Measuring surface and bulk relaxation in glassy polymers

We present a comprehensive study of gold nanoparticle embedding into polystyrene (PS) surfaces at temperatures ranging from T _g + 8 K to T _g − 83 K and times as long as 10⁵ minutes. This range in times and temperatures allows the first concurrent observation of and differentiation between surface and bulk behavior in the 20nm region nearest the free surface of the polymer film. Of particular importance is the temperature region near the bulk glass transition temperature where both surface and bulk processes can be measured. The results indicate that for the case of PS, enhanced surface mobility only exists at temperatures near or below the bulk T _g value. The surface relaxation times are only weakly temperature dependent and near T _g , the enhanced mobility extends less than 10nm into the bulk of the film. The results suggest that both the concept of a “surface glass transition” and the use of glass transition temperatures to measure local mobility near interfaces may not universally apply to all polymers. The results can also be used to make a quantitative connection to molecular dynamics simulations of polymer films and surfaces.     

Substrate and chain size dependence of near surface dynamics of glassy polymers

We use nanohole relaxation to study the surface relaxation of films of glassy isotactic poly (methyl methacrylate) (i-PMMA) films. These measurements allow us to obtain the time dependent relaxation function at a number of different sample temperatures for the first 2-3 nm of the free surface in a system often used as a model system for the effect of the substrate on thin film dynamics. The surface is observed to relax at temperatures up to 42 K below the bulk Tg value, even on systems where the thin film Tg is known to be greater than the bulk value. We are able to determine the range over which the substrate directly affects the free surface relaxation, and determine a surprisingly large (Mw independent) limiting thickness of approximately 180 nm where the free surface relaxation is not affected by the substrate. For thick films (h>200 nm) we find an unexpected linear Mw dependence of the near surface relaxation time.     

Depth-dependent spin dynamics of canonical spin-glass films: a low-energy muon-spin-rotation study

We have performed depth dependent muon-spin-rotation and -relaxation studies of the dynamics of single layer films of AuFe and CuMn spin glasses as a function of thickness and of its behavior as a function of distance from the vacuum interface (5-70 nm). A significant reduction in the muon-spin relaxation rate as a function of temperature with respect to the bulk material is observed when the muons are stopped near (5-10 nm) the surface of the sample. A similar reduction is observed for the whole sample if the thickness is reduced to, e.g., 20 nm and less. This reflects an increased impurity spin dynamics (incomplete freezing) close to the surface although the freezing temperature is only modestly affected by the dimensional reduction.     

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     

Direct imaging of nanoparticle embedding to probe viscoelasticity of polymer surfaces

Atomic force microscopy was used to study the embedding of gold nanoparticles into the surface of polystyrene films. The rate of embedding was determined at temperatures near the bulk glass transition temperature T(g) by measuring the apparent nanosphere height as a function of annealing time. In particular, relative height measurements of the adhered particles were made at temperatures below the bulk T(g) value. In the absence of enhanced surface dynamics or yield processes no embedding is expected to occur for T相似文献   

Self-confined polymer dynamics in miscible binary blends

Near-edge X-ray absorption fine-structure (NEXAFS) spectroscopy was used to measure simultaneously the relaxation rates of polystyrene (PS) molecules at the free surface and in the bulk. The samples were uniaxially stretched and annealed at temperatures below the bulk glass transition temperature of PS. The surface and bulk chain relaxation was monitored by measuring the partial-electron and the fluorescence NEXAFS yields, respectively, both parallel and perpendicular to the stretching direction. The decay of the optical birefringence was also measured to provide an independent measure of the bulk relaxation. Relaxation of PS chains was found to occur faster on the surface relative to the bulk. The magnitude of the surface glass transition temperature suppression over the bulk was estimated based on the information on the temperature dependence of the rates.Received: 1 January 2003, Published online: 14 October 2003PACS: 68.35.Ja Surface and interface dynamics and vibrations - 68.47.Mn Polymer surfaces     

Influence of surface interactions on the dynamics of the glass former ortho-terphenyl confined in nanoporous silica

We present results on investigations of the dynamics of the glass forming ortho-terphenyl (oTP) confined in nanoporous silica. Calorimetry experiments showed that the glass transition temperature of the confined liquid, T_gconf, has a non-trivial pore size dependence and is strongly affected by surface interactions. Fluid-wall interactions introduce gradients of structural relaxation times in the pores. The molecules at the surface of the pores are slowed down compared to those at the center of the pores. We focus here on a pore diameter range (7 σ< d < 12 σ, where σ is the molecular diameter), where a large variety of dynamical behavior were observed. Depending on surface properties of the confined media, T _gconf may be smaller or larger than the bulk one. In a quite attractive matrix with a pore size of around 7 nm, the structural relaxation times gradient is important enough to allow the observation of two glass transitions for the same liquid. Effects of fluid wall interactions on the short time dynamics at high temperature were also investigated by quasielastic neutron scattering. The self and collective motions exhibit well above the bulk melting point the same dependence on fluid-wall interactions as at T_g.     

NMR investigations of polymer dynamics in a partially filled porous matrix

The interior surface of well-defined porous alumina membranes (Anopore) of 20 nm and 200 nm pore diameter, respectively, was coated with polymer layers generated from solution by the solvent evaporation method. Deposits of poly(dimethyl siloxane) (PDMS) with nominal thicknesses ranging from 0.15 to 4.5 nm --corresponding to submonolayer to multilayer films--were investigated, and were compared to poly(butadiene) (PB) as an example for non-wetting polymers. Molecular weights below and above the critical value were studied since the bulk dynamics of such polymers are known to be qualitatively different. First results of NMR relaxation dispersion experiments on these systems are presented, supplemented by transverse relaxation times and double-quantum measurements obtained from high-field NMR. A systematic decrease of relaxation times at low fields with decreasing polymer amount is found for PDMS, but molecules retain a high degree of mobility irrespective of molecular weight. The relaxation dispersion results are supported by T2 data and 1H residual dipolar coupling (RDC) constants, and are discussed in terms of molecular order and reorientational dynamics.     

Investigation of structural relaxation and surface modification of ultrathin films of poly(ethylene terephthalate)

We studied dynamic properties of ultrathin films of poly(ethylene terephthalate) spincoated on different substrates, by means of dielectric spectroscopy and surface patterning experiments. We did not observe any variations of structural dielectric relaxation dynamics for films spincoated on aluminium substrate having thicknesses down to 40 nm. On the same substrate, 13 nm thick films are instead characterized by a reduction of the chains mobility. Surprisingly the chains dynamics as probed by a surface nanopatterning experiment evidenced a strong dependence on the substrate interaction even for 50 nm thick films, where dielectric relaxation dynamics is unaffected. It can be deduced that different length scales characterise dielectric relaxation dynamics and the processes related to the surface patterning, even if both are related to the chain mobility. Further experiments are wished to better understand this intriguing scenario.     

Reductions of the glass transition temperature in thin polymer films: probing the length scale of cooperative dynamics

We report measurements of the glass transition temperature, T(g), in free standing polymer films in a low M(n) limit where chain confinement effects are not observed. The measured T(g) values are accurately described by a layer model incorporating a mobile surface layer with a size determined by the length scale of cooperative dynamics. The analysis leads to a surface T(g) value and length scale of cooperative motion near bulk T(g) which quantitatively agree with independently determined values. The model and parameters provide a framework within which all previous measurements of T(g) values in thin supported films may be understood and provides values for the length scale of cooperative motion over an extended range of temperatures below the bulk T(g) value.     

Segmental dynamics of poly(methyl phenyl siloxane) confined to nanoporous glasses

The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DS), temperature modulated DSC (TMDSC) and neutron scattering (NS). Nanoporous glasses with pore sizes of 2.5–20 nm have been used. DS and TMDSC experiments show that for PMPS in 7.5 nm pores the molecular dynamics is faster than in the bulk which originates from an inherent length scale of the underlying molecular motions. For high temperatures the temperature dependence of the relaxation rates for confined PMPS crosses that of the bulk state. Besides finite states effects also the thermodynamic state of nano-confined PMPS is different from that of the bulk. At a pore size of 5 nm the temperature dependence of the relaxation times changes from a Vogel/Fulcher/Tammann like to an Arrhenius behavior where the activation energy depends on pore size. This is in agreement with the results obtained by NS. The increment of the specific heat capacity at the glass transition depends strongly on pore size and vanishes at a finite length scale between 3 and 5 nm which can be regarded as minimal length scale for glass transition to appear in PMPS.     

Confined viscous liquids: Interfacial <Emphasis Type="Italic">versus</Emphasis> finite size effects

Confining a supercooled liquid to spaces of several nanometer in diameter can lead to dramatic changes in the relaxation dynamics of the material. In many cases, the effect is reported as a confinement induced shift of the glass transition temperature T_g. Both positive and negative values for ΔT _g have been observed and the length scale of the confining geometry is considered the main variable. We review the dynamics of glass-forming liquids in both hard and soft confinement of <10 nm spaces, with focus on results from solvation dynamics experiments. It is shown that the interface is instrumental in determinig the dynamics, giving rise to reaxation time gradients across the cooperativity length scale of the liquid. Depending on the interfacial conditions, dynamics can become faster or slower for the same liquid, same size of confinement, and identical experimental technique used. No indications of true finite size effects are observed, and the pore or droplet size is relevant only indirectly through the relative number of molecules near the surface.     

Surface dynamics of polymer films

The dynamics of supported polymer films were studied by probing the surface height fluctuations as a function of lateral length scale using x-ray photon correlation spectroscopy. Measurements were performed on polystyrene (PS) films of thicknesses varying from 84 to 333 nm at temperatures above the PS glass transition temperature. Within a range of wave vectors spanning 10(-3) to 10(-2) nm(-1), good agreement is found between the measured surface dynamics and the theory of overdamped thermal capillary waves on thin films. Quantitatively, the data can be accounted for using the viscosity of bulk PS.     

Molecular dynamics in thin films of isotactic poly(methyl methacrylate)

The molecular dynamics in thin films (18 nm-137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the dynamic glass transition (α-relaxation) and a (local) secondary β-relaxation. While the latter does not depend on the dimensions of the sample, the dynamic glass transition becomes faster (≤2 decades) with decreasing film thickness. This results in a shift of the glass transition temperature T _g to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and a decrease of the dielectric strength is observed for the α-relaxation. This enables to deduce a model based on immobilized boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, T _g was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements yield a gradual increase of T _g with decreasing film thickness. The findings concerning the different thickness dependences of T _g are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the T _g shifts incorporates recently developed models to describe the glass transition in thin polymer films. Received 12 August 2001 and Received in final form 16 November 2001     

Probing interfacial dynamics of water in confined nanoporous systems by NMRD

The confined dynamics of water molecules inside a pore involves an intermittence between adsorption steps near the interface and surface diffusion and excursions in the pore network. Depending on the strength of the interaction in the layer(s) close to the surface and the dynamical confinement of the distal bulk liquid, exchange dynamics can vary significantly. The average time spent in the surface proximal region (also called the adsorption layer) between a first entry and a consecutive exit allows estimating the level of ‘nanowettablity’ of water. As shown in several seminal works, NMRD is an efficient experimental method to follow such intermittent dynamics close to an interface. In this paper, the intermittent dynamics of a confined fluid inside nanoporous materials is discussed. Special attention is devoted to the interplay between bulk diffusion, adsorption and surface diffusion on curved pore interfaces. Considering the nano or meso length scale confinement of the pore network, an analytical model for calculating the inter-dipolar spin–lattice relaxation dispersion curves is proposed. In the low-frequency regime (50?KHz–100?MHz), this model is successfully compared with numerical simulations performed using a 3D-off lattice reconstruction of Vycor glass. Comparison with experimental data available in the literature is finally discussed.     

Percolation model of interfacial effects in polymeric glasses

Recent observations using fluorophore probes of local dynamics in polymer films have provided new insight into the glass transition. Using a percolation model, we predict the local T_g in a polymer film, as a function of distance from a substrate or a free surface. Our predictions are in good agreement with the observed elevation of T_g near a substrate, whereas the observed reduction of T_g near a free surface is too strong to be accounted for by percolation effects.     

Substrate effect on mechanical relaxation of polystyrene in ultrathin films

Mechanical relaxation behavior in ultrathin polystyrene (PS) films supported on silicon oxide (SiO_x) and gold (Au) substrates has been studied by dynamic viscoelastic measurement. Based on the method, effects of free surface and substrate interface on the segmental dynamics were discussed. In the case of thin PS films with a thickness of approximately 200 nm, α_a-relaxation process corresponding to the segmental motion did not show any deviation from the bulk behavior. In contrast, for the films thinner than about 50 nm, the relaxation time distribution for the α_a-process became broader, probably due to a mobility gradient in the surface and interfacial regions. When we sandwiched an ultrathin PS film between SiO_x layers, another relaxation process, in addition to the original α_a-process, appeared at a higher temperature side that we assigned to the interfacial α_a-relaxation process. However, this was never seen for an ultrathin PS film between Au layers, implying that restriction from the substrate interface might be weak in this case.     

Dynamics of a polymer chain confined in a membrane

We present a Brownian dynamics theory with full hydrodynamics (Stokesian dynamics) for a Gaussian polymer chain embedded in a liquid membrane which is surrounded by bulk solvent and walls. The mobility tensors are derived in Fourier space for the two geometries, namely, a free membrane embedded in a bulk fluid, and a membrane sandwiched by the two walls. Within the preaveraging approximation, a new expression for the diffusion coefficient of the polymer is obtained for the free-membrane geometry. We also carry out a Rouse normal mode analysis to obtain the relaxation time and the dynamical structure factor. For large polymer size, both quantities show Zimm-like behavior in the free-membrane case, whereas they are Rouse-like for the sandwiched membrane geometry. We use the scaling argument to discuss the effect of excluded-volume interactions on the polymer relaxation time.     

Dynamics of plastic and liquid cyclohexane in bulk and in porous glasses studied by NMR methods

Cyclohexane was investigated both in bulk and in porous glasses with pore diameters between 4 and 208 nm in the temperature range 136 K≤T≤300 K. The methods involved were field-cycling NMR relaxometry, field-gradient NMR diffusometry, transverse-relaxation spectroscopy, and differential scanning calorimetry (DSC). The field-cycling data for the bulk material can best be described assuming translational modulation of intermolecular dipole-dipole coupling. This interpretation is confirmed by experiments with different degrees of deuteration, and is in accordance with diffusion coefficients determined with the aid of field-gradient diffusometry. The confinement in pores produces substantial changes in the phase behaviour and in molecular dynamics. For pore diameters of 30 nm and above, a non-frozen two monolayers thick film on the surface retains a diffusivity about one order of magnitude lower than in the bulk liquid, but two orders of magnitude larger than in the bulk plastic phase. Experiments indicate an exchange mechanism between this layer and the crystallite inside the pore. In glass with a pore diameter of 4 nm, all applied methods corroborate DSC results of the virtual absence of a phase transition and reveal a continuously decreasing translational mobility down to temperatures more than 100 K below the bulk liquid/cubic phase transition temperature.