Polymeric Dynamic Crosslinker for Upcycling of Fragile Low-Molecular-Weight Polypropylene

While tremendous progress has been made in the dynamic crosslinking of polypropylene (PP) for plastic upcycling, the efficacy in addressing low-molecular-weight (MW) PP waste remains untapped. In this work, we demonstrate a simple and scalable method to convert brittle low-MW PP to vitrimer materials with enhanced thermal and mechanical properties, enabling their use in circular upcycling. Different from most previous work employing small-molecule crosslinkers, we prepare PP vitrimers (PPv) using polymeric crosslinkers, containing polyethylene glycol segments, which leads to altered crystalline structures and network formation. Importantly, by increasing the MW of crosslinkers from 200 to 1000 Da, the PPv exhibit more than 50 times increase in their fracture energy with strong ductility, which can be attributed to combined effects of strengthened amorphous regions of semi-crystalline PP domains and the phase separation between soft polyethylene glycol segments and the PP matrix. Moreover, when blending the PPv with high MW PP (PPh), the PPh/PPv blends show comparable elastic modulus, yield strength, and stretchability to that of the PPh, in sharp contrast to the widely known embrittlement of low-MW PP/PPh blends. These results demonstrate the use of polymeric dynamic crosslinkers as an important strategy for upcycling low-MW PP waste to value-added products

Nature of Steady-State Fast Flow in Entangled Polymer Melts: Chain Stretching, Shear Thinning, and Viscosity Scaling

Understanding the nonequilibrium dynamics of topologically entangled polymers under strong external deformation has been a grand challenge in polymer science for more than half a century. Important deformation-induced single-polymer structural changes have been identified, such as chain orientation and stretching. But how these changes impact the physical entanglement network and bulk viscoelasticity remains largely elusive in the fast flow regime that involves highly oriented and stretched polymer chains. Here, through new experimental and theoretical developments, we establish a unified understanding of the steady-state shear viscosity, η, of entangled polymer melts at high Rouse Weissenberg numbers, WiR > 1. New capillary rheometry measurements in the absence of flow instabilities reveal a dramatic change in shear-thinning scaling from η ∼ γ̇–0.7 ± 0.1 at WiR N/γ̇)0.50 at WiR > 1, where N is the degree of polymerization and γ̇ is the shear rate. Moreover, the viscosity scaling exponent with polymer molecular weight decreases with applied shear stress, and a remarkable unentangled melt scaling η ∼ N emerges under ultrahigh constant stress conditions σ/Ge ≥ 2, where Ge is the equilibrium entanglement elastic modulus. These new observations are not consistent with existing molecular theories. We construct a dynamic scaling model based on tension blob concepts as extended to entangled polymers, resulting in a (near) universal expression for the shear-thinning behavior controlled by purely dissipative considerations associated with orientational stress. This physical picture is in sharp contrast to the predictions of various state-of-the-art tube-based models based on the widely adopted factorization approximation of the total stress into stretching and orientational contributions, and also qualitatively differs from predictions of non-tube-based slip-link models based on a transient network perspective

Thermodynamic Evaluation of the Cross-Current Moving-Bed Chemical Looping Configuration for Efficient Conversion of Biomass to Syngas

The rising chemical demand and its associated concern of climate change have put an impetus on converting diverse domestic sources to valuable products in a decarbonized manner. Lignocellulosic biomass, a viable feedstock, is garnering significant attention as a sustainable alternative to fossil fuels. However, challenges in handling biomass feed variability and effectively processing its char and tar contents have hampered its commercial deployment. However, the chemical looping-based biomass-to-syngas (BTS) technology being developed by The Ohio State University is among the most promising technologies for industrial biomass reforming. It utilizes proprietary iron oxide particles in a cocurrent moving-bed reactor, leveraging the flow dynamics to transform biomass to syngas, and has been proven to be more efficient than conventional processes. However, this cocurrent system suffers from a thermodynamic barrier, inhibiting the syngas yield. To overcome this barrier, a novel chemical looping cross-current system is developed and investigated through detailed thermodynamic ASPEN studies after accounting for practical constraints. The barrier in the cocurrent system can be attributed to the equilibrium between exiting syngas and solid streams, which limits the oxidation of oxygen carriers. The cross-current reactor system overcomes this issue by shifting the exit of the syngas stream to the middle of the reactor, thus not allowing the exiting syngas and solid streams to be in equilibrium and creating a cocurrent section above the syngas exit and a countercurrent section below it. Thermodynamic simulations conducted under autothermal conditions reveal that the cocurrent and cross-current systems perform similarly with steam and CO2 co-injection. However, under an isothermal condition, which is now feasible with cheaper and sustainable heating methods, the cross-current system achieves ∼34% higher syngas yield over the cocurrent system (∼0.074 in cross-current compared to ∼0.055 in cocurrent) for both steam and CO2 co-injection. The findings from this study justify the scale-up of the cross-current system and provide system-level insights into biomass valorization