Equivalence between polymer nanocomposites and thin polymer films: Effect of processing conditions and molecular origins of observed behavior

Recently we established a quantitative equivalence in thermomechanical properties between polystyrene-silica nanocomposites and planar freestanding polystyrene thin films. This equivalence was quantified by drawing a direct analogy between film thickness and an appropriate experimental particle spacing. Using these findings, here we unequivocally show that the glass transition process in confined geometries is controlled by the mean volume fraction of polymer that is affected by the presence of surfaces. Since separate signatures of the bulk and the surface layers are never found, we can clearly rule out any simple “two layer” model which postulates the existence of surfaces which are dynamically decoupled from the bulk. Rather, we argue that the modification of properties at the surfaces propagates into the bulk through a spatial gradient: macroscopic experimental techniques average over these gradients and yield a broadened signature relative to the bulk polymer. In a second aspect of this paper we focus on the role of processing conditions on the results obtained. We have developed a new method of processing the nanocomposites which results in a better dispersion of the nanoparticles in the matrix. However, these samples did not show the unique glass transition behavior seen in the first set of nanocomposites discussed above. This indicates that processing conditions can profoundly affect the nature of the particle-polymer interface which controls the macroscopic behavior of these important systems.     

Hydrophobic switching nature of methylcellulose ultra-thin films: thickness and annealing effects

We have studied the thermosensitive property of methylcellulose (MC) thin films supported on Si substrate by static sessile drop contact angle measurements, and their surface properties and thin film structure by x-ray reflectivity (XRR) and atomic force microscopy (AFM) techniques. From the static sessile drop contact angle measurements, the MC thin films showed the characteristic hydrophilic-to-hydrophobic transition at ～70?°C, which is the lower critical solution temperature of the bulk solution volume phase separation transition. For films with thickness d ≤ R(g), the onset of such a transition is affected by the film thickness while very thick films, d ? R(g), yielded higher contact angles. Annealing the MC thin films with thicknesses ～200 ? (near the radius of gyration, R(g), of the polymer) below the bulk glass transition temperature (T(g) ～ 195?° C) would not change the hydrophobic switch nature of the film but annealing 'at' and above the bulk T(g) would change its surface property. From surface topography images by AFM, there were no significant changes in either the roughness or the film texture before and after annealing. With XRR data, we were able to determine that such changes in the surface properties are highly correlated to the film thickness changes after the annealing process. This study, we believe, is the first to examine the thermal annealing affects on the thermal response function of a thermoresponsive polymer and is important for researching how to tailor the hydrophobic switching property of MC thin films for future sensing applications.     

Polymer thin films and surfaces: Possible effects of capillary waves

It is discussed how the proximity of a free surface or mobile interface may affect the strain relaxation behavior in a viscoelastic material, such as a polymer melt. The eigenmodes of a viscoelastic film are thus derived, and applied in an attempt to explain the experimentally observed substantial shift of the glass transition temperature of sufficiently thin polymer films with respect to the bulk. Based on the idea that the polymer freezes due to memory effects in the material, and exploiting results from mode-coupling theory, the experimental findings of several independent groups can be accounted for quantitatively, with the elastic modulus at the glass transition temperature as the only fitting parameter. The model is finally applied discussing the possibility of polymer surface melting. A surface molten layer is predicted to exist, with a thickness diverging as the inverse of the reduced temperature. A simple model of thin polymer film freezing emerges which accounts for all features observed experimentally so far. Received 8 August 2001     

Two simultaneous mechanisms causing glass transition temperature reductions in high molecular weight freestanding polymer films as measured by transmission ellipsometry

We study the glass transition in confined polymer films and present the first experimental evidence indicating that two separate mechanisms can act simultaneously on the film to propagate enhanced mobility from the free surface into the material. Using transmission ellipsometry, we have measured the thermal expansion of ultrathin, high molecular-weight (MW), freestanding polystyrene films over an extended temperature range. For two different MWs, we observed two distinct reduced glass transition temperatures (T(g)'s), separated by up to 60 K, within single films with thicknesses h less than 70 nm. The lower transition follows the expected MW dependent, linear T(g)(h) behavior previously seen in high MW freestanding films. We also observe a much stronger upper transition with no MW dependence that exhibits the same T(g)(h) dependence as supported and low MW freestanding polymer films.     

Molecular mobility in polymer thin films

Fluorescence recovery after photobleaching was used to measure in-plane dye-probe diffusion coefficients, D, in thin films of monodisperse polystyrene supported on fused quartz substrates. The substrates were prepared with a high density of surface hydroxyl groups which interact favorably with repeat units of the polymer. The effects of temperature and film thickness were investigated, at temperatures above the bulk glass transition of the polymer, T(g), and in the range of film thicknesses from 1-10(2) times the radius of gyration (R(g)) of individual polymer molecules. As the film thickness decreases towards R(g) the value of D increases above the bulk values, with significant effects first appearing in films approximately 20R(g). In the thinnest films studied, about 4R(g), the values of D lie as much as two orders over bulk values. At the same time, the temperature dependence of D becomes much weaker than in bulk. Analysis by free volume theory indicates that apparent values of both T(g) and the thermal expansion coefficient for liquid state, alpha(L), decrease as the film thickness decreases. The possible effects of surface segregation of the dye probe are discussed.     

Ordering Mechanisms in Confined Diblock Copolymers

We present several ordering mechanisms in diblock copolymers. For temperatures above the order-disorder temperature and in the weak segregation regime, a linear response theory is presented which gives the polymer density in the vicinity of confining flat surfaces. The surfaces are chemically patterned where different regions attract different parts of the copolymer chain. The surface pattern or template is decomposed into its Fourier modes, and the decay of these modes is analyzed. The propagation of the surface pattern into the disordered bulk is given for several types of patterns (e.g. uniform and striped surface). It is further shown that complex morphology can be induced in a thin film even though the bulk is disordered. We next consider lamellar diblock copolymers (low temperature regime) in the presence of a striped surface. It is shown that lamellae acquire a tilt with respect to the surface, if the surface periodicity is larger than the bulk one. The lamellae close to the surface are strongly distorted from their perfect shape. When the surface and lamellar periodicities are equal, the lamellae are perpendicular to the surface. Lastly, the transition from parallel to perpendicular lamellae in a thin film is presented. The transition between the two states depends on the surface separation and strength of surface interactions. We further calculate the phase diagram in the presence of perpendicular electric field favoring perpendicular ordering. In the strong segregation limit we introduce a simple model to calculate the phase diagram of the fully parallel, fully perpendicular and mixed (parallel and perpendicular) states.     

Pressure, temperature, and thickness dependence of CO2-induced devitrification of polymer films

The glass transition temperature is known to increase with decreasing film thickness h for sufficiently thin poly(methyl methacrylate) films supported by silicon oxide substrates. We show that this system undergoes a CO2 pressure-induced devitrification transition, P(g), which is film thickness dependent, P(g)(h)=DeltaP(g)+P(bulk)(g). P(bulk)(g) is the bulk glass transition and DeltaP(g) can be positive or negative depending on T and P. The phenomenon of retrograde vitrification, wherein the polymer exhibits a rubbery-to-glassy-to-rubbery transition upon changing temperature isobarically, is also shown to occur in this system and it is film thickness dependent.     

Chain entanglement in thin freestanding polymer films

When a thin glassy film is strained uniaxially, a shear deformation zone (SDZ) can be observed. The ratio of the thickness of the SDZ to that of the undeformed film is related to the maximum extension ratio, lambda, which depends on the entanglement molecular weight, M(e). We have measured lambda as a function of film thickness in strained freestanding films of polystyrene as a probe of M(e) in confinement. It is found that thin films stretch further than thick films before failure, consistent with the interpretation that polymers in thin films are less entangled than bulk polymers, thus the effective value of M(e) in thin films is significantly larger than that of the bulk. Our results are well described by a conceptually simple model based on the probability of finding intermolecular entanglements near an interface.     

Viscoelastic dynamics of polymer thin films and surfaces

The strain relaxation behavior in a viscoelastic material, such as a polymer melt, may be strongly affected by the proximity of a free surface or mobile interface. In this paper, the viscoelastic surface modes of the material are discussed with respect to their possible influence on the freezing temperature and dewetting morphology of thin polymer films. In particular, the mode spectrum is connected with mode coupling theory assuming memory effects in the melt. Based on the idea that the polymer freezes due to these memory effects, surface melting is predicted. As a consequence, the substantial shift of the glass transition temperature of thin polymer films with respect to the bulk is naturally explanied. The experimental findings of several independent groups can be accounted for quantitatively, with the elastic modulus at the glass transition temperature as the only fitting parameter. Finally, a simple model is put forward which accounts for the occurrence of certain generic dewetting morphologies in thin liquid polymer films. It demonstrates that by taking into account the viscoelastic properties of the film, a morphological phase diagram may be derived which describes the observed structures of dewetting fronts. It is demonstrated that dewetting morphologies may also serve to determine nanoscale rheological properties of liquids.Received: 1 January 2003, Published online: 14 October 2003PACS: 47.50. + d Non-Newtonian fluid flows - 68.47.Mn Polymer surfaces - 68.60.Dv Thermal stability; thermal effects     

Exact Calculation of Antiferromagnetic Ising Model on an Inhomogeneous Surface Recursive Lattice to Investigate Thermodynamics and Glass Transition on Surface/Thin Film

An inhomogeneous 2-dimensional recursive lattice formed by planar elements has been designed to investigate the thermodynamics of Ising spin system on the surface/thin film. The lattice is constructed as a hybrid of partial Husimi square lattice representing the bulk and 1D single bonds representing the surface. Exact calculations can be achieved with the recursive property of the lattice. The model has an anti-ferromagnetic interaction to give rise to an ordered phase identified as crystal, and a solution with higher energy to represent the amorphous/metastable phase.Free energy and entropy of the ideal crystal and supercooled liquid state of the model on the surface are calculated by the partial partition function. By analyzing the free energies and entropies of the crystal and supercooled liquid state,we are able to identify the melting and ideal glass transition on the surface. The results show that due to the variation of coordination number, the transition temperatures on the surface decrease significantly compared to the bulk system.Our calculation qualitatively agrees with both experimental and simulation works on the thermodynamics of surfaces and thin films conducted by others. Interactions between particles farther than the nearest neighbor distance are taken into consideration, and their effects are investigated.     

多晶铁电薄膜的相变——应力和晶粒尺寸效应

用掠入射X射线衍射法观察到钛酸铅多晶铁电薄膜表面层与体内的相变温度不同,表面层的结构参数也有异于体内;唯象地把多晶铁电薄膜抽象为一个表面层为细晶粒、低应变层,体内为粗晶粒、高应变层的两层结构,根据应力和晶粒尺寸效应对铁电相变的影响,解释了钛酸铅多晶铁电薄膜的相变特征 关键词：     

Mobility in thin polymer films ranging from local segmental motion, Rouse modes to whole chain motion: A coupling model consideration

Large increases of mobility of local segmental relaxation observed in polymer films as the film thickness is decreased, as evidenced by decreases of the glass temperature, are not found for relaxation mechanisms that have longer length scales including the Rouse relaxation modes and the diffusion of entire polymer chains. We show that the coupling model predictions, when extended to consider polymer thin films, are consistent with a large increase of the mobility of the local segmental motions and the lack of such a change for the Rouse modes and the diffusion of entire polymer chains. There are two effects that can reduce the coupling parameter of the local segmental relaxation in thin films. One is the chain orientation that is induced parallel to the surface when the film thickness h becomes smaller than the end-to-end distance of the chains and the other is a finite-size effect when h is no longer large compared to the cooperative length scale. Extremely thin ( ≈ 1.5 nm) films obtained by intercalating a polymer into layered silicates have thickness significantly less than the cooperative length scale near the bulk polymer glass transition temperature. As a result, the coupling parameter of the local segmental relaxation in such thin films is reduced almost to zero. With this plausible assumption, we show the coupling model can explain quantitatively the large decrease of the local segmental relaxation time found experimentally. Received 1 August 2001 and Received in final form 1 December 2001     

Molecular simulation of ultrathin polymeric films near the glass transition

Properties such as the glass transition temperature ( T(g)) and the diffusion coefficient of ultrathin polymeric films are shown to depend on the dimensions of the system. In this work, a hard-sphere molecular dynamics methodology has been applied to simulate such systems. We investigate the influence that substrates have on the behavior of thin polymer films; we report evidence suggesting that, depending on the strength of substrate-polymer interactions, the glass transition temperature for a thin film can be significantly lower or higher than that of the bulk.     

具有—U中心的非简单金属与BCS超导体的邻近效应(Ⅱ)

本文讨论了具有-U中心的非简单金属薄膜与BCS超导体薄膜夹层邻近效应的传导电子能隙函数、态密度及超导转变温度T_c随各参量的变化规律。结果表明,-U中心的存在使激发谱“能隙”内出现很小值的态密度。在一定参数范围内,这种夹层系统的T_c是有可能高于大块BCS超导体的转变温度。 关键词：     

Influence of Surface Transition Layers on Phase Transformation and Pyroelectric Properties of Ferroelectric Thin Film

Taking into account surface transition layers （STLs）, we study the phase transformation and pyroelectric properties of ferroelectric thin films by employing the transverse Ising model （TIM） in the framework of the mean field approximation. The distribution functions representing the intra-layer and inter-layer couplings between the two nearest neighbour pseudo-spins are introduced to characterize STLs. Compared with the results obtained by the traditional treatments for the thin films using only the single surface transition layer （SSL）, it is shown that the STL model reflects a more realistic and comprehensive situation of films. The effects of various parameters on the phase transformation properties have shown that STL can make the Curie temperature of the film higher or lower than that of the corresponding Sulk material, and the thickness of STL is a key factor influencing the film properties. For a film with definite thickness, there exists a critical STL thickness at which ferroelectricity will disappear when the intra-layer and inter-layer interactions are weak.     

Call for papers Special issue on "Properties of thin polymer films"

In recent years, there have been numerous reports of anomalous behavior of polymers in thin films. These observations, encompassing both structural and dynamical behavior, have attracted a great deal of interest. Measurements showing such anomalous behavior include density (irreversible and reversible changes after annealing below the bulk glass transition temperature), film stability and dewetting, glass transition temperature, diffusion coefficient and chain conformation and relaxation. The lack of a definitive M_w dependence in many of these studies means that it is not yet clear if the molecular size is a defining parameter in these observations. Alternatively, the surface to volume ratio may make such systems dependent on the (equally poorly understood) properties of polymer surfaces and interfaces. Despite a growing literature in this area, it is becoming increasingly obvious that a clear understanding of thin film properties has not yet been reached. In particular in glassy thin polymer films a possible cause for the anomalous behavior may originate from sample preparation. The widely used process of spin coating can potentially produce samples that are far from equilibrium. While most studies use thin films that have been annealed above the bulk glass transition, it is not clear if this represents sufficient annealing because the terminal relaxation times may be much larger than typical annealing times. One of the key questions is: Are these deviations from equilibrium important enough to cause any measurable effects? What are the consequences of film preparation and the resulting non-equilibrium conformations of the polymers on measurements of film stability, glass transition temperature, diffusion coefficient, etc.? Alternatively, if these non-equilibrium effects are not responsible for the anomalies observed in the structural and dynamical properties of thin polymer films, then other explanations must be considered. In the search for a unifying concept to explain all aspects simultaneously (i.e., density, film stability, glass transition temperature, ...) and in order to provide a forum for an open and possibly controversial discussion, The European Physical Journal E - Soft Matter invites regular articles and comments on this topic. In addition we invite submission of well-founded conjectures which may be published together with complementary views (solicited by the Editors). Papers to be considered for this special issue should be submitted by August 15, 2001 to the Editorial Office of the European Physical Journal E Véronique Condé, Editorial Office of The European Physical Journal E, Université Paris Sud, bâtiment 510, 91405 Orsay Cedex, France Tel.: 33 (0)1 69 15 59 76, Fax: 33 (0)1 69 15 59 75, e-mail: conde@edpsciences.org and marked clearly to the attention of either Günter Reiter (Editor) or James Forrest (Guest Editor). We look forward to receiving your submission.     

Hydrodynamic-flow-driven wetting in thin film polymer blends: growth kinetics and morphology

A thin film of deuterated poly(methyl methacrylate) (A) and poly(styrene-ran-acrylonitrile) at the critical composition is annealed in the two phase region to induce simultaneous phase separation and wetting of the A-rich phase at the surface. Using forward recoil spectrometry, the wetting layer thickness is found to grow linearly with time at 185 degrees C and 190 degrees C. After selective etching of A, atomic force microscopy reveals a depletion layer having a bicontinuous, phase separated morphology. The A-rich tubes in this layer provide a pathway for rapid transport of the wetting phase from the bulk to the surface via hydrodynamic flow. Taken together, fast wetting layer growth t(1) and connectivity between the wetting layer and bulk provide unambiguous support for hydrodynamic-flow-driven wetting in thin film polymer blends.     

Effect of Polymer-Substrate Interactions on the Surface Morphology of Polymer Blend Thin Films: Comparison between Simulation and Experiment

Microphase and macrophase separation phenomena can simultaneously appear in ABA/C copolymer blend systems due to the immiscibility among monomers A, B, and C. In this work, the surface morphologies and compositions of ABA/C blend thin films confined between two walls, which were used to mimic SEBS/PMMA films, have been simulated by a lattice Monte Carlo (MC) method. The effect of the polymer-wall interaction on the surface morphologies and compositions of thin films was investigated as a function of blend composition and film thickness. It is shown that the simulated surface morphologies of thin films resulting from the macrophase separation between copolymer ABA and homopolymer C and the microphase separation between block A and block B in ABA copolymer are similar to the experimental surface morphology of SEBS/PMMA polymer blend films observed by atomic force microscope (AFM). The effect of substrate on the surface morphologies by MC simulation is qualitatively consistent with the experimental results. The composition profiles of thin films are given to characterize the micro- and macrophase separation in thin films. It is indicated that the surface energy of the substrate (substrate/air) plays a crucial role on the surface composition. For a fixed surface, the adsorptions of polymer on the substrate and film thickness are also important.     

Crystal bridge formation marks the transition to rigidity in a thin lubrication film

We report that the rheological transition in a thin lubrication film of dodecane from bulk to high viscosity states as the thickness is decreased is the result of a novel structural transition. Using nonequilibrium molecular dynamics simulations we find that the initial increase in viscosity as the film thins is due to the formation of isolated crystalline bridges between the surfaces. As the thickness decreases further, these bridges increase in number and organize themselves into a tetratic order in the plane of the surface. We show that this ordered state melts at a temperature of 350 K.