Arresting Elevated-Temperature Creep and Achieving Full Cross-Link Density Recovery in Reprocessable Polymer Networks and Network Composites via Nitroxide-Mediated Dynamic Chemistry

Abstract

Thermosets and thermoset composites constitute an extraordinary challenge for recycling and participation in a circular economy because their permanent covalent cross-links prevent spent thermosets from being melt processed into new products. With annual world-wide production in the tens of billions of kilograms, the inability to recycle thermosets into high-value products represents major economic and sustainability losses. While recent research into polymer networks with dynamic covalent cross-links has indicated promise for reprocessability at common melt-state processing temperatures, a crucial shortcoming has been identified: such reprocessable networks and network composites commonly exhibit creep at use conditions due to their dynamic nature, which may prevent their use in applications that require long-term dimensional stability. Here, we use a strategy based on nitroxide-mediated polymerization to synthesize reprocessable networks and network composites containing alkoxyamine dynamic bonds. The resulting networks, including those synthesized from lab-grade polybutadiene and industrial-grade natural rubber/carbon black composites, exhibit full cross-link density recovery and essentially no creep at 80 °C, where alkoxyamine cross-links are nearly static, after multiple molding cycles at 140/160 °C, where alkoxyamine cross-links are dynamic. This capability to "turn on" and "arrest" dynamic chemistry over a relatively narrow temperature window is attributed to the high activation energy (∼120 kJ/mol) and thus strong temperature dependence of the alkoxyamine dissociation reaction. With this key element of high activation energy for the dissociation reaction in systems undergoing dynamic reversion or the dynamic exchange reaction in vitrimers, it is possible to design covalent network materials with acute temperature response allowing for reprocessability with outstanding elevatedtemperature creep resistance.

Covalent Adaptable Networks Using β-Amino Esters as Thermally Reversible Building Blocks

“…This is in line with reported studies, which showed that a synergetic effect is typically observed when multiple dynamic chemistries are implemented in a material. , Besides, when stress relaxation experiments were performed at different temperatures for the obtained materials, a high temperature dependence was observed, resulting in a flow activation energy ( E a(flow) ) of 110–240 kJ mol –1 (Table ). Such high temperature dependent dynamic behavior favors the creation of dynamic, yet creep resistant elastomers, as very recently reported with addition-type reprocessable elastomers …”

Reprocessing of Covalent Adaptable Polyamide Networks through Internal Catalysis and Ring-Size Effects

“…It is known that a higher temperature dependency should reduce irreversible deformation or creep at use temperatures. 34,35,71 While hardly any creep behavior could be observed up to 70 °C, a slight increase in creep rate could be determined starting from 80 °C. On the other hand, when observing the creep rate as a function of temperature in Figure S16c, a reduction in deformation could be found for PA-3 compared to PA-2, which was in accordance with stressrelaxation data.…”

“…In order to probe the long-term reactivity of these PA networks at lower temperatures, creep experiments without recovery were conducted by applying 2 kPa shear stress for 5000 s between 40 and 90 °C on PA-2 and PA-3 (Figure S16). It is known that a higher temperature dependency should reduce irreversible deformation or creep at use temperatures. ,, While hardly any creep behavior could be observed up to 70 °C, a slight increase in creep rate could be determined starting from 80 °C. On the other hand, when observing the creep rate as a function of temperature in Figure S16c, a reduction in deformation could be found for PA-3 compared to PA-2 , which was in accordance with stress-relaxation data.…”

“…Indeed, despite the significant progress in the development of catalyst-free CANs, their susceptibility to permanent deformation at service temperature significantly limits their application potential . As a consequence, polymer chemists have redirected their efforts toward the development of creep-resistant CANs where dissociation/exchange is governed by a high E a process. , To fulfill all the above-mentioned requirements, inert and diverse dynamic exchange systems, showing a sharp transition from the elastic to viscous state, are highly desirable.…”

“…33 As a consequence, polymer chemists have redirected their efforts toward the development of creepresistant CANs where dissociation/exchange is governed by a high E a process. 34,35 To fulfill all the above-mentioned requirements, inert and diverse dynamic exchange systems, showing a sharp transition from the elastic to viscous state, are highly desirable.…”