Monomer Diffusion Research Articles

Towards <i>in-situ</i> diagnostics of multi-photon 3D laser printing using optical coherence tomography

In recent years, multi-photon 3D laser printing has become a widely used tool for the fabrication of micro- and nanostructures for a large variety of applications. Typically, thorough sample characterisation is key for an efficient optimisation of the printing process. To date, three-dimensional microscopic inspection has usually been carried out on finished 3D printed microstructures, that is, using ex-situ approaches. In contrast, in-situ 3D characterization tools are desirable for quickly assessing the quality and properties of 3D printed microstructures. Along these lines, we present and characterise a Fourier-domain optical coherence tomography (FD-OCT) system that can be readily integrated into an existing 3D laser lithography setup. We demonstrate its capabilities by examining different 3D printed polymer microstructures immersed in a liquid photoresist. In such samples, local reflectivity arises from the (refractive-index) contrasts between the polymerised and non-polymerised regions. Thus, the refractive index of the printed material can be extracted. Furthermore, we demonstrate that the reflectivity of polymer-monomer transitions exhibits time-dependent behaviour after printing. Supported by transfer-matrix calculations, we explain this effect in terms of the time-dependent graded-index transition originating from monomer diffusion into the polymer matrix. Finally, we show exemplary 3D reconstructions of printed structures that can be readily compared with 3D computer designs.

Novel Poly(piperazinamide)/poly(m-phenylene isophthalamide) composite nanofiltration membrane with polydopamine coated silica as an interlayer for the splendid performance

Nanofiltration (NF) is a potential separation technology in the field of water treatment. However, NF membrane generally has the bottleneck problems of low permeability and poor selective separation. Therefore, this study proposed to use polydopamine coated silica microspheres as the intermediate layer to modify the poly(m-phenylene isophthalamide) (PMIA) substrate, then to prepare poly(piperazinamide) composite NF membrane via interfacial polymerization technique. Because both dopamine and silica contain a large number of hydroxyl groups, which can limit the diffusion of aqueous monomers to the water/oil boundary due to the hydrogen bonding force. The obtained NF membrane presented a nano-tubular Turing structure. As a result, the membrane exhibited extremely high permeability with the pure water flux of 31.37 ± 1.06 LMH/bar, and the corresponding rejection to Na2SO4 could reach to 97.0%, which was three times that of traditional polyamide NF membrane. Moreover, the performance of one-valent/bivalent salt was excellent. The composite nanofiltration membrane made of polydopamine coated silica as the intermediate layer provides a potential strategy to construct polyamide membrane with improved permeation and selectivity, which will be beneficial to save energy consumption during the actual application.

Combining network spread with protein aggregation correctly recapitulates empirical spatio-temporal progression of Alzheimer's tau pathology.

Alzheimer's disease involves widespread and progressive deposition of misfolded protein tau, likely via "prion-like" trans-neuronal transmission along the brain's structural connectome. It is not well understood how tau aggregation and spread leads to stereotypical progression in the Alzheimer brain. Here we extend our previous Network Diffusion Model (NDM, [1]) to include these critical processes (see Figure 1): a) Tau monomer seeding and production in the entorhinal cortex; b) aggregation of monomers into larger oligomers and then tangles using well-known Smoluchowski equations that were previously successful for amyloid beta and prions [2]. c) These processes were integrated into NDM, whereby anatomic connections govern the transmission of tau between two distant but connected brain regions. This extended model, which we call Aggregation-Network-Diffusion (AND), was then thoroughly tested on empirical data (Table 1): MRI-derived atrophy, AV1451-PET and CSF biomarkers (Ab42, pTau) from ADNI and Korea cohort [3]. AND model exhibits key hallmarks of AD, accurately capturing the spatial distribution of empirical tau (Korea cohort) as well as regional atrophy of ADNI cohort (Figure 2). R values shown in figure are all highly significant (p<0.0001). Next we interrogated different regions serving as seeding site; the best cortical seeding site was entorhinal cortex, while the best subcortical seeding was in amygdala followed by hippocampus. We also fitted the AND model's total tau PET against patients' cerebrospinal fluid phosphorylated tau (p-tau181) profiles as a function of disease progression; we obtained R = 0.37, p < 1e-36. Sensitivity analysis suggests that diffusion rate is far more important than monomer production rate in governing long-term pathology accumulation (Figure 3). This unified quantitative and testable model has the potential to explain observed phenomena. It may also serve as a test-bed for future hypothesis generation and testing in silico, e.g. to mathematically test whether monomer production, aggregation or diffusion is the key bottleneck.

Preparation of chiral polymer/cholesteric liquid crystals composite films with broadband reflective capability for smart windows and thermal management of buildings

In this study, a new method to prepare the chiral polymer/cholesteric liquid crystals composite films with broadband reflective capability was reported. A negative dielectric anisotropy liquid crystal material was utilized as the mesogenic solvents and a chiral mesogenic monomer was employed as the chiral dopant. The chiral mesogenic monomer not only induced the liquid crystal molecules to align to form the helical structure, but also as the polymerizable monomer to be crosslinked under UV light to stabilize the pitch gradient in the materials. The UV absorbing dye was used to generate the intensity gradient of UV light and promoted the diffusion of the chiral mesogenic monomer from the backlight side to the light side. The effect of the content of the chiral mesogenic monomer and the intensity of the UV light applied during the preparation process of the composite film on the broadband reflective capability was studied systematically. The chiral polymer/cholesteric liquid crystals composite film that can reflect the light covering the wavelength from 820 nm to 2100 nm was achieved successfully and was used as the functional film in the windows for shielding part of near-infrared light to achieve the thermal management effect of buildings. This work is expected to provide a new method for the preparation of smart windows and thermal management of buildings. • A composite film with broadband reflective capability was presented. • The composite film contributed to shielding part of near-infrared light. • The film can reflect the light covering the wavelength from 820 nm to 2100 nm. • The composite film can be applied in the thermal management effect of buildings. • The film was capable of reducing the thermal effect of sunlight in the house.

Diffusing Diffusivityin Dynamics of Unentangled Polymer Melts

AbstractThe exact results for Rouse chain dynamics in the crowded environment modeled bydiffusing diffusivity(N. Ahamad and P. Debnath,Phys. A2020,549, 124335) in explaining polymer melt dynamics is explored. Theory for mode accounts for anomalous and non‐Gaussian diffusion in the center of mass (c.m.) and monomer dynamics of united atom molecular dynamics simulation trajectories of unentangled polyethylene melts, with excellent agreement. Accurate estimates of scaling relations for c.m. and monomer mean square displacement with time interval are obtained. The corresponding c.m. and monomer diffusion coefficients as a function of confirmdiffusing diffusivityin polymer melts. It is shown shown that the dependent parameters of the modified Rouse model expressed in terms of all modes are reducible to an identical set of per mode parameter values on application to real data. Analysis of velocity autocorrelation functions indicates that the theory is tested on simulation trajectories of polymer melts with friction. The theory estimates for additional power‐law exponents in the exponential of displacement distributions agree with the data. The results show fairly convincingly that the Rouse model withdiffusing diffusivitymay find scope as an alternate theoretical formalism for polymeric liquids.

Multirole Regulations of Interfacial Polymerization Using Poly(acrylic acid) for Nanofiltration Membrane Development.

Effective control of monomer diffusion and reaction rate is the key to achieving a controlled interfacial polymerization (IP) and a high-performance nanofiltration (NF) membrane. Herein, an integration of multirole regulations was synchronously realized using poly(acrylic acid) (PAA) as an active additive in a piperazine (PIP) aqueous phase. Thanks to synergistic interactions, including hydrogen bonding, electrostatic interaction, and covalent bonding between PAA and PIP molecules, together with the increased viscosity of the solution, PIP diffusion was rationally controlled. Moreover, interfacial polycondensation was also restrained via the modestly reduced pH of the aqueous solution. These contribute to the formation of a thinner, looser, more hydrophilic, and higher negatively charged PAA-decorated polyamide selective layer with a unique nanostrand-nodule morphology. The harvested NF-PAA/PIP membrane showed an ∼70% rise in water permeability (up to 23.5 L·m-2·h-1·bar-1) while retaining high Na2SO4 and dye rejections. Furthermore, the optimized NF-PAA/PIP membrane presented a superior fouling resistance capability for typical pollutants, as well as long-term stability during successive filtration. Thus, this work offers a straightforward and impactful approach to regulating IP and promoting NF membrane properties.

Polyamide Nanofiltration Membranes from Surfactant‐Assembly Regulated Interfacial Polymerization: The Effect of Alkyl Chain

AbstractRegulating the monomer diffusion during interfacial polymerization (IP) process is the key for tailoring the pore structure and desalination performance of thin‐film composite polyamide nanofiltration (NF) membrane. Recently a surfactant‐assembly strategy to regulate IP process and NF membranes with sub‐Å separation precision is proposed. However, little is known for the role of the molecular structure of surfactant on IP process and membrane performance. In this work, five sulfate surfactants with different length of alkyl chains are used to construct surfactant monolayer assembly at the water/hexane interface to regulate IP process. The results show that for the sulfate surfactants, the longer the alkyl chain, the more uniform the pore size distribution of polyamide active layer is and the higher the ion separation selectivity of NF membrane is. Among them, the NF membrane prepared from sodium n‐hexadecyl sulfate (SHS) exhibits the highest separation performance with the rejection of Na2SO4, MgSO4, MgCl2, and CaCl2 up to 99.39%, 99.12%, 98.09%, and 97.38%, respectively. Overall, this study further confirms the regulatory role of surfactant‐assembled monolayer during IP process and provides important insights into how the polyamide structure and the resulting NF membranes are influenced by the alkyl chain length of the surfactants.

Kinetically driven island morphology in growth on strained Cu(100).

We study the effects of strain on the monomer and dimer diffusion mechanisms and island morphology during the growth of Cu on a biaxially strained Cu(100) substrate. We find an approximately linear dependence of the activation barriers on strain. In particular, while hopping is favored for compressive and/or small (<2%) tensile strain, for greater than 2% tensile strain, the exchange mechanism is favored. We then present the results of temperature-accelerated dynamics simulations of submonolayer growth at 200K. For the case of 2% compressive strain we find that, as in previous kinetic Monte Carlo simulations of Cu/Ni(100) growth, the competition between island growth and multi-atom relaxation ("pop-out") events leads to an island morphology with a mixture of open and closed steps. At slightly higher coverage, island coalescence then leads to elongated islands. However, annealing leads to a significant decrease in the number of open steps. In contrast, for the case of 8% tensile strain, only one large strongly anisotropic island is formed. Surprisingly, we find that despite the large strain, the island anisotropy is not due to energetics but is instead due to anisotropic attachment barriers that favor the exchange-mediated attachment of monomers to corners over close-packed step-edges. An explanation for the asymmetry in attachment barriers is provided. Our results provide a new general kinetic mechanism for the formation of anisotropic islands in the presence of isotropic diffusion and tensile strain.

Mechanism of Post-Radiation-Chemical Graft Polymerization of Styrene in Polyethylene.

Structural and morphological features of graft polystyrene (PS) and polyethylene (PE) copolymers produced by post-radiation chemical polymerization have been investigated by methods of X-ray microanalysis, electron microscopy, DSC and wetting angles measurement. The studied samples differed in the degree of graft, iron(II) sulphate content, sizes of PE films and distribution of graft polymer over the polyolefin cross section. It is shown that in all cases sample surfaces are enriched with PS. As the content of graft PS increases, its concentration increases both in the volume and on the surface of the samples. The distinctive feature of the post-radiation graft polymerization is the stepped curves of graft polymer distribution along the matrix cross section. A probable reason for such evolution of the distribution profiles is related to both the distribution of peroxide groups throughout the sample thickness and to the change in the monomer and iron(II) salt diffusion coefficients in the graft polyolefin layer. According to the results of electron microscope investigations and copolymer wettability during graft polymerization, a heterogeneous system is formed both in the sample volume and in the surface layer. It is shown that the melting point, glass transition temperature and degree of crystallinity of the copolymer decreases with the increasing proportion of graft PS. It is suggested that during graft polymerization a process of PE crystallite decomposition (melting) and enrichment of the amorphous phase of graft polymer by fragments of PE macromolecules occurs spontaneously. The driving force of this process is the osmotic pressure exerted by the phase network of crystallites on the growing phase of the graft PS.

Alginate Hydrogel Assisted Controllable Interfacial Polymerization for High-Performance Nanofiltration Membranes.

The deepening crisis of freshwater resources has been driving the further development of new types of membrane-based desalination technologies represented by nanofiltration membranes. Solving the existing trade-off limitation on enhancing the water permeance and the rejection of salts is currently one of the most concerned research interests. Here, a facile and scalable approach is proposed to tune the interfacial polymerization by constructing a calcium alginate hydrogel layer on the porous substrates. The evenly coated thin hydrogel layer can not only store amine monomers like the aqueous phase but also suppress the diffusion of amine monomers inside, as well as provide a flat and stable interface to implement the interfacial polymerization. The resultant polyamide nanofilms have a relatively smooth morphology, negatively charged surface, and reduced thickness which facilitate a fast water permeation while maintaining rejection efficiency. As a result, the as-prepared composite membranes show improved water permeance (~30 Lm−2h−1bar−1) and comparable rejection of Na2SO4 (>97%) in practical applications. It is proved to be a feasible approach to manufacturing high-performance nanofiltration membranes with the assist of alginate hydrogel regulating interfacial polymerization.

Full-Color See-Through Three-Dimensional Display Method Based on Volume Holography.

We propose a full-color see-through three-dimensional (3D) display method based on volume holography. This method is based on real object interference, avoiding the device limitation of spatial light modulator (SLM). The volume holography has a slim and compact structure, which realizes 3D display through one single layer of photopolymer. We analyzed the recording mechanism of volume holographic gratings, diffraction characteristics, and influencing factors of refractive index modulation through Kogelnik’s coupled-wave theory and the monomer diffusion model of photopolymer. We built a multiplexing full-color reflective volume holographic recording optical system and conducted simultaneous exposure experiment. Under the illumination of white light, full-color 3D image can be reconstructed. Experimental results show that the average diffraction efficiency is about 53%, and the grating fringe pitch is less than 0.3 μm. The reconstructed image of volume holography has high diffraction efficiency, high resolution, strong stereo perception, and large observing angle, which provides a technical reference for augmented reality.

Engineering novel thin-film composite membranes with crater-like surface morphology using rigidly-contorted monomer for high flux nanofiltration

Developing thin-film composite (TFC) nanofiltration (NF) membranes with high water permeability and salt rejection is highly desirable in water desalination. In this study, a new TFC membrane with high NF performances was developed to achieve such a goal by constructing active separation layer with a special crater-like surface morphology using a rigidly-contorted spirocyclic monomer, 5,5,6′,6′-tetrahydroxy-3,3,3′,3′-tetramethyl spirobisindane (TTSBI), as the co-monomer with piperazine (PIP) in aqueous phase during interfacial polymerization (IP) process. This design endowed the separation layer with the characteristics of low thickness, high microporosity, and high permeation surface area. The pure water permeability was therefore increased to 9.9 L m−2 h −1 bar−1, approximately two times higher than that of the typical TFC NF membrane prepared with PIP and trimesoyl chloride, without compromise in the rejection (the rejection of Na2SO4 up to 98%). The introduction of TTSBI tuned the diffusion and reaction behaviors of monomers at the aqueous-organic interface and controlled the special microporous structure and surface morphology, which are advantageous to the improvement of water permeability. This study provides new insights for synthesizing highly permeable TFC NF membranes with remarkable rejection characteristics to address the crisis of the current potable water scarcity.

Real-time molecular-level visualization of mass flow during patterned photopolymerization of liquid-crystalline monomers

Single-particle fluorescence imaging is used to monitor dynamic processes that occur during patterned photopolymerization of liquid-crystalline monomers. A spatial gradient of chemical potential can be created at the border of bright and dark regions by structured illumination in the photopolymerization process, leading to mutual diffusion of polymers and monomers. Analysis of the fluorescence from single quantum dots doped into the monomers at minute concentrations enables visualization of highly directional flow from the illuminated region where the photopolymerization proceeds toward a masked unpolymerized region. This directional mass flow causes flow-induced orientation of the polymers that is subsequently fixed by completion of the polymerization reaction, resulting in a mesoscopic aligned area of the polymer film.

Mosquito method based polymer tapered waveguide as a spot size converter.

We create a compact low-loss spot-size converter (SSC) which utilizes a tapered core polymer optical waveguide with circular cross-sectional graded-index (GI) core using the Mosquito method we developed. First, we theoretically analyze the mutual diffusion between the core and cladding monomers, which is a feature unique to the Mosquito method when forming GI cores. The monomer diffusion effect depends on the initial core diameter that is dispensed by a microdispenser and the diffusion time before UV curing: in a small core the monomer diffuses more rapidly than in a large core. Using this diffusion dependence on the initially dispensed core diameter, it is theoretically found that a tapered polymer waveguide based SSC can adiabatically convert the mode-field diameter between 4.0 and 8.6 μm at a 1550-nm wavelength waveguide as short as 4 mm. Next, the tapered waveguide based SSC with the designed structure is experimentally fabricated using the Mosquito method, and we confirm an 8-mm long tapered waveguide with an insertion loss of 1.83dB functions as a SSC that converts the MFD from 4.7 μm to 7.5 μm at 1550-nm wavelength.

Dynamically consistent coarse-grain simulation model of chemically specific polymer melts via friction parameterization.

Coarse-grained (CG) models of polymers involve grouping many atoms in an all-atom (AA) representation into single sites to reduce computational effort yet retain the hierarchy of length and time scales inherent to macromolecules. Parameterization of such models is often via "bottom-up" methods, which preserve chemical specificity but suffer from artificially accelerated dynamics with respect to the AA model from which they were derived. Here, we study the combination of a bottom-up CG model with a dissipative potential as a means to obtain a chemically specific and dynamically correct model. We generate the conservative part of the force-field using the iterative Boltzmann inversion (IBI) method, which seeks to recover the AA structure. This is augmented with the dissipative Langevin thermostat, which introduces a single parameterizable friction factor to correct the unphysically fast dynamics of the IBI-generated force-field. We study this approach for linear polystyrene oligomer melts for three separate systems with 11, 21, and 41 monomers per chain and a mapping of one monomer per CG site. To parameterize the friction factor, target values are extracted from the AA dynamics using translational monomer diffusion, translational chain diffusion, and rotational chain motion to test the consistency of the parameterization across different modes of motion. We find that the value of the friction parameter needed to bring the CG dynamics in line with AA target values varies based on the mode of parameterization with short-time monomer translational dynamics requiring the highest values, long-time chain translational dynamics requiring the lowest values, and rotational dynamics falling in between. The friction ranges most widely for the shortest chains, and the span narrows with increasing chain length. For longer chains, a practical working value of the friction parameter may be derived from the rotational dynamics, owing to the contribution of multiple relaxation modes to chain rotation and a lack of sensitivity of the translational dynamics at these intermediate levels of friction. A study of equilibrium chain structure reveals that all chains studied are non-Gaussian. However, longer chains better approximate ideal chain dimensions than more rod-like shorter chains and thus are most closely described by a single friction parameter. We also find that the separability of the conservative and dissipative potentials is preserved.

Molecular dynamics and structure of polyrotaxane in solution

Polyrotaxane (PR) is a supramolecular assembly in which ring molecules are threaded onto an axial polymer chain. This paper reviews the molecular structure and dynamics of a PR composed of polyethylene glycol (PEG) and α-cyclodextrin (CD) based on data obtained from X-ray and neutron scattering experiments on PR solutions. The dynamics of CD and the PEG monomers in the PR were evaluated separately by quasielastic neutron scattering with deuterium labeling. Inclusion complex formation with CD increases the stiffness of the PEG backbone and restricts the diffusion of the PEG monomers that are covered by or in close proximity to the CD molecules. The sliding motion of the CD molecules along the PEG axis was explored using full atomistic molecular dynamics (MD) simulations, showing that the sliding dynamics were retarded by the interaction between the CD cavities and PEG. This paper reviews our X-ray and neutron scattering experiments on the molecular structure and dynamics of a supramolecular assembly, polyrotaxane (PR) composed of polyethylene glycol (PEG) and α-cyclodextrin (CD). Inclusion complex formation with CDs increases the stiffness of the PEG backbone and restricts the diffusion of the PEG monomers that are covered by or in close proximity to the CD molecules.

Breaking through permeability–selectivity trade‐off of thin‐film composite membranes assisted with crown ethers

AbstractIn this study, we deployed a modified interfacial polymerization process to incorporate multifunctional crown ethers (CEs) into thin‐film composite (TFC) polyamide membranes. CE additives acted as both the phase‐transfer catalyst and co‐solvent to facilitate the diffusion of amine monomers into the organic phase via interacting with amine monomers (form the host‐guest inclusion complex), and enhanced the free volume content of the selective layer, therefore facilitating water transport and inhibiting the diffusion of salt ions. Various characterization techniques were employed to elucidate the modification mechanism as a function of CE chemical and physical properties on the microstructure of resultant TFC membranes and consequently separation performances. Compared to TFC membranes produced from traditional interfacial polymerization method, CE‐modified membranes exhibited a 146% water flux enhancement and 59% lower reverse salt fluxes with a suitable draw solution. CE‐modified membranes also showed the improved antifouling behavior and chemical stability in various harsh conditions.

Performance enhancement of nanofiltration membranes via surface modification with a novel acylation reagent

AbstractAcyl chloride monomers have been serving as the dominant acylation reagent for preparing thin‐film composite (TFC) nanofiltration (NF) membranes over the past few decades. Herein, a novel acylation reagent (trimellitic anhydride, TMA) was exploited in conjunction with trimesoyl chloride (TMC) to undergo interfacial polymerization with piperazine (PIP) on the polysulfone substrate membranes. The introduction of TMA enabled the deeper diffusion of PIP monomers into the organic phase, resulting in the creation of novel circular‐shaped protuberances on the top surface of the polyamide layer and the significant increase in the effective membrane area. Besides, abundant in‐situ carboxylic groups were generated in the polyamide layer, conducive to both the surface hydrophilicity and negative charge density. Consequently, with an addition of 0.03 wt% TMA, pure water flux reached up to 15.3 L m−2 hour−1 bar−1, almost 2.2 times that of the pristine membrane, and high rejection of Na2SO4 (97.3%) was maintained, evincing the superior desalination performance of the TMA‐modified membranes. The interaction mechanism between TMA, TMC, and PIP was described in detail. Furthermore, the TMA‐modified membranes exhibited a stable separation performance over the long‐running process.

Modelling nanocrystal growth via the precipitation method

A mathematical model for the growth of a single nanocrystal is generalised to deal with an arbitrarily large number of crystals. The basic model is a form of Stefan problem, describing diffusion of monomer over a moving domain. Various levels of approximation (an analytical solution, an ordinary differential equation model and an N particle model) are compared and shown to agree well. The N particle model and analytical solution are then shown to have excellent agreement with experimental data for the growth of CdSe nanocrystals. The theoretical solution clearly shows the effect of problem parameters on the growth process and, significantly, that there is a single controlling group. By increasing the value of N it is shown that in the absence of Ostwald ripening the single particle model may be considered as representing the average radius of a system with a large number of particles. Consequently a system with N=2 may represent either a two particle system or a bimodel initial distribution. The solution of the N=2 model provides an understanding of Ostwald ripening. In general if Ostwald ripening is expected some form of the N particle model should be employed. Finally it is shown how the analytical solution may be employed to represent a multi-stage growth process which can then guide and optimise crystal growth.

The two-state reversible kinetics of a long polymer molecule in solution with a delocalized coupling term. An exact analytical model

We develop a mathematical recipe for calculating the exact delocalized rate constant, for the end loop motion of a long polymer. It undergoes reversible reaction between two disjunct states, the open and the closed state, due to the effect of the surrounding solvent. The Smoluchowski-like equation mathematically represents the diffusion of the end monomers of the open polymer chain and the closed chain in terms of a free particle performing random motion under different harmonic potentials. The coupling term between the two potentials is assumed to be represented by a delocalized coupling term. For a favorable disposition towards the more realistic experimental behavior, the rate constant have effect incorporated from all the bond making and bond breaking phenomenon of the chemical reactions different from the end loop formation. It not only involves the end looping but can evaluate for reactions where only one of the ends of the open-chain is involved.