Dynamics of alpha and beta processes in thin polymer films: poly(vinyl acetate) and poly(methyl methacrylate)

Influence of Annealing on Polymer Optical Fiber Bragg Grating Inscription, Stability and Sensing: A Review

This article reviews recent research progress on the annealing effects on polymer optical fibers (POFs), which are of great importance for inscription, stability and sensing applications of fiber Bragg gratings (FBGs) in POFs due to their unique properties related to polymer molecular chains. In this review, the principle of annealing to reduce frozen-in stress in POFs drawing and different annealing timings are firstly summarized. Then, the annealing methods for POFs are introduced under several different conditions (temperature, humidity, strain, stress and solution). Afterwards, the principle of FBGs and several inscription techniques are reported. Subsequently, the annealing effects on the properties of POFs and polymer optical fiber Bragg gratings (POFBGs) quality are discussed. Finally, the influence of annealing on POFBG sensitivity is summarized. Overall, this paper provides a comprehensive overview of annealing techniques and their impact on both POFs and POFBGs. We hope that it will highlight the important progress made in this field.

Local Dynamics of the Hydration Water and Poly(Methyl Methacrylate) Chains in PMMA Networks

The dynamic behavior of water molecules and polymer chains in a hydrated poly(methyl methacrylate) (PMMA) matrix containing a small amount of water molecules was investigated. Water molecules have been widely recognized as plasticizers for activating the segmental motion of polymer chains owing to their ability to reduce the glass transition temperature. In this study, combined with judicious hydrogen/deuterium labeling, we conducted quasi-elastic neutron scattering (QENS) experiments on PMMA for its dry and hydrated states. Our results clearly indicate that the dynamics of hydrated polymer chains are accelerated, and that individual water molecules are slower than bulk water. It is therefore suggested that the hydration water affects the local motion of PMMA and activates the local relaxation process known as restricted rotation, which is widely accepted to be generally insensitive to changes in the microenvironment.

Interfacial Dynamics in Supported Ultrathin Polymer Films-From the Solid to the Free Interface

Molecular dynamics in ultrathin layers is investigated using nanostructured electrodes to perform broadband dielectric spectroscopy measurements, and by atomistic molecular dynamics simulations. Using poly(vinyl acetate) as the model system and taking advantage of access to the distribution of relaxation times in an extended temperature range above the glass transition temperature, T_g, we demonstrate that while the mean rates of the segmental relaxation remain bulklike down to 12 nm film thickness, modified molecular mobilities arise in the interfacial zones. Combining results from simulations and experiments, we show unambiguously that both the slow relaxations arising from adsorbed polymer segments and the faster modes attributed to segments in the vicinity of the free interface have non-Arrhenius temperature activation. These interfacial regions span thicknesses of ∼1.5 nm each just above the calorimetric T_g independent of molecular weight and film thickness. These deviations at interfaces are relevant for applications of polymers in adhesion, coatings, and polymer nanocomposites.

Direct observation of mobility of thin polymer layers via asymmetric interdiffusion using neutron reflectivity measurements

In this study, we investigated the diffusion dynamics at the interface between deuterated poly(methyl methacrylate) (d-PMMA) and protonated poly(methyl methacrylate) (h-PMMA) in two-layered thin films of d- and h-PMMA layers via neutron reflectivity (NR) measurements during isothermal annealing above the glass transition temperature T_g. When T_g of d-PMMA was higher than that of h-PMMA, the d-PMMA layer thickness increased with increasing annealing time t_a and, simultaneously, the h-PMMA layer thickness decreased. However, the opposite t_a dependence of the layer thicknesses was observed, if the T_g of d-PMMA was decreased by the increase in the fraction of the low-molecular weight d-PMMA: With increasing t_a, the d-PMMA layer thickness decreased and the h-PMMA layer thickness increased when T_g of d-PMMA was lower than that of h-PMMA. This change in the t_a dependence of the layer thickness was related to the change in the mobility of the d-PMMA layer accompanied by the change in the T_g value of d-PMMA. With the decrease in the d-PMMA layer thickness from 49 nm to 13 nm, when the h-PMMA layer thickness was maintained, the t_a dependence of the layer thickness changed and the mobility of the d-PMMA layer dramatically increased. These results suggest that the mobility of thin polymer films can be determined by the observation of interfacial dynamics via NR measurements.

Glassy state molecular mobility and its relationship to the physico-mechanical properties of plasticized hydroxypropyl methylcellulose (HPMC) films

Changes in tensile properties and the glass transition temperature (T_g) of plasticized polymer films are typically attributed to molecular mobility, often with no empirical data to support such an assertion. Herein solvent cast HPMC films containing varying amounts of PEG, as the plasticizer, were used to assess the dependence of tensile properties and the T_g on glassy state molecular mobility. Molecular mobility (molecular relaxation time and temperature) parameters were determined by Thermally Stimulated Current Spectroscopy (TSC). The tensile properties and T_g of the HPMC films were determined by texture analysis and DSC, respectively. Molecular mobilities detected by TSC were cooperative and occurred at temperatures (T_g′) well below (113 to 127 °C) the bulk T_g. The relaxation times (τ) were 71 ± 1, 46 ± 1, 42 ± 1, 36 ± 1 and 29 ± 1 s for HPMC films containing 0, 6, 8, 11 and 17% (w/w) PEG, respectively. The T_g and glassy state molecular mobility were found to be intimately linked and demonstrated a linear dependence. While tensile strength was found to be linearly related to molecular relaxation time, tensile elongation and elastic modulus exhibited a non-linear dependence on molecular mobility. The data presented in this work demonstrates the complex nature of the relationship between plasticizer content, molecular mobility, T_g and tensile properties for plasticized polymeric films. It highlights the fact that the dependence of the bulk physico-mechanical properties on glassy state molecular mobility, differ greatly. Therefore, empirical characterization of molecular mobility is important to fully understand and predict the thermo-mechanical behavior of plasticized polymer films. This work demonstrates the unique capability of TSC to provide key information relating to molecular mobility and its influence on the bulk properties of materials. Data generated using TSC could prove useful for stability and performance ranking, in addition to the ability to predict materials behavior using data generated at or below typical storage conditions in the pharmaceutical, food, and polymer industries.

Suppressed Surface Reorganization in a High-Density Poly(methyl methacrylate) Brush

A high-density poly(methyl methacrylate) (PMMA) brush (σ = 0.77 chain/nm²) with a lower molecular weight distribution was prepared onto a silicon wafer by surface-initiated atom transfer radical polymerization. The surface of the PMMA brush chains was characterized upon the process of the environmental change, from air to water, using contact angle measurements in conjunction with sum-frequency generation spectroscopy. The surface structure and properties altered less with the changing environment from air to water for the PMMA brush than for a spin-coated film; that is, the extent of surface reorganization could be suppressed by grafting densely-packed chains onto a substrate. Also, the water penetration into the brush surface was inhibited because of the densely packed chain structure.

Novel thermal annealing methodology for permanent tuning polymer optical fiber Bragg gratings to longer wavelengths

The Bragg wavelength of a polymer optical fiber Bragg grating can be permanently shifted by utilizing the thermal annealing method. In all the reported fiber annealing cases, the authors were able to tune the Bragg wavelength only to shorter wavelengths, since the polymer fiber shrinks in length during the annealing process. This article demonstrates a novel thermal annealing methodology for permanently tuning polymer optical fiber Bragg gratings to any desirable spectral position, including longer wavelengths. Stretching the polymer optical fiber during the annealing process, the period of Bragg grating, which is directly related with the Bragg wavelength, can become permanently longer. The methodology presented in this article can be used to multiplex polymer optical fiber Bragg gratings at any desirable spectral position utilizing only one phase-mask for their photo-inscription, reducing thus their fabrication cost in an industrial setting.

Interfacial interaction and glassy dynamics in stacked thin films of poly(methyl methacrylate)

Neutron reflectivity and dielectric permittivity of alternately stacked thin films of protonated and deuterated poly(methyl methacrylate) were measured to elucidate a correlation between the time evolution of the interfacial structure and the segmental dynamics in the stacked thin polymer films during isothermal annealing above the glass transition temperature. The roughness at the interface between two thin layers increases with the annealing time, whereas the relaxation rate and strength of the α-process decrease with an increase in the annealing time. A strong correlation between the time evolution of the interfacial structure and the dynamics of the α-process during annealing could be observed using neutron reflectivity and dielectric relaxation measurements.

Why is surface diffusion the same in ultrastable, ordinary, aged, and ultrathin molecular glasses?

The primitive/JG relaxation explains the same surface diffusion coefficient in ordinary, ultrastable and thin film glasses of OTP and TPD.

Effect of Interface Interaction on the Segmental Dynamics of Poly(vinyl acetate) Investigated by Local Dielectric Spectroscopy

The segmental dynamics of poly(vinyl acetate) (PVAc) thin films were measured in the presence of an aluminum interface and in contact with an incompatible polymer, poly(4-vinylpyridine). The local dielectric relaxation was found to be faster in thin films than in the bulk; however, no differences were observed for the various interfaces, including a PVAc/air interface. These results show that capping of thin films, even with a rigid material, does not necessarily affect the dynamics, the speeding up herein for capped PVAc was equivalent to that for the air interface. The insensitivity of the dynamics to the nature of the interface affords a means to engineer thin films while maintaining desired mechanical properties. Our findings for PVAc also may explain the discordant results that have been reported in general for the effect of air versus rigid interfaces on the local segmental relaxation of thin films.

Segmental and local dynamics of stacked thin films of poly(methyl methacrylate)

The glass transition temperature and the dynamics of the α and β processes have been investigated using differential scanning calorimetry and dielectric relaxation spectroscopy during successive annealing processes above the glass transition temperature for stacked thin films of poly(methyl methacrylate) (PMMA) of various thicknesses. The glass transition temperature and the dynamics of the α process (segmental motion) of as-stacked PMMA thin films exhibit thin-film-like behavior, insofar as the glass transition temperature is depressed and the dynamics of the α process are faster than those of the bulk system. Annealing at high temperature causes the glass transition temperature to increase from the reduced value and causes the dynamics of the α process to become slower approaching those of the bulk. Contrary to the segmental motion, the relaxation time of the β process (local motion) of the stacked PMMA thin films is almost equal to that of the bulk PMMA and is unaffected by the annealing process. However, the relaxation strengths of both the α process and β process show a strong correlation between each other. The sum of the relaxation strengths remains almost unchanged, while the individual relaxation strengths change during the annealing process. The fragility index of the stacked PMMA thin films increases with annealing, which suggests that the glassy state of the stacked thin films changes from strong to fragile.

A simple device for dielectric spectroscopy of polymers with temperature regulation close to 300 K based on a Peltier junction

We present a simple thermostat device for performing dielectric spectroscopy measurements on polymers close to their glass transition temperature. By using a vacuum chamber containing a Peltier junction with its regulator, we show that a very simple setup yields a temperature accuracy which is good enough for accurate studies of polymer dielectric properties. This technique is also more cost effective than standard setups using cryogenic fluids.

Glass transition dynamics of stacked thin polymer films

The glass transition dynamics of stacked thin films of polystyrene and poly(2-chlorostyrene) were investigated using differential scanning calorimetry and dielectric relaxation spectroscopy. The glass transition temperature T(g) of as-stacked thin polystyrene films has a strong depression from that of the bulk samples. However, after annealing at high temperatures above T(g), the stacked thin films exhibit glass transition at a temperature almost equal to the T(g) of the bulk system. The α-process dynamics of stacked thin films of poly(2-chlorostyrene) show a time evolution from single-thin-film-like dynamics to bulk-like dynamics during the isothermal annealing process. The relaxation rate of the α process becomes smaller with increase in the annealing time. The time scale for the evolution of the α dynamics during the annealing process is very long compared with that for the reptation dynamics. At the same time, the temperature dependence of the relaxation time for the α process changes from Arrhenius-like to Vogel-Fulcher-Tammann dependence with increase of the annealing time. The fragility index increases and the distribution of the α-relaxation times becomes smaller with increase in the annealing time for isothermal annealing. The observed change in the α process is discussed with respect to the interfacial interaction between the thin layers of stacked thin polymer films.

Anomalous increase in dielectric susceptibility during isothermal aging of ultrathin polymer films

The aging dynamics of poly(2-chlorostyrene) (P2CS) ultrathin films with thicknesses less than 10 nm were investigated using dielectric relaxation spectroscopy. The imaginary part of the dielectric susceptibility ϵ″ for P2CS ultrathin films with a thickness of 3.7 nm increased with an increase in isothermal aging time, while this was not the case for P2CS thin films thicker than 9.0 nm. This anomalous increase in ϵ″ for the ultrathin films is strongly correlated with the presence of a mobile liquidlike layer within the thin films.

The glass transition of thin polymer films in relation to the interfacial dynamics

As opposed to measurements on the glass transition of a polymer in the bulk, measurements of thin polymer layers reflect--due to the alterations of the glassy dynamics at the confining interfaces--several contributions acting together to give the net response of a polymer film. This fundamental difference is exemplified in detail for the particular case of broadband dielectric spectroscopy, an experimental tool extensively employed to investigate the glassy dynamics of polymers under condition of geometrical confinement.

Aging dynamics in the polymer glass of poly(2-chlorostyrene): Dielectric susceptibility and volume

Aging dynamics was investigated in the glassy states of poly(2-chlorostyrene) by measuring the complex electrical capacitance during aging below the glass transition temperature. The variations with time and temperature of the ac dielectric susceptibility and volume could be determined by simply measuring the variation in the complex electrical capacitance. Isothermal aging at a given temperature for several hours after an intermittent stop in constant-rate cooling is stored in the deviations of both the real and imaginary parts of the complex ac dielectric susceptibility and volume. During cooling after isothermal aging, the deviation of the ac dielectric susceptibility from the reference value decreases and almost vanishes at room temperature. By contrast, the deviation in volume induced during isothermal aging remains almost constant during cooling. The simultaneous measurement of ac dielectric susceptibility and volume clearly revealed that the ac dielectric susceptibility exhibits a full rejuvenation effect, whereas the volume does not show any rejuvenation effects. We discuss a plausible model that can reproduce the present experimental results.

Dielectric relaxation of ultrathin films of supported polysulfone

The dynamic glass transition (alpha relaxation, structural relaxation) of ultrathin polysulfone films prepared between aluminum electrodes is investigated by dielectric relaxation spectroscopy. As a main result, it is found that the glass transition temperature Tg does not depend on the thickness of the polymeric layer down to a thickness of 10 nm. For thicknesses lower than 10 nm, an increase of Tg is observed. A more detailed analysis of the temperature dependence of the relaxation rates reveals that the Vogel temperature increases and the fragility decreases systematically with decreasing film thickness d. Further, the dielectric strength Deltaepsilon decreases with decreasing d. This is discussed by the formation of a surface layer of adsorbed polysulfone segments having a reduced molecular mobility with regard to the time scale characteristic of the glassy dynamics of bulk polysulfone. Plotted versus inverse film thickness Deltaepsilon decreases linearly with 1/d and becomes zero for an extrapolated length scale of 10 nm. From that it is concluded that the thickness of the adsorbed boundary layer is about 5 nm. Contact-angle measurements were carried out to confirm the strong interaction between aluminum and polysulfone. It is also shown that preparation details like annealing conditions strongly influence the glass transition of supported ultrathin films.

A molecular dynamics study of nanoindentation on a methyl methacrylate ultrathin film on a Au(111) substrate: interface and thickness effects

The molecular dynamics simulation model of nanoindentation is proposed in order to study the mechanical and structural deformation properties of an ultrathin MMA (methyl methacrylate) film on a Au(111) surface. First, the significant differences in the structural arrangement of MMA thin films with different thicknesses are observed. Two layers are apparent in the thinnest MMA thin film next to the Au(111) surface, while three layer structures are apparent in the thicker film. Second, this study examines the indentation tip that penetrates the MMA thin film into the Au(111) substrate in order to understand the influence of the interface on the properties and deformation behavior in both the thin film and substrate. The result shows that the indentation force is influenced both by the layer structure and by the thickness of the MMA film. The thinnest case exhibits different deformation behavior from that of the thicker cases. In addition, the deformation of MMA molecules becomes significant at the interface between the MMA film and the Au(111) surface with the increase of film thickness, and detailed deformation behavior of the Au surface for different thicknesses of MMA film is reported in this paper. Finally, both the rigid and the active models for the indentation tip are utilized in the simulation to examine the interaction differences between the tip and the film and the deformation mechanism.

The effect of temperature on the nanoscale adhesion and friction behaviors of thermoplastic polymer films

Adhesion and friction tests were carried out in order to investigate the effect of temperature on the tribological characteristics of poly(methylmethacrylate) (PMMA) film using AFM. The pull-off and friction forces on the PMMA film were measured under a high vacuum condition (below 1 × 10(-4) Pa) as the temperature of the PMMA film was increased from 300 to 420 K (heating) and decreased to 300 K (cooling). Friction tests were also conducted in both high vacuum and air conditions at room temperature. When the temperature was 420 K, which is 25 K higher than the glass-transition temperature (T(g)) of PMMA, the PMMA film surface became deformable. Subsequently, the pull-off force increased in proportion to the maximum applied load during the pull-off force measurement. In contrast, when the temperature was under 395 K, the pull-off force did not depend on the maximum applied load. The friction force began to increase when the temperature rose above 370 K, which is 25 K lower than the T(g) of PMMA, and rapidly increased at 420 K. Decrease of the PMMA film stiffness and plastic deformation of the PMMA film were observed at 420 K in force-displacement curves. After the heating to 420 K, the coefficient of friction was measured under air condition at room temperature and was found to be lower than that measured before the heating. Additionally, the RMS roughness increased as a result of heating.

Dynamic anisotropy and heterogeneity of polystyrene thin films as studied by inelastic neutron scattering

We studied the dynamic anisotropy and heterogeniety of polystyrene thin films in glassy state with inelastic neutorn scattering. Adjusting the scattering vector to the directions parallel and perpendicular to the film surface, we observed the elastic scattering intensities as a function of temperature. It was found for the 200 A film that the elastic intesity decreased with increasing temperature more rapidly in the perpendicular direction than in the pararell direction, showing the higher mobility in the perpendicular direction. However, such dynamical anisotropy was not observed in the 1000 A film. The decrease in the mobility was observed with the film thickness in both the directions. These results were explained in terms of an interface hard layer. We also evelauated the dynamical heterogeniety from the non-Gaussian parameter A0, which increased with decreasing the film thickness, showing the increase in the dynamical heterogeneity. Assuming a simple bi-layer model consisting of the interface hard layer and the bulk-like layer, we analyzed the thickness dependence of the non-Gaussian parameter A0 and the mean square displacement (u2) to find that the hard layer has a thickness of approximately 130 A and a mean square displacement of approximately 0.018 A2 at 230 K.