Session D17: Sustainable Polymers: Physics of New Materials, Design for Sustainability, and End-of-Life

Seven Simple Dynamic Covalent Chemistries to Transform Thermosets into Thermoplastics: Sustainable Chemical Recycling of Traditionally Non-Recyclable Materials

The production of conventional crosslinked polymer networks and their composites, i.e., thermosets and thermoset composites, was estimated to consume more than 40 billion kg of polymer in 2020. Unfortunately, thermosets cannot be melt-reprocessed into moderate- to high-value products because permanent crosslinks prevent melt flow. We have developed seven simple one-step or two-step chemistries to produce networks and their composites with dynamic covalent crosslinks that are robust at use conditions but allow for melt-state reprocessing at high temperature. Unique to our research group, we have developed three approaches for melt-state reprocessing of addition-type polymer networks and network composites, including those synthesized directly from monomers containing carbon-carbon double bonds and those synthesized from combined polymer and monomer with both containing carbon-carbon double bonds. These approaches allow for full crosslink density recovery after multiple reprocessing steps. We have also demonstrated for the first time with four dynamic chemistries the ability to make PU and PU-like networks, e.g., polyhydroxyurethane and polythiourethanes networks, reprocessable with full recovery of crosslink density. An "Achilles' heel" has been identified regarding the application of dynamic covalent networks, i.e., such networks are subject to creep at elevated or sometimes even room temperature, which is often highly undesirable. We have successfully addressed this issue in several ways. Brief highlights of each of these accomplishments will be presented. Implications for making major gains in the sustainability of networks and network composites will be discussed as will the potential benefits of such chemistry even with regards to sustainability    

A High-Throughput Approach to Chemically Recyclable Polyolefins

The lack of a “closed loop” in the plastics carbon economy has driven the vast increase in the quantity of waste plastics with low recyclability and detrimental environmental impacts. This is in part due to the lack of chemical functionality in the structure of commodity polyolefin plastics, making their (chemical) degradation into useful materials a significant chemical challenge. This presentation will detail our studies into the design of polyolefin-like polymers that can be broken down into macromonomers and re-polymerized through the incorporation of chemically-labile chemistries. We are specifically interested in accessing polyethylene-like materials with incorporated functionality that can be tuned to enable the depolymerization of the material to macromonomer and subsequently re-polymerized to “close the loop”. This presentation will detail the discovery of structure-property relationships of these polyolefin-like materials utilizing a high-throughput synthetic approach. An investigation of depolymerization/repolymerization processes and kinetics will also be presented.   

Thermophysical Property Prediction of Polyethylene-like Materials and Their Blends

The accurate determination of bulk thermophysical properties of polymers using only molecular structure and composition is a topic of critical academic and industrial interest. However, the ability to predict multi-molecule processes in polymer systems remains challenging for molecular modeling.  In this context, polyethylene (PE) constitutes not only the major fraction of global plastic production but also an excellent system to develop structure-property and composition-property relationships. In this work, we propose a workflow to understand and predict thermophysical properties of PE-like materials and their blends using supervised Machine Learning (ML) techniques. We generate a data set from atomistic molecular dynamics [RA1] simulations and train a ML model in order to obtain accurate predictions. Here we present two important case studies in the context of chemical recycling where this workflow can be used to accelerate both circular polymer design and commercial polyolefin hydrocracking. For the former, we study ester-linked PE and its blends. For the latter, we modeled PE chains with smaller alkane chains in the presence of hydrogen. Our results show how quantitative predictive models can aid the design of sustainable solutions for plastic innovation.   

Sustainably sourced discrete macromolecule design in materials science

Precise sequence and structural control is critical to the development of new functional, responsive, and programmable polymeric materials. However, attempts to synthesize unimolecular polymers and precise networks with well-defined sequences are hampered by scale-up limitations. These limitations in scale-up have left research into the impact of sequence on materials largely underexplored. Motivated by these opportunities and the need for sequence-control and structural diversity in polymeric materials research, I will present a versatile strategy for the assembly of sustainable cross-linkable sequence-defined macromolecules. This new synthetic functional oligocarbamate platform overcomes the scalability issue that plagues the iterative assembly of sequence-defined macromolecules and enables the assembly of oligocarbamate macromers at the gram-scale. Data highlighting the effect of sequence on network topology, optical and mechanical properties will be discussed. Overall, this body of work should provide the foundation for future studies exploring the tunability of bulk material properties via sequence control.   

Strengthening protein-based materials through protein self-assembly and processing

The development of sustainable polymeric materials with comparable performance to petroleum-derived ones is urgently needed, especially in light of the irreversible environmental and resource impacts associated with conventional plastics. While a great deal of work has focused on sugar-based materials, proteins offer a promising feedstock to mimic and replace petroleum-derived polymers such as nylon and polyurethane. However, proteins are usually either too brittle (neat materials) or too weak (plasticized materials) to serve as suitable substitutes. Our group has previously demonstrated that this limitation can be overcome by designing protein copolymers in which proteins act as ‘hard blocks’ and low-T_g acrylates as ‘soft blocks’. There are other scientific challenges to achieve broader applications, for example, the reported range of mechanical properties does not yet match that of high-strength synthetic polyurethanes. Here, we found that the mechanical properties of subsequent materials can be greatly improved by controlling the unfolding-assembly process of proteins, i.e. the reordering of the "hard block". This is a result of the additional, dense intermolecular forces generated by the reassembly between protein molecules. Importantly, this toughening mechanism is generalized among the proteins examined, such as whey protein and bovine serum albumin. This result provides a scalable and sustainable strategy for improving the performance of protein materials and replacing polyurethanes-like materials.   

Sustainable Thermoplastic Elastomers with Ionic Interactions

Thermoplastic elastomers (TPEs) are ABA triblock copolymers, in which A represents glassy end-blocks and B a rubbery midblock. Commercial TPEs are derived from petroleum whose manufacturing and disposal have undesired environmental impacts, motivating the development of TPEs from sustainable sources. However, polymers with bulky constituents, such as the long alkyl side-chains of vegetable oil-derived polymers, exhibit poor mechanical performance due to lack of entanglements in the rubbery matrix. Transient networks were incorporated into the midblock through either hydrogen bonding or ionic interactions to improve mechanical performance. ABA triblock copolymers were synthesized with poly(n-butyl acrylate–co–acrylic acid) or poly(lauryl methacrylate–co–methacrylic acid) copolymer midblocks and poly(methyl methacrylate) endblocks. Enhancement of tensile strength and strain at break in these systems with varying acid and ion content showed a collapse onto a master curve when plotted vs. relaxation time of the rubbery midblock. However, it was seen that when all acid sites were participating in ionic interactions, there was strong deviation in enhancement from master curve. This implies crucial role of free acid sites in stress relaxation mechanism in ion containing midblocks.   

Bio-based composites of poly(ethylene glycol)-grafted cellulose nanocrystals in poly(lactic acid)

Cellulose nanocrystals (CNCs) are bio-based nanofillers that allow access to sustainable composites with enhanced property profiles.  However, obtaining the CNC dispersion required to achieve optimal composite reinforcement can be a challenge. Incorporation of polymer-grafted CNCs into bio-based polymers is one way to address this issue; however, the question remains: how do we optimize the variables associated with CNC polymer grafting (e.g. molecular weight, grafting density, polymer volume fraction) to maximize the properties of these sustainable materials? In this work, we use poly(ethylene glycol)-grafted CNCs to analyze the effects of polymer molecular weight and grafting density on the properties of poly(lactic acid) (PLA) composites and develop structure/properties relationships help to inform future sustainable polymer composite design.   

Swelling Behavior of Sustainable Chitosan/Graphene Oxide Nanocomposite Ultrathin Films

Humidity sensors play an essential role in monitoring product quality in the manufacturing and pharmaceutical industries. Lately, the use of thermoplastic polymers and inorganic semiconductors in sensors is raising environmental concerns owing to non-degradability despite their performances. Biodegradable and renewably-sourced chitosan-based nanocomposite films provide a more sustainable alternative. Chitosan has good thermal properties and can be chemically modified by blending polar nanofillers like graphene oxide (GO), improving its mechanical properties. The oxygen-rich groups of GO and protonation of NH₂ groups of chitosan increase the affinity of these blend films towards moisture. Here, ultrathin chitosan/GO nanocomposite films of 30-500nm thickness were fabricated on Si substrate. The thin films swelled rapidly in a humid environment, with visible changes in color. Over the full relative humidity range of 95%, film thicknesses increase 50% compared to dry state, confirmed by in-situ interferometry. The absorption-desorption of moisture was fast and repeatable. This highly sensitive humidity colorimetric property of chitosan/GO nanocomposite films enable its potential as a biodegradable sensor for monitoring systems benefiting several industries.   

Star-to-Bottlebrush Transitional Behavior of Model Graft Polymers in Fast Extensional Flows

Polylactide (PLA) is a bio-based, industrially compostable polymer and represents one of the most prevalent sustainable materials in the plastics industry. PLA has poor melt strength (i.e., extensional viscosity), which limits its utility in processing methods that require uniaxial extension, such as film blowing. We report the synthesis and rheological characterization of poly((±)-lactide) with a graft molecular architecture with the goal of creating melt strain hardening to improve the melt strength. We demonstrate that a critical value of the backbone degree of polymerization (N_bb,c) is necessary for achieving melt strain hardening; which we attribute to a star-to-bottlebrush transition, where the graft polymer behaves star-like below N_bb,c and recovers bottlebrush behavior as N_bb increases. Ring-opening metathesis polymerization (ROMP) was employed to synthesize a library of polymers with varied N_bb and well-defined grafting densities and side-chain degrees of polymerization. Extensional rheological properties of the melts were determined using an ARES-G2 extensional viscosity fixture. The findings of this work will guide the design of graft polymers that exhibit melt strain hardening, enabling an expanded window for the processing of sustainable plastics.   

Atomistic Simulations on Mechanical Properties of Lignin

Mechanical properties of lignin, an aromatic heteropolymer constituting 20%-30% of plant biomass, are important to the fabrication and processing of lignin-based sustainable polymeric materials. Atomistic simulations are performed to provide microscopic insight into the mechanics of lignin. Representative samples of miscanthus, spruce, and birch lignin are studied. At temperature below the glass transition temperature, the stress-strain curve for lignin under uniaxial compression exhibits initial elastic response, yielding, and post-yield plastic response with increasing strain. The decomposition of the overall stress shows that the energetic component contributes to the elastic response and yielding, but remains low in the post-yield plastic regime until strain hardening sets in. The dissipative component dominates the post-yield regime before strain hardening. In addition to the three real lignin samples, minimalist model systems of monodisperse linear polymers consisting of only guaiacyl units and β-O-4 linkages are simulated. While the elastic response, yielding, and the plastic flow under compression do not depend on the molecular weight of the model lignin_, the strain hardening under compression is enhanced as lignin molecular weight increases.    

Investigating the effect of Co-solvent Pretreatment on Acid-Based Hydrolysis of Cellulose for Biofuels through Molecular Dynamics Simulations

The increased need for utilization of sustainable energy sources with reduced environmental impact has led to advancements in deconstruction of lignocellulosic biomass for the economic production of biofuels and bioproducts. Cellulose is a renewable biopolymer that can be converted into biofuels and the choice of solvents for pre-treatment is important for processing and modification of cellulose fibrils for conversion into simple sugars. Co-solvent enhanced lignocellulosic fractionation pre-treatment with tetrahydrofuran (THF)-water (H₂O) has shown to significantly improve cellulose deconstruction by increased solubilization for acid-based hydrolysis. While atomistic molecular dynamics simulations have provided insights on the preferential aggregation behavior of THF-H₂O and decrystallization of cellulose fibrils, little is known about the local solvent structure around hydrolyzed sites and how it facilitates the disintegration of cellulose fibrils. Here, we perform all-atom replica- exchange umbrella sampling (REUS) simulations of an 18-chain cellulose fibril with degree of polymerization (DP) = 20, and a hydrolyzed cellobiose unit at the reducing end of one of the chains, in two solvent systems: (1) 1:1 THF: H₂O by volume and (2) H₂O at 298 K and 423 K. The free energy of binding (ΔG_binding) of the detached cellobiose unit from the fibril are calculated from potential mean force (PMF) profiles. Furthermore, we investigate the local solvent structure around the hydrolyzed sites and the fibril through radial distribution functions, solvent contact numbers, and conformational analysis of cellobiose, and the results are compared with findings from acid-based hydrolysis experiments. Understanding the competing physico-chemical interaction between solvents and cellulose after hydrolysis will provide new insights for the development of improved cellulose solubilization using co-solvents.   

Fabrication and Characterization of Lignin-Based, Thermo-Responsive Soft Composite Materials

Lignin has gained significant attention due to its biocompatibility and antimicrobial properties, which make it an ideal candidate for biomedical applications. Thermo-responsive polymers, such as poly(N-isopropylacrylamide) (PNIPAm), in hydrogels can prove advantageous for bioseparations as PNIPAm exhibits a volume phase transition at a temperature below that of the human body. In this study, we fabricated two series of soft composites containing lignin, PNIPAm, and poly(vinyl alcohol) (PVA) with mass ratios of 1:1:1 and 2:2:1 (lignin:PNIPAm:PVA). Three types of lignin were used – raw lignin and two fractionated cuts of lignin with prescribed molecular weights (MWs) – and the concentration of free radical accelerator was varied between 5 and 10 wt%. The Young's modulus of each membrane was measured using mechanical indentation at room temperature and 40 °C. Young's moduli were higher for membranes at 40 °C, highlighting a volume phase transition. The permeation of methylene blue, a model pollutant, was measured by ultraviolet-visible spectroscopy and decreased with the addition of lignin, with a reduction of approximately 45:1 for membranes containing high MW lignin and 10 wt% accelerator in the 2:2:1 series. Higher water uptake was observed for the 2:2:1 membrane series.   

Spinnability and Rheology of PEO Solutions in Water/Acetonitrile Mixtures via Centrifugal Force Spinning

Centrifugal force spinning has recently emerged as a highly promising alternative technique for the production of nonwoven, ultrafine fiber mats.  Due to its high production rate and ability to utilize a wider range of solvents, it can provide a more technologically relevant fiber spinning technique than electrospinning.  In this contribution, we investigate the influence of polymer concentration and solvent properties on the centrifugal spinning process and fiber morphology.  We also correlate spinnability to the processing conditions and material properties using shear and extensional rheology carried out using torsional rheometry and dripping-onto-substrate (DoS) rheometry.  We eventually find that increasing polymer concentration will allow for continuous fibers to form after a beaded fiber regime.  We also find that fiber diameter is concentration-dependent, and that solvent choice and processing conditions affect the fiber morphology and mechanical properties of fiber mats.