Polymer Diffusion Research Articles

(Invited) Decoding the Symbiotic Relationship between Ionomer and Water in Cathode Catalyst Layers of PEM Fuel Cells

The pressing need for highly-performing and environmentally benign energy technologies continues to spur on research on polymer electrolyte fuel cells (PEFC). Among the components that are needed for a well-functioning, durable, and affordable PEFC, the cathode catalyst layer (CCL) continues to stand out. The main function to be provided by the CCL is to facilitate the oxygen reduction reaction (ORR). However, this function entails an intricate interplay of microscopic kinetics with the transport of electrons, protons, oxygen molecules, and water. A hierarchy of structural effects must be considered, as illustrated in Figure 1. Due to the complex composition and multiscale nature of the CCL, physical modeling has gained high importance in efforts to rationalize the dynamic interplay of structure, properties, and performance. The first part of the presentation will briefly review the capabilities of model-based analyses of experimental data to deconvolute and quantify voltage loss contributions,1 extract kinetic and transport parameters from fitting and discover systematic trends and correlations among these parameters,2 provide an activity map of the layer, and evaluate the overall effectiveness factor of Pt utilization.3 Macrohomogeneous modelling can propose or predict the optimal CCL thickness or macroscopic effective composition (for the target range of operating conditions).Recognizing the importance of an optimal water-distribution for a well-functioning CCL, recent efforts in CCL modeling have been focusing on the consistent treatment of aspects like pore size distributions and pore network morphologies as well as wettability properties. The ionomer inclusions in the CCL play a crucial role in this context. At the microscopic scale, the ionomer film that forms an interface with the water-covered catalyst-support surface strongly impacts the local reaction environment that determines the rate of the ORR as well as that of platinum dissolution. Moreover, the structure and distribution of ionomer inclusions determine the wetting behaviour of pores and thus the water sorption properties of the porous composite CCL, which in turn affect the transport properties for oxygen and water. Recent forays in modelling that strive to unravel the intertwined impacts of ionomer and water will be presented. As a final consideration, a CCL cannot be understood and optimized as a stand-alone component. Overarching balances at the PEFC level in terms of reactant, charge, water, and heat fluxes must be considered. Comprehensive modeling approaches must account for the coupling of the corresponding local equilibria and transport phenomena across the whole cell, including polymer electrolyte membrane, diffusion media and flow fields. Efforts focusing on the coupled water fluxes across the cell are underway with promising results to look out for. References. 1 M. Baghalha, J. Stumper and M. Eikerling, ECS Transactions 28, 159–167 (2010). 2 T. Muzaffar, T. Kadyk and M. Eikerling, Sustainable Energy & Fuels 2, 1189–1196 (2018). 3 M. Eikerling and A.A. Kulikovsky, Polymer Electrolyte Fuel Cells – Physical Principles of Materials and Operation, CRC Press Taylor & Francis Group, 2014. Figure 1

The Role of Ion-Correlation in Reducing the Lithium Transference Number in Lithium-Ion Polyelectrolyte Solutions

Non-aqueous polyelectrolyte solutions (PESs) have been suggested as a promising route to high conductivity, high transference number (t+) electrolytes.1–3 State-of-the-art liquid electrolytes suffer from low t+, meaning the majority of ionic conductivity results from motion of the anion rather than the electrochemically active Li+ ion. Increasing t+ requires decreasing the mobility of the anion, which is dictated by both the diffusion of the anion as well as its net charge. PESs are intuitively appealing because anchoring the anion to a polymer backbone slows down the motion of the electrochemically inactive anion while maintaining higher ion conductivity through improved ion dissociation and solvent-mediated Li+ transport. However, in polyelectrolyte systems, increasing molecular weight both decreases polymer diffusion and increases charge, which will act as competing effects for t+.Recent molecular dynamic simulations of PESs have highlighted the critical importance of correlated ion motion in these systems and have called into question oligomeric PESs as a feasible strategy to achieving high t+ and conductivity electrolytes4,5 In this work we discuss complete studies of transport properties in lithium-ion and lithium metal battery-relevant PESs- specifically lithium triflimide appended polystyrene (PS-LiTFSI) and polymethacrylate (PM-LiTFSI) dissolved in carbonate blends. All prior PES experimental work in the literature has relied on ideal solution assumptions for measuring transport properties. This work represents the first rigorous characterization of transport properties for a battery-relevant polyelectrolyte solution. Using electrophoretic NMR and electrochemical experiments, we characterized the transport properties, including the electrophoretic ion mobilities, conductivity, diffusion coefficients, and t+ of these model PESs. While previous studies that rely on ideal assumptions predict that PESs will have higher t+ than monomeric solutions, we demonstrate that below the entanglement limit, t+ decreases with increasing degree of polymerization. For higher degrees of polymerization, we directly observe Li+ move in the “wrong direction” in an electric field, evidence of a negative transference number due to correlated motion through ion clustering. Using calculated Onsager transport coefficients and insights from molecular dynamics modeling, we demonstrate that despite selectively slowing anion motion using polyanions, anion-anion correlation through the polymer backbone and cation-anion correlation through ion aggregates reduce the t+ in non-entangled PESs.References Diederichsen, K. M. et al. ACS Energy Lett. (2017).Diederichsen, K. M. et al. Macromolecules (2018).Dewing, B. L., et al. Chem. Mater. (2020).Fong, K. D. et al. ACS Cent. Sci. (2019).Fong, K. D., et al. Macromolecules (2020).

Extraction of Fickian water diffusion in polymers using terahertz time-domain spectroscopy

The dynamic behavior of polycarbonate and poly(methyl methacrylate) upon water immersion at temperatures of 40 °C and 70 °C was studied by gravimetric measurements and by terahertz time-domain spectroscopy. Time-resolved measurements of the terahertz refractive index and absorption coefficient show a good agreement to the gravimetric measurements. Assuming a Fickian diffusion mechanic the diffusion coefficients were extracted from both methods. It was found that the terahertz measurements provide a reliable non-contact way to study the water diffusion in polymers. Especially, the THz absorption coefficient seems to be a robust method to determine the diffusion coefficients. • Extraction of Fickian diffusion constants in polymers by THz time domain spectroscopy. • Terahertz optical constants are in good agreement against gravimetric analysis. • A contactless technique to investigate water mobility in polymers.

Dispersion and Diffusion Mechanism of Nanofillers with Different Geometries in Bottlebrush Polymers: Insights from Molecular Dynamics Simulation.

The dispersion and diffusion mechanism of nanofillers in polymer nanocomposites (PNCs) are crucial for understanding the properties of PNCs, which is of great significance for the design of novel materials. Herein, we investigate the dispersion and diffusion behavior of two geometries of nanofillers, namely, spherical nanoparticles (SNPs) and nanorods (NRs), in bottlebrush polymers by utilizing coarse-grained molecular dynamics simulations. With the increase of the interaction strength between the nanofiller and polymer (εnp), both the SNPs and NRs experience a typical "aggregated phase-dispersed phase-bridged phase" state transition in the bottlebrush polymer matrix. We evaluate the validity of the Stokes-Einstein (SE) equation for predicting the diffusion coefficient of nanofillers in bottlebrush polymers. The results demonstrate that the SE predictions are slightly larger than the simulated values for small SNP sizes because the local viscosity that is felt by small SNPs in the densely grafted bottlebrush polymer does not differ much from the macroscopic viscosity. The relative size of the length of the NRs (L) and the radius of gyration (Rg) of the bottlebrush polymer play a key role in the diffusion of NRs. In addition, we characterize the anisotropic diffusion of NRs to analyze their translational and rotational diffusion. The motion of NRs in the direction perpendicular to the end-to-end vector is more hindered, indicating that there is a strong coupling between the rotation of NRs and the motion of the polymer. The NR motion shows stronger anisotropic diffusion at short time scales because of the steric effects generated by side chains of the bottlebrush polymer. In general, our results provide a fundamental understanding of the dispersion of nanofillers and the microscopic mechanism of nanofiller diffusion in bottlebrush polymers.

Impact of Star Polyacid Branching on Polymer Diffusion within Multilayer Films

A series of linear and star poly(acrylic acids) (LPAA and star PAAs) were synthesized to explore the effect of molecular architecture on the stratification and polymer dynamics of electrostatic layer-by-layer (LbL) films. Studies of LbL deposition of LPAA and star PAAs with poly[2-(dimethylamino)ethyl methacrylate] (PDMAEMA) at acidic pH showed an ∼30% increase in film dry thickness with increased polymer branching. Consistent with a greater mass of star polymer deposited within the films, in situ ellipsometric measurements of PAA uptake from solution revealed ∼3.5-fold greater diffusion coefficients for 8-arm PAA in comparison to linear PAA. For comparison, the dynamics of the linear PDMAEMA partner was explored via neutron reflectometry (NR) studies of stacked multilayers containing hydrogenated and deuterated polycations, hPDMAEMA and dPDMAEMA. The stacked multilayers deposited from low-ionic-strength solutions were stratified, exhibiting interfacial widths between hydrogenated and deuterated stacks of ∼15 and 10 nm for films constructed with star and linear PAAs, respectively, suggesting relatively low mobility of the polycation in both assemblies. Further exposure of these films to 0.5 M sodium chloride solutions enhanced the mobility of PDMAEMA, revealing an order magnitude faster diffusion of PDMAEMA in films of 8-arm PAA relative to linear PAA. The faster diffusion of polymers within films of star polyacids was correlated not only with the compactness of star polymers but also with an ∼2-fold lower ionization of assembled 8-arm PAA as determined by Fourier transform infrared spectroscopy and thus a lower number of polycation–polyacid ionic contacts in the case of star polymers as compared to their linear counterparts. The significant influence of molecular architecture on the number of polymer–polymer contacts was further confirmed by isothermal titration calorimetry studies of polyelectrolyte complexes in solution.

Kinetics of Surface Enrichment of a Polymer in a Glass-Forming Molecular Liquid

X-ray photoelectron spectroscopy has been used to measure the surface concentration and the surface enrichment kinetics of a polymer in a glass-forming molecular liquid. As a model, the bulk-miscible system of maltitol-polyvinylpyrrolidone (PVP) was studied. The PVP concentration is significantly higher at the liquid/vapor interface than in the bulk by up to a factor of 170, and the effect increases with its molecular weight. At a freshly created liquid/vapor interface, the concentration of PVP gradually increases from the bulk value at a rate controlled by bulk diffusion. The polymer diffusion coefficient obtained from the kinetics of surface enrichment agrees with that calculated from viscosity and the Stokes-Einstein equation. Our finding allows prediction of the rate at which the surface composition equilibrates in an amorphous material after milling, fracture, and a change in ambient temperature.

Deteriorating dispersibility of flushable wet wipes during storage: Role of fibre swelling and ionic shielding

Wet wipes are everyday products with the purpose to be flushed down the toilet after usage and dispersed into single fibres and small fragments thereby. During wet storage, these wipes can deteriorate their dispersibility over time, characterized by the slosh box disintegration test, recently named dispersibility ageing. This effect in flushable wet wipes, usually made of long man-made cellulose fibres and short wood pulp fibres, can be reduced by using unbleached softwood kraft pulp as the short fibre component. The aim of this work is to analyse mechanisms that could contribute to dispersibility ageing. Therefore, we will discuss three mechanisms that are able to explain this effect, that occurs in cellulosic fabrics when stored in water. Swelling (1) is demonstrated to be a long-term effect, but altering swelling with pH and salt addition has no effect on dispersibility ageing, making it unlikely that swelling contributes to it. With ionic shielding (2) a mechanism has been introduced that could impact dispersibility ageing by cations leaching from the wood pulp, which was demonstrated for an unbleached softwood kraft pulp by conductivity measurements over wet storage time. These cations neutralize the negatively charged pulp fibre fibrils and restrict their possibility to entangle with each other, which could contribute to decreasing dispersibility ageing. We will discuss the mechanism of interdiffusion (3) in dispersibility ageing, although we were not able to find a method that quantifies the contribution of cellulosic polymer diffusion to the time-dependent deterioration in dispersibility.

Investigation of the effects of a pre-deposition heating system on the interfacial temperature and interlayer bonding strength for fused filament fabrication

PurposeThe purpose of this study is to propose the use of a pre-deposition heating system for fused filament fabrication (FFF) as a means to enhance interlayer bonding by elevating the substrate temperature. The effects of the heating on thermal profile at the bonding interface and the mechanical properties of three-dimensional printed parts are investigated.Design/methodology/approachA 12-W laser head is integrated to a commercial printer as the pre-deposition heating system. The laser beam heats up substate before the deposition of a fresh filament. Effects of laser powers are investigated and the thermal profile is measured with thermocouple, infrared camera and finite element model. The correlation between the temperature at the bonding interface and the bonding quality is investigated by conducting tensile testing and neck width measurement with microscope.FindingsThe pre-deposition heating system is proven to be effective in enhancing the inter-layer strength in FFF parts. Tensile testing of specimens along build direction (Z) shows an increase of around 50% in ultimate strength. A linear relationship is observed between the pre-deposition temperature at bond interface and bonding strength. It is evident that elevating the pre-deposition temperature promotes interlayer polymer diffusion as shown by the increased neck width between layers.Originality/valueThermocouples that are sandwiched between layers are used to achieve accurate measurement of the interfacial temperature. The temperature profiles under pre-deposition heating are analyzed and correlated to the interlayer bonding strengths.

Elucidating atomistic mechanisms underlying water diffusion in amorphous polymers: An autonomous basin climbing-based simulation method

Conventional molecular simulations of diffusive molecules in amorphous polymers lead to discrepancies in understanding diffusion mechanisms at low temperatures due to the short timescales of the simulations. This work proposes a combined scheme of the autonomous basin climbing (ABC) method accompanied with kinetic Monte Carlo and reactive molecular dynamics (MD) simulations to investigate water molecules’ diffusion pathways in amorphous epoxy resins across a wide range of temperatures. The ABC method, along with a reactive force field, provides an accurate potential energy surface sampling approach to elucidate diffusion mechanisms affected by hydrogen bonding interactions between water molecules and an epoxy network. The simulation framework allows the capture of thermal effects on the diffusion coefficients and offers a predictive simulation tool for predicting diffusion coefficients of water molecules in amorphous polymers. The simulation results show that the activation energy of the diffusion coefficient is in agreement with experimental data. The proposed simulation framework not only estimates kinetic properties of water diffusion in epoxy resins that are consistent with experimental observations, but also provides a predictive tool for studying the diffusion of small molecules in other amorphous polymers.

Controlled synthesis of charged lignin nanocarriers by electrospray

An experimental investigation of synthesis of lignin nanoparticles using an electrospray system was systematically performed. This investigation demonstrated the control of size and charge distribution of lignin nanoparticles in the single-step electrospray process by tuning the operation parameters and precursor properties. The most important parameters include the liquid flow rate, the initial precursor concentration, and the electrical conductivity. A generalized theoretical model to predict the size and charge developed in our previous work is then verified for a variety of experimental conditions. Finally, we explored the formation mechanism of lignin nanoparticles with three different morphologies: solid sphere, hollow sphere, and elliptical solid. The lignin nanoparticle morphology depends on the competition between the solvent evaporation, the polymer diffusion inside the droplet, and the Coulombic fission processes. This work provides significant insights for fabricating charged nanocarriers for drug delivery applications.

Predicting Solute Diffusivity in Polymers Using Time-Temperature Superposition.

We demonstrate a novel application of the time-temperature superposition (TTS) principle to predict solute diffusivity D in glassy polymers using atomistic molecular dynamics simulations. Our TTS approach incorporates the Debye-Waller factor ⟨u2⟩, a measure of solute caging, along with concepts from thermodynamic scaling methods, allowing us to balance contributions to the dynamics from temperature and ⟨u2⟩ using adjustable parameters. Our approach rescales the solute mean-squared displacement curves at several temperatures into a master curve that approximates the diffusive dynamics at a reference temperature, effectively extending the simulation time scale from nanoseconds to seconds and beyond. With a set of "universal" parameters, this TTS approach predicts D with reasonable accuracy in a broad range of polymer/solute systems. Using TTS greatly reduces the computational cost compared to standard MD simulations. Thus, our method offers a means to rapidly and routinely provide order-of-magnitude estimates of D using simulations.

Solute Diffusivity and Local Free Volume in Cross-Linked Polymer Network: Implication of Optimizing the Conductivity of Polymer Electrolyte.

The diffusion of small molecules or ions within polymeric materials is critical for their applications, such as polymer electrolytes. Cross-linking has been one of the common strategies to modulate solute diffusivity and a polymer’s mechanical properties. However, various studies have shown different effects of cross-linking on altering the solute transports. Here, we utilized coarse-grained molecular dynamics simulation to systematically analyze the effects of cross-linking and polymer rigidity of solute diffusive behaviors. Above the glass transition temperature , the solute diffusion followed the Vogel–Tammann–Fulcher (VTF) equation, D = D e. Other than the conventional compensation relation between the activation energy and the pre-exponential factor D, we also identified a correlation between and Vogel temperature T. We further characterized an empirical relation between T and cross-linking density. Integrating the newly identified correlations among the VTF parameters, we formulated a relation between solute diffusion and the cross-linking density. The combined results proposed the criteria for the optimal solute diffusivity in cross-linked polymers, providing generic guidance for novel polymer electrolyte design.

Salt and ion transport in a series of crosslinked AMPS/PEGDA hydrogel membranes

Salt permeability and ionic conductivity are critical properties for membranes in water purification and energy applications. Both properties depend on individual ion sorption and diffusion coefficients, which are significantly influenced by polymer chemical and physical parameters such as fixed charge concentration and membrane water content. However, systematic studies connecting polymer structure to ion transport properties are still lacking. In this study, a series of uncharged and charged membranes were synthesized using poly(ethylene glycol) diacrylate (PEGDA) as a cross-linker and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) as a charged monomer. Membrane fixed charge concentration and water uptake were systematically varied by adjusting AMPS content in the pre-polymerization mixture. Salt sorption and permeability coefficients and ionic conductivity of these membranes were measured as a function of NaCl solution concentration (0.01–1 M). Combining the solution-diffusion model and Nernst-Planck equation, individual ion diffusion coefficients were calculated. Experimental Na+ diffusion coefficients for all materials were well described by the Mackie and Meares tortuosity model, highlighting the strong influence of water content on ion diffusivity in both uncharged and charged polymers. Model predictions for Cl− diffusion coefficients agree reasonably well with experimental values, with some deviations occurring in more highly charged membranes. This discrepancy might result from interactions not captured by the Mackie and Meares model (e.g., fixed charge-ion interactions).

Visualizing polymer diffusion in hydrogel self-healing

Self-healing hydrogels are attractive to extend material lifetime by rapid recovery from damage; the underlying healing mechanism regarding polymer diffusion are of broad research interest. However, intuitive and convenient characterization of polymer diffusion remains challenging due to the complex and dynamic features of hydrogels. Herein, we have constructed a dually-crosslinked hydrogel system to decouple complex factors for direct visualization of polymer diffusion and quantified study of healing dynamics. The successively formed dually-crosslinked hydrogel networks are designated for purposes of self-healing/visualization and tunable constraining effects (varied crosslinking density), respectively. As a result, we observed direct polymer diffusion across the crack interface and calculated the diffusion speed ranging from 0.51 to 0.04 μm/s depending on varied constraining degree. The corresponding self-healing performance is consistent with other conventional characterizations (e.g., dynamic mechanical properties, surface morphology changes). The above method has enabled facile visualization of dynamic healing processes with flexible adjustment of polymeric systems, which could inspire novel designs of high-performance self-healing materials.

Digital Light Processing 3D Printing of Enhanced Polymers via Interlayer Welding.

Digital light processing (DLP) 3D printing is advantageous in high printing efficiency and printing resolution for fabricating complex structures across various applications. However, the layer-by-layer curing manner of DLP leads to weak interlayer adhesion and the anisotropic mechanical properties of printed objects. Here, linear polymers are introduced into commercial resins to weld the interlayer by the diffusion and entanglement of linear polymers after DLP printing via heat treatment. This introduction of linear polymers not only shows a strengthening and toughening effect on the printed objects, but also has no negative impact on the DLP printability. The tensile strengths of objects containing 4.7wt% polycaprolactone can reach up to ≈500% of that of neat samples in any printing direction. This simple strategy by adding linear polymers into printing resins provides an effective access to prepare DLP printed objects with improved mechanical properties as well as ensure printing resolution and printing efficiency.

Influence of intraparticle cross-linking on polymer diffusion in latex films prepared from secondary dispersions

In this work, we synthesized surfactant-free latex dispersions of nanoparticles (NPs) based upon emulsification of a preformed proprietary BASF polymer (MnGPC = 5000 g/mol, Ð = 3, denoted as Pol), in which the —COOH groups were partially neutralized by ammonia. To obtain useful coatings with polymers of such low molecular weight (MW), we approached this problem through the reaction of carboxylated polymer dispersions with neopentyl glycol diglycidyl ether (NGDE, 20 to 60 mol% based on carboxyl groups) to increase the MW and to introduce gel content. We employed AFM to monitor the influence of cross-link density on the surface roughness and found the presence of cross-links increases the resistance of particles to deformation. Fluorescence resonance energy transfer (FRET) measurements were carried out to determine the extent of polymer mixing upon film formation. Our most important finding is that the gel content substantially limited the extent of molecular mixing at long annealing times or in corresponding solvent-cast films. In parallel, we carried out a series of similar reactions on the secondary dispersion particles with a monoepoxide, consisting of a mixture of dodecyl and tetradecyl glycidyl ether. This reaction maintained the size and size distribution of the latex particles. These polymers had no gel content, but the reaction with the epoxide lowered the Tg of the polymer. FRET studies showed a pronounced influence of Tg on the rates of polymer diffusion in these films.

Multilayer nonwoven lining materials made of wool and cotton for clothing and footwear

Five types of multilayer nonwovens for clothing and footwear parts were obtained by the adhesive bonding method. The thickest middle layer of the material consists of evenly laid coarse camel or sheep fibers or of reconstituted cotton fibers from flaps, the upper and lower layers consist of knitwear, and polymer adhesive is located between the layers. The layers are bonded by thermal pressing at a temperature of 150 ± 5°C for 2.0 ± 0.2 min. The microstructure and morphology of fibers, polymer adhesive, and multilayer nonwoven fabric were investigated by FT-IR spectroscopy, SEM, and X-ray phase analysis. The chemical interaction between wool fibers and polymer adhesive, the geometric dimensions and shape of the fibers, the structure and morphology of the cross section of the layers of the material, and the change in the degree of crystallinity of the material have been established. The investigated coarse and thick fibers of camel and sheep wool are more suitable for the production of nonwoven textile material. In the process of thermal exposure, the molten polymer diffuses into the structure of the nonwoven layer and knitted fabric. The diffusion and excellent adhesion of the molten polymer to the fibers ensures the solidity and strength of the composite. The developed design provides high strength of the material as a whole and adhesive strength between layers, high heat-retaining properties, and the use of a mesh adhesive film provides sufficient air and vapor permeability.

Investigation on Electrical, Thermal, and Mechanical Properties of Silicone Rubber ATH Nanocomposites

In the present study, investigations have been carried out by choosing the nano-sized ATH (30 – 50 nm) particles as filler content in silicone rubber to acquire a deep insight to understand the use of nano ATH fillers in silicone rubber on its performance, to use it as insulation structure as an outdoor insulator. Variation in the surface condition of silicone rubber nanocomposites due to corona aging and the recovery characteristics analysis carried out through contact angle measurement. Nano ATH added in silicone rubber, up to 3 wt% of filler, shows increased dielectric constant with less loss factor. Water droplet-initiated discharges on silicone rubber nano ATH composites under DC voltage measured using UHF technique and the damage caused zone analyzed through AFM studies and by surface potential measurement, which indicates higher surface roughness and increased charge trap depth. It is observed that negative DC voltage has higher CIV followed by positive DC and AC voltage. Laser Induced breakdown studies (LIBS) show a direct correlation between plasma temperature and material surface hardness. Also, on laser shining on the surface of the specimen, its temperature was measured on the backside of the specimen through thermal imaging confirming increased diffusivity of temperature. Laser flash analysis shows a direct correlation between filler content in base polymer and thermal conductivity and diffusivity. Higher storage modulus and activation energies measured from the dynamic mechanical analysis indicate the significance of prepared composites as an excellent mechanical support structure for the power system network.

Motif-pattern dependence of biomolecular phase separation driven by specific interactions.

Eukaryotic cells partition a wide variety of important materials and processes into biomolecular condensates—phase-separated droplets that lack a membrane. In addition to nonspecific electrostatic or hydrophobic interactions, phase separation also depends on specific binding motifs that link together constituent molecules. Nevertheless, few rules have been established for how these ubiquitous specific, saturating, motif-motif interactions drive phase separation. By integrating Monte Carlo simulations of lattice-polymers with mean-field theory, we show that the sequence of heterotypic binding motifs strongly affects a polymer’s ability to phase separate, influencing both phase boundaries and condensate properties (e.g. viscosity and polymer diffusion). We find that sequences with large blocks of single motifs typically form more inter-polymer bonds, which promotes phase separation. Notably, the sequence of binding motifs influences phase separation primarily by determining the conformational entropy of self-bonding by single polymers. This contrasts with systems where the molecular architecture primarily affects the energy of the dense phase, providing a new entropy-based mechanism for the biological control of phase separation.

Reduced-order kinetic Monte Carlo model to simulate water diffusion in biodegradable polymers

Water is a driving factor in the degradation process of biodegradable polymers. However, water diffusion is typically not incorporated in the kinetic models of biodegradation such as kinetic Monte Carlo (kMC), because water diffusion events occur at a much shorter time scale than the hydrolysis reactions. As such, there is a need to improve the computational efficiency of water diffusion in the kMC models of diffusion processes in biodegradation. In this work, a new dimensionality reduction scheme for kMC diffusion models is developed to significantly reduce the computation time, where a two-dimensional kMC diffusion model for porous microstructures is reduced to one-dimensional ones. The dimensionality reduction is accomplished by calibrating the model parameters with multi-objective Bayesian optimization. The reduced-order diffusion model shows a 675-fold faster computation compared to the original two-dimensional model.