Session W26: Physics-Informed Design of Recycled, Upcycled, and Sustainable Polymers: Additives and Processing Conditions

Controlled phase behavior of polymer-grafted nanoparticles (PGNP) blend thin films.

Polymer blend thin films are essential materials for emerging technologies and applications such as optoelectronics, energy storage, and coatings. The microstructure and molecular interactions of polymer thin films influence their properties and suitability for various applications. This study reveals how the presence of grafted nanoparticles influences the morphology and properties of blends of polymethylmethacrylate grafted nanoparticle silica (PMMA-SiO2) and polymers of different chemical classes. A comprehensive investigation of the phase separation at the nanoscale was done using Atomic force microscopy (AFM), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS), and Grazing-Incidence Wide-Angle X-ray Scattering (GIWAXS) to characterize the surface topography, morphology and structure of the thin films of various thicknesses at different temperatures, and processing pathways including thermal annealing (TA) and direct immersion annealing (DIA). The measurement results combined show the lateral phase separation kinetics and the vertical phase separation in the polymer blend thin films. The changes in mechanical and thermal properties will also be discussed.   

Shear Flow/Complex flow-induced polymer chain scission in entangled melts

In this study, shear flow and complex flow-induced polymer scission are analyzed to enhance depolymerization processes for upcycling. Chemical upcycling of commercial plastics is intended to produce intermediate molecules through depolymerization, often catalytically. However, catalytic reactions are hindered in polymeric systems by high viscosity and low transport to catalytic sites. An enhancement is to induce polymer scission either mechanically or thermally as a pre-process step or in parallel to catalytical upcycling. Polymer processing operations like extrusion are well known to degrade polymers, due to a combination of flow types (shear and extension). However, flow-induced scission is not well-understood since it is hard to distinguish nor is the quantitative impact of flow type. Here, we strive to decouple the impact of flow type and duration on overall chain scission. Particularly, we analyse shear-induced scission and complex (shear + extension) flow-induced scission with rotational rheometry and micro-compounders. We focus on entangled polymer melts and the impact of entanglement molecular weight. Optimal flow conditions (total strain, flow rate and the flowing time) are studied for hydrogentaed-PI. Nonlinear shear flows of varying type and duration are applied while complex flows of varying screw speed and duration are applied. The impact of shear flow on overall molecular weight is tracked in situ through SAOS and compared to ex situ detailed measurement of molecular weight distribution. Scaling of scission with work of flow, flow type and flow duration are considered.   

Chemically Inert Nanoparticles Enhance Mechanical Degradation in Flowing Polymer Nanocomposites

Flow can be used to mechanically degrade polymer chains by transmitting macroscopic strain into the tension of chain backbones. Established models for flow-induced scission work well for highly dilute polymer solutions where chain scission events are uncorrelated, but scalable mechanical recycling processes must be designed for concentrated and messy plastics with significant intermolecular entanglement, particulate contamination, and catalytic material. This study applies molecular dynamics simulations to explore the flow-induced degradation of highly entangled polymer melts and polymer nanocomposites. Simulations show that incorporating nanoparticles into melts can significantly enhance flow-induced chain scission and substantially lower the specific work required to degrade the melt. This enhancement is due to a nonlinear coupling between chain alignment and nanoparticle ordering. Aligning chains induces nanoparticles to organize into aggregates, which in turn enhances the local stretching of chains squeezed between aggregates. This feedback loop rapidly builds chain tension at the entanglement length scale, causing chains to break prior to full extension and at lower strain rates than are needed to degrade neat melts.   

Enhanced Mechancial Properties in Uniaxially Stretched Films of Diblock Polymer Modified Poly(L-lactide)

Semicrystalline poly(L-lactide) (PLLA) is a bio-sourced, compostable alternative to conventional plastics

but lacks the toughness required for many applications. Chain alignment via uniaxially stretching

significantly toughens PLLA in the machine direction (MD) but leaves the transverse direction (TD)

brittle. This work reports uniaxially stretched films of PLLA blended with 3 wt% poly(ethylene oxide)-b-

poly(butylene oxide) (PEO-PBO/PLLA) with increased toughness in both MD and TD. Tensile testing in

the MD showed toughness in both PLLA and PEO-PBO/PLLA, with shear yielding as the primary

mechanism of deformation. In the TD, by contrast, PEO-PBO/PLLA films outperformed PLLA and

deformed uniformly by crazing. Faster stretching was shown to enhance the mechanical properties of

both MD and TD and was correlated with greater degrees of crystallinity and crystallite alignment

determined by wide-angle X-ray scattering. Increasing the stretching rate from 2 %s ^-1 to 100 %s ^-1

increased the yield stress in the MD from 76 MPa to 95 MPa, while in the TD elongation at break was

increased from 12 % to 36 %, exceeding that of the MD. In summary, this work presents uniaxially-

stretched but biaxially-toughened PEO-PBO/PLLA as a more sustainable alternative to conventional

plastics.   

Influence of Mixing Protocol on Block Copolymer Compatibilized PE/iPP Blends

Compatibilization of poly(ethylene) (PE) and isotactic poly(propylene) (iPP) using block copolymers is a promising approach for recycling mixed polyolefin plastic waste. An EXE triblock copolymer was premixed into commercial PE and iPP followed by addition in a subsequent mixing step of the second homopolymer, where X denotes an iPP melt-miscible poly(ethylene-ran-ethylethylene) random copolymer and E represents hydrogenated 1,4-poly(butadiene). Through transmission electron microscopy, EXE was found to produce micelles in PE and larger aggregates in iPP. The 2-stage blending process led to materials that consistently underperformed in uniaxial tensile testing blend specimens prepared by mixing all three components in a single step. Annealing both PE-rich and iPP-rich 2-stage blended specimens above the melting temperature followed by cooling to room temperature led to recovery of the exceptional toughness (ca. > 400% strain at break) obtained with the single-stage mixing process, but over surprisingly different annealing times. These results reveal distinct process-dependent differences in interfacial localization of EXE in melt blended PE and iPP.    

Immiscible Polymer Blend Compatibilization through Pendant Ionic Interactions

Polymer blending provides a simple recycling platform for waste commodity plastics, yet its limited applicability is based on the immiscibility between chemically dissimilar polymers that results in macrophase separated and poorly performing materials. Prior theoretical work has suggested that small numbers of ionic interactions can be used for immiscible polymer blend compatibilization. Here, we demonstrate that low levels of incorporation of ionic bonds (1, 7 and 10 mol% pendant functionalization of a polymer backbone) formed via sulfonic acid to imidazole base proton transfer mechanism results in the formation of optically clear blends in an otherwise incompatible poly(dimethylsiloxane) and poly(n-butyl acrylate) mixture. Oscillatory rheology and small angle X-ray scattering reveal the unique blend viscoelastic and microstructural behavior at various ionic functionalization levels, highlighting the underlying rich physics of immiscible polymer blends with sparse incorporation of charged groups. This work is opening numerous avenues in exploiting ionic interactions as a tool towards alleviating the current plastic waste crisis.   

Microscopic insights into compatibilized semicrystalline blends

Recent work shows certain compatibilizers can enhance polyolefin blend toughness, potentially reducing the cost of mixed plastic waste recycling. While researchers have proposed several toughening mechanisms, their validation is challenging. Experiments to resolve how compatibilizers interact with crystal or amorphous domains are difficult to perform. Employing a coarse-grained model that forms crystal lamellae, we use molecular dynamics simulations to investigate the interfacial structure of semicrystalline polymer blends with and without compatibilizer. These simulations offer insights into how compatibilizers interact with the lamellae, including whether they co-crystallize or form trapped entanglements. Such findings could aid in compatibilizer architecture optimization.   

Polymorphism and Stretch-Induced Transformations of Long-Spaced Sustainable Aliphatic Polyesters

Polyethylene-like bioderived, biodegradable, and fully recyclable unbranched aliphatic polyesters, such as PE-2,18, develop hexagonal crystal structures upon quenching from the melt to temperatures < ~50 °C, and orthorhombic-like packing at higher quenching temperatures, or after isothermal crystallization. Both crystal types are layered. While all-trans CH₂ packing characterizes the structure of the orthorhombic-like form, there is significant conformational disorder in the staggered long CH₂ sequences of the hexagonal crystals. On heating, the hexagonal crystals transform to the orthorhombic type at ~ 60°C via melt-recrystallization, but no change is apparent during heating samples with the orthorhombic form up to the melting point (~ 95°C). The hexagonal structure is of interest not only because it develops under very rapid quenching from the melt, but also because under uniaxial tensile deformation it undergoes a stretched-induced transformation to the orthorhombic structure. Compared to deformation of orthorhombic specimens that maintain the same crystal type, such transformation derives in larger strains and enhanced strain hardening, thus representing a desired toughening mechanism for this type of polyethylene-like materials.   

Understanding and Controlling Polymer-Porous Solid Interactions for Polymer Upcycling

Recent reports have shown that metal-functionalized high surface area porous materials can catalyze the deconstruction of various polymers. To enable efficient and selective polymer upcycling reactions, it is critical to understand and control the interactions between polymers and porous catalyst materials. In this presentation, I will describe our collaborative efforts to understand these interactions by modifying silica-based porous solids with atomic layer deposition (ALD). Based on capillary rise infiltration measurement of polyethylene and polystyrene into ALD-modified porous solids and calorimetry characterization of hexane and benzene adsorption onto such porous solids, we show that there is a strong correlation between the polymer-solid interfacial energy and the heat of gas adsorption. This trend is rationalized by calculating the interaction energy between small molecules and surfaces using density functional theory (DFT) calculations. Moreover, coarse-grained (CG) simulations also corroborate that the monotonic decreasing relationship between the polymer melt-solid interfacial tension and the solid-gas heat of adsorption. These new insights potentially provides guidelines on designing new catalyst materials for efficient polymer upcycling reactions.   

Crystalline Structures and Transitions in Chemically Recyclable Poly(oligocyclobutane): Monomer Sequence and Tacticity

Chain extension of telechelic (1,n′-divinyl)oligocyclobutane (DVOCB) oligomers using acyclic diene metathesis yields pDVOCB, a new class of semi-crystalline polyolefin. At temperatures below its melting point (Tm), pDVOCB exhibits a crystal-crystal transition (Tcc) in which pDVOCB chains become rotationally disordered around the chain axis. These transitions are examined in a series of pDVOCB polymers of varying oligomeric repeat unit, manifesting as a reduction in both Tm and Tcc with decreasing oligomer length. The influence of cis and trans content on pDVOCB phase behavior is investigated through saturation of the pDVOCB backbone to achieve HpDVOCB. Hydrogenation leads to improved stability of the HpDVOCB polymorphs, as reflected by a significant increase in both Tcc and Tm. These experimental results are complemented using simulations, providing insights into the effects of chain sequence and dispersity on pDVOCB crystalline behavior. Their crystallization behavior is key to their chemical solubility, which in turn determines the depolymerization efficacy. Thus, understanding these behaviors may guide polymer design and expand our control over the depolymerization potential of these chemically recyclable polyolefins.   

Reactive Additives for Mechanical Recycling of Polyethylene terephthalate-Polyethylene Mixed Waste

Mechanical recycling of polymers is challenging because physical sorting into pure streams is imperfect, often resulting in mixed waste. Due to the thermodynamic immiscibility of commonly used materials, such as polyethylene terephthalate (PET) and polyethylene (PE), the melt reprocessing step of mechanical recycling produces phase separated blends from such mixtures that are usually brittle. Carefully designed compatibilizers can improve the mechanical properties of polymer blends, improving the value of recycled plastics and preventing downcycling. Reactive additives (e.g., hydroxy-telechelic PE) have shown promise in improving mechanical properties of blends at low loadings by forming compatibilizers in-situ during melt processing. This work aims to better understand the effect of reactive functional group location. Ring-opening metathesis polymerization was used to control the overall molar mass and spacing between functional groups along a PE precursor backbone. By melt-mixing the reactive additives in PET/PE blends at 1 wt%, an optimal molar mass between reactive sites around 1000 g/mol was identified producing a strain at break of 300%, a 12x increase over neat blend analogs.   

Recyclable biobased self-blown non-isocyanate polyurethane foams: Influence of blowing agent structure and concentration

Polyhydroxyurethanes (PHUs), made by the aminolysis of cyclic carbonates, are non-isocyanate polyurethane materials with promising potential as benign alternatives to isocyanate-based polyurethanes. Recently, thiols were shown to react with cyclic carbonates to liberate CO₂, presenting a simple pathway to produce self-blowing PHU foams. We recently developed a rheology-guided, rapid preparation method for self-blowing PHU foams, allowing for dramatic reduction of reaction times to achieve foams. This method unlocks the potential to employ less reactive monomers, such as biobased monomers that are important for sustainability. Here, we have studied the effects of blowing agent concentration and thiol functionality to understand the structure-property relationships in PHU foams. We demonstrate that morphology depends on thiol concentration but is independent of the thiol functionality. In contrast, compressive mechanical properties depend greatly on both concentration and functionality of blowing agents. To address the sustainability challenges of network foams, the PHU foams can be melt-reprocessed into bulk films with full recovery of crosslink density. We further elaborate the dynamic behaviors of these dynamic network foams via stress relaxation, observing tunable relaxation times and activation energies that depend on structure and concentration of the thiols. Lastly, we show excellent elevated-temperature creep resistance of the bulk PHU films, highlighting potential applications of the recycled foams as elastomers.   

Decrystallization Free Energy in Polyesters: Insights from Molecular Dynamics Simulations

Continued breakthroughs in catalytic deconstruction strategies for commodity semicrystalline plastics are essential to addressing the global crisis of plastics pollution. We have developed a molecular dynamic-based protocol to estimate the potential of mean force to decrystallize a single chain from the polymer crystal surface in water. The decrystallization work is an essential element of many depolymerization strategies for semi-crystalline polymers including for hydrolase enzymes prior to chain cleavage. Five synthetic polyesters of commercial and scientific interest (PET, PBT, PEF, PEN, and PTT) were examined including PET, the world's most consumed polyester. Our calculations indicate free energy barriers in the range from ~15 kcal/mol (PEN) to ~8 kcal/mol (PEF) per monomer. Our results give insight into the molecular interactions that form the structural basis of semi-crystalline synthetic polyesters, providing guidance to experimentalists pursuing more efficient plastic recycling and upcycling strategies, which could include catalyst development, fine-tuning conditions during the recycling process including pretreatment, enzyme and chemical selections, and design of new materials.   

Investigating the internal structure of methylcellulose fibrillar assembly in aqueous solutions using multiscale modeling and simulations

Methylcellulose (MC) is a cellulose derivative where some or all hydroxyl groups are replaced by methoxy groups. Aqueous solutions of MC demonstrate a unique phase behavior where MC chains are soluble in water at lower temperatures but assemble into semi-flexible fibrils and fibrillar networks at elevated temperatures. Structural analysis via small-angle scattering (Macromolecules, 2018, 51, pp7767) further reveals consistent MC fibril diameters at varying molecular weights and concentrations. How the MC chains assemble into consistent fibril diameter has been debated by various groups and yet no conclusive answer has been found. To answer this question, we combine coarse-grained (CG) and atomistic molecular dynamics (MD) simulations. CG MD simulations show how MC chains assemble into fibrils with consistent diameters; chains align parallel to each other during assembly into fibrils with a select few outermost MC chains twisting around the fibril. Atomistic MD simulations of MC chains in explicit water show increased twisting of MC chains in the fibrils at elevated temperatures as compared to room temperature suggesting that the increased hydrophobicity and bulkiness of the methoxy groups as compared to hydroxyl groups is the driving force for such twisted MC chain configurations.   

Changes to the morphology, density and mechanical stiffness of phytoglycogen nanoparticles subjected to acid hydrolysis

Phytoglycogen (PG) is a glucose-based polymer with a dendritic architecture produced as compact nanoparticles in sweet corn. Its deformability, hydration, biocompatibility and digestibility make PG nanoparticles ideal for applications in personal care, nutraceuticals, and drug delivery. The particles can also be modified chemically to tune their physical properties, opening up possibilities for new applications. We performed acid hydrolysis, a process in which glycosidic bonds between the glucose subunits are broken by exposure to heated dilute acid. Dynamic light scattering and rheology measurements have shown that acid hydrolysis produces smaller, less dense particles [1]. We have used size-exclusion chromatography with multi-angle light scattering and atomic force microscopy force spectroscopy [2] to measure the dependence of the radius, molar mass and mechanical stiffness of the particles on hydrolysis time. These measurements allow us to quantify changes to the morphology, average density and the stiffness distribution within the PG particles with acid hydrolysis.

[1] H. Shamana and J.R. Dutcher, Biomacromolecules 23, 2040 (2022).

[2] B. Baylis et al., Biomacromolecules 22, 2985 (2021).