Session Q50: Focus Session: Dynamics of Polymers: Phenomena due to Confinement II

New insight into adsorption of polymer melts onto impenetrable surfaces

We report the novel structures of irreversibly adsorbed polystyrene (PS) layers composed of six different molecular weights ranging from 30k to 2,000k. Spin cast PS films (originally $\sim $ 100 nm in thickness) prepared onto hydrogen-passivated silicon substrates were annealed at 170 \r{ }C for about 50h under vacuum and subsequently rinsed with toluene (a good solvent for PS) thoroughly. X-ray reflectivity results show that the adsorbed layers are well described by a two-layer model: the one is a higher density layer relative to the bulk adjacent to the substrate and the other is a nearly bulk density layer on top of the bottom layer. On the other hand, a single-layer model with the higher density layer is valid for the adsorbed layers composed of low molecular weights PS. We will reveal the origin of the difference, shedding light on a new pathway for the formation of the equilibrium adsorbed polymer layers at the impenetrable interfaces.   

Effect of the interfacial interaction on the relaxation of polymer melts in nanofilms

The relaxation of polymer melts in nanofilms could be orders of magnitude slower than in bulk. To date, the governing mechanism remains unclear on the role of spatial confinement and interfacial interaction. Here we report the experimental results indicating that the polymer-substrate interfacial interaction plays a key role in the relaxation. Two perfluoropolyethers (PFPEs) with the same backbone and different endgroups, one polar and the other non-polar, have been studied. The relaxation of the nanofilms on silicon wafers was characterized by the contact angle measurement. For the PFPE with polar endgroups, the contact angle ``relaxes'' with time and the relaxation time constant, obtained from KWW model, is ten orders of magnitude higher than that of bulk polymer. However, for the PFPE with non-polar endgroups, the contact angle relaxation was not observed. The experimental results indicate that the relaxation is thermodynamically driven by the attractive interaction between the polar endgroups of the polymers and the polar sites on the solid substrate. The very slow kinetics of the relaxation has been attributed to the heterogeneity of the polymer-solid interfacial interaction and the cooperative nature of the molecular motions during the relaxation.   

Differential AC chip calorimeter for in situ investigation of vapor deposited thin films

Physical vapor deposition (PVD) can be used to produce thin films with particular material properties like extraordinarily stable glasses of organic molecules. We describe an AC chip calorimeter for in-situ heat capacity measurements of as-deposited nanometer thin films of organic glass formers. The calorimetric system is based on a differential AC chip calorimeter which is placed in the vacuum chamber for physical vapor deposition. The sample is directly deposited onto one calorimetric chip sensor while the other sensor is protected against deposition. The device and the temperature calibration procedure are described. The latter makes use of the phase transitions of cyclopentane and the frequency dependence of the dynamic glass transition of toluene and ethylbenzene. Sample thickness determination is based on a finite element modeling (FEM) of the sensor sample arrangement. A layer of toluene was added to the sample sensor and its thickness was varied in an iterative way until the model fits the experimental data.   

Methyl Methacrylate Polymerization in Nanoporous Matrix: Reactivity and Resulting Properties

Nanoconfinement is well known to affect the properties of polymers, including changes in the glass transition temperature (Tg). In this work, the focus is on the influence of nanoconfinement on free radical polymerization reaction kinetics and the properties of the polymer produced. Controlled pore glass (CPG) is used as a nanoconfining matrix for methyl methacrylate (MMA) polymerization with pore diameters of 13 nm, 50 nm, and 110 nm. The reaction is followed by measuring heat flow as a function of reaction time during isothermal polymerization at temperatures ranging from 60 \r{ }C to 95 \r{ }C using differential scanning calorimetry (DSC). After reaction, the properties of the polymer are measured, including Tg, molecular weight, and tacticity. Nanoconfiment is found to result in earlier onset of autoacceleration, presumablely due to a decrease in the rate of termination arising from decreases in chain diffusivity in the confined state. In addition, Tg and molecular weight of the resulting PMMA are found to increase. A model of the nanoconfined reaction is able to quantitatively capture these effects by accounting for changes in chain diffusivity, and in native pores, also accounting for changes in intrinsic reaction rates.   

Polystryene films confined between gold surfaces

The properties of thin short-chain polystyrene films between two parallel Au(111) surfaces are studied using a combination of density functional theory (DFT) and classical molecular dynamics (MD) simulations. The chemical interaction with the surface is calculated with DFT and the results are used to develop accurate atomistic classical surface potentials. These potentials are used in the MD simulations to investigate several systems with various film thickness and the effect of increasing confinement on the structural and dynamical properties of the films will be presented. A coarse-grained model is developed and used to study longer-chain polystyrene films and larger systems.   

Surface Mobility of Polymeric Systems of Varying Chain Lengths

Thin films of polymeric material can exhibit drastically altered glass transition temperatures, mechanical responses, and overall dynamics as compared to the bulk. Differences in surface and bulk mobility are often cited as a primary explanation for such differences, but it has remained difficult to quantify local mobility as within the film. We have recently shown that decay constants from Mullins' surface diffusion model correspond directly to surface mobility for small-molecule glasses. In the current work, we examine long-chain polymers via two types of systems: polymeric pillars where the cross sections evolve from square to circular, and large particles which sink into polymeric thin films. For the pillars, we use Mullins' model to interpret changes in curvature. It is found that Mullins' decay constants correspond to relaxation times that can be identified with distinct segmental relaxation processes. Such decay constants exhibit a strong dependence on chain length and temperature. For the particle-thin film systems, we relate the rate of particle sinking to various measures of local mobility in the film. This setup is a direct analog to recent experimental work. The results presented here provide a connection between surface shape transformations, mobility, and diffusion.   

Shear thinning near the rough boundary in a viscoelastic flow

It was first noticed by de Gennes, that because of huge difference between viscosities of entangled polymeric liquid and monomeric liquid, the boundary conditions for a polymer flow near microscopically smooth boundary should be assumed as ``slip''. Nevertheless, in the presence of surface roughness, or undulations, the flow is characterized by mixed boundary conditions. We had shown that at certain slip velocities the deformation of the melt near the rough (undulated) boundary might became more elastic than viscous. This results in ``shear thinning'' of the roughness of boundary. At higher velocities, near the slip boundary, the chains can be considered simply trapped in an entangled mesh of other chains. They are subjected to oscillating strain rate, comparable to frequencies of internal Rouse modes of these chains. We calculate the total dissipation of energy due to oscillating strain and calculate the slippage of the polymer melt near the boundary as function of velocity, undulation wavelength and amplitude.   

Relaxation of Capillary Wrinkles

We have investigated the relaxation of a wrinkle pattern on a thin viscoelastic film. The films are made from spin-coated PS (polystyrene) of thickness ranging from 40 to 240 nm that were floated on the surface of water. Viscoelastic behavior is introduced to the film by depressing the glass transition of PS with a soluble plasticizer, dioctyl phthalate. Wrinkle patterns are formed by placing a small droplet ($\sim $1$\mu $L) at the center of the floating disc. Due to the differential tension generated across the film, radial wrinkles form around the drop where the compressive axial force buckles the membrane. Thereafter, length of the wrinkles decays, and so does their wavelength. We have studied the relaxation of wrinkles as a function of PS molecular weight and plasticizer content, in order to understand the relationship with the bulk glass transition temperature.   

New insight into the melting behavior of nanoconfined semicrystalline polymers -The effect of an immobile interfacial layer at the substrate-

It is known that when semicrystalline polymer chains are confined on a nanometer length scale, the crystalline structures and dynamics differ from bulks, the so-called ``nanoconfinement effects.'' In this talk, we will report the anomalous melting behavior of nano-confined polyethylene spin cast films prepared on Si substrates by integrating various in-situ grazing incidence scattering techniques. We found that a very thin adsorbed layer at the weakly interactive substrate interface plays a crucial role in the melting behavior.   

Solvent Swelling as a Means to Modify the Properties of Polymer Thin Films

It has been observed that sample preparation can influence certain properties of polymer films. In particular, spin-coating from solutions of different solvent qualities result in films with different chain conformations. We surmise that upon formation by spin-coating, the chain conformation of a film is still adjustable by means of solvent swelling, resulting in modifications to the amount of entanglement and free volume. Initial measurements of thermal expansion upon heating after swelling suggest that there is a difference between polystyrene films swelled with a good solvent and a $\Theta$ solvent. We have begun a more detailed investigation by studying the effect of swelling on the dewetting behaviors. Preliminary data indicates that the quality of the solvent affects both the dewetting hole size and aging rate of the film.   

Equilibration of Polymer Films Cast from Solutions with Different Solvent Qualities

The preparation history can affect the physical properties of polymer thin films. In spin coating, films are made from a polymer solution. Due to rapid evaporation of solvent in this process, the polymer chains in the films cannot fully interpenetrate, resulting in a non-equilibrium conformation with reduced entanglement density. These in turn can affect the film's equilibration process and viscoelastic properties. On the basis that the chain conformation and entanglement density in a film depend on the conformation of the chains while in solution before spin-coating. We modify the structural properties of the films by adjusting the quality of the solvent used in spin-coating. We examine in detail how these adjustments affect the way polystyrene films approach equilibrium on annealing above the glass transition temperature. It is found that the equilibration time of the film is significantly increased as the solvent quality is decreased towards the $\Theta $ condition. We attribute this observation to reduced entanglement in the films with decreasing solvent quality.   

Tuning confinement effects at constant film thickness

We show experimental evidence confuting the commonly accepted idea that the deviation from bulk behavior can be explained in terms of finite size effects and interfacial interactions. By reproducing Guiselin's experiment and upon variation of the molecular weight, we could prepare films of polystyrene spincoated on aluminum and annealed for the same time, having constant thickness but different glass transition temperature and tracer diffusivity. The results can be rationalized in terms of t*, a dimensionless parameter obtained by the ratio of the annealing time and the adsorption time [1], quantifying the equilibrium character of the films. Further evidence on the relevance of t* on understanding the behavior of polymers at the nanoscale is provided [2].\\[4pt] [1] Napolitano, S. and W\"{u}bbenhorst, M., Nature Communications, 2, 260 (2011).\\[0pt] [2] Rotella, C.; W\"{u}bbenhorst, M. and Napolitano, S., Soft Matter, 7, 5260 (2011).   

Capillary levelling in thin polymer films as a nano-rheological tool to probe interface dynamics

Entanglement of polymer chains in confinement is modified as a result of altered chain conformations. According to Silberberg's principle, chain segments are reflected at an interface causing a reduction of the inter-chain entanglement density. If the interface is transient, local polymer conformation changes can be inferred from a temporal change in flow properties: over time polymer chains become more entangled, thus there is more resistance to flow. Here, we measure the gradual disappearance of an entropic interface between two melts of identical polymer chains during the flow of stepped bilayer polymer films. Samples are prepared in the glassy state and, when in the melt, flow to relieve the Laplace pressure gradient induced by a step in the topography (McGraw \emph{et al}., Soft Matter, 2011). Our results reveal the dynamics of re-entanglement across the transient entropic interface.   

Slow physical aging of thin polymer films of varying chain architecture

The physical aging rate of supported polystyrene (PS) films is influenced by film thickness, H, and by macromolecular architecture. For linear PS films, in the thickness range 300 nm to 50 nm, supported by silicon oxide substrates, the aging rate decreased by 15 percent. On the other hand, star-shaped PS, with functionality f=8 and with an average molecular weight per arm of Marm=25 kg/mol. exhibited a 25 percent decrease throughout the same thickness range. When Marm was decreased to 10 kg/mol the depression in aging rate was 45 percent. We reconcile these changes in physical aging in terms of model that accounts for gradients in the local Tg of the film in the vicinity of interfaces. These findings have important implications for the processing and function of thin polymer films for different applications.   

Diffusion of adsorbed theta-solvent polymers at a solid-liquid interface

We study how surface diffusion depends on temperature when this is varied below the theta temperature. In the polystyrene-cyclohexane system, we use FRAP (fluorescence recovery after photobleaching) to measure over times over 4 orders of magnitude, from 10 sec to 10$^{5}$ seconds. A fast component of motion is attributed to chains loosely bound to the surface. A slower component of motion is retained after rinsing; it is subdiffusive. At temperatures below the bulk coexistence temperature, the surface layer is thicker than a monolayer. We show that bulk phase separation of polymers in dilute solution produces a dense surface layer of emulsion and foamy near-surface structure.