Solvent-Dependent Light-Induced Structures in Gem-Dichlorocyclopropanated Polybutadiene Solutions.

The formation of permanent structures upon mild red laser illumination in transparent polydiene solutions is examined in the case of gem-dichlorocyclopropanated polybutadiene ( gDCC-PB) polymers bearing 15% functional units of the dichlorocyclopropane groups. The response was found to be distinct from the precursor PB. Whereas fiber-like patterns were clearly observed in both precursor and gDCC-PB solutions in cyclohexane, these were absent in the case of gDCC-PB/chloroform but were present in the precursor PB/chloroform solutions. The involved mechanical stresses were not sufficient for the gDCC activation to be detected by NMR spectroscopy. Remarkably, addition of even 10 wt % gDCC-PB into the latter solution sufficed to suppress the light-induced patterning. The importance of the chemical environment on the response to light irradiation was further checked and confirmed by use of other PB copolymers. Different diameter patterns and kinetics were observed. The strong solvent and comonomer mediated effect was reflected neither in solvency nor in optical polarizability differences of the polymers solvent couples.

Kinetics of Light-Induced Concentration Patterns in Transparent Polymer Solutions.

When exposed to weak visible laser light, solutions of common polymers like poly(isoprene) and poly(butadiene) respond by local concentration variations, which in turn lead to refractive index changes. Various micropatterns have been recently reported, depending mostly on the solvent environment and the irradiation conditions. Here, we focused on the simpler case of single polymer-rich filaments and we employed phase contrast microscopy to systematically investigate the influence of laser illumination and material parameters on the kinetics of the optically induced local concentration increase in the polydiene solutions. The refractive index contrast of the formed filaments increased exponentially with the laser illumination time. The growth rate exhibited linear dependence on the laser power and increased with polymer chain length in semidilute solutions in good solvents. On the contrary, the kinetics of the formed filaments appeared to be rather insensitive to the polymer concentration. Albeit the origin of the peculiar light field-polymer concentration coupling remains yet elusive, the new phenomenology is considered necessary for the elucidation of its mechanism.

Microsecond-resolved SDR-based cavity ring down ellipsometry.

We present an experimental apparatus that allows microsecond-resolved ellipsometric and absorption measurements. The apparatus is based on an optical cavity containing a Dove prism, in which light undergoes total internal reflection (TIR), while the data acquisition is based on software defined radio technology and custom-built drivers. We demonstrate the ability to sense rapid variations in the refractive index above the TIR interface for arbitrarily long times with a temporal resolution of at least 2 µs.

Near-field laser Doppler velocimetry measures near-wall velocities.

A new near-wall velocimetry technique is proposed, based on evanescent wave dynamic light scattering, which allows for the measurement of near-wall velocity profile (characterized by an apparent slip velocity and a shear rate) with a resolution of tens of nanometers. A full theoretical expression of the correlation function is derived for the case of linear flow with negligible Brownian motion. The technique is demonstrated for latex spheres dispersed in water-glycerol mixtures.

Characterization of nanoparticles in diluted clear solutions for Silicalite-1 zeolite synthesis using liquid 29Si NMR, SAXS and DLS.

Clear solutions for colloidal Silicalite-1 synthesis were prepared by reacting tetraethylorthosilicate in aqueous tetrapropylammonium hydroxide solution. A dilution series with water resulting in clear solutions with a TEOS ratio TPAOH ratio H2O molar ratio of 25 : 9 : 152 up to 25 : 9 : 15,000 was analysed using liquid 29Si nuclear magnetic resonance (NMR), synchrotron small angle X-ray scattering (SAXS) and dynamic light scattering (DLS). Particle sizes were derived independently from DLS and from the combination of SAXS and NMR. NMR allowed quantitative characterization of silicon distributed over nanoparticles and dissolved oligomeric silicate polyanions. In all samples studied, the majority of silicon (78-90%) was incorporated in the nanoparticle fraction. In concentrated suspensions, silicate oligomers were mostly double-ring species (D3R, D4R, D5R, D6R). Dilution with water caused their depolymerisation. Contrarily, the internal condensation and size of nanoparticles increased with increasing dilution. SAXS revealed a decrease of effective nanoparticle surface charge upon dilution, reducing the effective particle interactions. With DLS, the reduction of nanoparticle interactions could be confirmed monitoring the collective diffusion mode. The observed evolution of nanoparticle characteristics provides insight in the acceleration of the Silicalite-1 crystallization upon dilution, in view of different crystallization models proposed in the literature.

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.

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.

Static and dynamic solution properties of monodisperse oligofluorenes.

Dynamic light scattering techniques were employed for the study of molecular and collective properties of newly synthesized oligomers (n = 3-7) of the polyfluorene in toluene solutions. At low concentrations, the molar mass, interaction parameter, optical anisotropy and the transport coefficients for translational and rotational diffusion provide a consistent description using a rigid rod model with thickness b = 0.6 nm and length L = nl (l = 0.75 nm). This description is in agreement with the persistence length (6-7 nm) reported for the fluorene polymers. In the non-dilute regime, the estimated concentration for the lyotropic transition further supports the proposed molecular shape and the second slow process in the intermediate scattering function relates to the formation of almost spherical clusters above about 30%.

Orientation dynamics in isotropic phases of model oligofluorenes: glass or liquid crystal.

Orientation molecular dynamics were investigated in a series of "defect-free" oligofluorenes by depolarized dynamic light scattering and dynamic NMR spectroscopy. Typical liquid crystalline pretransitional dynamics were observed upon cooling the isotropic phase to the liquid crystalline phase with strong increase of the scattered intensity and slowing down of the characteristic time of the probed collective relaxation. This is well accounted for by the Landau-de Gennes theory, however, with a strong temperature dependence of the viscosity coefficient, reflecting the proximity of the glass transition. For the trimer the two transitions almost overlap and the molecular orientation coincide with the alpha-relaxation associated with the glass transition. The NMR measurements confirm that the time scale of the dynamics is completely governed by the glass process, yet the geometry of the motion is anisotropic, yielding order parameters ranging from 0.15 to 0.25 for the long axis in the liquid crystalline phase. The glass transition is therefore geometrically restricted with poorly ordered mesophase which is consistent with the weak transverse phonons in the light scattering experiment down to Tg+20 K.

Collective dynamics of an end-grafted polymer brush in solvents of varying quality.

The dynamic structure of a chemically end-grafted polystyrene brush bathed in solvents of varying interactions was studied by evanescent wave dynamic light scattering. It reveals distinct behavior under good and poor solvent conditions. The cooperative diffusion is a generic feature of a good solvent environment, whereas a second slow relaxation mode appears in the theta solvent regime. Its characteristics resemble self-diffusion of clusters in a gel while weak concentration fluctuations in the polymer brush decay similarly to a semidilute polymer solution.

On the origin of the heterogeneous orientation dynamics of semiflexible polymers in solutions.

Collective rotational motion in nondilute isotropic solutions of semirigid chains is sensitively probed by depolarized light scattering over a broad time range. The bimodal shape and the peculiar dependence of the orientational relaxation function on the scattering angle might arise from the coupling between orientational and shear modes of molecular motion (P.G. de Gennes, Mol. Cryst. Liq. Cryst. 12, 193 (1971)). The dynamic heterogeneity, i.e. the separation of the time scales and the shape of the relaxation functions appears to be system specific.

Kinetic arrest of crowded soft spheres in solvents of varying quality.

Crowded solutions of multiarm star polymers, representing model colloidal spheres with ultrasoft repulsive interactions, undergo a reversible gelation transition upon heating in solvents of intermediate quality (between good and Theta). This unusual phenomenon is due to the kinetic arrest of the swollen interpenetrating spheres at high temperatures, forming clusters, in analogy to the colloidal glass transition. In this work we demonstrate that the choice of the solvent has a dramatic effect on the gelation transition, because of the different degree of star swelling (at the same temperature) associated with the solvent quality. We construct a generic kinetic phase diagram for the gelation of different stars in different solvents (gelation temperature against effective volume fraction, phi) and propose a critical "soft sphere close packing" volume fraction phi(c) distinguishing the temperature-induced (for phiphi(c)) glass-like gelation. We conclude that appropriate selection of the solvent allows for manipulation of the sol-gel transition in such ultrasoft colloids.