Correlation between fragility and surface glass transition temperature of polymers.

The fragility of glass describes how rapidly its molecules slow down as it is cooled near its glass transition temperature. In nanoscale films, polymer glasses with higher fragility experience larger reductions in their Tg compared to those with lower fragility. We investigated whether this is due to the free surface of the polymers, which can cause the surface Tg (Tgsurf) to decrease relative to the bulk Tg. By measuring Tgsurf of various polymers, we found that the shift in Tgsurf relative to the bulk Tg increased with fragility. This suggests that more fragile polymers are more susceptible to the free surface effect. We explain this using the concept of energy landscape, as it is used to explain the different slowdown rates between strong and fragile glass formers at Tg.

Loosely Adsorbed Chains Expedite the Desorption of Flattened Polystyrene Chains on Flat Silicon Surface.

Herein, the desorption of the adsorbed chains (including two regions of flattened chains and loosely adsorbed chains) was examined by monitoring the chain exchange kinetics between the adsorbed chains and the top-free chains in a bilayer system by using fluorine-labeled polystyrene (PS). The results indicated that the exchange behavior of PS-flattened chains with the top-free chains is much slower than that of PS-loose chains and has a strong molecular weight (MW) dependence. Interestingly, in the presence of loosely adsorbed chains, the desorption of flattened chains was accelerated greatly and had weaker MW dependency. We attribute the MW-dependent desorption phenomena to the average number of contact sites between polymer adsorbed chains and the substrate, which rapidly increased with increasing MW. Likewise, the desorption of loosely adsorbed chains may provide extra conformational energy to accelerate the desorption of flattened chains.

Microscale mobile surface double layer in a glassy polymer.

This study examines the origin of the widely different length scales, ht-nanometers to micrometers-that have been observed for the propagation of the near-surface enhanced mobility in glassy polymers. Mechanical relaxations of polystyrene films with thicknesses, h, from 5 nm to 186 µm have been studied. For h < ~1 µm, the films relaxed faster than the bulk and the relaxation time decreased with decreasing h below ~100 nm, consistent with the enhanced dynamics originating from a near-surface nanolayer. For h > ~1 µm, a bulk-like relaxation mode emerged, while the fast mode changed to one that extended over ~1 µm from the free surface. These findings evidence that the mobile surface region is inhomogeneous, comprising a nanoscale outer layer and a slower microscale sublayer that relax by different mechanisms. Consequently, measurements probing the enhanced mobility of different mechanisms may find vastly different ht's as shown by the literature.

Mechanical Responses of Breast Cancer Cells to Substrates of Varying Stiffness Revealed by Single-Cell Measurements.

How cancer cells respond to different mechanical environments remains elusive. Here, we investigated the tension in single focal adhesions of MDA-MB-231 (metastatic breast cancer cells) and MCF-10A (normal human breast cells) cells on substrates of varying stiffness using single-cell measurements. Tension measurements in single focal adhesions using an improved FRET-based tension sensor showed that the tension in focal adhesions of MDA-MB-231 cells increased on stiffer substrates while the tension in MCF-10A cells exhibited no apparent change against the substrate stiffness. Viscoelasticity measurements using magnetic tweezers showed that the power-law exponent of MDA-MB-231 cells decreased on stiffer substrates whereas MCF-10A cells had similar exponents throughout the whole stiffness, indicating that MDA-MB-231 cells change their viscoelasticity on stiffer substrates. Such changes in tension in focal adhesions and viscoelasticity against the substrate stiffness represent an adaptability of cancer cells in mechanical environments, which can facilitate the metastasis of cancer cells to different tissues.

Glass transition temperature of single-chain polystyrene particles end-grafted to oxide-coated silicon.

A method based on the PeakForce QNM atomic force microscopic (AFM) adhesion measurement is employed to investigate the glassy dynamics of polystyrene (PS) single-chain particles end-grafted to SiO2-Si substrates with different diameters, D0, of 3.4 nm-8.8 nm and molar masses, Mn, of 8-123 kg/mol. As temperature was increased, the adhesion force, Fad, experienced by the AFM tip on pulling off the single chains after loading demonstrated a stepwise increase at an elevated temperature, which we identified to be Tg based on previous works. Our result shows that Tg of our grafted single chains increases with Mn in a manner consistent with the Fox-Flory equation, but the coefficient quantifying the Mn dependence of Tg is only (36 ± 6)% the value of bulk PS. In addition, the value of Tg in the Mn → ∞ limit is about 25 °C below the bulk Tg but more than 15 °C above that of (untethered) PS nanoparticles with D0 ≈ 100 nm suspended in a solution. Our findings are consistent with Tg of our single chains being dominated by simultaneous effects of the interfaces, which depress Tg, and end-grafting, which enhances Tg. The latter is believed to exert its influence on the glass transition dynamics by a mechanism reliant on chain connectivity and does not vary with chain length.

Strain Rate and Thickness Dependences of Elastic Modulus of Free-Standing Polymer Nanometer Films.

Elastic moduli, E, of free-standing polystyrene (PS) single-layers and polystyrene-polydimethylsiloxane (PS-PDMS) bilayers are measured by uniaxial tensile testing at room temperature under different strain rates, γ̇, and for PS thicknesses, h, from 8 to 130 nm. As γ̇ increases, E increases initially, then approaches the bulk value, Ebulk, when γ̇ exceeds a characteristic value (≡ τ-1) that decreases with increasing h. The noted variation of E with γ̇ shows that stress relaxation occurs in the films during measurement when γ̇τ ≪ 1, while the noted variation of τ-1 with h shows that thinner films relax faster. Consequently, E decreases with decreasing h if γ̇ is small, but displays independence of h if γ̇ is large. Visually, the crossover takes place at around γ̇ = 0.0015 s-1, where at γ̇τ > 1 for all films.

Thermal-induced slippage of soft solid films.

The dynamics of interfacial slippage of entangled polystyrene (PS) films on an adsorbed layer of polydimethylsiloxane on silicon was studied from the surface capillary dynamics of the films. By using PS with different molecular weights, we observed slippage of the films in the viscoelastic liquid and rubbery solid state, respectively. Remarkably, all our data can be explained by the linear equation, J=-M∇P and a single friction coefficient, ξ, where J is the unit-width current, M is mobility, and P is Laplace pressure. For viscous films, M is accountable by using conventional formulism. For rubbery films, M takes on different expressions depending on whether the displacements associated with the slip velocity, v_{s}(∼∇P/ξ), dominate or elastic deformations induced by ∇P dominate. For viscoelastic liquid films, M is the sum of the mobility of the films in the viscous and rubbery states.

Tg Confinement Effect of Random Copolymers of 4-tert-Butylstyrene and 4-Acetoxystyrene with Different Compositions.

We observe that the Tg confinement effect of polymer films can saturate with polymer-substrate interaction. Thickness dependences of the glass transition temperature, Tg(h0), of random copolymer films of 4-tert-butylstyrene (TBS) and 4-acetoxystyrene (AS) supported by silica (SiOx) were measured for different TBS concentrations, XTBS. For 0 ≤ XTBS ≤ 0.47, Tg(h0) displays identical enhancements, independent of XTBS. For XTBS > ∼0.66; however, Tg(h0) decreases steadily with XTBS. The XTBS > 0.66 result is in keeping with expectations that TBS interacts less strongly with SiOx than AS does, and weaker polymer-substrate interaction renders greater dominance of the air surface over substrate surface on Tg, and thereby Tg reduction. We propose that saturation in Tg(h0) found for XTBS ≤ 0.47 is caused by the maximization in polymer-substrate-specific bond formation. Further experiments and a calculation support this proposition.

Unexpected thermal annealing effects on the viscosity of polymer nanocomposites.

The effects of thermal annealing, 12-50 K above the glass transition temperature, on the zero-shear viscosity, η, of polymer nanocomposites (PNCs) and the corresponding host polymers were studied. For all specimens, including neat and 4 wt% dioctyl phthalate (DOP)-plasticized polystyrene (PS), neat poly(methyl methacrylate) (PMMA), and PNCs containing bare and grafted silica nanoparticles in neat and DOP-plasticized PS, the η increased with time initially, and only asymptotically approached a steady-state value after thermal annealing for ∼100 to ∼200 h. We found that this phenomenon occurred regardless of the solvent used to prepare the sample although the fractional changes in η (δη/η) are visibly bigger for tetrahydrofuran (THF). Moreover, the PNCs not plasticized by DOP showed bigger δη/η than their host polymers while the plasticized ones behave essentially the same as the neat hosts. Interestingly, some unplasticized PNCs prepared from THF exhibited smaller viscosities than the host polymer, but this anomaly disappeared on thermal annealing. By correlating the viscosity measurements with the evolution of the solvent content, average NP aggregate size and the amount of adsorbed PS on silica for samples prepared from different solvents, we infer that the temporal viscosity evolution originates from out-of-equilibrium chain conformations produced during sample preparation. Because these relaxations are limited by the rearrangement of the polymer chains adsorbed on the NP or sample substrate surface, the timescales over which η changes can be much longer than the polymer reptation time, as observed.

A novel method to encapsulate a Au nanorod array in 15 nm radius multiwalled carbon nanotubes.

In this paper we demonstrate a novel complex array structure comprising well-aligned Au nanorods (10 nm in diameter) encapsulated inside 15 nm radius multiwalled carbon nanotubes (MWCNTs). A pre-aligned and open-ended nanoporous MWCNT membrane is used as the starting material. Au nanorods are precisely deposited and aligned inside the hollow channels of CNTs by inter-diffusing the HAuCl4 precursor and the reductant solution. Ultra-long Au nanowires and spherical Au nanoparticles are also observed in the CNT cavity with the same diameter in special cases. Using high-resolution TEM (HRTEM), scanning transmission electron microscopy (STEM), 3-dimensional TEM (3D-TEM) and energy dispersive X-ray spectroscopy (EDX), the precise location and composition of the encapsulated Au components with various structures are confirmed. This aligned Au@CNT endohedral material has important potential applications in nanocatalysis, waveguides, as well as in novel plasmonic devices.

Crossover to surface flow in supercooled unentangled polymer films.

We study the driven flow of an unentangled glassy polymer film with a free upper surface and supported below by a substrate using nonequilibrium molecular dynamics simulations based on a bead-spring model. Above the glass transition temperature T(g), simple Poiseuille laminar flow is observed with the film mobility defined as the flow current density per unit pressure gradient scaling as h(3) with the film thickness h. Below T(g), the film mobility becomes independent of h, signifying surface transport. This is in full agreement with recent experiments on the time evolution of capillary waves in polystyrene films supported by silica. A mobile layer is found responsible for the surface transport, as previously conjectured. Our result also shows that it has a velocity profile decaying exponentially into the bulk.

Power spectral density of free-standing viscoelastic films by adiabatic approximation.

In a previous study, we calculated the surface dynamics of noisy viscoelastic supported films by using an adiabatic approximation. An expression was derived for the time-dependent power spectral density (PSD), which was found to produce good agreement with experiment. In this study, we extend the treatment to viscoelastic free-standing films. Two sets of surface capillary normal modes, namely, the squeezing and bending modes, were found. The frequency dispersion relation of the former resembles that of supported films. The latter is distinctively different and diverges at long wavelengths. By incorporating the experimental conditions, we obtained satisfactory agreement between theory and experiment.

Surface dynamics of noisy viscoelastic films by adiabatic approximation.

Surface dynamics is sometimes used to determine the rheological properties of soft materials. In typical data analyses, surface capillary waves are included without incorporating thermal noise. A phenomenological expression for the time-dependent power spectral density has been proposed to account for thermal noise and shown to agree well with experiment. In this paper, we investigate the surface dynamics of viscoelastic films with thermal noise by using an adiabatic approximation involving fast quasi-equilibrium elastic vibrations to derive the power spectral density. Our result justifies the use of the phenomenological expression.

Glass transition dynamics and surface layer mobility in unentangled polystyrene films.

Most polymers solidify into a glassy amorphous state, accompanied by a rapid increase in the viscosity when cooled below the glass transition temperature (T(g)). There is an ongoing debate on whether the T(g) changes with decreasing polymer film thickness and on the origin of the changes. We measured the viscosity of unentangled, short-chain polystyrene films on silicon at different temperatures and found that the transition temperature for the viscosity decreases with decreasing film thickness, consistent with the changes in the T(g) of the films observed before. By applying the hydrodynamic equations to the films, the data can be explained by the presence of a highly mobile surface liquid layer, which follows an Arrhenius dynamic and is able to dominate the flow in the thinnest films studied.

Unconventional spinodal surface fluctuations on polymer films.

We study the temporal growth pattern of surface fluctuations on a series of spinodally unstable polymer films where the instability can be adjusted with the film thickness, h0. For the most unstable film studied (whose /(h0 - h(sp))/h(sp)/ = 0.988; h(sp) is the thickness where the second derivative of the interfacial potential of the film equals zero), the growth rate function of the surface modes as a function of the wavevector fits well to the mean-field theory. When the film thickness is increased such that /(h0 - h(sp))/h(sp)/ < or = 0.977, the mean-field theory demonstrates marked disagreement with experiment, notwithstanding the provision of the known corrections from nonlinear effects and thermal noise. We show that the deviations arise from large-amplitude fluctuations induced by homogeneous nucleation, which are not considered in the conventional treatments.

Liquid crystal pretilt control by inhomogeneous surfaces.

We consider the pretilt alignment of a nematic liquid crystal (LC) on inhomogeneous surface patterns comprising patches of homeotropic alignment domains in a matrix favoring homogeneous alignment, or vice versa. We found that the resultant LC pretilt generally increases continuously from the homogeneous limit to the homeotropic limit as the area fraction of the homeotropic region increases from 0 to 1. For any given homeotropic area fraction, the variations are qualitatively different depending on how the distance between adjacent patches compares to the extrapolation lengths of the anchoring domains. Our results agree with those previously found in stripe patterns. The present finding provides useful guidelines for designing inhomogeneous alignment surfaces for variable LC pretilt control.

Dewetting induced by complete versus nonretarded van der Waals forces.

Spontaneous rupture of some polymer films upon heating is commonplace. The very criterion for this instability is the system free energy, G(L), possessing a negative curvature. In films that are apolar with h < or = 100 nm van der Waals (vdW) interactions usually constitute a major contribution to G(L) for which the approximate form G(L) = -A/12piL(2) (where A is the Hamaker constant), ignoring retardation, has been widely used. In this work, we investigate the limits to this approximation by calculating the complete vdW interactions for popular polymer film systems in dewetting experiments including air-polystyrene-SiO2-Si, air-polystyrene-poly(methyl methacrylate)-Si, and air-poly(methyl methacrylate)-polystyrene-Si based on the theory of Dzyaloshinskii, Lifshitz, and Pitaevskii (DLP). We found that retardation effects could produce significant modifications to G(L) even when the thickness of the polymer and/or the interlayer is only 1-2 nm, contrary to conventional presumption.

First-order liquid crystal orientation transition on inhomogeneous substrates.

In a recent experiment, we uncovered an unconventional liquid crystal (LC) orientation transition on microtextured substrates consisting of alternating horizontal and vertical corrugations. When the period of alternation was decreased toward approximately 1 microm, the LC alignment underwent an abrupt transition from inhomogeneous planar to a more uniform configuration with a large pretilt angle ( approximately 40 degrees ). With the aid of a model based on the competition between the Frank-Oseen elastic energy and a phenomenological surface potential of the form W(theta,phi)=(1/2)W((2))(theta) sin(2) theta+(1/4)W((4))(theta) sin(4) theta+(1/2)W(phi) cos(2) theta sin(2) phi(x,y) (where theta and phi are, respectively, the pretilt and azimuthal angles of the LC director and W((2))(theta), W((4))(theta), and W(phi) are constants) that demonstrated good agreement with experiment, we investigated the microscopic origin of the observed transition. It was found that this transition comprises two steps. First, the LC director homogenizes toward the phi=45 degrees azimuthal direction in the plane to relax the elastic energy. The resulting rise in azimuthal anchoring energy subsequently drives the LC to adopt a finite pretilt. The values of the W's deduced from the model reveal that the polar anchoring energy is about approximately 1/10 of the typical values, with the sin(4) theta term dominating the sin(2) theta term. We present a possible explanation for this unexpected finding.