Effects of nanoscopic-confinement on polymer dynamics

Nonmonotonic polymer translocation kinetics through nanopores under changing surface-polymer interactions

Understanding the dynamics of polymers in confined environments is pivotal for diverse applications ranging from polymer upcycling to bioseparations. In this study, we develop an entropic barrier model using self-consistent field theory that considers the effect of attractive surface interactions, solvation, and confinement on polymer kinetics. In this model, we consider the translocation of a polymer from one cavity into a second cavity through a single-segment-width nanopore. We find that, for a polymer in a good solvent (i.e., excluded volume, u0 > 0), there is a nonmonotonic dependence of mean translocation time (τ) on surface interaction strength, ɛ. At low ɛ, excluded volume interactions lead to an energetic penalty and longer translocation times. As ɛ increases, the surface interactions counteract the energetic penalty imposed by excluded volume and the polymer translocates faster through the nanopore. However, as ɛ continues to increase, an adsorption transition occurs, which leads to significantly slower kinetics due to the penalty of desorption from the first cavity. The ɛ at which this adsorption transition occurs is a function of the excluded volume, with higher u0 leading to an adsorption transition at higher ɛ. Finally, we consider the effect of translocation across different size cavities. We find that the kinetics for translocation into a smaller cavity speeds up while translocation to a larger cavity slows down with increasing ɛ due to higher surface contact under stronger confinement.

Disentangling Component Dynamics in an All-Polymer Nanocomposite Based on Single-Chain Nanoparticles by Quasielastic Neutron Scattering

We have investigated an all-polymer nanocomposite (NC) consisting of single-chain nanoparticles (SCNPs) immersed in a matrix of linear chains of their precursors (25/75% composition in weight). The SCNPs were previously synthesized via “click” chemistry, which induces intramolecular cross-links in the individual macromolecules accompanied by a slight shift (5–8 K) of the glass transition temperature toward higher values and a broadening of the dynamic response with respect to the raw precursor material. The selective investigation of the dynamics of the NC components has been possible by using properly isotopically labeled materials and applying quasielastic neutron scattering techniques. Results have been analyzed in the momentum transfer range where the coherent scattering contribution is minimal, as determined by complementary neutron diffraction experiments with polarization analysis. We observe the development of dynamic heterogeneity in the intermediate scattering function of the NC components, which grows with increasing time. Local motions in the precursor matrix of the NC are accelerated with respect to the reference bulk behavior, while the displacements of SCNPs’ hydrogens show enhanced deviations from Gaussian and exponential behavior compared with the pure melt of SCNPs. The resulting averaged behavior in the NC coincides with that of the pure precursor, in accordance with the macroscopic observations by differential scanning calorimetry (DSC) experiments.

Dissipative particle dynamics simulations in colloid and Interface science: a review

Dissipative particle dynamics (DPD) is one of the most efficient mesoscale coarse-grained methodologies for modeling soft matter systems. Here, we comprehensively review the progress in theoretical formulations, parametrization strategies, and applications of DPD over the last two decades. DPD bridges the gap between the microscopic atomistic and macroscopic continuum length and time scales. Numerous efforts have been performed to improve the computational efficiency and to develop advanced versions and modifications of the original DPD framework. The progress in the parametrization techniques that can reproduce the engineering properties of experimental systems attracted a lot of interest from the industrial community longing to use DPD to characterize, help design and optimize the practical products. While there are still areas for improvements, DPD has been efficiently applied to numerous colloidal and interfacial phenomena involving phase separations, self-assembly, and transport in polymeric, surfactant, nanoparticle, and biomolecules systems.

Local orientation of chains at crystal/amorphous interfaces buried in isotactic polypropylene thin films

A better understanding of the aggregation states of polymer chains in thin films is of pivotal importance for developing thin film polymer devices in addition to its inherent scientific interest. Here we report the preferential orientation of the crystalline lamellae for isotactic polypropylene (iPP) in spin-coated films by grazing incidence of wide-angle X-ray diffraction in conjunction with sum frequency generation vibrational spectroscopy, which provides information on the local conformation of chains at crystal/amorphous interfaces buried in a thin film. The crystalline orientation of iPP, which formed cross-hatched lamellae induced by lamellar branching, altered from a mixture of edge-on and face-on mother lamellae to preferential face-on mother lamellae with decreasing thickness. The orientation of methyl groups at the crystal/amorphous interfaces in the interior region of the iPP films changed, accompanied by a change in the lamellar orientation.

Molecular mobility and crystallization of renewable poly(ethylene furanoate) in situ filled with carbon nanotubes and graphene nanoparticles

We investigate the thermal transitions and molecular mobility in new nanocomposites of biobased poly(ethylene furanoate) (PEF), by calorimetry and dielectric spectroscopy, supplemented by X-ray diffraction, Fourier transform infra-red spectroscopy and polarized light microscopy. The emphasis is placed on the facilitation of the crystallization of PEF, which is in general low and slow due to structural limitations that result in poor nucleation. Tuning of the crystalline fraction (CF) and semicrystalline morphology are important for optimization of the mechanical performance and manipulation of the permeation of small molecules (e.g., in packaging applications). The nucleation and CF are successfully improved here by the in situ filling of PEF with 0.5-2.5 wt% of carbon nanotubes (CNTs) and graphene nanoplatelets (GNPs). The improvements are discussed in connection with weak or absent interfacial polymer-filler interactions. CNTs were found to be more effective in facilitating crystallization, as compared with GNPs, possibly due to their larger aspect ratio. The segmental dynamics of PEF are both accelerated and decelerated by the addition of GNP and CNT, respectively, with complex phenomena contributing to the effects, namely, nucleation, changes in molar mass and changes in the free volume. The molecular mobility of PEF is moderately affected 'directly' by the particles, whereas stronger effects are induced by crystallization (an indirect effect) and, furthermore, by the increase in the length of alkylene sequences on the chain. Local dynamics exhibit time scale disturbances when the temperature approaches that of the glass transition, which is proposed here to be a common characteristic in the case of mobilities originating from the polymer backbone for these as well as different polyesters. Despite the weak effects on molecular mobility, the role of the fillers as nucleating agents seems to be further exploitable in the frame of envisaged applications, as the use of such fillers in combination with thermal treatment offer possibilities for manipulating the semicrystalline morphology, ion transport and, subsequently, permeation of small molecules.

Molecular Dynamics in Nanocomposites Based on Renewable Poly(butylene 2,5-furan-dicarboxylate) In Situ Reinforced by Montmorillonite Nanoclays: Effects of Clay Modification, Crystallization, and Hydration

This study deals with poly(butylene 2,5-furan-dicarboxylate), PBF, a renewable bio-based polyester expected to replace non-eco-friendly fossil-based homologues. PBF exhibits excellent gas barrier properties, which makes it promising for packaging applications; however, its rather low and slow crystallinity affects good mechanical performance. The crystallization of this relatively new polymer is enhanced here via reinforcement by introduction in situ of 1 wt % montmorillonite, MMT, nanoclays of three types (functionalizations). We study PBF and its nanocomposites (PNCs) also from the basic research point of view, molecular dynamics. For this work, we employ the widely used combination of techniques, differential scanning calorimetry (DSC) with broad-band dielectric relaxation spectroscopy (BDS), supplemented by polarized light microscopy (PLM) and thermogravimetric analysis (TGA). In the PNCs, the crystalline rate and fraction, CF, were found to be strongly enhanced as these fillers act as additional crystallization nuclei. The improvements in crystallization here correlate quite well with those on the mechanical performance recorded recently; moreover, they occur in the same filler order, in particular, with increasing MMT interlayer distance (from ∼1 to ∼3 nm). In the amorphous fraction of the polymer, the chain diffusion (calorimetric T_g and dynamic α process) is easier in the PNCs due to their slightly smaller length, while in the semicrystalline state, it decelerates by crystal-induced constraints. The local polymer dynamics (β process, below T_g) was found to be independent of the PNC composition, however, sensitive to structural changes of the matrix. Finally, a filler-induced dynamics was additionally recorded in the PNCs (α* process), arising possibly from the polymer located at the MMT surfaces. α* follows the changes in polymer chain length and decelerates with crystallization, whereas its activation energy decreases with mild hydration. The combined results on α* with the DSC and TGA findings, provide proof for weak MMT-PBF interactions. Overall, our results, along with data from the literature, suggest that such furan-based polyesters reinforced with properly chosen nanofillers could potentially serve well as tailor-made PNCs for targeted applications.

A Rotavirus Virus-Like Particle Confined Palladium Nanoreactor and Its Immobilization on Graphene Oxide for Catalysis

Abstract

In this work, a new viral protein cage based nanoreactor was successfully constructed via encapsulating Tween 80 stabilized palladium nanoparticles (NPs) into rotavirus capsid VP2 virus-like particles (i.e. Pd@VP2). The effects of stabilizers including CTAB, SDS, Tween 80 and PVP on controlling the particle size of Pd NPs were investigated. They were further immobilized on graphene oxide (i.e. Pd@VP2/GO) by a simple mixing method. Some characterizations including FT-IR and XPS were conducted to study adsorption mode of Pd@VP2 on GO sheets. Their catalytic performance was estimated in the reduction of 4-nitrophenol (4-NP). Results showed that Tween 80 stabilized Pd NPs with the molar ratio of Pd to Tween 80 at 1:0.1 possessed the smallest size and the best stability as well. They were encapsulated into viral protein cages (mean size 49 ± 0.26 nm) to assemble confined nanoreactors, most of which contained 1-2 Pd NPs (mean size 8.15 ± 0.26 nm). As-prepared Pd@VP2 indicated an enhanced activity (apparent reaction rate constant k _app  = (3.74 ± 0.10) × 10^-3 s^-1) for the reduction of 4-NP in comparison to non-confined Pd-Tween80 colloid (k _app  = (2.20 ± 0.06) × 10^-3 s^-1). It was logically due to confinement effects of Pd@VP2 including high dispersion of Pd NPs and high effective concentration of substrates in confined space. Pd@VP2 were further immobilized on GO surface through C-N bond. Pd@VP2/GO exhibited good reusability after recycling for four runs, confirming the strong anchoring effects of GO on Pd@VP2.

Graphic Abstract

Controlling water structure and behavior: design principles from metal organic nanotubular materials

Water exhibits unique and unexpected behavioral and structural changes when confined to the nanoscale, notably within the pores of metal–organic nanotubes.

Polymer dynamics under confinement

We review recent neutron scattering work and related results from simulation and complementary techniques focusing on the microscopic dynamics of polymers under confinement. Confinement is either realized in model porous materials or in polymer nanocomposites (PNC). The dynamics of such confined polymers is affected on the local segmental level, the level of entanglements as well as on global levels: (i) at the segmental level the interaction with the surface is of key importance. At locally repulsive surfaces compared to the bulk the segmental dynamics is not altered. Attractive surfaces slow down the segmental dynamics in their neighborhood but do not give rise to dead, glassy layers. (ii) Confinement generally has little effect on the inter-chain entanglements: both for weakly as well as for marginally confined polymers the reptation tube size is not changed. Only for strongly confined polymers disentanglement takes place. Similarly, in PNC at higher NP loading disentanglement phenomena are observed; in addition, at very high loading a transition from polymer caused topological constraints to purely geometrical constraints is observed. (iii) On the more global scale NSE experiments revealed important information on the nature of the interphase between adsorbed layer and bulk polymer. (iv) Polymer grafts at NP mutually confine each other, an effect that is most pronounced for one component NP. (v) Global diffusion of entangled polymers both in weakly and strongly attractive PNC is governed by the ratio of bottle-neck to chain size that characterizes the 'entropic barrier' for global diffusion.

Polymer-Polymer Interfacial Perturbation on the Glass Transition of Supported Low Molecular Weight Polystyrene Thin Films

Clarifying interfacial perturbation on polymer relaxation is important for polymer material development. Herein we investigated polymer-polymer interfacial perturbation on low molecular weight (MW) polystyrene (PS) thin film (15-180 nm) glass transition by depositing various polymers atop PS films. Overall, rubbery topcoats induced T_g depression of PS thin film (below 60 nm), while glassy topcoats induced T_g elevation of PS thin film (below 30 nm). Importantly, for the rubbery topcoat, T_g perturbation strength is largely dependent on the T_g difference between interfacial polymers and a larger T_g difference would induce stronger perturbation, while for the glassy topcoat this dependence is inconspicuous. Meanwhile, the interfacial perturbation length during PS glass transition by rubbery topcoats is estimated to be around 8 nm, while it is considered to be about 3.5 nm for glassy topcoats. The different interfacial perturbation length induced by disparate topcoats was accounted for by their different perturbation strength on adjacent PS molecules and disparate interfacial roughness. The results can promote the understanding of polymer interfacial perturbation and benefit the design and development of polymer-based materials.

Differentiation Capacity of Monocyte-Derived Multipotential Cells on Nanocomposite Poly(e-caprolactone)-Based Thin Films

Background

Μonocyte-derived multipotential cells (MOMCs) include progenitors capable of differentiation into multiple cell lineages and thus represent an ideal autologous transplantable cell source for regenerative medicine. In this study, we cultured MOMCs, generated from mononuclear cells of peripheral blood, on the surface of nanocomposite thin films.

Methods

For this purpose, nanocomposite Poly(e-caprolactone) (PCL)-based thin films containing either 2.5 wt% silica nanotubes (SiO2ntbs) or strontium hydroxyapatite nanorods (SrHAnrds), were prepared using the spin-coating method. The induced differentiation capacity of MOMCs, towards bone and endothelium, was estimated using flow cytometry, real-time polymerase chain reaction, scanning electron microscopy and fluorescence microscopy after cells' genetic modification using the Sleeping Beauty Transposon System aiming their observation onto the scaffolds. Moreover, Wharton's Jelly Mesenchymal Stromal Cells were cultivated as a control cell line, while Human Umbilical Vein Endothelial Cells were used to strengthen and accelerate the differentiation procedure in semi-permeable culture systems. Finally, the cytotoxicity of the studied materials was checked with MTT assay.

Results

The highest differentiation capacity of MOMCs was observed on PCL/SiO2ntbs 2.5 wt% nanocomposite film, as they progressively lost their native markers and gained endothelial lineage, in both protein and transcriptional level. In addition, the presence of SrHAnrds in the PCL matrix triggered processes related to osteoblast bone formation.

Conclusion

To conclude, the differentiation of MOMCs was selectively guided by incorporating SiO2ntbs or SrHAnrds into a polymeric matrix, for the first time.

Structure and Dynamics of Biobased Polyester Nanocomposites

The structure and the dynamics of two bio-based polyester polyols are investigated in the bulk and close to surfaces in polymer/layered silicate nanocomposites. The morphology of the neat polymers as well as the structure of the nanohybrids are investigated with X-ray diffraction and their thermal properties are studied by differential scanning calorimetry. One of the investigated polyesters is amorphous, whereas the second one is a semicrystalline polymer with intriguing thermal behavior. Hybrids have been synthesized over a broad range of compositions and intercalated structures are always obtained. The thermal transitions in the nanocomposites are observed only when the polymers are in excess outside the completely filled galleries. The glass transition, whenever it can be resolved, appears insensitive to the presence of the inorganic material, whereas the way the crystallization takes place depends on the composition of the nanohybrid. Dielectric relaxation spectroscopy was utilized to study the polymer dynamics. It revealed multiple relaxation processes for the neat polymers both below and above their glass transition temperatures, whereas in the nanocomposites, similarities and differences are observed depending on the specific mode of the dynamic process.

Formation of Oriented Polar Crystals in Bulk Poly(vinylidene fluoride)/High-Aspect-Ratio Organoclay Nanocomposites

We have investigated the formation of lamellar crystals of poly(vinylidene fluoride) (PVDF) in the presence of oriented clay particles with different aspect ratios (ARs) and surface properties. Hot-melt screw extrusion of PVDF with 5 wt % of montmorillonite (AR ≈ 12) or fluoromica (AR ≈ 27) resulted in formation of phase-separated blends. Replacing the clays with their organoclay derivatives, organomontmorillonite or organofluoromica, resulted in the corresponding intercalated nanocomposites. The organoclays induced formation of polar β- and γ-polymorphs of PVDF in contrast to the α-polymorph, which dominates in the pure PVDF and the PVDF/clay blends. Solid-state nuclear magnetic resonance revealed that the content of the α-phase in the nanocomposites was never higher than 7% of the total crystalline phase, whereas the β/γ mass ratio was close to 1:2, irrespective of the AR or crystallization conditions. X-ray diffraction showed that the oriented particles with a larger AR caused orientation of the polar lamellar crystals of PVDF. In the presence of the organofluoromica, PVDF formed a chevron-like lamellar nanostructure, where the polymer chains are extended along the extrusion direction, whereas the lamellar crystals were slanted from normal to the extrusion direction. Time-resolved X-ray diffraction experiments allowed the identification of the formation mechanism of the chevron-like nanostructure.

Anomalous toluene transport in model segmented polyurethane-urea/clay nanocomposites

The kinetics of liquid solvent sorption in polymeric systems and their nanocomposites often deviate from normal Fickian behaviour. This needs to be understood and interpreted, in terms of their underlying mechanistic origins. In the present study, the results of time dependent toluene sorption measurements in model segmented polyurethane-urea/clay nanocomposites have been analysed at room temperature. The studies revealed pronounced S-shaped sorption curves and unusually higher swelling of the nanocomposites compared to the neat polyurethane-urea matrix. Dynamic mechanical analysis (DMA) and small angle X-ray scattering (SAXS) measurements on the nanocomposites in the dry and liquid toluene saturated state have been carried out. The DMA studies revealed a significant decrease in the α relaxation temperature and storage modulus of the nanocomposites in the swollen state compared to the dry samples. The SAXS results showed that the nanoclay dispersion morphology transformed from intercalation in the dry state to exfoliation in the swollen state and the interdomain distance between hard segments increased upon swelling. Thermodynamic analysis of the Flory-Huggins interaction parameter (χ) of nanocomposite/toluene systems revealed increasingly negative χ values with increased clay loading. These results imply a significant plasticization effect of toluene on the nanocomposites. An interpretation of these data, which relates the abovementioned results, is presented in the framework of differential swelling stress (DSS) induced deviation from Fickian transport characteristics. We expect that these findings and methods may provide new insight into the analysis of the solvent diffusion process in heterogeneous polymers and their nanocomposites.

Effects of polymer-nanoparticle interactions on the viscosity of unentangled polymers under extreme nanoconfinement during capillary rise infiltration

We explore the effect of confinement and polymer-nanoparticle interactions on the viscosity of unentangled polymers undergoing capillary rise infiltration (CaRI) in dense packings of nanoparticles. In CaRI, a polymer is thermally induced to wick into the dense packings of nanoparticles, leading to the formation of polymer-infiltrated nanoparticle films, a new class of thin film nanocomposites with extremely high concentrations of nanoparticles. To understand the effect of this extreme nanoconfinement, as well as polymer-nanoparticle interactions on the polymer viscosity in CaRI films, we use two polymers that are known to have very different interactions with SiO2 nanoparticles. Using in situ spectroscopic ellipsometry, we monitor the polymer infiltration process, from which we infer the polymer viscosity based on the Lucas-Washburn model. Our results suggest that physical confinement increases the viscosity by approximately two orders of magnitude. Furthermore, confinement also increases the glass transition temperature of both polymers. Thus, under extreme nanoconfinement, the physical confinement has a more significant impact than the polymer-nanoparticle interactions on the viscosity of unentangled polymers, measured through infiltration dynamics, as well as the glass transition temperature. These findings will provide fundamental frameworks for designing processes to enable the fabrication of CaRI nanocomposite films with a wide range of nanoparticles and polymers.

Glass transition of polymers in bulk, confined geometries, and near interfaces

When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass-a solid lacking long-range order. Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers. We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye-Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature. We end by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.

Chain relaxation in thin polymer films: turning a dielectric type-B polymer into a type-A' one

A molecular dynamics simulation study of chain relaxation in a thin polymer film is presented, studying the dielectric response of a random copolymer of cis and trans 1,4-polybutadiene, a type B polymer without net chain dipole moment, confined between graphite walls. We stress the orientational effect of the attractive walls, inducing polarization in the vicinity of the walls, while the center of the film stays bulk-like. This polarization leads to a net dipole moment of the adsorbed chains, which is perpendicular to their end-to-end vector, which we termed as type A' behavior. In this situation, the dipole moment relaxes only upon desorption of the chains from the wall, a dynamic process which occurs on timescales much longer than the bulk relaxation time of the polymer.

Polymer Conformation under Confinement

The conformation of polymer chains under confinement is investigated in intercalated polymer/layered silicate nanocomposites. Hydrophilic poly(ethylene oxide)/sodium montmorillonite, PEO/Na⁺-MMT, hybrids were prepared utilizing melt intercalation with compositions where the polymer chains are mostly within the ~1 nm galleries of the inorganic material. The polymer chains are completely amorphous in all compositions even at temperatures where the bulk polymer is highly crystalline. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) is utilized to investigate the conformation of the polymer chains over a broad range of temperatures from below to much higher than the bulk polymer melting temperature. A systematic increase of the gauche conformation relatively to the trans is found with decreasing polymer content both for the C⁻C and the C⁻O bonds that exist along the PEO backbone indicating that the severe confinement and the proximity to the inorganic surfaces results in a more disordered state of the polymer.

Clusters of polyhedra in spherical confinement

Dense particle packing in a confining volume remains a rich, largely unexplored problem, despite applications in blood clotting, plasmonics, industrial packaging and transport, colloidal molecule design, and information storage. Here, we report densest found clusters of the Platonic solids in spherical confinement, for up to [Formula: see text] constituent polyhedral particles. We examine the interplay between anisotropic particle shape and isotropic 3D confinement. Densest clusters exhibit a wide variety of symmetry point groups and form in up to three layers at higher N. For many N values, icosahedra and dodecahedra form clusters that resemble sphere clusters. These common structures are layers of optimal spherical codes in most cases, a surprising fact given the significant faceting of the icosahedron and dodecahedron. We also investigate cluster density as a function of N for each particle shape. We find that, in contrast to what happens in bulk, polyhedra often pack less densely than spheres. We also find especially dense clusters at so-called magic numbers of constituent particles. Our results showcase the structural diversity and experimental utility of families of solutions to the packing in confinement problem.