to model charged physical cross-links found in mussels. 38 These results collectively reveal another apparently general behavior of GF materials, changes in T g and fragility are related to factors altering the cohesive interaction strength of the material.…”

Glass Formation in Ring Polymer Melts Having Variable Knot Complexity and Molecular Mass

“…Experimental studies for systems in which the cohesive interaction strength increases through the introduction of charged and polymer groups have likewise indicated a tendency of T g to increase while fragility decreases as the molecular parameter influencing the cohesive interaction strength of the fluid increases. These systems include melts of polyzwitterion materials having increasing charge density, polymer networks with strong hydrogen bonding, − weakly coordinating ionic physical cross-links, and ionomers of increasing charge density, where the decreased fragility is implied qualitatively by a significant broadening of the breadth of the glass transition. The variations of T g and fragility in the knotted ring and many-arm star polymer melts appear to follow this general pattern of behavior.…”