Microscopic temperature-dependent structural dynamics in polymer nanocomposites: role of the graft-matrix chain interfacial entropic effect.

We present a systematic study of the structural dynamics in bulk entropic polymer nanocomposites (PNCs) with deuterated-polymer-grafted nanoparticles (DPGNPs) using quasi-elastic neutron scattering (QENS). We observe that the wave-vector-dependent relaxation dynamics depend on the entropic parameter f as well as the length scale being probed. The entropic parameter can be defined in terms of the grafted-to-matrix polymer molecular weight ratio and controls the extent of matrix chain penetration into the graft. Dynamical cross-over from Gaussian to non-Gaussian behavior at the wave vector Qc, which depends on temperature and f, was observed. Further insight into the underlying microscopic mechanism responsible for the observed behavior revealed that when interpreted using a jump-diffusion model, in addition to the speeding-up in local chain dynamics, the elementary distance over which sections of the chain hop is strongly dependent on f. Interestingly, we also observe dynamic heterogeneity (DH) in the studied systems, characterized by the non-Gaussian parameter α2, which reduces for a high-f (f = 0.225) sample compared with the pristine host polymer, indicating reduced dynamical heterogeneity, while it is mostly unchanged for the low-f sample. The results highlight that, unlike enthalpic PNCs, entropic PNCs with DPGNPs can modify the host polymer dynamics due to the subtle balance of interactions that occur at different length scales in the matrix.

Enhanced efficiency of water desalination in nanostructured thin-film membranes with polymer grafted nanoparticles.

Polyamide composite (PA-TFC) membranes are the state-of-the-art ubiquitous platforms to desalinate water at scale. We have developed a novel, transformative platform where the performance of such membranes is significantly and controllably improved by depositing thin films of polymethylacrylate [PMA] grafted silica nanoparticles (PGNPs) through the venerable Langmuir-Blodgett method. Our key practically important finding is that these constructs can have unprecedented selectivity values (i.e., ∼250-3000 bar-1, >99.0% salt rejection) at reduced feed water pressure (i.e., reduced cost) while maintaining acceptable water permeance A (= 2-5 L m-2 h-1 Bar-1) with as little as 5-7 PGNP layers. We also observe that the transport of solvent and solute are governed by different mechanisms, unlike gas transport, leading to independent control of A and selectivity. Since these membranes can be formulated using simple and low cost self-assembly methods, our work opens a new direction towards development of affordable, scalable water desalination methods.

Kinetics of high density functional polymer nanocomposite formation by tuning enthalpic and entropic barriers.

High density functional polymer nanocomposites (PNCs) with high degree of dispersion have recently emerged as novel materials for various thermo-mechanical, optical and electrical applications. The key challenge is to attain a high loading while maintaining reasonable dispersion to attain maximum possible benefits from the functional nanoparticle additives. Here, we report a facile method to prepare polymer grafted nanoparticle (PGNP)-based high density functional polymer nanocomposites using thermal activation of a high density PGNP monolayer to overcome entropic or enthalpic barriers to insertion of PGNPs into the underlying polymer films. We monitor the temperature-dependent kinetics of penetration of a high density PGNP layer and correlate the penetration time to the effective enthalpic/entropic barriers. The experimental results are corroborated by coarse-grained molecular dynamics simulations. Repeated application of the methodology to insert nanoparticles by appropriate control over temperature, time and graft-chain properties can lead to enhanced densities of loading in the PNC. Our method can be engineered to produce a wide range of high density polymer nanocomposite membranes for various possible applications including gas separation and water desalination.

Engineering interfacial entropic effects to generate giant viscosity changes in nanoparticle embedded polymer thin films.

Thin polymer and polymer nanocomposite (PNC) films are being extensively used as advanced functional coating materials in various technological applications. Since it is widely known that various properties of these thin films, especially their thermo-mechanical behavior, can be considerably different from the bulk depending on the thickness as well as interaction with surrounding media, it is imperative to study these properties directly on the films. However, quite often, it becomes difficult to perform these measurements reliably due to a dearth of techniques, especially to measure mechnical or transport properties like the viscosity of thin polymer or PNC films. Here, we demonstrate a new method to study the viscosity of PNC thin films using atomic force microscopy based force-distance spectroscopy. Using this method we investigated viscosity and the glass transition, Tg, of PNC thin films consisting of polymer grafted nanoparticles (PGNPs) embedded in un-entangled homopolymer melt films. The PGNP-polymer interfacial entropic interaction parameter, f, operationally controlled through the ratio of grafted and matrix molecular weight, was systematically tuned while maintaining good dispersion even at very high PGNP loadings, φ. We observed both a significant reduction (low f) and giant enhancement (high f) in the viscosity of the PNC thin films with the effect becoming more prominent with increasing φ. Significantly, none of the established theoretical models for viscosity changes observed earlier in suspensions or polymer nanocomposites can explain the observed viscosity variation. Our results thus not only demonstrate the tunability of the interfacial entropic effect to facilitate a dramatic change in the viscosity of PNC coatings, which could be of great utility in various applications of these materials, but also suggest a new regime of viscosity variation in athermal PNC films indicating the possible need for a new theoretical model.

Thermal stability and dynamics of soft nanoparticle membranes: role of entropy, enthalpy and membrane compressibility.

Nanoparticle based ultra-thin membranes have been shown to have remarkable mechanical properties while also possessing novel electrical, optical or magnetic properties, which could be controlled by tailoring properties at the level of individual nanoparticles. Since in most cases the ultra-thin membranes are coupled to some substrates, the role of membrane-substrate interactions, apart from nanoparticle-nanoparticle interactions become very crucial in understanding their mechanical and thermal stability, as well as their plethora of applications. However, systematic studies in this direction have been conspicuously absent. Here we report thermal stability and the corresponding microscopic dynamics of polymer supported ultra-thin membranes comprising of self-assembled, ordered grains of polymer grafted nanoparticles having tunable mechanical properties. The initially ordered membranes show distinct pathways for temperature induced disordering depending on membrane flexibility as well as on interfacial entropic and enthalpic interactions with the underlying polymer thin film. We also observe contrasting temperature dependence of microscopic dynamics of these membranes depending on whether the graft polymer-substrate polymer interactions are predominantly entropic or enthalpic in nature. Our results suggest that apart from their varied applications, the soft nanoparticle-polymer hybrid membranes are a playground for rich physics involving subtle entropic and enthalpic effects along with the nanoparticles softness, which eventually determine their thermo-mechanical stability.

Viscosity and fragility of confined polymer nanocomposites: a tale of two interfaces.

Viscosity and fragility are key parameters determining the processability and thermo-mechanical stability of glassy polymers and polymer nanocomposites (PNCs). In confined polymers, these parameters are largely dominated by the long relaxation times of the polymers adsorbed at the substrate-polymer interface. On the other hand, for polymer nanocomposites, the interface layer (IL) between the nanoparticles and the surrounding matrix chains often control not only the morphology and dispersion but also various parameters like viscosity and glass transition temperature. Confined PNCs, hence, present a unique opportunity to study the interplay of these two independent interfacial effects. Here, we report the results of X-ray scattering based dynamics measurements of PNC thin films, with a two IL width, unraveling the subtle interplay of these two interfaces on the measured viscosity and fragility. Coupled with coarse-grained molecular dynamics (MD) simulations, our experimental results demonstrate that the viscosity of the PNC films increases with both the IL width and the thickness of the polymer layer adsorbed at the substrate interface. However, while both pristine PS and PNCs with a higher IL width become stronger glasses, as estimated by their fragility, the PNC with a lower IL width shows an increase in fragility with increasing confinement. Our results suggest a novel method to control thermo-mechanical properties and stability of PNC coatings by independently controlling the two interfacial effects in athermal glassy PNCs.

Nanoparticle-polymer interfacial layer properties tune fragility and dynamic heterogeneity of athermal polymer nanocomposite films.

Enthalpic interactions at the interface between nanoparticles and matrix polymers are known to influence various properties of the resultant polymer nanocomposites (PNC). For athermal PNCs, consisting of grafted nanoparticles embedded in chemically identical polymers, the role and extent of the interface layer (IL) interactions in determining the properties of the nanocomposites are not very clear. Here, we demonstrate the influence of the interfacial layer dynamics on the fragility and dynamical heterogeneity (DH) of athermal and glassy PNCs. The IL properties are altered by changing the grafted to matrix polymer size ratio, f, which in turn changes the extent of matrix chain penetration into the grafted layer, λ. The fragility of PNCs is found to increase monotonically with increasing entropic compatibility, characterised by increasing λ. Contrary to observations in most polymers and glass formers, we observe an anti-correlation between the dependence on IL dynamics of fragility and DH, quantified by the experimentally estimated Kohlrausch-Watts-Williams parameter and the non-Gaussian parameter obtained from simulations.

Correlation between grafted nanoparticle-matrix polymer interface wettability and slip in polymer nanocomposites.

Controlling and understanding the flow properties of polymer nanocomposites (PNC) is very important in realising their potential for various applications. In this study we report molecular dynamics simulation studies of slip between a rotating polymer-grafted nanoparticle and the surrounding free linear matrix chains. By varying the interface wettability between the nanoparticle and matrix chains defined by the parameter f, the ratio of the graft to the matrix chain length, or the graft chain density, Σ, we were able to tune the interface slip, δ, significantly. Both f and Σ alter the interface wettability by changing the matrix chain penetration depth, λ, into the graft chain layer. We observed a large value of δ at smaller f or Σ which reduces with an increasing value of the respective parameters. Since interface slip is also likely to affect other properies of PNCs, like viscosity and the glass transition, we suggest that these parameters could become useful tools to control the flow and mechanical properties of PNCs made with grafted nanoparticles.

Applications of STED fluorescence nanoscopy in unravelling nanoscale structure and dynamics of biological systems.

Fluorescence microscopy, especially confocal microscopy, has revolutionized the field of biological imaging. Breaking the optical diffraction barrier of conventional light microscopy, through the advent of super-resolution microscopy, has ushered in the potential for a second revolution through unprecedented insight into nanoscale structure and dynamics in biological systems. Stimulated emission depletion (STED) microscopy is one such super-resolution microscopy technique which provides real-time enhanced-resolution imaging capabilities. In addition, it can be easily integrated with well-established fluorescence-based techniques such as fluorescence correlation spectroscopy (FCS) in order to capture the structure of cellular membranes at the nanoscale with high temporal resolution. In this review, we discuss the theory of STED and different modalities of operation in order to achieve the best resolution. Various applications of this technique in cell imaging, especially that of neuronal cell imaging, are discussed as well as examples of application of STED imaging in unravelling structure formation on biological membranes. Finally, we have discussed examples from some of our recent studies on nanoscale structure and dynamics of lipids in model membranes, due to interaction with proteins, as revealed by combination of STED and FCS techniques.

Broadband room temperature strong coupling between quantum dots and metamaterials.

Herein, we report the first demonstration of room temperature enhanced light-matter coupling in the visible regime for metamaterials using cooperative coupled quasi two dimensional quantum dot assemblies located at precise distances from the hyperbolic metamaterial (HMM) templates. The non-monotonic variation of the magnitude of strong coupling, manifested in terms of strong splitting of the photoluminescence of quantum dots, can be explained in terms of enhanced LDOS near the surface of such metamaterials as well as the plasmon mediated super-radiance of closely spaced quantum dots (QDs). Our methodology of enhancing broadband, room temperature, light-matter coupling in the visible regime for metamaterials opens up new possibilities of utilising these materials for a wide range of applications including QD based thresholdless nanolasers and novel metamaterial based integrated photonic devices.

Pattern formation in thin films of polymer solutions: Theory and simulations.

Self-assembly has been recognized as an efficient tool for generating a wide range of functional, chemically, or physically textured surfaces for applications in small scale devices. In this work, we investigate the stability of thin films of polymer solutions. For low concentrations of polymer in the solution, long length scale dewetting patterns are obtained with wavelength approximately few microns. Whereas, for concentrations above a critical value, bimodal dispersion curves are obtained with the dominant wavelength being up to two orders smaller than the usual dewetting length scale. We further show that the short wavelength corresponds to the phase separation in the film resulting in uniformly distributed high and low concentration regions. Interestingly, due to the solvent entropy, at very high concentration values of polymer, a re-entrant behaviour is observed with the dominant length scale now again corresponding to the dewetting wavelength. Thus, we show that the binary films of polymer solutions provide additional control parameters that can be utilized for generating functional textured surfaces for various applications.

Suspensions of polymer-grafted nanoparticles with added polymers-Structure and effective pair-interactions.

We present the results of combined experimental and theoretical (molecular dynamics simulations and integral equation theory) studies of the structure and effective interactions of suspensions of polymer grafted nanoparticles (PGNPs) in the presence of linear polymers. Due to the absence of systematic experimental and theoretical studies of PGNPs, it is widely believed that the structure and effective interactions in such binary mixtures would be very similar to those of an analogous soft colloidal material-star polymers. In our study, polystyrene-grafted gold nanoparticles with functionality f = 70 were mixed with linear polystyrene (PS) of two different molecular weights for obtaining two PGNP:PS size ratios, ξ = 0.14 and 2.76 (where, ξ = Mg/Mm, Mg and Mm being the molecular weights of grafting and matrix polymers, respectively). The experimental structure factor of PGNPs could be modeled with an effective potential (Model-X), which has been found to be widely applicable for star polymers. Similarly, the structure factor of the blends with ξ = 0.14 could be modeled reasonably well, while the structure of blends with ξ = 2.76 could not be captured, especially for high density of added polymers. A model (Model-Y) for effective interactions between PGNPs in a melt of matrix polymers also failed to provide good agreement with the experimental data for samples with ξ = 2.76 and high density of added polymers. We tentatively attribute this anomaly in modeling the structure factor of blends with ξ = 2.76 to the questionable assumption of Model-X in describing the added polymers as star polymers with functionality 2, which gets manifested in both polymer-polymer and polymer-PGNP interactions especially at higher fractions of added polymers. The failure of Model-Y may be due to the neglect of possible many-body interactions among PGNPs mediated by matrix polymers when the fraction of added polymers is high. These observations point to the need for a new framework to understand not only the structural behavior of PGNPs but also possibly their dynamics and thermo-mechanical properties as well.

Bimodality and re-entrant behaviour in the hierarchical self-assembly of polymeric nanoparticles.

We show that a film of a suspension of polymer grafted nanoparticles on a liquid substrate can be employed to create two-dimensional nanostructures with a remarkable variation in the pattern length scales. The presented experiments also reveal the emergence of concentration-dependent bimodal patterns as well as re-entrant behaviour that involves length scales due to dewetting and compositional instabilities. The experimental observations are explained through a gradient dynamics model consisting of coupled evolution equations for the height of the suspension film and the concentration of polymer. Using a Flory-Huggins free energy functional for the polymer solution, we show in a linear stability analysis that the thin film undergoes dewetting and/or compositional instabilities depending on the concentration of the polymer in the solution. We argue that the formation via 'hierarchical self-assembly' of various functional nanostructures observed in different systems can be explained as resulting from such an interplay of instabilities.

Kinetics of dispersion of nanoparticles in thin polymer films at high temperature.

We report the first detailed study of the kinetics of dispersion of nanoparticles in thin polymer films using temperature dependent in situ X-ray scattering measurements. We show a comparably enhanced dispersion at higher temperatures for systems which are otherwise phase segregated at room temperature. Detailed analysis of the time dependent X-ray reflectivity and diffuse scattering data allows us to explore the out-of-plane and in-plane mobility of the nanoparticles in the polymer films. While the out-of-plane motion is diffusive with a diffusion coefficient almost two orders of magnitude lower than that expected in bulk polymer, the in-plane one is found to be super-diffusive resulting in significantly larger in-plane displacement at similar time scales. We discuss the origin of the observed highly anisotropic motion of nanoparticles due to their slaved motion with respect to the anisotropic chain orientation and consequent diffusivity anisotropy of matrix chains. We also suggest strategies to utilize these observations to kinetically improve dispersion in otherwise thermodynamically segregated polymer nanocomposite films.

A new microscopic insight into membrane penetration and reorganization by PETIM dendrimers.

Dendrimers are highly branched polymeric nanoparticles whose structure and topology, largely, have determined their efficacy in a wide range of studies performed so far. An area of immense interest is their potential as drug and gene delivery vectors. Realizing this potential, depending on the nature of cell surface-dendrimer interactions, here we report controlled model membrane penetration and reorganization, using a model supported lipid bilayer and poly(ether imine) (PETIM) dendrimers of two generations. By systematically varying the areal density of the lipid bilayers, we provide a microscopic insight, through a combination of high resolution scattering, atomic force microscopy and atomistic molecular dynamics simulations, into the mechanism of PETIM dendrimer membrane penetration, pore formation and membrane re-organization induced by such interactions. Our work represents the first systematic observation of a regular barrel-like membrane spanning pore formation by dendrimers, tunable through lipid bilayer packing, without membrane disruption.

Confinement enhances dispersion in nanoparticle-polymer blend films.

Polymer nanocomposites constitute an important class of materials whose properties depend on the state of dispersion of the nanoparticles in the polymer matrix. Here we report the first observations of confinement-induced enhancement of dispersion in nanoparticle-polymer blend films. Systematic variation in the dispersion of nanoparticles with confinement for various compositions and matrix polymer chain dimensions has been observed. For fixed composition, strong reduction in glass transition temperature, Tg, is observed with decreasing blend-film thickness. The enhanced dispersion occurs without altering the polymer-particle interactions and seems to be driven by enhanced matrix-chain orientation propensity and a tendency to minimize the density gradients within the matrix. This implies the existence of two different mechanisms in polymer nanocomposites, which determines their state of dispersion and glass transition.

Morbidity from unsafe termination of pregnancy in South Africa.

The Termination of Pregnancy (TOP) Act in South Africa (SA) allows women to request TOP for social reasons. A retrospective record review was done on all (85) women who were admitted with complications arising from TOP over a 1-year period in 2008, at the Charlottee Maxeke Johannesburg Academic (CMJA) Hospital, SA. The demography, methods and place of TOP, presenting complications and final management was discussed. Morbidity such as severe blood loss (39%), sepsis (26%), multiple organ dysfunctions (4%) and organ injuries (5%) were observed. A correct guideline for TOP was followed in only 33% of women. Self-induced TOP remained common practice. There is a need for accelerated training among healthcare workers providing TOP in SA to prevent this costly and preventable maternal morbidity.

Microscopic dynamics of nanoparticle monolayers at air-water interface.

We present results of surface mechanical and particle tracking measurements of nanoparticles trapped at the air-water interface as a function of their areal density. We monitor both the surface pressure (Π) and isothermal compression modulus (ϵ) as well as the dynamics of nanoparticle clusters, using fluorescence confocal microscopy while they are compressed to very high density near the two dimensional close packing density Φ∼0.82. We observe non-monotonic variation in both ϵ and the dynamic heterogeneity, characterized by the dynamical susceptibility χ4 with Φ, in such high density monolayers. We provide insight into the underlying nature of such transitions in close packed high density nanoparticle monolayers in terms of the morphology and flexibility of these soft colloidal particles. We discuss the significance our results in the context of related studies on two dimensional granular or colloidal systems.

Variation in glass transition temperature of polymer nanocomposite films driven by morphological transitions.

We report the variation of glass transition temperature in supported thin films of polymer nanocomposites, consisting of polymer grafted nanoparticles embedded in a homopolymer matrix. We observe a systematic variation of the estimated glass transition temperature T(g), with the volume fraction of added polymer grafted nanoparticles. We have correlated the observed T(g) variation with the underlying morphological transitions of the nanoparticle dispersion in the films. Our data also suggest the possibility of formation of a low-mobility glass or gel-like layer of nanoparticles at the interface, which could play a significant role in determining T(g) of the films provided.

Demographic and clinical variables influencing gestational age at booking among South African pregnant women.

A retrospective record review was made of 758 women who delivered over a 2-month period at the Charlottee Maxeke Johannesburg Academic Hospital. Women booking early (up to 14 weeks) were compared with women booking late (from 15 weeks onwards). A total of 45 (6%) and 713 (94%) women booked in the early and late groups, respectively. Women of white ethnicity and women who experienced previous fetal loss booked significantly early. Primigravidae booked significantly late.