Advances in Colloid and Interface Science (v.94, #1-3)

Foreword (ix-x).

Surface energy and surface composition of end-fluorinated polystyrene by Ralf Mason; Claire A Jalbert; Patricia A.V O'Rourke Muisener; Jeffrey T Koberstein; James F Elman; Tim E Long; Binnur Z Gunesin (1-19).
Contact angle analysis was applied to characterize the compositions, surface enrichment and surface energies for a homologous series of fluorosilane-terminated polystyrenes (PS-F) ranging from 5300 to 148 000 g/mol in molecular weight. Surface enrichment of fluorinated end groups was found to decrease with increasing molecular weight but the relative enrichment of end groups (the ratio of the surface to the bulk concentration) increased with increasing molecular weight. Calculations showed that surface segregation greatly affected surface properties with a 3% concentration of end groups yielding a 39% decrease in surface tension and a 0.1% concentration leading to an 8% decrease. The scaling of polymer surface tension with molecular weight for this system was compared with theory and was found to vary with molecular weight, the power law dependence increasing with molecular weight.
Keywords: Fluorosilane-terminated polystyrenes; Polymer surface tension; Molecular weight; Contact angle analysis; Surface segregation; Surface enrichment;

The most common observation is that the surface tension of molten polymer liquids is an increasing function of the molecular weight of the chains. However, when chain ends have been chemically modified, the opposite trend is found. In this article, we discuss theoretically, the interplay between segregation effects of chain ends to the surface tension which are enthalpic dominated (that contribution originated from the small chemical shift between end-groups and monomers from the backbone), and an entropic contribution originated from the self-organisation of the chains in the immediate vicinity of the interface (formation of loops). We find that enthalpic contributions are not able to account for the decrease of surface tension with molecular weight, and plausibly enough, the entropic contribution that we have considered is responsible for this feature.
Keywords: Entropic; Enthalpic; Surface tension; Polymer melts;

We utilize exact thermodynamic relationships to separate the role of density variations and chain end segregation to surfaces in determining the molecular weight dependence of the surface tensions of polymer melts. By utilizing standard mean-field treatments we find that density effects dominate, except in special cases, where the chain ends are strongly attracted or repelled from the surfaces. These results stress that PVT measurements are generally sufficient to properly capture the molecular weight dependence of γ for normally encountered polymer melts.
Keywords: Density; Surface tensions; Polymer melts;

The stick–slip transition in highly entangled poly(styrene-butadiene) melts by L. Léger; H. Hervet; T. Charitat; V. Koutsos (39-52).
We study the interfacial slippage of highly viscous polymer melts in a plane Couette cell by combining stress measurements and optical determination of sliding velocities (Fluorescence Recovery After Photobleaching). This enables us to observe the apparition of stick–slip instabilities in a narrow shear rate range whose limits are governed by the molecular characteristics of both the bulk and surface anchored polymer chains. A qualitative interpretation of these results will be given in terms of extraction and re-entanglement of the surface anchored chains from the bulk ones as well as a necessary criterion for the apparition of these instabilities. Experiments exploring the kinetics of the stick–slip regime appear to be a simple and promising way to investigate interfacial dynamics and to estimate the characteristic times involved.
Keywords: Stick–slip; Polymer solid friction; Interfacial slip; Chain pull out;

Block copolymer thin films: pattern formation and phase behavior by Peter F Green; Ratchana Limary (53-81).
During the last decade, research on thin, sub-micron thick, block copolymer films was devoted toward understanding and controlling microstructural and topographical features at temperatures T<T ODT, where T ODT is the order–disorder transition temperature below which thermodynamic interactions favor the formation of ordered (phase separated) microstructures. Symmetric diblock copolymers, the subject of this review, undergo an isotropic to lamellar transition when T<T ODT. Topographical features, ‘islands’ or ‘holes,’ of these films typically reflect the underlying phase separation; and the dimension of these features, normal to the substrate is equal to the interlamellar spacing, L. Two aspects of block copolymer thin films that have not received much attention are discussed in this paper: (1) pattern formation in symmetric block copolymers under conditions of T>T ODT; and (2) phase behavior of thin film symmetric diblock copolymer/homopolymer mixtures, when T<T ODT.
Keywords: Block copolymers thin films; Pattern formation; Phase behavior;

Influence of nanoparticles and polymer branching on the dewetting of polymer films by Kathleen A. Barnes; Jack F. Douglas; Da-Wei Liu; Alamgir Karim (83-104).
Previous studies have shown that spun-cast films of unentangled synthetic polymers commonly dewet inorganic or organic substrates, leading to technologically detrimental results for many applications. We illustrate two strategies for influencing polymer film dewetting on inorganic and organic substrates. First, the addition of small amounts of C60 fullerene nanoparticles to the spin-casting polymer solutions of model synthetic polymers [polystyrene (PS) and polybutadiene (PB)] leads to a significant inhibition of film dewetting on Si. This effect is associated with the formation of a diffuse fullerene layer near the solid substrate that frustrates the dewetting hole growth process. Next, we consider polymer branching effects on the dewetting of various generations of hypergraft polymer poly(2-ethyl-2-oxazoline) (PEOX) films cast on high molecular weight polystyrene substrates. The early stage of dewetting is found to be similar in a zeroth generation G0 hypergraft (a comb polymer) and a G2 hyper-graft (resembling a spherical ‘micro-gel’ particle). The late stage of dewetting in the G2 films, however, differs significantly from the low generation films because of an inhibition of hole coalescence in the intermediate stage of film dewetting. This behavior resembles previous observations of dewetting in ‘entangled’ polystyrene films. Thus, the viscoelasticity of the polymer film can have an inhibitory effect on film dewetting, leading to changes in the dewetted film morphology rather than a suppression of film dewetting.
Keywords: Nanoparticles; Polymer branching; Dewetting;

We extend the one-dimensional self-consistent field (SCF) scheme of Scheutjens-Fleer to three dimensions (3D) and use this three-dimensional SCF model to investigate the adsorption of A–B copolymers from A homopolymer matrices onto planar substrates composed of two chemically distinct sites (C and D), one of which has a preferential affinity for the B segments of the copolymer. To address the role of the substrate chemical heterogeneity on copolymer adsorption, we keep the fraction of the C and D sites constant (50% of each site) and vary their spatial distribution on the substrate. The interplay between the surface chemical heterogeneity and the chain microstructure is examined for A–B diblock, A–B–A, B–A–B triblock, and A–alt–B alternating copolymers. Our results indicate that regardless of the type of the surface chemical heterogeneity, the A–B diblock and triblock copolymers adopt ‘brush’-like and ‘bridge’-like structures, respectively, with the B block being anchored to the substrate. In contrast, the A–alt–B macromolecule is found to be ‘zipped’ to the substrate. For a fixed chemical potential of the copolymer in the A–B/A mixture and the surface adsorption energy of B, the amount of the adsorbed copolymer depends on: (1) the number of the B segments; (2) the copolymer microstructure; and (3) the distribution of the C and D regions on the substrate. Three-dimensional maps of the spatial density of copolymer segments provide insight into copolymer conformation at the mixture/substrate interface and also the ability of the copolymer to recognize and mimic the substrate pattern. In addition, we discuss the circumstances under which the substrate pattern is transferred deep inside the A–B/A mixture and those, which lead to strong damping of the substrate motif as one moves away from the substrate/mixture interface.
Keywords: Copolymer; Adsorption; Self-consistent field; Modeling; Heterogeneous substrate; Sequence distribution;

We find an unexpected analogy between polymer melts compressed between strongly-attractive solid surfaces, and end-attached polymers in near-theta-solvent. End-grafted polystyrene (PS) chains with various graft density were produced by immersing mica, coated with adsorbed diblock copolymers of PS/PVP, PS/polyvinylpyiridine, into trans-decalin at 24°C (this solvent is a near-theta-solvent for PS but a non-solvent for PVP), and the force–distance relations were measured using a surface forces apparatus. The end-grafted PS chains repelled one another in spite of the theta-solvent situation. Repulsive forces began at a thickness, per adsorbed layer, equivalent to 4–5 times the unperturbed radius of gyration of the PS chain. These cases of symmetrically-opposed PS layers are compared to the asymmetric case (PS on one mica surface, the other mica surface bare) and to the case of adsorbed PS homopolymer. Force–distance profiles in the presence of unattached PS homopolymer in solution at concentration c* (the overlap concentration) displayed repulsion beginning at considerably larger separation than in the presence of pure solvent. Finally, we compare to confined melts of linear polydimethylsiloxane (PDMS), cyclic PDMS and a linear perfluoropolyether (Demnum). For these cases of polymer melts confined between adsorbing surfaces, force–distance relations could be described by the same functional relations as for end-attached chains in near-theta-solvent.
Keywords: Force–distance; Polystyrene (PS) chains; Theta-solvent; Surface; Polymer melt; Adsorption;

Structure and buckling of charged diblock copolymer films at liquid interfaces by F. Dubreuil; G. Romet-Lemonne; P. Guenoun; J. Yang; J.W. Mays; J. Daillant (151-165).
Adsorbed and spread charged diblock copolymer layers are studied at flexible interfaces. In both cases, equilibrium situations can be reached and the structure and fluctuations in height of the interface are examined by different experimental means. In particular, buckled deformations of the air–water interface are evidenced by optical and X-ray scattering techniques. Preliminary results at the oil–water interface with similar copolymers are discussed.
Keywords: Diblock copolymer films; Liquid interfaces;

The glass transition in thin polymer films by James A. Forrest; Kari Dalnoki-Veress (167-195).
In this article, we present a detailed account of important recent developments in the rapidly evolving area of glass transitions in thin polymer films. We review the case of polymer films supported by substrates, and show that a definite experimental consensus exists. We consider recent results from experimental studies of free-standing films of polystyrene. These studies have provided a thorough quantification of the behavior of glass transition anomalies in free-standing polymer films, and have served to motivate recent attempts at theoretical descriptions. We introduce and examine models, which have been proposed to explain the experimental observations and discuss the significance of these models.
Keywords: Glass transition; Thin polymer films; Dynamics in confinement;

The present paper reviews recent attempts to study the development of glassy behavior in thin polymer films by means of Monte Carlo simulations. The simulations employ a version of the bond-fluctuation lattice model, in which the glass transition is driven by the competition between an increase of the local volume requirement of a bond, caused by a stiffening of the polymer backbone and the dense packing of the chains in the melt. The melt is geometrically confined between two impenetrable walls separated by distances that range from once to approximately fifteen times the bulk radius of gyration. The confinement influences static and dynamic properties of the films: Chains close to the walls preferentially orient parallel to it. This orientation tendency propagates through the film and leads to a layer structure at low temperatures and small thicknesses. The layer structure strongly suppresses out-of-plane reorientations of the chains. In-plane reorientations have to take place in a high density environment which gives rise to an increase in the corresponding relaxation times. However, local density fluctuations are enhanced if the film thickness and the temperature decrease. This implies a reduction of the glass transition temperature with decreasing film thickness.
Keywords: Monte Carlo simulations; Polymer films; Gyration tensor; Dynamic correlation functions; Glass transition;

Comb-like polymers inside nanoscale pores by C. Gay; E. Raphaël (229-236).
A new method of polymer characterization, based on permeation studies using nanoscale pores, was recently proposed by Brochard and de Gennes. In the present paper, we study (at the level of scaling laws) how this method, initially developed for star polymers, can be extended to comb-like polymers.
Keywords: Comb-like polymers; Nanoscale pores;

‘Intrinsic’ profiles and capillary waves at interfaces between coexisting phases in polymer blends by Kurt Binder; Marcus Müller; Friederike Schmid; Andreas Werner (237-248).
Lateral fluctuations in the local position of the center of the interface between coexisting phases in unmixed polymer blends lead to a broadening of interfacial widths; comparing self-consistent field predictions for the ‘intrinsic’ profile to simulations (or experiments), this ‘capillary wave’ broadening needs consideration. This problem has been studied by extensive Monte Carlo simulations of the bond fluctuation model for symmetrical polymer mixtures, both for free interfaces (between bulk phases) and for confined interfaces (in thin films between parallel walls). While the capillary wave predictions at large length scales are confirmed, the extraction of the ‘intrinsic’ profile remains a problem. Related experiments are also briefly discussed.
Keywords: Polymer blends; Intrinsic profile; Capillary wave; Bond fluctuation model;