Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session B16: Physics of Polymer RecyclingFocus Recordings Available
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Sponsoring Units: DPOLY Chair: Wenlin Zhang, Dartmouth College Room: McCormick Place W-184A |
Monday, March 14, 2022 11:30AM - 12:06PM |
B16.00001: Cascading Crystallization in Mixed Polyolefins Invited Speaker: Kalman B Migler Polyolefins constitute the largest share of plastics that enter the waste stream but have recycling rates less than 10 percent. The underlying difficulty is thermodynamic in nature. The post-consumer polyolefins – polyethylenes and polypropylenes - typically enter the recycling stream in mixed form and are processed together. Due to thermodynamic incompatibilities the various polyolefins mix and micro-separate into domain morphologies, and upon subsequent cooling, crystallize separately in a cascading fashion, frequently with poor interfacial adhesion - causing brittleness. Such cascading crystallization is critical to the final mechanical properties yet is poorly understood. Here we employ the rheo-Raman microscope, phase contrast microscopy, and differential scanning calorimetry to study the crystallization and the associated rheology in immiscible blends of high density polyethylene and isotactic polypropylene. We find that the crystallization kinetics and rheology are a strong function of domain structure and composition. In particular, we examine the compositional dependence of the continuity of the iPP domain and its impact on the crystallization kinetics. Our results indicate the importance of rheology and processing on the structure and properties of mixed waste-stream crystallizing polymers. |
Monday, March 14, 2022 12:06PM - 12:18PM |
B16.00002: Understanding the Physics of Polymer Depoloymerization for Polymer Recycling Whitney C Blocher McTigue, Charles E Sing To reduce the global accumulation of plastics in the environment, recyclable polymers have been a topic of increasing interest. One principle strategy is to develop polymers with low ceiling temperatures, which can be reversibly deconstructed (depolymerized) to yield pure monomer following an applied stimulus. It is then possible to re-polymerize recycled materials from the recovered monomer. In this talk, we use a hybrid Brownian Dynamics and Monte Carlo simulations to model the interaction of a polymer with a surface that can initiate depolymerization of linear and cyclic polymers. This can occur either through subsequent "unzipping" events or successive chain scission events. We consider both single- and multi-chain systems and quantify and develop models to predict the kinetic competitions between different depolymerization mechanisms. |
Monday, March 14, 2022 12:18PM - 12:30PM |
B16.00003: Molecular Design of Polymeric Compatibilizers for Improved Mechanical Recycling Mark D Dadmun Managing end-of-life plastics is a challenging problem for a variety of reasons, including the complexity of recycling mixed waste streams. Most polymer pairs don’t mix, and any product made from such a mixture will exhibit poor properties. These polymer mixtures can be enhanced by their compatibilization with proper polymeric interfacial modifiers. Our group has developed a fundamental understanding of polymeric compatibilizers for amorphous polymer blends. However, most polymers that end up in the waste stream are crystalline. In crystalline polymers, the compatibilizer may co-crystallize with either homopolymer providing a crucial mechanism to strengthen the biphasic interface. However, the importance of this mechanism, and understanding the compatibilizer molecular characteristics required to improve blend properties via co-crystallization, is poorly understood. This presentation will provide the current state of our understanding of the role of co-crystallization in the compatibilization of crystalline polymer blends, and provide insight into the molecular characteristics of effective copolymeric compatibilizers for phase separated crystalline polymer blends, including those that are most relevant for mixed waste streams in polymer recycling. |
Monday, March 14, 2022 12:30PM - 12:42PM |
B16.00004: Improved toughness of incompatible polymer mixtures via sequence-defined compatibilizers Robert J Ivancic, Chase B Thompson, Sara Orski, Debra J Audus Compatibilizers can be used as part of a broader strategy to upcycle incompatible plastic mixtures. Recent experiments demonstrate that using tetrablock copolymers as compatibilizers increases the toughness of incompatible polymer blends significantly, while using diblock copolymers does not. This observation leads to the question: how does the compatibilizer sequence determine the toughness of incompatible polymer mixtures? Here, we examine this question through the use of coarse-grain molecular dynamics simulations in which we add various sequence-defined copolymer compatibilizers to an incompatible polymer blend. Analogously to experiments, we equilibrate these systems in the melt, cool well below the glass transition temperature, and strain them until failure. We seek to understand how sequence affects structure at the interface and how this leads to enhanced toughness of the polymer blend. |
Monday, March 14, 2022 12:42PM - 12:54PM |
B16.00005: Upcycling of Polypropylene through Mechano-Chemical Functionalization Ramanan Krishnamoorti, Amy N Le Chemical recycling of polypropylene (PP) is a promising path to restore or add value into polypropylene waste upcycling. Low temperature thermal pyrolysis of PP has been tuned to selectively yield a solid degraded product with controlled molecular weight and therefore preservation of physical and mechanical properties. However, during conventional high temperature pyrolysis, low molecular weight residues result in higher concentrations of functional end groups. In order to develop functionalization and molecular weight control as independent parameters of the degradation process, we assess the efficiency of functional group formation through mechano-chemical pathways. Our work here describes the understanding of the combination of thermal, mechanical and chemical degradation of polypropylene to result in functional controlled molecular weight PP moieties to be integrated in upcycling of PP. These studies aim to investigate the properties of chemically recycled functionalized polypropylene and establish a foundation for the design of a new class of polymers. |
Monday, March 14, 2022 12:54PM - 1:06PM Withdrawn |
B16.00006: Modeling PET Solvolysis Sanat K Kumar We use order of magnitude analysis to show that the solvolysis of PET is not transport limited - rather the restrictions stem from reaction kinetics, and also from the distribution of loop, tail and train lengths in this semicrystalline morphology. Kinetic Monte Carlo simulations coupled to detailed information on chain legnth distributions in the amorphous phase provide predictions that can be directly compared to experiment. |
Monday, March 14, 2022 1:06PM - 1:18PM |
B16.00007: Addressing plastic waste through novel upcycling enabled by cold-sintering Po-Hao Lai Plastic products are ubiquitous and indispensable in our lives. Although plastics are easily available and have beneficial uses, their disposal has created a critical problem associated with the every growing plastic waste generation. Traditional recycling fails to address a majority of the plastic waste and nascent technologies to enable full circularity are not fully developed and economic feasibility questions remain. Here, we demonstrate a new approach that could incorporate plastic waste into long lifetime durable composites through cold-sintering, which is capable of sintering inorganic materials near 100 degrees Celsius. One potential avenue for these composites is structural materials. We show that polypropylene (PP) can be cold-sintered with gypsum to create composites from plastic waste and construction waste at compositions that are between 10% to 50% polymer. The mechanical properties of PP/gypsum cold-sintered composites can exhibit improved properties over their individual components. This approach could address problems associated with waste accumulation through the co-sintering and re-processing of polymers with inorganic materials. |
Monday, March 14, 2022 1:18PM - 1:30PM |
B16.00008: Refractory Plasmonic Nanoparticles for Visible Light Mediated Recycling of Epoxy Kavon Mojtabai, Samantha Lindholm, Brandon T McReynolds, John D McCoy, Youngmin Lee, Sanchari Chowdhury Epoxy polymers exhibit excellent mechanical properties and have wide applications ranging from adhesives to structural materials. However, recycling of epoxy is difficult because of their irreversible cross-linked network structure. We have developed a reversible epoxy which can be recycled using visible light. Diels-Alder chemistry is studied to develop thermo-reversible epoxy. At higher temperatures (above~ 120°C), the Diels-Alder reaction reverses, resulting in depolymerization of the epoxy and can polymerize again as it cools down. Photothermal refractory plasmonic titanium nitride (TiN) nanoparticles are incorporated in the reversible epoxy. These nanoparticles can efficiently absorb visible light and convert that to heat to promote the retro Diels-Alder reaction. The nanoscale heating can efficiently break the chemical bonds increasing rate of depolymerization. TiN nanoparticles were well dispersed in the epoxy matrix using optimized probe sonication method. The photothermal reversible epoxy composite was characterized using FTIR, UV-Vis spectroscopy and rheometer. The depolymerization of reversible epoxy could be achieved with 0.1 wt% loading of TiN at light intensity of 1420 W/cm2. Funding: U.S. Department of Energy, Office of Basic Energy Sciences, DE-SC0022261. |
Monday, March 14, 2022 1:30PM - 1:42PM |
B16.00009: Rapid Structural Classification of Post-Consumer Polyolefins by Multimodal Measurement Correlations Shailja Goyal, Tyler B Martin, Peter Beaucage, Debra J Audus, Sara Orski Infrared (IR) spectroscopy is the rapid, workhorse technique used by recycling facilities for materials classification; however, as currently used it does not provide complete identification of many recycled materials. While techniques such as multidetector chromatography and small-angle scattering can measure chain composition, topology, and conformation in the laboratory, they are too slow to be of use in a sorting facility. We will outline our approach and present recent results from our effort to develop machine learning tools that leverage the limited chemistry of polyolefins to connect the infrared chemical signature to structural parameters from more time-consuming analytical techniques. We will also describe developments in automated instrumentation to generate reference IR and SAXS datasets to enable development of the machine learning models. Such models will allow IR classification of polyolefin materials with unprecedented fidelity at production speed and will provide FAIR datasets for future development of improved commercial algorithms for advanced sorting and processing equipment. |
Monday, March 14, 2022 1:42PM - 1:54PM |
B16.00010: Modeling welding of semicrystalline polymers Sanat K Kumar, Sabin Adhikari
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Monday, March 14, 2022 1:54PM - 2:06PM |
B16.00011: Stress-induced dynamics in glassy vitrimers Alessandro Perego, Fardin Khabaz Vitrimers are a class of polymers that bring together desirable mechanical properties of thermosets with the recyclability of thermoplastics. This ability arises from the rearrangement of the vitrimer network via a bond shuffling mechanism while its cross-link density remains preserved. As the properties of the material are directly affected by the dynamics of the polymer chains, it is critical to understand the link between the macroscopic behavior and the microscopic dynamic of the molecules. Molecular dynamics simulations can provide detailed molecular mechanisms of the system under macroscopic stress-induced deformations. We present a simulation methodology that utilizes coarse-grained molecular dynamics in conjunction with a Monte-Carlo method to capture the bond exchange in vitrimers. Constant stress is applied to the system below its glass transition temperature while monitoring the strain (e.g., creep). Vitrimer shows accelerated dynamics under applied shear stress compared to that of thermoset. This different behavior between the two networks is reflected in the mean squared displacement of the crosslinker. In addition, the motion of the crosslinker shows non-affine displacement, resulting in regions of enhanced mobility (e.g., dynamic heterogeneities). |
Monday, March 14, 2022 2:06PM - 2:18PM |
B16.00012: An Experimental and Computational Study of Thermomechanical Recycling of Flame-Retardant PET Jiuke Chen, Christopher Bascucci, Rudolf Hufenus, Ali Gooneie Polyethylene terephthalate (PET), well known for its excellent recyclability via thermomechanical recycling, has a wide variety of applications in packaging and textiles due to its outstanding properties. In many PET products, flame retardants (FRs) are blended in as additives by extrusion to extend the time-of-escape from fires. However, PET/FR products often are not designed for recycling and downgrade during thermomechanical recycling. A crucial step toward improving recyclability is to understand the physio-chemical interactions between FRs and PET during melt (re-)processing. To address this issue, a series of rheological, thermal, and chemical experiments were performed on various PET/FR compounds containing a DOPO derivative (DOPO-PEPA, shortened DP) or a phosphonate compound (Aflammit PCO 900, shortened AF). Furthermore, thermomechanical recycling of PET/FR compounds was simulated by multiple extrusion and injection molding cycles. To study the synergistic properties of DP, the chemical degradation mechanism of DP was investigated by molecular dynamics (MD) simulations using the reactive force field ReaxFF, combined with detailed chemical analysis. The results of this study make it possible to improve the recyclability of PET/FR materials by designing new phosphorus FRs. |
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