Probing Collective Motions of Terminally Anchored Polymers

Ultrastable Glassy Polymer Films with an Ultradense Brush Morphology

Glassy polymer films with extreme stability could enable major advancements in a range of fields that require the use of polymers in confined environments. Yet, from a materials design perspective, we now know that the glass transition temperature (T_g) and thermal expansion of polymer thin films can be dramatically different from those characteristics of the bulk, i.e., exhibiting confinement-induced diminished thermal stability. Here, we demonstrate that polymer brushes with an ultrahigh grafting density, i.e., an ultradense brush morphology, exhibit a significant enhancement in thermal stability, as manifested by an exceptionally high T_g and low expansivity. For instance, a 5 nm thick polystyrene brush film exhibits an ∼75 K increase in T_g and ∼90% reduction in expansivity compared to a spin-cast film of similar thickness. Our results establish how morphology can overcome confinement and interfacial effects in controlling thin-film material properties and how this can be achieved by the dense packing and molecular ordering in the amorphous state of ultradense brushes prepared by surface-initiated atom transfer radical polymerization in combination with a self-assembled monolayer of initiators.

Large T g Shift in Hybrid Bragg Stacks through Interfacial Slowdown

Studies of glass transition under confinement frequently employ supported polymer thin films, which are known to exhibit different transition temperature T g close to and far from the interface. Various techniques can selectively probe interfaces, however, often at the expense of sample designs very specific to a single experiment. Here, we show how to translate results on confined thin film T g to a "nacre-mimetic" clay/polymer Bragg stack, where periodicity allows to limit and tune the number of polymer layers to either one or two. Exceptional lattice coherence multiplies signal manifold, allowing for interface studies with both standard T g and broadband dynamic measurements. For the monolayer, we not only observe a dramatic increase in T g (∼ 100 K) but also use X-ray photon correlation spectroscopy (XPCS) to probe platelet dynamics, originating from interfacial slowdown. This is confirmed from the bilayer, which comprises both "bulk-like" and clay/polymer interface contributions, as manifested in two distinct T g processes. Because the platelet dynamics of monolayers and bilayers are similar, while the segmental dynamics of the latter are found to be much faster, we conclude that XPCS is sensitive to the clay/polymer interface. Thus, large T g shifts can be engineered and studied once lattice spacing approaches interfacial layer dimensions.

Transfer of functional thermoresponsive poly(glycidyl ether) coatings for cell sheet fabrication from gold to glass surfaces

Thermoresponsive polymer coatings can facilitate cell sheet fabrication under mild conditions by promoting cell adhesion and proliferation at 37 °C. At lower temperatures the detachment of confluent cell sheets is triggered without enzymatic treatment. Thus, confluent cell sheets with intact extracellular matrix for regenerative medicine or tissue engineering applications become available. Herein, we applied the previously identified structural design parameters of functional, thermoresponsive poly(glycidyl ether) brushes on gold to the more application-relevant substrate glass via the self-assembly of a corresponding block copolymer (PGE-AA) with a short surface-reactive, amine-presenting anchor block. Both, physical and covalent immobilization on glass via either multivalent ionic interactions of the anchor block with bare glass or the coupling of the anchor block to a polydopamine (PDA) adhesion layer on glass resulted in stable coatings. Atomic force microscopy revealed a high degree of roughness of covalently attached coatings on the PDA adhesion layer, while physically attached coatings on bare glass were smooth and in the brush-like regime. Cell sheets of primary human dermal fibroblasts detached reliably (86%) and within 20 ± 10 min from physically tethered PGE-AA coatings on glass when prepared under cloud point grafting conditions. The presence of the laterally inhomogeneous PDA adhesion layer, however, hindered the spontaneous temperature-triggered cell detachment from covalently grafted PGE-AA, decreasing both detachment rate and reliability. Despite being only physically attached, self-assembled monolayer brushes of PGE-AA block copolymers on glass are functional and stable thermoresponsive coatings for application in cell sheet fabrication of human fibroblasts as determined by X-ray photoelectron spectroscopy.

Polymer Surface Transport Is a Combination of in-Plane Diffusion and Desorption-Mediated Flights

Previous studies of polymer motion at solid/liquid interfaces described the transport in the context of a continuous time random walk (CTRW) process, in which diffusion switches between desorption-mediated "flights" (i.e., hopping) and surface-adsorbed waiting-time intervals. However, it has been unclear whether the waiting times represented periods of complete immobility or times during which molecules engaged in a different (e.g., slower or confined) mode of interfacial transport. Here we designed high-throughput, single-molecule tracking measurements to address this question. Specifically, we studied polymer dynamics on either chemically homogeneous or nanopatterned surfaces (hexagonal diblock copolymer films) with chemically distinct domains, where polymers were essentially excluded from the low-affinity domains, eliminating the possibility of significant continuous diffusion in the absence of desorption-mediated flights. Indeed, the step-size distributions on homogeneous surfaces exhibited an additional diffusive mode that was missing on the chemically heterogeneous nanopatterned surfaces, confirming the presence of a slow continuous mode due to 2D in-plane diffusion. Kinetic Monte Carlo simulations were performed to test this model and, with the theoretical in-plane diffusion coefficient of D_2D = 0.20 μm²/s, we found a good agreement between simulations and experimental data on both chemically homogeneous and nanopatterned surfaces.

Near-wall dynamics of concentrated hard-sphere suspensions: comparison of evanescent wave DLS experiments, virial approximation and simulations

In this article we report on a study of the near-wall dynamics of suspended colloidal hard spheres over a broad range of volume fractions. We present a thorough comparison of experimental data with predictions based on a virial approximation and simulation results. We find that the virial approach describes the experimental data reasonably well up to a volume fraction of ϕ≈ 0.25 which provides us with a fast and non-costly tool for the analysis and prediction of evanescent wave DLS data. Based on this we propose a new method to assess the near-wall self-diffusion at elevated density. Here, we qualitatively confirm earlier results [Michailidou, et al., Phys. Rev. Lett., 2009, 102, 068302], which indicate that many-particle hydrodynamic interactions are diminished by the presence of the wall at increasing volume fractions as compared to bulk dynamics. Beyond this finding we show that this diminishment is different for the particle motion normal and parallel to the wall.

Nanostructures and Dynamics of Macromolecules Bound to Attractive Filler Surfaces

We report in situ nanostructures and dynamics of polybutadiene (PB) chains bound to carbon black (CB) fillers (the so-called "bound polymer layer (BPL)") in a good solvent. The BPL on the CB fillers was extracted by solvent leaching of a CB-filled PB compound and subsequently dispersed in deuterated toluene to label the BPL for small-angle neutron scattering and neutron spin echo techniques. The results demonstrate that the BPL is composed of two regions regardless of molecular weights of PB: the inner unswollen region of ≈ 0.5 nm thick and outer swollen region where the polymer chains display a parabolic profile with a diffuse tail. In addition, the results show that the dynamics of the swollen bound chains can be explained by the so-called "breathing mode" and is generalized with the thickness of the swollen BPL.

Translational and rotational near-wall diffusion of spherical colloids studied by evanescent wave scattering

In this article we extend recent experimental developments [Rogers et al., Phys. Rev. Lett., 2012, 109, 098305] by providing a suitable theoretical framework for the derivation of exact expressions for the first cumulant (initial decay rate) of the correlation function measured in Evanescent Wave Dynamic Light Scattering (EWDLS) experiments. We focus on a dilute suspension of optically anisotropic spherical Brownian particles diffusing near a planar hard wall. In such a system, translational and rotational diffusion are hindered by hydrodynamic interactions with the boundary which reflects the flow incident upon it, affecting the motion of colloids. The validity of the approximation by the first cumulant for moderate times is assessed by juxtaposition to Brownian dynamics simulations, and compared with experimental results. The presented method for the analysis of experimental data allows the determination of penetration-depth-averaged rotational diffusion coefficients of spherical colloids at low density.

Microscopic analysis of the surface functionalization of polymer particles and subsequent grafting of polymer chains from the surface

The characterization of the surface functionalization of polymer particles and subsequent grafting of hydrated polymer chains from their surface by microscopic techniques are essential to obtain reliable data about the actual morphology of the system. Since the size range of morphological features of functionalized polymer surfaces has long ago reached the lower end of the nanometer scale, classical light microscopy and dynamic light scattering have been replaced by electron and atomic force microscopy techniques which provide sufficient resolution for the visualization of nano-sized structures. Moreover, only polymer particle aggregates and fine organization of hydrated polymer chains which are not efficiently characterized by particle size measurements can be detected accurately with microscopy methods. Both solid and hydrated systems can be characterized by transmission electron microscopy and scanning electron microscopy (inc. cryo-electron microscopy (EM)) after appropriate sample preparation. Moreover, analytical EM methods allow not only for the size, shape and internal structure characterization, but also for the chemical composition with high spatial resolution.

Resonance enhanced dynamic light scattering

We present a novel light scattering setup that enables probing of dynamics near solid surfaces. An evanescent wave generated by a surface plasmon resonance in a metal layer is the incident light field in the dynamic light scattering experiment. The combination of surface plasmon resonance spectroscopy and dynamic light scattering leads to a spatiotemporal resolution extending a few hundred nanometers from the surface and from microseconds to seconds. The comparison with evanescent wave dynamic light scattering identifies the advantages of the presented technique, e.g., surface monitoring, use of metal surfaces, and biorelevant systems. For both evanescent wave geometries, we define the scattering wave vector necessary for the analysis of the experimental relaxation functions.

Evanescent-wave dynamic light scattering at an oil-water interface: diffusion of interface-adsorbed colloids

A light-scattering goniometer for evanescent-wave dynamic light scattering (EWDLS) measurements at a liquid-fluid interface is introduced, and used for measurements on two charge-stabilized polystyrene colloid systems adsorbed to alkane-water interfaces. The goniometer allows an independent variation of the penetration depth and the scattering vector components parallel and perpendicular to a liquid-fluid interface. The possible illumination geometries are compared. Ellipsometry at the liquid-fluid interface is implemented as a complementary tool. In EWDLS measurements, the absence of diffusive motion perpendicular to the interface is demonstrated, which confirms the adsorption of the particles. The two-step decay of the autocorrelation function is interpreted in terms of diffusion within a two-dimensional interface lattice of colloidal particles, stabilized by repulsive electrostatic interactions, and a desorption process. A significant slowing down of the in-plane diffusion of the colloids as compared to the bulk diffusion is observed.

Probing dynamics at interfaces: resonance enhanced dynamic light scattering

Experiments addressing supramolecular dynamics at interfaces are of paramount importance for the understanding of the dynamic behaviour of polymers, particles, or cells at interfaces, transport phenomena to and from surfaces, thin films or membranes. However, there are only few reports in the literature due to the paucity of experimental methods that offer the required spatial and time resolution. Evanescent wave dynamic light scattering originally developed to meet these needs has limited sensitivity and is restricted to glass substrates. Here we report the first experimental realization of a dynamic light scattering experiment close to an interface using surface plasmon polaritons as light source offering a strong increase in the signal to noise ratio and allowing for the use of metallic interfaces. As a proof of concept, we consider the diffusion of particles with radii down to 10nm in dilute dispersions close to a gold surface.

Convergence of dissipation and impedance analysis of quartz crystal microbalance studies

A quartz crystal microbalance (QCM) consists of a resonator, which measures the resonance frequency of the quartz slab. When coupled with a network analyzer or coupled with impulse excitation technology, QCM gives additional impedance or dissipation information, respectively. This report provides a set of equations that bring the QCM community a convergence of the dissipation and impedance analysis. Equations derived from the complex frequency shift were applied to quantitatively analyze the dissipation data of polymer brushes obtained from QCM-D. The obtained viscoelastic properties of polymer brushes were then compared with those obtained by the Voigt model method. We believe that these equations will be useful in quantitative studies of interfacial phenomena accompanied with mass or viscoelasticity changes.

Dynamics of concentrated hard-sphere colloids near a wall

We investigate the Brownian motion of hard-sphere colloids near a solid wall by Evanescent Wave Dynamic Light Scattering (EWDLS). We carried out measurements for various volume fractions of sterically stabilized poly(methyl methacrylate) (PMMA) particles over a range of scattering wave vectors, q. While in the dilute regime, the near wall short-time diffusion is significantly slowed down due to particle-wall hydrodynamic interactions (HI); as volume fraction increases, the wall effect is progressively diminished at all q's. We present a new analysis for the EWDLS short-time self- and collective diffusivities applicable to all volume fractions and a simple model for the self-diffusion describing the interplay between particle-wall and particle-particle HI. Moreover, a weaker decay of the near-wall self-diffusion coefficient with volume fraction is predicted by Stokesian dynamics simulations.

Particle dynamics within a wetting layer in a colloid-polymer mixture

The near-wall dynamics at the top and bottom phases of a phase-separated colloid-polymer mixture were measured by evanescent-wave dynamic light scattering. The short-time dynamics near the wall were found to be liquidlike in both phases, confirming the presence of a liquid wetting layer. The short-time diffusion within the wetting layer was slower than in the bulk liquid phase. Similarly, the near-wall dynamics in both phases of the colloid-polymer mixture were also slower compared to the near-wall colloidal dynamics in a pure concentrated suspension at the same volume fraction. These effects highlight the role of interparticle attractions and specific wall-induced hydrodynamic interactions in slowing down the colloidal motion in confinement.

Observation of slow down of polystyrene nanogels diffusivities in contact with swollen polystyrene brushes

The diffusion of dilute colloids in contact with swollen polymer brushes has been studied by evanescent wave dynamic light scattering. Two polystyrene nanogels with 16 nm and 42 nm radius were put into contact with three polystyrene brushes with varying grafting densities. Partial penetration of the nanogels within the brushes was revealed by the evanescent wave penetration depth-dependent scattering intensities. The experimental short-time diffusion coefficients of the penetrating particles were measured and found to strongly slow down as the nanoparticles get deeper into the brushes. The slow down is much more marked for the smaller (16 nm) nanogels, suggesting a size exclusion type of mechanism and the existence of a characteristic length scale present in the outer part of the brush.

Unexpected slow near wall dynamics of spherical colloids in a suspension of rods

In this paper, we will show the influence of an additional rodlike component, that is, fd-virus, on the diffusion of spherical polystyrene colloids close to a wall. The sphere diffusivity normal to the wall, D perpendicular, is strongly affected by the presence of the rods, while the effect on the parallel diffusivity, D||, is less pronounced except in the immediate vicinity of the wall. We show that this observation cannot be explained by describing the effect of the rods as a simple mean field depletion potential alone.

Molecular Motion at Soft and Hard Interfaces: From Phospholipid Bilayers to Polymers and Lubricants

Spatially resolved and time-resolved understanding of complex fluid situations compose a new frontier in physical chemistry. Here we draw attention to the significance of spatially resolving systems whose ensemble average differs fundamentally from the spatially resolved individual elements. We take examples from the field of fluid phospholipid bilayers, to which macromolecules adsorb; the field of polymer physics, when flexible chains adsorb to the solid-liquid interface; and from the field of lubrication, when two solids are squeezed close together with confined fluid retained between them.

Brownian diffusion close to a polymer brush

In an effort to control particle diffusion near surfaces, we have studied the dynamics of colloidal hard spheres and soft compliant star copolymers on surfaces coated with polymer brushes using evanescent wave dynamic light scattering. The same experiments provide information on the brush structure and confined particle motion. The penetration into dense polydisperse brushes is size- and solvent-dependent.

Accessing the Dynamics of End-Grafted Flexible Polymer Chains by Atomic Force-Electrochemical Microscopy. Theoretical Modeling of the Approach Curves by the Elastic Bounded Diffusion Model and Monte Carlo Simulations. Evidence for Compression-Induced Lateral Chain Escape

The dynamics of a molecular layer of linear poly(ethylene glycol) (PEG) chains of molecular weight 3400, bearing at one end a ferrocene (Fc) label and thiol end-grafted at a low surface coverage onto a gold substrate, is probed using combined atomic force-electrochemical microscopy (AFM-SECM), at the scale of approximately 100 molecules. Force and current approach curves are simultaneously recorded as a force-sensing microelectrode (tip) is inserted within the approximately 10 nm thick, redox labeled, PEG chain layer. Whereas the force approach curve gives access to the structure of the compressed PEG layer, the tip-current, resulting from tip-to-substrate redox cycling of the Fc head of the chain, is controlled by chain dynamics. The elastic bounded diffusion model, which considers the motion of the Fc head as diffusion in a conformational field, complemented by Monte Carlo (MC) simulations, from which the chain conformation can be derived for any degree of confinement, allows the theoretical tip-current approach curve to be calculated. The experimental current approach curve can then be very satisfyingly reproduced by theory, down to a tip-substrate separation of approximately 2 nm, using only one adjustable parameter characterizing the chain dynamics: the effective diffusion coefficient of the chain head. At closer tip-substrate separations, an unpredicted peak is observed in the experimental current approach curve, which is shown to find its origin in a compression-induced escape of the chain from within the narrowing tip-substrate gap. MC simulations provide quantitative support for lateral chain elongation as the escape mechanism.

Anisotropy of Brownian motion caused only by hydrodynamic interaction with a wall

The diffusivity of spherical colloidal particles close to a planar hard wall is studied by dynamic light scattering with evanescent illumination. A novel setup allows us to independently vary the scattering vector components parallel Q parallel and normal Q perpendicular to the wall. An expression for the initial decay rate Gamma of the time autocorrelation functions is derived as a function of both Q parallel and Q perpendicular, as well as the penetration depth of the evanescent wave, where hydrodynamic interactions of particles with the wall are included. This makes it possible to study the viscous wall drag effect quantitatively for particles as small as 85 nm in radius.

Stimuli responsive surfaces through recognition-mediated polymer modification

Specific three-point hydrogen bonding between diamidopyridine (DAP) and thymine (Thy) was employed to reversibly anchor "brush-like" Tri-DAP end-functionalized polystyrene onto Thy-modified silica surfaces.