Polymer Diffusion Research Articles

Enhanced efficiency in plastic waste upcycling: the role of mesoporosity and acidity in zeolites.

By modulating zeolite confinement and improving pore diffusion properties, addressing a significant limitation in current plastic waste upcycling methodologies is essential. In this work, we have developed mesoporous zeolites that exhibit enhanced diffusion capabilities for long-chain polymers without compromising the crystalline structure. The mesopore volume doubled from 0.14 cm3 g-1 (CBV720) to 0.28 cm3 g-1 (M7203h) after zeolite modification. This has enabled to overcome the inefficiencies associated with polymer diffusion in conventional zeolites, significantly advancing the catalytic conversion of plastic waste into valuable products. Catalytic pyrolysis experiments on various polyethylenes underline the superior performance of mesoporous zeolites, especially for highly branched polymer structures where degradation temperatures are reduced by 29 °C compared to conventional zeolites, highlighting the importance of pore arrangement. Detailed analysis using NH3-TPD and in situ DRIFT spectroscopy reveals the crucial role of Brønsted acid sites in enhancing degradation efficiency. The optimized mesoporous zeolite catalyst, M720cit, showed excellent effectiveness in reducing degradation temperatures for a wide range of daily-use plastic waste. The T 10 values were significantly reduced for various plastic wastes: food packaging dropped to 208 °C (from 354 °C), plastic bottles to 349 °C (from 381 °C), and milk packets to 277 °C (from 409 °C), among others. Moreover, the well-retained microstructure of the M720 catalyst yielded a very similar product distribution despite the introduction of mesoporosity. This study not only surmounts crucial obstacles in the modulation of zeolite confinement and the enhancement of pore diffusion properties but also augments the economic and environmental sustainability of plastic waste conversion processes.

A multi-scale framework for predicting α-cyclodextrin assembly on polyethylene glycol axles.

Controlling the distribution of rings on polymer axles, such as α-cyclodextrin (αCD) on polyethylene glycol (PEG), is paramount in imparting robust mechanical properties to slide-ring gels and polyrotaxane-based networks. Previous experiments demonstrated that the functionalization of polymer ends could modulate the coverage of αCDs on PEG. To explore the design rule, we propose a multi-scale framework for predicting αCD assembly on bare and functionalized PEG. Our approach combines all-atom molecular dynamics with two-dimensional (2D) umbrella sampling to compute the free energy landscapes of threading αCDs onto PEG with ends functionalized by various moieties. Together with the predicted free energy landscapes and a lattice treatment for αCD and polymer diffusion in dilute solutions, we construct a kinetic Monte Carlo (kMC) model to predict the number and intra-chain distribution of αCDs along the polymer axle. Our model predicts the effects of chain length, concentration, and threading barrier on the supramolecular structure of end-functionalized polypseudorotaxane. With simple modifications, our approach can be extended to explore the design rule of polyrotaxane-based materials with advanced network architectures.

All-atom molecular dynamics simulation of solvent diffusion in an unentangled polystyrene film.

The diffusion behavior of low molecular weight solvents within an unentangled polystyrene film below and above its glass transition temperature is investigated. The diffusion behavior in the glassy state exhibits a distinct behavior known as case II or class II diffusion, noticeably diverging from conventional Fickian diffusion observed above the glass transition temperature of the polymer film. In the context of case II diffusion, the primary experimental observation entails the emergence of a well-defined concentration front moving at a constant speed, delineating a swollen, rubbery region from a glassy region within the polymer system. Despite the prevalence of this phenomenon in experimental settings, simulating case II diffusion has posed a significant challenge, primarily due to the computationally intensive nature of the diffusion process. To address this, we have developed an all-atom molecular dynamics simulation approach for the observation of case II diffusion in glassy polymers. This method aims to unravel the intricacies of the diffusion process, providing valuable insights into the dynamic interactions between solvents and the polymer matrix.

Probing the Molecular Mechanism of Viscoelastic Relaxation in Transient Networks.

Hydrogels, which have polymer networks through supramolecular and reversible interactions, exhibit various mechanical responsibilities to its surroundings. The influence of the reversible bonds on a hydrogel's macroscopic properties, such as viscoelasticity and dynamics, is not fully understood, preventing further innovative material development. To understand the relationships between the mechanical properties and molecular structures, it is required to clarify the molecular understanding of the networks solely crosslinked by reversible interactions, termed "transient networks". This review introduces our recent progress on the studies on the molecular mechanism of viscoelasticity in transient networks using multiple methods and model materials. Based on the combination of the viscoelasticity and diffusion measurements, the viscoelastic relaxation of transient networks does not undergo the diffusion of polymers, which is not explained by the framework of conventional molecular models for the viscoelasticity of polymers. Then, we show the results of the comparison between the viscoelastic relaxation and binding dynamics of reversible bonds. Viscoelastic relaxation is primarily affected by "dissociation dynamics of the bonds" and "network structures". These results are explained in the framework that the backbone, which is composed of essential chains supporting the stress, is broken by multiple dissociation events. This understanding of molecular dynamics in viscoelasticity will provide the foundation for designing transient networks.

Kinetics of Light-Responsive CNT / PNIPAM Hydrogel Microactuators.

Light-responsive microactuators composed of vertically aligned carbon nanotube (CNT) forests mixed with poly(N-isopropylacrylamide) (PNIPAM) hydrogel composites are studied. The benefit of this composite is that CNTs act as a black absorber to efficiently capture radiative heating and trigger PNIPAM contraction. In addition, CNT forests can be patterned accurately using lithography to span structures ranging from a few micrometers to several millimeters in size, and these CNT-PNIPAM composites can achieve response times as fast as 15ms. The kinetics of these microactuators are investigated through detailed analysis of high-speed videos. These are compared to a theoretical model for the deswelling dynamics, which combines thermal convection and polymer diffusion, and shows that polymer diffusion is the rate-limiting factor in this system. Applications of such CNT/hydrogel actuators as microswimmers are discussed, with light-actuating micro-jellyfish designs exemplified, and >1500 cycles demonstrated.

Properties of Low-Exothermic polymer grouting materials and its application on highway

Frost heaving and thawing settlement are the main problems in highways in permafrost regions. Non-aqueous reacting polymer is a relatively effective and environmentally friendly material to repair highways. However, its expansion and heat release influence on different strata in perma-frost areas remains unclear. Thus, a low-exothermic polymer was used to conduct grouting tests in different strata of frozen soil. The heat release characteristics and thermal conductivity of low exothermic polymers during the reaction were studied by laboratory experiments. Then it was applied to the protection of freeze-heave and thaw subsidence of highway in frozen soil area. Sensors were arranged to dynamically monitor the influence of polymer grouting, diffusion, expansion, and exothermic processes on different layers of frozen soil. The results showed that the thermal conductivity of low exothermic polymerwas affected by density, the higher the density, the higher the thermal conductivity. The maximum temperature, maximum displacement, and maximum pressure of polymer grouting on each layer were less than 36 °C, 1.4 mm, and 280 kPa, respectively, and the soil of each layer recovered to its initial state 60 min after grouting was completed. Polymer grouting had significant effects on the artificial road bases with a depth of more than 80 cm, and the affected depth of semi-rigid bases was less than 40 cm. Because of serious damage in the driving direction, the grout preferentially diffused to the driving side. Therefore, repairing roadbed diseased in the permafrost region using a low-exothermic polymer is feasible.

Programming hydrogel adhesion with engineered polymer network topology

Hydrogel adhesion that can be easily modulated in magnitude, space, and time is desirable in many emerging applications ranging from tissue engineering and soft robotics to wearable devices. In synthetic materials, these complex adhesion behaviors are often achieved individually with mechanisms and apparatus that are difficult to integrate. Here, we report a universal strategy to embody multifaceted adhesion programmability in synthetic hydrogels. By designing the surface network topology of a hydrogel, supramolecular linkages that result in contrasting adhesion behaviors are formed on the hydrogel interface. The incorporation of different topological linkages leads to dynamically tunable adhesion with high-resolution spatial programmability without alteration of bulk mechanics and chemistry. Further, the association of linkages enables stable and tunable adhesion kinetics that can be tailored to suit different applications. We rationalize the physics of polymer chain slippage, rupture, and diffusion at play in the emergence of the programmable behaviors. With the understanding, we design and fabricate various soft devices such as smart wound patches, fluidic channels, drug-eluting devices, and reconfigurable soft robotics. Our study presents a simple and robust platform in which adhesion controllability in multiple aspects can be easily integrated into a single design of a hydrogel network.

Transforming Cl-Containing Waste Plastics into Carbon Resource for Steelmaking: Theoretical Insight.

The accumulation of waste plastics poses a significant environmental challenge, leading to persistent pollution in terrestrial and aquatic ecosystems. A practical approach to address this issue involves the transformation of postconsumer waste plastics into industrially valuable products. This study focuses on an example of harnessing the carbon content in these polymers for carbon-demanding industrial processes, thereby reducing waste plastics from the environment and alleviating the demand for mined carbon resources. Employing quantum simulations, we examine the viability of polychloroprene as a carburizing agent in the steelmaking process. Our simulations reveal that polychloroprene exhibits excellent carbon diffusivity in molten iron, with a theoretical diffusion coefficient of 8.983 × 10-5cm2 s-1. This value competes favorably with that of metallurgical coke and surpasses the carbon diffusivity of other polymers, such as polycarbonate, polyurethane, and polysulfide. Additionally, our findings demonstrate that the chlorine content in polychloroprene does not permeate into molten iron but instead remains confined to the molten iron and slag interface.

Roles of specific hydrogen-bonding interactions in the crystallization kinetics of polymers

Polyamides hold specific hydrogen-bonding interactions as the hierarchical thermodynamic driving forces for crystallization. Their roles in crystallization kinetics are worthy of further investigation. We performed dynamic Monte Carlo simulations to compare crystallization kinetics among 16-mer melts holding hydrogen-bonding interactions with variable strengths specifically assigned to 2, 4 and 8 monomers on the chain, respectively. The results demonstrate higher densities and strengths of hydrogen-bonding interactions generally accelerating the cooling and isothermal crystallization as well as the lateral growth of lamellar crystals, consistent with the experimental observations of polyamide crystallization. Too strong hydrogen-bonding interactions will slow down polymer diffusion and thus bring retardation to crystallization kinetics. Furthermore, hydrogen-bonding interactions at the central 8–9 monomers make higher crystal growth rates than those at the evenly distributed 5–12 monomers, demonstrating the dominant roles of specific interactions in the hierarchical parallel packing of polymer chains at the lateral growth front of lamellar crystals.

Mechanism of bonding, surface property, electrical behaviour, and environmental friendliness of carbon/ceramic composites produced via the pyrolysis of coal waste with polysiloxane polymer

A simple mixing-pressing followed by thermal curing and pyrolysis process was used to upcycle coal waste into high-value composites. Three coal wastes of different physicochemical properties were investigated. The hypothetical mechanisms of bonding between the coal particles and the preceramic polymer are presented. The textural properties of the coals indicated that the lowest volatile coal waste (PCD) had a dense structure. This limited the diffusion and reaction of the preceramic polymer with the coal waste during pyrolysis, thereby leading to low-quality composites. The water contact angles of the composites up to 104° imply hydrophobic surfaces, hence, no external coating might be required. Analysis of the carbon phase confirmed that the amorphous carbon structure is prevalent in the composites compared to the coal wastes. The dc volume resistivity of the composites in the range of 22 to 82 Ω-cm infers that the composites are unlikely to suffer electrostatic discharge, which makes them useful in creating self-heating building parts. The leached concentrations of heavy metal elements from the composites based on the end-of-life scenario were below the Toxicity Characteristic Leaching Procedure regulatory limits. Additionally, the release potential or mobility of the metals from the composites was not influenced by the pH of the eluants used. On the basis of the reported results, these carbon/ceramic composites show tremendous prospects as building materials due to these properties.Graphical

Nonequilibrium thermodynamic modeling of case II diffusion in glassy polymers

In this article, we formulate a nonequilibrium thermodynamic theory for case II diffusion in a glassy polymer. In the model, we integrate the three-dimensional elasto-visco-plasticity of a glassy polymer, the Flory-Rehner theory for polymer network swelling, and the kinetics law for solvent migration with the concentration-dependent diffusivity. Our theory is given in a general 3D tensorial form, which, therefore, can be used to model case II diffusion in glassy polymers with arbitrary geometries and under different mechano-chemical loading conditions. An intrinsic length determined by the combination of the polymer viscosity and solvent diffusivity arises from the theory, which dictates the size-dependent case II diffusion in glassy polymers. The theory developed in the article may provide important guidance for designing semi-permeable membranes, waterproof coating, and drug-delivery systems, where case II diffusion can commonly occur.

Liquid state theory study of the phase behavior and macromolecular scale structure of model biomolecular condensates.

Biomolecular condensates can form through the liquid-liquid phase separation (LLPS) of proteins and RNAs in cells. However, other states of organization, including mesostructured network microstructures and physical gels, have been observed, the physical mechanism of which are not well understood. We use the Polymer Reference Interaction Site Model liquid state integral equation theory to study the equilibrium behavior of (generally aperiodic in sequence) biomolecular condensates based on a minimal sticker-spacer associating polymer model. The role of polymer packing fraction, sequence, and the strength and range of intermolecular interactions on macromolecular scale spatial organization and phase behavior is studied for typical sticker-spacer sequences. In addition to the prediction of conventional LLPS, a sequence-dependent strongly fluctuating polymeric microemulsion homogeneous state is predicted at high enough concentrations beyond the so-called Lifshitz-like point, which we suggest can be relevant to the dense phase of microstructured biomolecular condensates. New connections between local clustering and the formation of mesoscopic microdomains, the influence of attraction range, compressibility, and the role of spatial correlations across scales, are established. Our results are also germane to understanding the polymer physics of dense solutions of nonperiodic and unique sequence synthetic copolymers and provide a foundation to create new theories for how polymer diffusion and viscosity are modified in globally isotropic and homogeneous dense polymeric microemulsions.

Robust estimates of solute diffusivity in polymers for predicting patient exposure to medical device leachables

AbstractMedical devices often include polymeric components, which contain additives or contaminants that may leach into patients and pose a health risk. Previously, we proposed a mass transport model that conservatively estimates the leaching kinetics and only requires the solute's diffusion coefficient in the polymer, , to be specified. Because determining experimentally is time‐consuming, we also parameterized empirical models to estimate worst‐case values using only the solute molecular weight, . These models were based on a modest database and were limited to 19 polymers and larger solutes ( g/mol). Here, we assemble a much larger database, which enables us to construct more accurate models using a robust statistical approach, expanding the coverage to 50 device‐relevant polymers and smaller solutes ( g/mol). Then, we demonstrate several applications of these bounds, including modeling the release kinetics. Finally, we observe an interesting phenomenon, a discontinuous drop in of up to 25,000× for solutes with g/mol in glassy polymers. Using molecular simulations and cheminformatics tools, we propose a novel definition of the effective diameter of free volume channels in polymers, and we show that solutes larger than this channel size diffuse much more slowly.

Computer Simulation Insight into the Adsorption and Diffusion of Polyelectrolytes on Oppositely Charged Surface.

In the present work, by means of computer simulation, we studied the adsorption and diffusion of polyelectrolyte macromolecules on oppositely charged surfaces. We considered the surface coverage and the charge of the adsorbed layer depending on the ionization degree of the macromolecules and the charge of the surface and carried out a computer experiment on the polymer diffusion within the adsorbed layers, taking into account its strong dependency on the surface coverage and the macromolecular ionization degree. The different regimes were distinguished that provided maximal mobility of the polymer chains along with a high number of charged groups in the layer, which could be beneficial for the development of the functional coatings. The results were compared with those of previous experiments on the adsorption of polyelectrolyte layers that may be applied as biocidal renewable coatings that can reversibly desorb from the surface.

Kinetics Modelling of Vitamin B12 Release in an Agar/κ-Carrageenan Hydrogel Blend

A phycocolloidal hydrogel patch is studied as a potential material for the transdermal delivery device for vitamin B12. The vitamin release kinetics from an agar/κ-carrageenan hydrogel blend as a function of mass ratio and vitamin loading. Concentration measurements were done using a colorimetric method, and the experimental data were fitted into the Korsmeyer-Peppas model, Peppas-Sahlin model, and Berens-Hopfenberg model. From the curve fitting, parameters such as first-order polymer relaxation constant and diffusivity constant were obtained. The results showed that for the Korsmeyer-Peppas model and the Peppas-Sahlin model, the release mechanism followed Fickian diffusion predominantly. On the other hand, the Berens-Hopfenberg model fit shows that the release mechanism predominantly follows non-Fickian diffusion and may need to be modified.

Deconvoluting the roles of polyolefin branching and unsaturation on depolymerization reactions over acid catalysts

Developing a circular economy for plastics is a societal priority with a range of stakeholders engaged in developing pathways to mechanical and chemical recycling. Acid catalysis has been proposed as an attractive strategy to chemically recycle or upcycle plastic waste. In principle, the energetics that govern polyolefin decomposition reactions over Brønsted acid sites should be the same ones known for alkane cracking. However, the nature of polymers as feedstock imposes new challenges for these well-known processes, e.g., strong mass transfer limitations. To better understand the role of polymer structure and diffusion on cracking rates, the effect of tertiary carbons, chain unsaturation, and catalyst active sites distribution were studied. Our findings confirm that higher concentrations of tertiary carbons will govern decomposition reactions, regardless of diffusion limitations. Also, we unveil that LLDPE reactions mostly take place on the external surface of ZSM-5 crystals while HDPE conversion takes place within the zeolite pores.

Tuning High-Density Polyethylene Hydrocracking through Mordenite Zeolite Crystal Engineering

We investigate the hydrocracking of high-density polyethylene using a bifunctional Pt/Al2O3 and modified mordenite acid catalyst. Mass transport limitations impact polymer diffusion into the mordenite pore complex. Initial reaction intermediates are formed on the zeolite’s outer surface. Intercrystallite open-end mesopores improve the diffusion of reaction intermediates deeper into the crystal. Recrystallization and desilication of mordenite lead to a higher polymer conversion and shift the product distribution maximum from pentanes to hexanes and heptanes. The nature of mesopores (occluded or open) and total Brønsted acidity significantly impact zeolite activity and selectivity.

Effect of process parameters on interlaminar properties of thermoplastic composite: Molecular dynamics simulation and experimental verification

Interlaminar properties are one of the most important indicators of thermoplastic composite quality. In this paper, the unidirectional carbon fibre reinforced polyether ether ketone (CF/PEEK) thermoplastic composite was taking as the research object, an all-atom model constructing method of the interlaminar interface was proposed, the influence mechanism of process parameters including forming pressure and temperature on interlaminar properties were studied by molecular dynamics (MD) simulation and experimental verification. The simulation results showed that a reasonable forming pressure could promote the non-bonded interaction between polymer molecules, and an appropriate raising of forming temperature could promote the thermal motion of polymer molecules, which was conducive to the interlaminar diffusion and penetration of polymer, thereby improving the interlaminar bonding strength, the interlaminar properties were more sensitive to the changes of forming temperature than pressure. The experimental results were in good agreement with the MD simulation result in terms of the significance and the trend of process parameters influence on the interlaminar properties. Illustrating that it is effective to explore the forming mechanism of thermoplastic composite and guide the forming process by MD simulation.

Diffusion NMR and Rheology of a Model Polymer in Bacterial Cell Lysate Crowders.

The intracellular milieu is crowded and heterogeneous, and this can have profound consequences for biomolecule motions and biochemical kinetics. Macromolecular crowding has been traditionally studied in artificial crowders like Ficoll and dextran or globular proteins such as bovine serum albumin. It is, however, not clear if the effects of artificial crowders on such phenomena are the same as the crowding that is experienced in a heterogeneous biological environment. Bacterial cells, for example, are composed of heterogeneous biomolecules with different sizes, shapes, and charges. Using crowders composed of one of three different pretreatments of bacterial cell lysate (unmanipulated, ultracentrifuged, and anion exchanged), we examine the effects of crowding on the diffusivity of a model polymer. We measure the translational diffusivity, via diffusion NMR, of the test polymer polyethylene glycol (PEG) in these bacterial cell lysates. We show that the small (Rg ∼ 5 nm) test polymer shows a modest decrease in self-diffusivity with increasing crowder concentration for all lysate treatments. The corresponding self-diffusivity decrease in the artificial Ficoll crowder is much more pronounced. Moreover, a comparison of the rheological response of biological and artificial crowders shows that while the artificial crowder Ficoll exhibits a Newtonian response even at high concentrations, the bacterial cell lysate is markedly non-Newtonian; it behaves like a shear-thinning fluid with a yield stress. While at any concentration the rheological properties are sensitive to both lysate pretreatment and batch-to-batch variations, the PEG diffusivity is nearly unaffected by the type of lysate pretreatment.

Improvement of the glass transition temperature in novel molybdenum carbide doped polyaniline nanocomposites

Several studies have revealed that the glass transition temperature (Tg) of polymer nanocomposites (PNCs) may change upon the loading of even a small number of nanoparticles (NPs). However, the exact phenomenon behind such shifting in Tg is still not well known. This study uses experimental and MD simulation approaches to explore the physics of the Tg of polyaniline (PANI) nanocomposite with molybdenum carbide (Mo2C) nanoparticles at different weight percentage loading of NPs. The experimental study shows that, at a lower wt-% loading of NPs, the Tg of the resulting PNCs decreases slowly, and above a particular loading wt-%* (30 wt-%), the depression in Tg becomes more serious. The SEM images show that for wt-% < wt-%*, NPs remain diffuse in the polymer matrix, and for wt-%t > wt-%* a large number of NPs surround the polymer. The MD simulation reveals a slow increase in the diffusion of polymer and NPs for wt-% < wt-%* and a sudden increase for wt-%t > wt-%*. The measurement of the radius of gyration of the polymer reveals swelling of the polymer for wt-% < wt-%* and contraction of the polymer for wt-%t > wt-%*. Moreover, the variations of the diffusion constants and the radius of gyration of the polymer are consistent with the Tg behaviour of PNCs.