Session T26: Physics-Informed Design of Recycled, Upcycled, and Sustainable Polymers: Recycling and Upcycling via Molecular Design

Functional Upcycling of Crosslinked Polyurethanes via Reactive Extrusion Decrosslinking through Catalyzed Carbamate Exchange

The thermoset nature of many commercial polyurethanes (PUs) prohibit common melt reprocessing methodologies. Dynamic carbamate exchange catalyzed by reactive extrusion emerges as a novel reprocessing pathway for covalently crosslinked PUs, however, is confined to network-to-network reprocessing. The introduction of monofunctional urethanes (serving as end-capping reagents) into a model PU network effectively decreases crosslink density and eventually deconstructs the network into a solvent-soluble, highly branched PU oligomer. The small-molecule urethane loading dictates the final material structures after reactive extrusion, consistent with the Flory-Stockmayer gelation theory. In addition, synthesis of a library of small-molecule decrosslinkers with variable molecular weight, functionality, and aromaticity valorizes the robustness of the depolymerization process. Furthermore, functional end groups installed onto PU oligomers by the urethane decrosslinkers provides post-decrosslinking reactivity. Altogether, this work seeks to develop an effective catalytic reactive-extrusion mediated depolymerization process where the feedstock materials are crosslinked PU networks, low molecular weight urethane end capping agents, and catalysts. We then seek to propose the decrosslinking technique as a platform to reprocess other polymer networks synthesized by step-growth reactions, such as polyureas.   

Thermomechanical Recycling of Polymers via Reversible Dynamic Crosslinking

Most of the industrially used polymers are immiscible and incompatible and do not form a homogeneous mixture. Stabilizing these immiscible mixed plastics could increase their lifespan and enable previously unrecoverable mixed plastic wastes to be reprocessed and reused. Here, we study how reversible dynamics covalent bonds can reactivate mixed plastic "dead" chains into compatibilized multiblock copolymers. We develop a phenomenological bead-spring model and carry out large-scale hybrid molecular dynamics (MD) – Monte Carlo (MC) simulations of an incompatible homopolymer blend. These simulations show a clear transition from an immiscible blend to a progressively more miscible one via dynamic crosslinking when thermally activated. The creation of a "living" gMBCPs, is found to be the underpinning driver for the increased miscibility. They enhance the local density of microphase-separated domains and compatibilize the interfaces of the blend. The work provides fundamental insights into the thermomechanical recycling of polymers.

Reference:

Clarke et al., Nature 616, 731 (2023)   

Engineering photo-responsive recyclability into polymer networks for sustainable 3D printing

The 3D printing industry is growing quickly, with projections that over 10% of manufactured products will be 3D printed within this decade. A consequence of this rapid growth is rapid waste production: over 30% of 3D-printed material is discarded immediately after printing. To enable recycling of scrap materials, we report photo-reversible polymer networks that are crosslinked and un-crosslinked using different wavelengths of light. Light-driven un-crosslinking has potential as a low-cost, low-energy, on-demand recycling technology. We designed photo-reversible polymer networks comprising multi-arm star polyethylene glycol functionalized with anthracene (PEG-anthracene). PEG-anthracene undergoes crosslinking upon irradiation with ultraviolet light (UV, 365 nm) and un-crosslinking upon irradiation with deeper UV light (265 nm). The un-crosslinking of PEG-anthracene with 3, 4, 6, or 8 arms (5 kg/mol per arm) was compared using in situ dynamic rheology. During network un-crosslinking, polymers with fewer arms exhibited “recycling windows,” where the sample transitioned from solid-like to liquid-like, during which re-crosslinking could occur. These findings demonstrate an opportunity for on-demand recycling of photo-reversible polymers to increase 3D printing sustainability.    

Mesoscale modeling of random chain scission in polymer melts

The traditional approach to recycling polymers involves thermal degradation of the material to recover the monomer. However, such a process creates unwanted side products. Controlled degradation to oligomers could potentially overcome challenges in recycling to usable products with sufficiently high yield. Herein we develop a mesoscale framework to model random scission in polyethylene melts at high temperatures. Dissipative particle dynamics (DPD) approach is used along with mSRP (modified segmental repulsive potential) formulation, which allows one to capture the effects of entanglements in polymer melts. We account for the temperature dependence of the probability of bond breaking. We systematically characterize weight fraction, number fraction, degree of polymerization, and dispersity of polymer fragments during random scission at a range of high temperatures. We identify the conditions at which the weight fraction and number fraction distribution of polymer fragments approximately follows the Flory-Schulz distribution. Modeling thermal degradation of polymers on mesoscale can potentially assist in developing alternative strategies for polymer recycling and upcycling.   

Molecular Insights into Cleavable Bond-Modified Polyethylene: High-Throughput Simulations for Circular Polymer Design

Polyethylene represents the largest fraction of materials used in global plastics production. However, chemical recycling of polyethylene ineffective in its current state due to the high energetic cost associated with breaking down the carbon-carbon sigma bond in polyethylene. We performed a systematic exploration aimed at establishing structure-property relationships in polyethylene-based polymers containing strategically incorporated cleavable bonds, with the aim of enhancing their chemical recyclability. Employing high-throughput molecular dynamics simulations guided by Gaussian process regression, we investigated six distinct telechelic functionalities across varying chain lengths: ester, aromatic ester, anhydride, carbonate, urethane, and amide linkages. Our study focused on elucidating the effects of cleavable bonds on heat of vaporization, density, diffusion, and viscosity, key parameters for polymer processing, within a range of temperatures in the melt phase. By comparing these properties with those of linear polyethylene, we establish a framework that provides crucial insights into polymer behavior. Our findings provide a roadmap for chemists engaged in sustainable polymer synthesis, guiding the design of circular polymers that align with principles of environmental responsibility.   

Upcycling Plastic Waste into Tough, Fully Recyclable Composites

Plastics, with their low cost, offer significant societal benefits, including applications in medicine, food preservation, and fuel efficiency. However, the widespread use of plastics has led to a pressing waste management issue. Traditional recycling methods face challenges in collecting and reusing post-consumer plastics. We propose an innovative approach that enables direct reprocessing of waste materials into durable composites via cold sintering. This process consolidates inorganic powders at lower temperatures than conventional sintering, making it compatible with plastics. Our method circumvents many costly steps associated with advanced recycling. We create structural materials from plastic waste (polypropylene, PP) and construction waste (gypsum) through cold sintering, resulting in inorganic-matrix composites with significantly improved tensile strength and toughness compared to neat gypsum. X-ray Computed Tomography analysis reveals critical mesoscale structures enhancing mechanical properties. Remarkably, these composites are fully recyclable with just the addition of water, maintaining tensile properties over tens of cycles. Life Cycle Analysis (LCA) shows that these recyclable composites demand significantly less energy, have a lower global warming potential, and reduced water usage compared to common construction products. Cold sintering offers a transformative way to convert waste into fully recyclable composites with tunable properties.   

Title:Upcycling Polymers into Functional Coatings through SLED

In the realm of advanced materials, there is a critical demand for polymer composite coatings, driven by the escalating demand for high-performance materials that balance environmental sustainability and cost-effectiveness. One such way of addressing this demand is to create polymers through electrospray deposition (ESD). ESD is a manufacturing technique that makes use of electric fields to guide electrically charged droplets towards a grounded substrate. This potential becomes even greater in the self-limiting electrospray deposition (SLED) regime, which makes use of the accumulated charge in the deposited polymer to repel and redirect the spray droplets to uncoated areas of the substrate. This leads to a self-limiting effect, creating a polymer coating that is relatively uniform in thickness and conformally wraps complex topologies. These coatings are on the order of microns, and therefore can coat a large area of material with minimal material, and the ESD process also minimizes overspray. Here, we explore the use of SLED as a means of upcycling polymer waste into functional porous and dense coatings of polymers and polymer composites. Specifically, we are able to make solutions in sustainable solvents from plexiglass and styrofoam waste, and then spray these solutions to create these coatings, which have superhydrophobic, barrier, or composite-derived functionality.   

Tunable Functionalization and Upcycling of Polyolefins to Polyurethanes

Polyolefins represent the largest contribution to plastic production, use, and generated waste worldwide, yet their recycling rates are low. More efficient methods of recycling polyolefins have been underexplored, due to significant technological, scientific and economic challenges. We are exploring chemical functionalization and processing strategies for converting waste polyolefins to high value materials for advanced manufacturing and use. In this work, maleic anhydride grafted polypropylene was hydroxylated and subsequently cured with a diisocyanate to form thermoset polyurethane films. The physical properties of the resulting polyurethane films were evaluated and benchmarked to conventional polyurethanes. The complementary roles of the crosslinked network and crystallization behavior on the mechanical properties of the films were examined.   

Modeling Polyolefin Catalytic Cracking with Zeolite Catalysts Under Flow

The plastic feedstock for recycling is typically constituted by over 90% polyolefins. However, so far there have not been any attempts to explore low-temperature, catalytic reactive extrusion of polyolefin waste, despite its potential energy, cost savings and scalability potential. Our experimental and computational study addresses a novel recycling method of polyolefin incorporating zeolite catalyst within the extrusion process. Herein, using Machine Learning (ML) coupled with Dissipative Particle Dynamics (DPD), we develop a coarse-grained model to simulate the reaction-extrusion process of polyolefin with catalyst under flow. We associate bond breaking rate with polymer local velocity related to diffusivity and investigate the effect of catalyst structure and flow rates on polymer conformation and dynamics. We also track the evolution of polydispersity and average molecular weight and compare it successfully with experimental results.   

effect of initial molecular weight distribution in polyethylene melts on degradation process at high temperatures

The molecular weight distribution of synthetic polymers is an important factor to consider when synthesizing, processing, and manufacturing industrial plastics. The breadth of this distribution, or dispersity, is determined by the ratio of the number average molecular weight to weight average molecular weight, and is correlated with material properties. While most theoretical studies consider uniform polymer melts, monodispersity is not practically achieved in experiments. There is a pressing need to explore the effect of initial dispersity on the degradation process. We use dissipative particle dynamics (DPD) to probe the effects of initial molecular weight distribution on random scission in linear polymer melts. To capture the effects of entanglements in melts, we use modified segmental repulsive potential (mSRP) formulation of DPD. We consider melts with initial dispersities of 1.05 to 1.3. We use the Schulz-Zimm initial distribution of chains, which is known to be an accurate predictor of the distribution of polymer chains in polyethylene. We track weight fraction, number fraction, and dispersity of polymer fragments during random scission at high temperatures and quantify the effects of initial dispersity.   

Mapping pore-level activity of catalysts for polymer upcycling through dielectric spectroscopy

Polymer upcycling by catalytic scission provides a powerful route to convert waste plastic to valuable chemicals. Despite the discovery of many promising catalytic reaction schemes that cleave polyolefins to form selective products, reaction engineering of upcycling processes is hindered by a lack of understanding of pore-level macromolecular deconstruction. Change in segmental dynamics can serve as a marker for the progress of chain scission in upcycling reactions. In this talk, we present a method to monitor the progress of a scission-based reaction inside porous catalysts by tracking the evolving dielectric signature of the reacting polymer. Nanoporous membranes of anodic aluminum oxide are filled with poly(n-butyl) methacrylate as a model for polymer-filled catalytic pores. Change in the measured dielectric properties upon thermal depolymerization is correlated to the rate of scission of polymeric species. To highlight the applicability of this technique to catalytic upcycling of commercial polyolefins, the scission of poly(ethylene-alt-propylene) is tracked inside functionalized nanopores. Nanorheological characterization of pore-level catalytic activity will help guide the design of catalyst morphology for efficient upcycling reactions.