Dynamically Cross-Linked Polyolefin Elastomers with Highly Improved Mechanical and Thermal Performance

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.

Cationic chitosan enables eutectogels with high ionic conductivity for multifunctional applications in energy harvesting and storage

Eutectogels are popular as an emerging material in the field of flexible electronics. However, limited mechanical properties and ionic conductivity restrict their multifunctional application expansion. Herein, cationic chitosan quaternary ammonium salt (CQS) was evenly embedded into the three-dimensional porous framework of eutectogel to build ion migration channels. And a simple solvent replacement process enhanced the crystallization of polyvinyl alcohol matrix and hydrogen bonding, preparing composite eutectogels with high toughness, environmental tolerance and conductivity. The prepared gel exhibites excellent mechanical properties (1.72 MPa, 413 %) and conductivity (0.40 S·m-1). Under external force, three-dimensional porous network with cationic polysaccharide distribution can achieve effective piezoionic effect. Moderate CQS significantly enhances the piezoionic voltage output to 270 mV, which is 4.5 times that of the pure eutectogel. Further, the prepared composite eutectogels was used for capacitor energy storage and wearable sensing devices, and has good charge/discharge stability (94 % capacitance retention) and fast response time (292 ms). This design is typically suitable for preparing advanced multifunctional ion conductors using various natural polysaccharides with sustainable application potential.

Biomimetic Nanostructured Polyimine Aerogels with Graded Porosity, Flame Resistance, Intrinsic Superhydrophobicity, and Closed-Loop Recovery

Polymer aerogels, with their porous and lightweight features, excel in applications such as energy storage, absorption, and thermal insulation, making them a sought-after new material. However, the covalent cross-linking networks of current polymer aerogels result in unsustainable manufacturing and processing practices, persistently depleting our finite natural resources and causing significant global environmental impacts. Herein, we have constructed a high-performance dynamic covalent cross-linking aerogel network using biobased materials, with its structure and green sustainability akin to those of plants in nature. Abundant reversible cross-linking points endow the aerogel with ultrafast degradation capabilities, enabling allow for closed-loop chemical monomer recovery and reprocessing. Furthermore, utilizing the highly active reversible network, net-zero emission material reuse and reprocessing can be achieved. Additionally, the controlled dynamic aerogel network features a multilevel roughness nanostructured surface similar to lotus leaf and a biomimetic pore structure, contributing to significant anisotropy. The distinctive structure and composition endow the dynamic aerogel with high compressive strength (2.2 MPa) vertically, low thermal conductivity (0.0257 W/(m·K)) horizontally, and outstanding fire resistance (LOI is as high as 36%). Notably, the aerogel demonstrates the highest hydrophobicity among polyimine materials, with a contact angle of 154°. Furthermore, those dynamic aerogels have excellent performance in a variety of potential applications such as oil-water separation, directional transport, and phase change energy storage, and it is anticipated that these applications will greatly benefit from systematic upgrades in recyclability and reprocessing.

Comparative Study of Polyethylene, Polypropylene, and Polyolefins Silyl Ether-Based Vitrimers

Polyolefins (POs), which constitute over 50% of all plastics, predominantly end up in landfills. To date, there have been no reports on mixtures of PO vitrimers. This study reports the successful synthesis of PO vitrimers from a mixture of 27.7% high-density polyethylene (HDPE), 36.3% linear low-density polyethylene (LLDPE)/low-density polyethylene (LDPE), and 36.0% polypropylene (PP), which is similar to that of Municipal Solid Waste (MSW). This is achieved by using silyl ether-based chemistry, both with and without nitroxides. Additionally, these PO vitrimers are compared with individual vitrimers made of HDPE, LDPE, LLDPE, and PP, as well as vitrimers made from PE blends (comprising HDPE, LLDPE, and LDPE). All vitrimers were prepared via melt extrusion. Their cross-linking density, storage modulus, tensile properties, and reprocessability were assessed. For PO vitrimers, a storage modulus of 0.61 MPa was achieved, indicating a cross-linked network while also maintaining complete melt reprocessability. This study not only provides fundamental insights but also presents a sustainable pathway for recycling PEs and POs into useful materials, hence helping to minimize waste.

Functionalization and Repurposing of Polypropylene to a Thermoset Polyurethane

Developing effective recycling pathways for polyolefin waste, enabling a move to a circular economy, is an imperative that must be met. Postuse modification has shown promising results in upcycling polyolefins, removing the limitation of inertness, and improving the final physical properties of the recycled material while extending its useful lifetime. Grafting of maleic anhydride groups to polypropylene is an established industrial process that enhances its reactivity and provides a convenient route to further functionalization and upcycling. In this work, maleic anhydride grafted polypropylene was hydroxylated and subsequently cured with a diisocyanate to form a thermoset polyurethane (PU). The crystal structure (unit cell and lamellar structure) of the polypropylene (PP) was preserved in the PU. At room temperature, the PU showed a high modulus due to the crystallization behavior of the PP; upon increasing the temperature above the melting temperature, the modulus decreased to a rubbery plateau, consistent with formation of a network. The resulting PU showed a higher glass transition temperature and lower degree of crystallinity than its PP predecessor due to the crosslinked nature of the polymer. The mechanical integrity of the PU was maintained through several reprocessing cycles due to the melt processability enabled by the presence of a urethane exchange catalyst. This functionalization and upcycling route thus offers a promising alternative to repurposing PP waste in which the creation of melt-processable thermoset polymers expands applications for the materials.

Mechanical Property of Thermoplastic Polyurethane Vascular Stents Fabricated by Fused Filament Fabrication

Vascular stents have many applications in treating arterial stenosis and other vascular-related diseases. The ideal vascular stent for clinical application should have radial support and axial bending mechanical properties that meet the requirements of vascular deformation coordination. The materials used for vascular stents implanted in the human body should have corresponding biocompatibility to ensure that the stents do not cause coagulation, hemolysis, and other reactions in the blood. This study fabricated four types of vascular stents, including inner hexagon, arrowhead, quadrilateral, and outer hexagonal, using fused filament fabrication technology and thermoplastic polyurethane (TPU) as materials. By evaluating the effects of edge width and wall thickness on the radial support and axial bending performance, it was found that the inner hexagonal stent exhibited the best radial support and axial bending performance under the same conditions. The design and fabrication of vascular stents based on 3D printing technology have promising application prospects in personalized customized vascular repair therapy.

Mechanically Ultra-Robust Fluorescent Elastomer for Elaborating Auxetic Composite

Fluorescent elastomers are predominantly fabricated through doping fluorescent components or conjugating chromophores into polymer networks, which often involves detrimental effects on mechanical performance and also makes large-scale production difficult. Inspired by the heteroatom-rich microphase separation structures assisted by intensive hydrogen bonds in natural organisms, an ultra-robust fluorescent polyurethane elastomer is reported, which features a remarkable fracture strength of 87.2 MPa with an elongation of 1797%, exceptional toughness of 678.4 MJ m-3 and intrinsic cyan fluorescence at 445 nm. Moreover, the reversible fluorescence variation with temperature could in situ reveal the microphase separation of the elastomer in real time. By taking advantage of mechanical properties, intrinsic fluorescence and hydrogen bonds-promoted interfacial bonding ability, this fluorescent elastomer can be utilized as an auxetic skeleton for the elaboration of an integrated auxetic composite. Compared with the auxetic skeleton alone, the integrated composite shows an improved mechanical performance while maintaining auxetic deformation in a large strain below 185%, and its auxetic process can be visually detected under ultraviolet light by the fluorescence of the auxetic skeleton. The concept of introducing hydrogen-bonded heteroatom-rich microphase separation structures into polymer networks in this work provides a promising approach to developing fluorescent elastomers with exceptional mechanical properties.

Terpolymerization of Ethylene with Hexene and Styrene Derivatives by Half-Sandwich Scandium Catalyst

The terpolymerization of ethylene with hexene and styrene derivatives was achieved with a rare earth metal catalyst (C5Me4SiMe3)Sc(CH2C6H4NMe2-o)2 to prepare functional polyethylene. The catalyst system exhibited high activity in the terpolymerization of ethylene with hexene and amine-substituted styrene, affording terpolymers a moderate molecular weight and a unimodal molecular weight distribution. In addition, the comonomer content of the terpolymers can be controlled by changing the feeding ratio of monomers. The aliphatic region of the 13C NMR spectra reveals that the structural units of the comonomers are separately incorporated into the polyethylene backbone. Terpolymers containing styrene derivatives exhibit enhanced tensile strength and significantly improve hydrophilic properties.

Ultrastable, Superrobust, and Recyclable Supramolecular Polymer Networks

Supramolecular polymer networks (SPNs), crosslinked by noncovalent bonds, have emerged as reorganizable and recyclable polymeric materials with unique functionality. However, poor stability is an imperative challenge faced by SPNs, because SPNs are susceptible to heat, water, and/or solvents due to the dynamic and reversible nature of noncovalent bonds. Herein, the design of a noncovalent cooperative network (NCoN) to simultaneously stabilize and reinforce SPNs is reported, resulting in an ultrastable, superrobust, and recyclable SPN. The NCoN is constructed by multiplying the H-bonding sites and tuning the conformation/geometry of the H-bonding segment to optimize the multivalence cooperativity of H-bonds. The rationally designed H-bonding segment with high conformational compliance favors the formation of tightly packed H-bond arrays comprising higher-density and stronger H-bonds. Consequently, the H-bonded crosslinks in the NCoN display a covalent crosslinking effect but retain on-demand dynamics and reversibility. The resultant ultrastable SPN not only displays remarkable resistance to heat up to 120 °C, water soaking, and a broad spectrum of solvents, but also possesses a superhigh true stress at break (1.1 GPa) and an ultrahigh toughness (406 MJ m-3 ). Despite the covalent-network-like stability, the SPN is recyclable through activating its reversibility in a high-polarity solvent heated to a threshold temperature.

Achieving high molecular weight alternating copolymers of 1-octene with methyl acrylate via Lewis acid catalyzed copolymerization

The radical (co)polymerization of long-chain α-olefins (C4+) to produce high molecular weight (Mw) polymers is of great importance. However, this process is currently faced with significant challenges due to the presence of less reactive allylic radicals during radical (co)polymerization, leading to oligomers or polymers with extremely low Mw (less than 1 × 104 g mol-1). Using copolymerization of 1-octene with methyl acrylate (MA) as a proof-of-concept for addressing this challenge, we present a feasible method for synthesizing high Mw α-olefin copolymers via scandium trifluoromethanesulfonate (Sc(OTf)3)-mediated radical copolymerization. In this case, copolymers of 1-octene and MA (poly(1-octene-alt-MA)) with a Mw exceeding 3 × 104 g mol-1 were successfully synthesized in the presence of Sc(OTf)3. Meanwhile, the presence of alternating 1-octene-MA sequential structures was observed. To further enhance the Mw of poly(1-octene-alt-MA), a difunctional comonomer, 1,7-octadiene, was introduced to copolymerize with 1-octene and MA. The results indicate that the incorporation of difunctional comonomer leads to a significant increase in the Mw of the copolymers synthesized. The addition of 1 mol% of 1,7-octadiene resulted in a copolymer with a remarkably high Mw of up to 13.45 × 104 g mol-1 while still maintaining a high degree of the alternating 1-octene-MA sequence (41%). The influence of polymerization parameters on the molecular weight were also investigated. Increasing the monomer concentration, reducing the dosage of initiator, and lowering the polymerization temperature have been found to be advantageous in enhancing the molecular weight. This approach has also been successfully applied to the synthesis of high molecular weight alternating copolymers of other long-chain α-olefins, including 1-hexene, 1-decene and 1-tetradecane, with methyl acrylate. In summary, this study provides a feasible method for converting "less activated" α-olefins into high Mw olefin copolymers. This approach holds significant potential for the production of value-added polyolefins, thus offering promising prospects for future applications.

Vitrimeric Polylactide by Two-step Alcoholysis and Transesterification during Reactive Processing for Enhanced Melt Strength

Because of its rather low melt strength, polylactide (PLA) has yet to fulfill its promise as advanced biobased and biodegradable foams to replace fossil-based polymer foams. In this work, PLA vitrimers were prepared by two-step reactive processing from commercial PLA thermoplastics, glycerol, and diphenylmethane diisocyanate (MDI) using Zn(II)-catalyzed addition and transesterification chemistry. The transesterification reaction of PLA and glycerol occurs with zinc acetate as the catalyst, and chain scission will take place due to the alcoholysis of the PLA chains by the free hydroxyl groups from the glycerol. Long-chain PLA with hydroxyl groups can be obtained and then cross-linked with MDI. Rheological analysis shows that the formed cross-linked network can significantly improve melt strength and promote strain hardening under extensional flow. PLA vitrimers still maintain the ability of thermoplastic processing via extrusion and compression. The enhanced melt strength and the rearrangement of network topology facilitate the foaming processing. An expansion ratio as large as 49.2-fold and microcellular foam with a uniform cell morphology can be obtained for PLA vitrimers with a gel fraction of 51.8% through a supercritical carbon dioxide foaming technique. This work provides a new way with the scale-up possibility to enhance the melt strength of PLA, and the broadened range of PLA applicability brought by PLA vitrimers is truly valuable in terms of the realization of a sustainable society.

Molecularly Defined Polyolefin Vitrimers from Catalytic Insertion Polymerization

Vitrimers can combine the advantageous properties of cross-linked materials with thermoplastic processability. For the prominent case of polyethylene, established post-polymerization introduction of cross-linkable moieties results in extremely heterogeneous compositions of the chains. Here, we report the generation of functionalized polyethylenes directly by catalytic insertion polymerization, with incorporated cross-linkable aryl boronic esters or alternatively acetal-protected groups suited for cross-linking with difunctional boronic esters. In addition to the desired homogeneous in-chain distribution, the reactive cross-linkable groups are enriched at the chain ends. This enables the incorporation of all chains in the network, as also supported by simulations of all chains' compositions. The uniform molecular composition of the chains reflects in resulting vitrimers' material properties, particularly lack of leaching with solvents. At the same time, cross-linking is indeed fully reversible and the vitrimers can be recycled.