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SoftComp Highlight 2013

Effect of nanoconfinement on polymer dynamics: Surface layers and interphases

M. Krutyeva, A. Wischnewski, M. Monkenbusch, L. Willner, J. Maiz, C. Mijangos, A. Arbe, J. Colmenero, D. Richter
SoftComp partner: FZJ-Richter, Univ. Basque Country
PRL 110 (2013) 108303

The presence of a solid surface reduces the number of possible conformations of a polymer chain and influences the dynamics, in particular for those macromolecules which are close to the surface. In this work we demonstrate that the attractive polymer-surface interaction leads to the formation of the frequently discussed interphase in a confined linear polymer melt using neutron spin-echo (NSE) spectroscopy. The dynamics of polydimethylsiloxane (PDMS) confined in cylindrical nanopores of anodic aluminium oxide is strongly affected by the confinement and characterized by two phases, one fully equal to the bulk polymer and another one that is partly anchored at the surface. By topological interaction, the anchored part confines further chains with no direct contact to the surface. They are forming the often invoked interphase. The confined phase is internally highly mobile and not glassy as so far promoted heavily in the literature. These results are inferred from the space-time dependent chain dynamics that is observed in terms of the single chain dynamic structure factor and represent the first direct and quantitative observation of the interphase.

Left figure: The NSE results for confined PDMS. Solid lines present the fitting with the two-phase model. In the insert the sketch of the two-phase model is shown. Right figure: Schematic representation of the artificial surface-induced entanglements in the confined polymer melt. Black line represents the chain adsorbed on the surface of AAO nanopore, red lines show entangled chains in confined phase.

Multi-scale filler structure in simplified industrial nanocomposite silica/SBR systems studied by SAXS and TEM

G.P.Baeza, A.-C.Genix, C.Degrandcourt, L.Petitjean, J..Gummel, M.Couty, J.Oberdisse
SoftComp partner: CNRS-Montpellier
Macromolecules 46 (2013) 317-329

Simplified silica (Zeosil 1165MP) and Styrene-Butadien-Rubber (SBR) have been formulated by mixing of a reduced number of ingredients with respect to industrial applications. The filler structure on large scales up to micrometers was studied by transmission electron microscopy (TEM) and very small-angle X-ray scattering (SAXS). A complete quantitative model extending from the primary silica nanoparticle (10 nm), to nanoparticles aggregates (40-50 nm), up to micrometer-sized branches with typical lateral dimension of 150 nm is proposed. A comparison between the model and the experimental data leads to the determination of the compacity of the aggregates allowing us to represent in a coherent way the whole geometry of the silica branched formed tridimensional network and using it to quantitatively describe the reinforcement of the pure polymer matrix as a function of the filler fraction. We pay a particular attention on the determination of the critical volume fraction in aggregate leading to the full percolation of the network synonymous of a much higher reinforcement regime.

a) SAXS (samples loaded from 8.4 to 21.1%v) and TEM (8.4%v) data leading to the quantitative description of a 3D silica network through the SBR matrix. b) Artist view of the network build up from aggregates made of nanoparticles and its schematic structure factors modeling the interactions between objects at different length scales.

A direct observation of non-affine tube deformation in strained polymer networks

W.Pyckhout-Hintzen, S.Westermann, A.Wischnewski, M.Monkenbusch, D.Richter, E.Straube, B.Farago and P.Lindner
SoftComp partner: FZJ-Richter, ILL
PRL 110 (2013) 196002

The topology in entangled melts and rubberlike polymer networks is governed by the uncrossability of chains. Crosslinking now further freezes in the chain configurations. Their reptational motion becomes quenched and only locally perpendicular dynamics remains. The quenched disorder and topological constraints can be theoretically accounted for by harmonic potentials, which are found to be anisotropic in the main axes of the deformation tensor. The fluctuation scales show thereby a non-trivial square-root power law dependence on strain.
In a first one-to-one comparison of tube diameters, measured by small angle neutron scattering (SANS) in networks with those obtained by neutron spin echo (NSE) spectroscopy we achieved an independent proof of this strain dependence and ruled out non-deformed and affinely deformed tubes. The Des Cloizeaux approach provided direct access to the tube diameter in terms of the single chain dynamic structure factor and unequivocally identified the coupling of chain fluctuations to the strain. The dependence is essential in constitutive material laws, which presently are rated as most powerful for engineering applications and continuum mechanics of rubber materials.

The high sensitivity of NSE clearly resolves the anisotropic tube diameters along both principal axes of deformation. Solid and dashed lines correspond to sub-affine resp. affine deformation.

Structural Arrest and Texture Dynamics in Suspensions of Charged Colloidal Rods

K.Kang and J.K.G.Dhont
SoftComp partner: FZJ-Dhont
PRL 110 (2013) 015901

Very little is known about the glass transition of very anisometric colloidal particles. We discuss structural arrest of very long and thin rod-like colloids at low ionic strength. A glass is formed due to “cages” that result from overlap of electrical double layers (so-called “Wigner glasses”). As a model system for rods we use fd-virus particles (880 nm long, diameter 6.8 nm). They form a nematic at 3.0 mg/ml. Structural particle arrest is probed by means of dynamic light scattering, and occurs at 11.7 mg/ml. This is far into the full nematic state, where there is a texture of many (chiral) nematic domains. The dynamics of the texture is quantified with “video correlation spectroscopy”. Image-correlation functions have a time-decay constant (of 250 hours) that is independent of concentration, up to structural particle arrest, above which the time constant discontinuously drops to zero. The glass transition where particle arrest occurs thus coincides with the point where the macroscopic dynamics of the nematic texture freezes. The figure shows images of the texture in a cuvette of 2 cm diameter, for two concentrations: just below and above the glass concentration. Right after filling the cuvette, the nematic texture is flow-aligned (upper two images). The shear-aligned texture breaks down into smaller domains after about 50 hours for the concentration below the glass transition (left lower image). Just above the glass concentration, however, the shear-aligned texture does not relax (lower image on the right).

Images of the nematic texture taken between crossed polarizers of a cuvette of 1mm thickness and 2 cm diameter. The left two images are for a concentration just below the glass transition concentration, while the two images on the right are for a concentration just above. The time indicated is the time after filling the cuvette.

Capillarity-induced ordering of spherical colloids on an interface with anisotropic curvature

D.Ershov, J.Sprakel, J.Appel, M.A.Cohen Stuart, and J.van der Gucht
SoftComp partner: Univ. Wageningen
PNAS 110 (2013) 9220

Objects floating at a liquid interface, such as breakfast cereals floating in a bowl of milk or bubbles at the surface of a soft drink, clump together in space-saving hexagons to minimize the disruption of the liquid interface. Micrometer-sized colloidal particles embedded in a liquid interface normally do not disrupt the interface, so that such clustering does not occur. Here, we show that this is different when the interface has a curvature that is anisotropic. We find that in this case the condition of constant contact angle along the three-phase contact line can only be satisfied when the interface is deformed. We present experiments and numerical calculations that demonstrate how this leads to quadrupolar capillary interactions between the particles, giving rise to organization into regular square lattices. We demonstrate that the strength of the governing anisotropic interactions can be rescaled with the deviatoric curvature alone, irrespective of the exact shape of the liquid interface. Our results suggest that anisotropic interactions can easily be induced between isotropic colloids through tailoring of the interfacial curvature.

Square lattice organization of colloids on a droplet with anisotropic curvature. Particles are labelled with a fluorescent dye to make them visible.

Emergence of Metachronal Waves in Cilia Arrays

J.Elgeti and G.Gompper
SoftComp partner: FZJ-Gompper
PNAS 110 (2013) 4470

Propulsion by cilia is a fascinating and universal mechanism in biological organisms to generate fluid motion on the cellular level. Cilia are hair-like organelles, which are found in many different tissues and many uni- and multi-cellular organisms. Assembled in large fields, cilia beat neither randomly nor completely synchronously -- instead they display self-organization in the form of metachronal waves (MCWs). The main questions are how the individual cilia interact with the flow field generated by their neighbors and synchronize their beats for the metachronal wave to emerge, and how the properties of the metachronal wave are determined by the geometrical arrangement of the cilia, like cilia spacing and beat direction.
We address these issues by large-scale computer simulations of a mesoscopic model of two-dimensional cilia arrays in a three-dimensional fluid medium. We show that hydrodynamic interactions are indeed sufficient to explain the self-organization of MCWs, and study beat patterns, stability, energy expenditure and transport properties. We find that MCW strongly increase both propulsion velocity and efficiency -- compared to cilia all beating in phase. This can be a vital advantage for ciliated organisms, and may be interesting to guide biological experiments as well as the design of efficient microfluidic devices and artificial microswimmers.

Cilia beat by alternating power and recovery strokes, which are characterized by nearly extended and curled-up shapes, respectively. Hydrodynamic interactions lead to the formation of metachronal waves.

Skin - a marvelous functional barrier

C.Das, M. Noro and P. Olmsted
SoftComp partner: Unilever, Univ. Leeds
PRL 111 (2013) 148101

The outermost part of skin (the stratum corneum, SC) keeps us from drying up, protects us from dangerous chemicals and is the first line of defence against foreign pathogens. While pliable, it resists mechanical and dehydration stresses [2]. New computer simulations show how the glue that holds the SC together is structured and gives skin its remarkable properties. The SC comprises water-retaining “bricks” of proteinaceous dead cells (corneocytes) [3] surrounded by an impermeable “mortar” made of roughly twenty solid layers of lipids (soap-like molecules, oils, and cholesterol) [4]. This mortar is responsible for the skin’s extraordinary barrier properties.
We have run massive computer simulations on realistic SC lipid mixtures. We find that the spontaneous phase of SC lipids in water is not layered; rather, the lipids encase water droplets in a so-called “inverted micelle” structure, matching electron microscopy observations from larger irregular spaces between corneocytes. However, the simulations show that adjacent SC corneocyte walls actually induce layers, through an envelope of covalently-bound lipids whose molecular corrugation encourage the low-permeability ‘mortar’ layers between the ‘brick’ faces.
These simulations relate the sometimes paradoxical biochemical pathways for lipid synthesis to the physical lipid structures found in the SC, and shed light on the reason for the relatively long time for skin barrier healing. The simulations also suggest how the lipid mixture provides plasticity and allows the skin to absorb energy, as well as enabling three dimensional packing of corneocytes without compromising the barrier. Hence, this work can help with future efforts to make, heal, and nourish skin; moreover, the fundamental physics is relevant for making new functional nanomaterials based on self-assembly.

Simulation of a the evolution of a hydrated double bilayer, showing that an unconstrained layered state is unstable to the formation of water drops encased within inverse micelles. The bilayer is a polydisperse mixture of ceramides, free fatty acids, and cholesterol, which is representative of lipid bilayers in the stratum corneum.

Direct evidence of two equilibration mechanisms in glassy polymers

D.Cangialosi, V.M.Boucher, A.Alegría and J.Colmenero3
SoftComp partner: Univ. Basque Country
PRL 111 (2013) 095701

The dynamics and thermodynamics of glass-forming systems have been the subject of intense research in the last decades. Among the variety of aspects that have been analyzed, the following can be included: i) the dramatic slowing down of the dynamics when decreasing temperature often described by a Vogel-Fulcher-Tammann (VFT) law; ii) the possible connection between such slowing down and the thermodynamics of the glass-former. These aspects have been deeply investigated above the laboratory glass transition temperature (Tg). It has been speculated that mere extrapolation of the dynamics and thermodynamics to low temperatures produces a singularity at a finite temperature. In particular, extrapolating the behavior above Tg to lower temperatures would imply that: (i) the relaxation time associated to the glassy dynamics shows a divergence; (ii) the entropy of the glass equals that of the crystal. In this study the temperature range of dynamics and thermodynamics is extended to temperatures as low as Tg - 40 K by performing enthalpy recovery experiments on glassy polymers for times as large as 10^7-10^8 seconds. We find a single stage recovery behavior for temperatures larger than about Tg - 10 K. Interestingly, a double stage recovery is observed for T < Tg - 10 K, as shown in Fig. 1 for high molecular weight polystyrene (PS85k) as a showcase. In all cases the enthalpy recovered after the two-stage decay approximately equals that extrapolated from the melt, whereas partial enthalpy occurs in the first decay. The time to reach each equilibrium contains information on the dynamics below Tg. The following scenario emerges analyzing its temperature dependence (see Fig. 2): i) The equilibration time corresponding to the first stage recovery exhibits relatively low activation energy (several times smaller than that of the segmental relaxation process at Tg); ii) The equilibration time of the second decay exhibits activation energy similar to that of the polymer segmental relaxation at Tg - 10 K. These results indicate a complex scenario of the dynamics and thermodynamics below Tg with multiple equilibration steps and leave open the question of the presence of a singularity at a finite temperature.

Left figure: Time evolution of the recovered enthalpy at the indicated temperatures for high molecular weight polystyrene (PS85k). Right figure: Logarithm of equilibration times corresponding to the first (circles) and the second (triangles) plateau as a function of the inverse temperature obtained from enthalpy recovery data and derived from the temperature dependence of the segmental relaxation according to the VFT equation (lines).

Solvent-induced Division of Plasmonic Clusters

M.Grzelczak, A.Sánchez-Iglesias and L.Liz-Marzán
SoftComp partner: CIC BiomaGUNE
Soft Matter 9 (2013) 9094

According to biomimetic principles, self-organization of organic/inorganic matter is a fundamental step toward novel materials with life-like characteristics. Thus, structural changes of supramolecular entity in the presence of environmental stimulus offer chemical control in the design of dynamically changing materials. In a recent article, the group of Prof. Luis Liz-Marzán reported on a composite polymeric system that spontaneously undergoes division at specific solvent compositions. Such a composite comprises clusters of polystyrene-capped gold nanoparticles, which are formed through hydrophobic interactions between the building blocks. Because of the reversible nature of these binding forces, such clusters can behave as self-assembled amphiphiles and thus display dynamic size variations. Copolymer micelles carrying a cargo of gold nanoparticles show a progressive decrease in size, in the presence of a solvent that is compatible with the hydrophobic micellar core. Structural changes of the polymeric micelles revealed that they spontaneously undergo fission to produce smaller units with homogenously distributed gold nanoparticles of two different sizes. This system is a suitable experimental model for future studies on fundamental understanding of dynamic self-assembly processes and for design of adaptive materials with biomedical relevance.

Top panel: Schematic representation of the fission of binary plasmonic clusters stabilized within block-copolymer micelles, upon addition of dioxane as a good solvent. Bottom panel: Transmission electron micrographs of binary clusters before and after fission, showing that the concentration of larger nanoparticles remains constant.

Patterning Polymer–Fullerene Nanocomposite Thin Films with Light

H.C.Wong, A.M.Higgins, A.R.Wildes, J.F.Douglas, J.T.Cabral
SoftComp partner: Imperial College London and ILL
Advanced Materials 25 (2013) 985

Trace amounts of nanoparticles, including fullerenes, can impart stability to thin polymer films against dewetting by the combined effects of pinning the contact lines of dewetting holes and by effectively altering the polymer-substrate interaction. Polymer nanocomposite thin films stable to dewetting eventually yield well defined morphologies from uniform to spinodal-like, via spontaneous polymer–nanoparticle phase separation and crystallisation. In this paper, we show that UV-visible, and even background, light exposure, can finely tune the morphology of dewetting and phase separating polymer-fullerene thin films. Neutron reflectivity allows us to locate the various constituents within the film. We find a coupling of fullerene photo-sensitivity and both self-assembly processes which results in controlled pattern formation, and we illustrate the potential with a model polymer–fullerene circuit pattern. We believe this approach opens new opportunities in soft matter lithography via the directed assembly of polymer nanocomposites and underscores their photoactive nature, an effect of great interest to material performance and stability of organic photovoltaics (OPV) and aerospace materials under long-term radiation exposure.

Left panel: optical profilometry of a fullerene-polymer circuit fabricated by lithography and directed assembly; Central panel: film stability (against dewetting) and phase separated morphology can be quantitatively tuned by light exposure; Right panel: neutron reflectivity demonstrates that no fullerene displacement, specifically surface segregation, occurs during exposure.

Performance Enhancement of fullerene-based solar cells by light processing

Z.Li, H.C.Wong, Z.Huang, H.Zhong, C.H.Tan, W.C.Tsoi, J.S.Kim, J.R.Durrant and J.T.Cabral
SoftComp partner: Imperial College London
Nature Communications 4 (2013) 2227

A key challenge to the commercialisation of organic solar cells remains the achievement of morphological stability, particularly under thermal stress conditions. Bulk heterojunctions, BHJ, benefit from a bicontinuous inter-percolated morphology with large interfacial area and approximately ~10 nm characteristic lengthscale. In this paper we demonstrate the directed assembly a blend polymer:PC60BM solar cells via a simple light processing step can result in a 10-fold increase in device thermal stability and, under certain conditions, enhanced device performance. The enhanced stability is linked to the light-induced oligomerisation of PC60BM that effectively hinders diffusion and crystallisation in the blend. This effect appears to be general and promises to be an effective and cost-effective strategy to optimise fullerene-based solar cell performance.

Top panel: schematic of the light processing step to improve thermal stability and increase lifetime of fullerene-based solar cells. Bottom-left panel: fullerene crystallisation is supressed upon light-induced oligomerisation. Bottom right: the lifetime of solar cells, as measured by the time corresponding to a 0% efficiency drop, upon low intensity visible illumination. A lifetime increase of 10-100 fold has been achieved.

Plasmonic Mesoporous Composites as Molecular Sieves for SERS Detection

V.López-Puente, S.Abalde-Cela, P.C.Angelomé, R.A.Alvarez-Puebla and L.M. Liz-Marzán
SoftComp partner: CIC BiomaGUNE
J. Phys. Chem. Lett. 4 (2013) 2715

Detection of small organic molecules in biological samples is often complicated because of the presence of other (larger) biomolecules such as proteins and nucleic acids. These biomolecules often interfere with the analysis methods by contaminating the transducer signal. The group of Luis Liz-Marzán recently reported the design and fabrication of a thin-film material that acts as a molecular sieve, filtering out the large molecules and allowing small ones through for detection by surface-enhanced Raman scattering spectroscopy. In SERS, the molecules to be analyzed must be adsorbed to the surface of the particles, so that they can be affected by the electromagnetic field enhancement related to localized surface plasmon resonances. Such field enhancement leads to huge Raman signals that are characteristic of each molecule. In biological fluids, large proteins and nucleic acids also can adsorb onto the metal and their Raman signals can shadow those from smaller molecules. In the new approach, a mesoporous material is grown on gold nanoparticles that had been deposited on a glass substrate. The pores are then used as templates to grow thin tips from the nanoparticles so that they provide larger SERS enhancement. Additionally, the pores can act as molecular sieves to prevent the diffusion of large biomolecules and only allow smaller molecules to reach the metal underneath. To performance of the films was tested by using a simplified biological fluid containing bovine serum albumin (BSA) and 4-nitrobenzenethiol (NBT), as a model small molecule with a high Raman cross section. The results of SERS measurements indeed indicated that NBT could diffuse through the 6-nm-diameter pores of mesoporous titania, whereas BSA could not.

Transmission electron micrograph showing gold nanoparticles embedded in a mesoporous titania film. The nanoparticles were used as seed catalysts to grow thin branches for SERS enhancement. The inset shows a schematic view of the molecular sieving effect, where small MBA molecules diffuse through the pores but larger BSA do not because of size restriction.

Computer-aided molecular design of solvents for optimum reaction kinetics

H.Struebing, Z.Ganase, P.G.Karamertzanis, E.Siougkrou, P.Haycock, P.M.Piccione, A.Armstrong, A.Galindo, C.S.Adjiman
SoftComp partner: Imperial College London
Nature Chemistry DOI: 10.1038/NCHEM.1755 (2013)

What is the best solvent for a given chemical reaction? Given that the rate and selectivity of chemical reactions can vary by several orders of magnitude in different solvents, this question has important ramifications for the exploration of novel reaction routes and the development of industrial processes. To address this challenge, a systematic methodology is proposed that quickly identifies improved reaction solvents by combining quantum mechanical computations of the reaction rate constant in a few solvents with a computer-aided molecular design (CAMD) procedure. The approach allows the identification of a high-performance solvent within a large set of possible molecules. The validity of our CAMD approach is demonstrated through application to a classical nucleophilic substitution reaction for the study of solvent effects, the Menschutkin reaction. The results are successfully validated via in-situ kinetic experiments. A space of 1341 solvents is explored in silico, but requiring quantum mechanical calculations of the rate constant in only 9 solvents, and uncovering a solvent that increases the rate constant by 40%.

A numerical algorithm relies on ab initio calculations and a faster surrogate model to identify the best solvent in a liquid-phase organic reaction.

Last modified: 21/05/2013