Vitrimers form a promising class of dynamic polymer networks, but they have an Achilles’ heel: elastomeric vitrimers exhibit significant creep under conditions where permanently cross-linked, elastomeric networks exhibit little or no creep. We demonstrate that vitrimers can be designed with strongly suppressed creep and excellent reprocessability by incorporating a substantial yet subcritical fraction of permanent cross-links. This critical fraction of permanent cross-links, which has little or no detrimental effect on reprocessability, is defined by the gelation point of only permanent cross-links leading to a percolated permanent network. Via a modification of classic Flory–Stockmayer theory, we have developed a simple theory that quantitatively predicts an approximate limiting fraction. To test our theory, we designed vitrimers with controlled fractions of permanent cross-links based on thiol–epoxy click chemistry. We characterized the rubbery plateau modulus before and after reprocessing as well as stress relaxation of our original vitrimers. Our experimental results strongly support our theoretical prediction: as long as the fraction of permanent cross-links is insufficient to form a percolated permanent network, the vitrimer can be reprocessed with full recovery of cross-link density. In particular, with a predicted limiting fraction of 50 mol %, a vitrimer system designed with 40 mol % permanent cross-links achieved full property recovery associated with cross-link density after reprocessing as well as 65–71% creep reduction (for both original and reprocessed samples) relative to a similar vitrimer without permanent cross-links. In contrast, a system with 60 mol % permanent cross-links could not be reprocessed into a well-consolidated sample, nor did it recover full cross-link density; it failed by breaking at early stages of creep tests. The ability to predict an approximate limiting fraction of permanent cross-links leading to enhanced creep resistance and full reprocessability represents an important advance in the science and design of vitrimers.

We developed reprocessable polyhydroxyurethane (PHU) networks with full property recovery and incorporating both associative and dissociative dynamic chemistry.

Polythiourethane (PTU) can be synthesized by a type of click chemistry involving the reaction of thiols with isocyanates. To our knowledge, thiourethane dynamic chemistry has not been significantly explored from a fundamental standpoint and has only begun to be studied regarding its use in producing recyclable, reprocessable substitutes for traditional cross-linked polyurethane networks. Using model compounds, we demonstrated the dual nature of the mechanism associated with catalyzed thiourethane dynamic chemistry: at elevated temperature, thiourethane groups undergo exchange reactions with free thiol groups and thermal reversion to thiols and isocyanates. We used this chemistry to synthesize cross-linked PTU elastomers which, upon optimization, achieve full recovery of cross-link density and tensile properties after multiple, relatively rapid remolding cycles. We characterized stress relaxation as a function of temperature and stoichiometric imbalance to provide insight into the mechanism of the network structure change. A small level (10 mol %) of excess thiol groups reduces reversion, thereby suppressing undesired side reactions during reprocessing and promoting thiol–thiourethane exchange reactions, leading to excellent property recovery after multiple recycling steps. With a proof-of-concept demonstration, we also revealed the potential of recovering thiol monomer by solvolysis from PTU networks, which provides a second route for sustainable recycling. In addition to introducing thiourethane dynamic chemistry as a simple way to achieve high-value recyclability of polyurethane-type networks by reprocessing and/or monomer recovery, our study shows that tuning of the reaction stoichiometry may be a facile approach to optimize property recovery after reprocessing for some dynamic networks that exhibit property loss when at stoichiometric balance.

A nitroxide-mediated polymerization strategy allows one-step synthesis of recyclable crosslinked polymeric materials from any monomers or polymers that contain carbon-carbon double bonds amenable to radical polymerization. The resulting materials with dynamic covalent bonds can show full property recovery after multiple melt-reprocessing recycles. This one-step strategy provides for both robust, relatively sustainable recyclability of crosslinked polymers and design of networks for advanced technologies.

Conventional polymer network composites cannot be recycled for high-value applications because of the presence of permanent covalent cross-links. We have developed reprocessable polyhydroxyurethane network nanocomposites using silica nanoparticles with different surface functionalities as reinforcing fillers. The property recovery after reprocessing is a function of the interaction between the filler surface and the network matrix during the network rearrangement process. When nonreactive silica nanoparticles lacking significant levels of surface functional groups are used at 4 wt % (2 vol %) loading, the resulting network composite exhibits substantial enhancement in mechanical properties relative to the neat network and based on values of rubbery plateau modulus is able to fully recover its cross-link density after a reprocessing step. When nanoparticles have surface functional groups that can participate in dynamic chemistries with the reprocessable network matrix, reprocessing leads to losses in mechanical properties associated with cross-link density at potential use temperatures, along with faster rates and lower apparent activation energies of stress relaxation at elevated temperature. This work reveals the importance of appropriate filler selection when polymer network composites are designed with dynamic covalent bonds to achieve both mechanical reinforcement and excellent reprocessability, which are needed for the development of recyclable polymer network composites for advanced applications.

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.

Simultaneous manipulation of topological and chemical structures to induce ionic nanochannel formation within solid electrolytes is a crucial but challenging task for the rational design of high-performance electrochemical devices including proton exchange membrane fuel cell. Herein, a novel generic approach is presented for the construction of tunable ion-conducting nanochannels via direct assembly of graphene oxide (GO)/poly(phosphonic acid) core-shell nanosheets prepared by surface-initiated precipitation polymerization. Using this simple and rapid approach to engineer GO/ polymer nanosheets at the molecular-level, ordered and continuous nanochannels with interconnected hydrogen-bonded networks having a favorable water environment can be created. The resulting membranes exhibit proton conductivities up to 32 mS cm −1 at 51% relative humidity, surpassing stateof-the-art Nafi on membrane and all previously reported GO-based materials.

Polymer-tethered nanoparticles provide a strategy to improve particle dispersion in polymer nanocomposites and as materials themselves can exhibit self-healing behavior and enhanced mechanical properties. The few studies that previously characterized the glass transition temperature (T g) behavior of neat polymer-grafted nanoparticles in the absence of a polymer matrix largely focused on average T g response. We synthesized polystyrene-grafted silica nanoparticles (Si-PS) via ARGET ATRP, achieving the densely grafted state. Using differential scanning calorimetry, we investigated the brush molecular weight (MW) dependence of T g, T g breadth, heat capacity jump (ΔC p ), and fragility from 12 to 98 kg/mol. Compared with free PS chains of the same MW, brush T g increases by 1–2 °C, brush T g breadth remains unchanged within error down to 36 kg/mol and increases by 3–4 °C at brush MWs of 12 and 13 kg/mol, and brush ΔC p and fragility remain unchanged within error down to 52 kg/mol and then decrease with decreasing MW. Evidence of a significant T g gradient from near the nanoparticle graft interface to near the free chain end was obtained for the first time via fluorescence of a pyrenyl dye labeled at specific regions along the brush chain length. In relatively high MW brushes, T g = ∼116 °C near the graft interface and T g = ∼102 °C near the chain end. Comparisons are made to results recently reported for similar PS brushes densely grafted to a flat substrate, which indicate that a larger T g gradient is evident in a grafting geometry involving a flat interface as compared with a spherical nanoparticle interface. Other comparisons are also made with glass transition and fragility behaviors reported in the flat substrate geometry. Results of this study and others will help to better understand nanocomposites and tailor them for optimal properties.