Elastomeric polyolefin vitrimer: Dynamic imine bond cross-linked ethylene/propylene copolymer

Composition-structure-property relationships of polyethylene vitrimers crosslinked by 8-arm polyhedral oligomeric silsesquioxane

Transforming polyolefins (POs), such as polyethylene (PE), into vitrimers is a promising research field due to their low cost, high availability, and excellent chemical resistance and mechanical properties. In these systems, the introduction of dynamic crosslinking can affect the degree of crystallinity in POs and may lead to phase separation due to incompatibility between the PO matrix and crosslinking agents, both of which can impact mechanical performance. This study investigates the relationship between crystallinity, crosslinking, and thermal-mechanical properties in commodity PE-derived vitrimers utilizing reactive 8-arm polyhedral oligomeric silsesquioxane (POSS) nanoparticles by deconvoluting the crosslinked and non-crosslinked components. Specifically, the insoluble crosslinked components displayed a lower modulus and increased brittleness, while the non-crosslinked phase performed similarly to neat PE. Together, the PE-vitrimer, crosslinked with 8-arm POSS, exhibited reduced toughness, elongation at break, and a slight increase in ultimate tensile strength. These behaviors were consistent when comparing the crosslinking density and gel fraction with a bifunctional crosslinker analogue. This work demonstrates the influence of multi-arm, nanoparticle-based crosslinker content on the mechanical properties of semi-crystalline PO-vitrimers, elucidating the roles of network density and crystallinity in determining their performance.

Bio-Based and Solvent-Free Epoxy Vitrimers Based on Dynamic Imine Bonds with High Mechanical Performance

Conventional epoxy thermosets, with irreversible crosslinking networks, cannot be reprocessed and recycled. Furthermore, the utilization of petroleum-based materials accelerates the depletion of non-renewable resources. The introduction of dynamic covalent bonds and the use of bio-based materials for thermosets can effectively address the above issues. Herein, a series of bio-based epoxy vitrimers with dynamic covalent imine bonds were synthesized via a simple solvent-free, one-pot method using vanillin-derived aldehyde monomers, 4,4-diaminodiphenylsulfone (DDS) and bisphenol F diglycidyl ether (BFDGE) as raw materials. The effect of crosslinking density, crosslinking structure and imine bond content on the resulting bio-based vitrimers was studied, demonstrating their excellent thermal properties, UV shielding and solvent resistance, as well as outstanding mechanical properties compared to those of the previously reported vitrimers. In particular, the cured neat resin of vitrimer had a maximum tensile strength of 109 MPa and Young's modulus of 6257 MPa, which are higher than those of previously reported imine-based vitrimers. The dynamic imine bonds endow these vitrimers with good reprocessability upon heating (over 70% recovery) and degradation under acidic conditions, enabling recycling by physical routes and gentle degradation by chemical routes. This study demonstrates a simple and effective process to prepare high-performance bio-based and recycled epoxy thermosets.

Circular Cross-Linked Polyethylene Enabled by In-Chain Ketones

Cross-linked polyethylenes (PEs) are widely employed, but the permanent links between the chains impede recycling. We show that via imine formation with diamines keto-functionalized polyethylenes from both free-radical (keto-low-density PE, keto-LDPE) and catalytic (keto-high-density PE, keto-HDPE) nonalternating ethylene-CO copolymerization can be cross-linked efficiently in the melt, resulting in gel fractions of the formed cross-linked PEs of up to 85% and improved tensile properties. The imine-based cross-links in the material can be hydrolyzed at 140 °C to recycle up to 97% of the initial thermoplastic keto-polyethylene. Low keto contents of ≤1.5 mol % are found ideal to retain PE-like thermal properties, achieve sufficient cross-link density, and maintain circular recyclability.

Cross-Linked Polyolefins: Opportunities for Fostering Circularity Throughout the Materials Lifecycle

Cross-linked polyolefins (XLPOs) constitute a significant portion of the plastics commercial market, with a market size of a similar order of magnitude to those of polystyrene and polyethylene terephthalate. However, few aspects of XLPO materials circularity have been examined relative to thermoplastic polyolefins. The cross-linking of polyolefins imparts superior performance properties, such as impact strength, chemical and electrical resistance, and thermal stability vs thermoplastic analogues, but it also makes the reprocessing of XLPOs to valuable products more challenging, as XLPOs cannot be molten. Thus, most XLPOs are incinerated or landfilled at the end of the first lifecycle, even though XLPO products are commonly collected as a relatively clean waste stream-providing a unique opportunity for valorization. In this review, we discuss approaches to improve XLPO circularity throughout the entire materials lifecycle by examining biobased feedstocks as alternative olefinic monomer sources and by assessing both traditional mechanical and advanced XLPO recycling methods based on industrial feasibility and potential product value. We also consider how advancing materials longevity can reduce environmental impacts and lifecycle costs and how recyclable-by-design strategies can enable better end-of-life opportunities for future generations of XLPO materials. Throughout this review, we highlight XLPO circularity routes that have the potential to balance the performance, circularity, and scalability necessary to impart economic and environmental viability at an industrial scale.

Circularity in polymers: addressing performance and sustainability challenges using dynamic covalent chemistries

The circularity of current and future polymeric materials is a major focus of fundamental and applied research, as undesirable end-of-life outcomes and waste accumulation are global problems that impact our society. The recycling or repurposing of thermoplastics and thermosets is an attractive solution to these issues, yet both options are encumbered by poor property retention upon reuse, along with heterogeneities in common waste streams that limit property optimization. Dynamic covalent chemistry, when applied to polymeric materials, enables the targeted design of reversible bonds that can be tailored to specific reprocessing conditions to help address conventional recycling challenges. In this review, we highlight the key features of several dynamic covalent chemistries that can promote closed-loop recyclability and we discuss recent synthetic progress towards incorporating these chemistries into new polymers and existing commodity plastics. Next, we outline how dynamic covalent bonds and polymer network structure influence thermomechanical properties related to application and recyclability, with a focus on predictive physical models that describe network rearrangement. Finally, we examine the potential economic and environmental impacts of dynamic covalent polymeric materials in closed-loop processing using elements derived from techno-economic analysis and life-cycle assessment, including minimum selling prices and greenhouse gas emissions. Throughout each section, we discuss interdisciplinary obstacles that hinder the widespread adoption of dynamic polymers and present opportunities and new directions toward the realization of circularity in polymeric materials.

Fused Filament Fabrication of a Dynamically Crosslinked Network Derived from Commodity Thermoplastics

A massive carbon footprint is associated with the ubiquitous use of plastics and their afterlife. Greenhouse gas (GHG) emissions from plastics are rising and increasingly consuming the global "carbon budget". It is, hence, paramount to implement an effective strategy to reclaim postconsumer plastic as feedstock for technologically innovative materials. Credible opportunity is offered by advances in materials chemistry and catalysis. Here, we demonstrate that by dynamically crosslinking thermoplastic polyolefins, commodity plastics can be upcycled into technically superior and economically competitive materials. A broadly applicable crosslinking strategy has been applied to polymers containing solely carbon-carbon and carbon-hydrogen bonds, initially by maleic anhydride functionalization, followed by epoxy-anhydride curing. These dynamic networks show a distinct rubber modulus above the melting transition. We demonstrate that sustainability and performance do not have to be mutually exclusive. The dynamic network can be extruded into a continuous filament to be in three-dimensional (3D) printing of complex objects, which retain the mechanical integrity of vitrimers. Being covalently crosslinked, these networks show a thermally triggered shape-memory response, with 90% recovery of a programmed shape. This study opens up the possibility of reclaiming recycled thermoplastics by imparting performance, sustainability, and technological advances to the reprocessed plastic.