Molecular Weight Dependence of Spreading Rates of Ultrathin Polymeric Films

We study experimentally the molecular weight $M$ dependence of spreading rates of molecularly thin precursor films, growing at the bottom of droplets of polymer liquids. In accord with previous observations, we find that the radial extension R(t) of the film grows with time as R(t) = (D_{exp} t)^{1/2}. Our data substantiate the M-dependence of D_{exp}; we show that it follows D_{exp} \sim M^{-\gamma}, where the exponent \gamma is dependent on the chemical composition of the solid surface, determining its frictional properties with respect to the molecular transport. In the specific case of hydrophilic substrates, the frictional properties can be modified by the change of the relative humidity (RH). We find that \gamma \approx 1 at low RH and tends to zero when RH gets progressively increased. We propose simple theoretical arguments which explain the observed behavior in the limits of low and high RH.Comment: 4 pages, 2 figures, to appear in PR

Dewetting, partial wetting and spreading of a two-dimensional monolayer on solid surface

We study the behavior of a semi-infinite monolayer, which is placed initially on a half of an infinite in both directions, ideal crystalline surface, and then evolves in time due to random motion of the monolayer particles. Particles dynamics is modeled as the Kawasaki particle-vacancy exchange process in the presence of long-range attractive particle-particle interactions. In terms of an analytically solvable mean-field-type approximation we calculate the mean displacement X(t) of the monolayer edge and discuss the conditions under which such a monolayer spreads (X(t) > 0), partially wets (X(t) = 0) or dewets from the solid surface (X(t) < 0).Comment: 4 pages, 2 figures, to appear in PRE (RC

Force-velocity relation and density profiles for biased diffusion in an adsorbed monolayer

In this paper, which completes our earlier short publication [Phys. Rev. Lett. 84, 511 (2000)], we study dynamics of a hard-core tracer particle (TP) performing a biased random walk in an adsorbed monolayer, composed of mobile hard-core particles undergoing continuous exchanges with a vapor phase. In terms of an approximate approach, based on the decoupling of the third-order correlation functions, we obtain the density profiles of the monolayer particles around the TP and derive the force-velocity relation, determining the TP terminal velocity, V_{tr}, as the function of the magnitude of external bias and other system's parameters. Asymptotic forms of the monolayer particles density profiles at large separations from the TP, and behavior of V_{tr} in the limit of small external bias are found explicitly.Comment: Latex, 31 pages, 3 figure

Growth of two dimensional domains in copolymer thin films

Quenching experiments on very thin films (≃ 1 000 Å) of symmetric diblock copolymer deposited on solid substrates result in the formation of circular relief domains at the free surface of the sample. It is due to the quantization of the possible thicknesses of the sample which accompanies the lamellar ordering of the two species parallel to the substrate. Depending on the average thickness of the film, the domains can be depressions or elevations. Based on the conjecture of a simple dislocation structure of the domain edge, equations governing the time evolution of the domain size distribution are proposed. The flows involved in the domain growth are shown to be confined into a bilayer which is disrupted at the dislocation lines. The hypothesis is made that interlayer frictions dominate. The growth kinetics of a single hole in a finite film is finally solved explicitely. Its radius is found to decay as a 1/3 power law of its life time.Lorsqu'on trempe des films très minces (1 000 Å d'épaisseur) de copolymères déposés sur un substrat solide, des domaines circulaires apparaissent en relief à la surface de l'échantillon. Ce phénomène est dû à la formation d'un ordre lamellaire dans le film qui s'accompagne d'une quantification de son épaisseur locale. Les domaines peuvent correspondre à des dépressions ou des élévations, selon l'épaisseur du film initial. Après avoir postulé que la structure des bords des domaines est une simple dislocation, on établit les équations qui gouvernent leur croissance. On montre en particulier que les écoulements de polymère qui permettent cette évolution doivent prendre place dans une bicouche incomplète enfermée entre deux monocouches complètes, et on fait l'hypothèse que la dissipation provient essentiellement du frottement entre couches. Le cas d'un trou unique dans un film de surface finie est finalement examiné en détail. Son rayon évolue comme la puissance 1/3 de sa durée de vie

Depletion layers in polymer solutions : influence of the chain persistence length

We propose a simple model for the concentration profile induced by a non-adsorbing solid wall in dilute solutions of semi-flexible chains. Our approach is based on the assumption that the restriction in the chain local curvature of the chain creates a non-zero surface concentration. The monomer concentration profile ΦSF(Z) is deduced from the mean-field model for flexible chains, ΦF(Z) = Φb tanh 2(Z/RG2), by introducing an extrapolation distance D proportional to the persistence length q. We show that ΦSF(Z) = Φ b tanh2[(Z + D)/RG2] where RG is the chain radius of gyration and Φb is the bulk polymer concentration. This analytical expression provides a very good fit with recent experimental data obtained by the evanescent wave induced fluorescence method (EWIF) in aqueous xanthan solutions.Nous proposons un modèle simple permettant de décrire le profil de concentration dans une solution diluée de chaînes semi-flexibles au voisinage d'une paroi solide non-adsorbante. Notre approche est basée sur l'hypothèse d'une concentration de surface non nulle due à la rigidité partielle de la chaîne. Le profil de concentration ΦSF(Z) est obtenu à partir de celui des chaînes flexibles ΦF(Z) = Φ tanh 2(Z/RG2) à condition d'introduire une longueur d'extrapolation D proportionnelle à la longueur de persistance q de la chaine. Nous montrons que Φ SF(Z) = Φb tanh2[(Z + D)/RG 2] avec RG rayon de gyration et Φb concentration en volume. Cette formule décrit très bien des résultats expérimentaux récents obtenus par la méthode de Fluorescence Induite par Ondes Evanescentes, sur des solutions de xanthane, un polysaccharide hydrosoluble

Composites copolymer-nanoparticule : courbure

The compression coefficient and the mean curvature coefficient of a composite of an A-B diblock copolymer charged with nanoparticles depend on the concentration of nanoparticles and on the dissymmetry of the copolymer chains. Concerning the curvature coefficient, a distinction must be done between an adiabatic and an isotherm coefficient when the copolymer chains are not symmetric or when $\Phi$ is not zero. Finally, the AB interfaces of the composite film present a spontaneous curvature that we calculate in the approximation of “small” nanoparticles.Les coefficients de compression et de courbure moyenne d'un composite de copolymères diblocs A-B et de nanoparticules dépendent de la concentration $\Phi$ en nanoparticules et de la dissymétrie du copolymères. Pour la courbure, on doit faire une distinction entre un coefficient adiabatique et un coefficient isotherme dès que le polymère n'est plus symétrique ou que $\Phi$ n'est plus nulle. Enfin, et surtout, les interfaces AB du composite présentent une courbure spontanée que nous calculons dans l'approximation des “petites” nanoparticules

Composites copolymères-nanoparticules : I. Période lamellaire dans le régime des “petites nanoparticules"

In the new composite materials made of lamellar diblock copolymer and ferrofluidics, the lamellar period $L$ is experimentally shown to depend on the volume fraction $\Phi$ of nanoparticles according to: $L=L_0(1+\Phi/3)$, where $L_0=L(\Phi=0)$, while $L=L_0(1+\Phi)$ was rather expected. Knowing that the particles are exclusively located into one of the two polymeric components, we show that these two laws can be established in a simple way from two different assumptions about the spatial distribution of nanoparticles in the host layers. The hypothesis of particles confined in the center of the layers is finally rejected.Une étude expérimentale nous montre que dans les nouveaux matériaux composites élaborés récemment à partir de copolymères diblocs lamellaires et de ferrofluides, la période lamellaire $L$ varie avec la fraction volumique de nanoparticules d'une façon à peu près bien décrite par une loi $L=L_0(1+\Phi/3)$, où $L_0=L(\Phi=0)$, alors qu'on attendait plutôt $L=L_0(1+\Phi)$. Sachant les nanoparticules localisées dans l'une des deux espèces du copolymère, nous montrons comment ces deux lois s'obtiennent théoriquement à partir de deux hypothèses fortes très différentes sur la distribution des particules à l'intérieur des domaines hôtes. Celle qui conduit à des prédictions infirmées par l'expérience est finalement écartée

Hole nucleation in thin diblock copolymer films

This paper reports an experimental study of a homogeneous nucleation process in the direct space, using an atomic-force microscope. Nucleated patterns are holes in a thin diblock copolymer film. The formation of nuclei from the metastable majority phase is followed either by the dissolution or by the growth of the hole. It is also preceded by the unexpected presence of a bump at the free surface. From the study of the nucleus surface topography at different stages of the evolution, we infer a description of the nucleation mechanism at a molecular level. It involves the existence of ordered metastable states characterized by the presence of compressed internal layers, the formation of pores in the center of the film, and the existence of interfacial flows across these pores. The presence of compressed internal layers was directly demonstrated using Neutron Reflectivity experiments, Our scheme is also consistent with all existing experimental data. After hole nucleation, we identify two distinct growth regimes: the first one is individual and the second one is interactive