Glassy Dynamics and Glass Transition in Nanometric Thin Layers of Polystyrene

The glass transition and enthalpy recovery of polystyrene nanorods using Flash differential scanning calorimetry

The glass transition (Tg) behavior and enthalpy recovery of polystyrene nanorods within an anodic aluminum oxide (AAO) template (supported nanorods) and after removal from AAO (unsupported nanorods) is studied using Flash differential scanning calorimetry. Tg is found to be depressed relative to the bulk by 20 ± 2 K for 20 nm-diameter unsupported polystyrene (PS) nanorods at the slowest cooling rate and by 9 ± 1 K for 55 nm-diameter rods. On the other hand, bulk-like behavior is observed in the case of unsupported 350 nm-diameter nanorods and for all supported rods in AAO. The size-dependent Tg behavior of the PS unsupported nanorods compares well with results for ultrathin films when scaled using the volume/surface ratio. Enthalpy recovery was also studied for the 20 and 350 nm unsupported nanorods with evolution toward equilibrium found to be linear with logarithmic time. The rate of enthalpy recovery for the 350 nm rods was similar to that for the bulk, whereas the rate of recovery was enhanced for the 20 nm rods for down-jump sizes larger than 17 K. A relaxation map summarizes the behavior of the nanorods relative to the bulk and relative to that for the 20 nm-thick ultrathin film. Interestingly, the fragility of the 20 nm-diameter nanorod and the 20 nm ultrathin film are identical within the error of measurements, and when plotted vs departure from Tg (i.e., T - Tg), the relaxation maps of the two samples are identical in spite of the fact that the Tg is depressed 8 K more in the nanorod sample.

Dynamics of polylactic acid under ultrafine nanoconfinement: The collective interface effect and the spatial gradient

Polymers under nanoconfinement can exhibit large alterations in dynamics from their bulk values due to an interface effect. However, understanding the interface effect remains a challenge, especially in the ultrafine nanoconfinement region. In this work, we prepare new geometries with ultrafine nanoconfinement ∼10nm through controlled distributions of the crystalline phases and the amorphous phases of a model semi-crystalline polymer, i.e., the polylactic acid. The broadband dielectric spectroscopy measurements show that ultrafine nanoconfinement leads to a large elevation in the glass transition temperature and a strong increment in the polymer fragility index. Moreover, new relaxation time profile analyses demonstrate a spatial gradient that can be well described by either a single-exponential decay or a double-exponential decay functional form near the middle of the film with a collective interface effect. However, the dynamics at the 1-2 nm vicinity of the interface exhibit a power-law decay that is different from the single-exponential decay or double-exponential decay functional forms as predicted by theories. Thus, these results call for further investigations of the interface effect on polymer dynamics, especially for interfaces with perturbed chain packing.

Tracing the slow Arrhenius process deep in the glassy state-quantitative evaluation of the dielectric relaxation of bulk samples and thin polymer films in the temperature domain

The slow Arrhenius process (SAP) is a dielectric mode connected to thermally activated equilibration mechanisms, allowing for a fast reduction in free energy in liquids and glasses. The SAP, however, is still poorly understood, and so far, this process has mainly been investigated at temperatures above the glass transition. By employing a combination of methods to analyze dielectric measurements under both isochronal and isothermal conditions, we were able to quantitatively reproduce the dielectric response of the SAP of different polymers and to expand the experimental regime over which this process can be observed down to lower temperatures, up to 70 K below the glass transition. Employing thin films of thicknesses varying between 10 and 800 nm, we further verified that the peak shape and activation energy of the SAP of poly(4-bromostyrene) are not sensitive to temperature, nor do they vary upon confinement at the nanoscale level. These observations confirm the preliminary trends reported for other polymers. We find that one single set of parameters-meaning the activation barrier and the pre-exponential factor, respectively, linked to the enthalpic and entropic components of the process-can describe the dynamics of the SAP in both the supercooled liquid and glassy states, in bulk and thin films. These results are discussed in terms of possible molecular origins of the slow Arrhenius process in polymers.

Recent Progress in Vacuum Engineering of Ionic Liquids

Since the discovery of ionic liquids (ILs) as a new class of liquid that can survive in a vacuum at room temperature, they have been aimed at being characterized with vacuum analysis techniques and used in vacuum processes for the last two decades. In this review, our state-of-the-art of the vacuum engineering of ILs will be introduced. Beginning with nanoscale vacuum deposition of IL films and their thickness-dependent ionic conductivity, there are presented some new applications of the ellipsometry to in situ monitoring of the thickness of IL films and their glass transitions, and of the surface thermal fluctuation spectroscopy to investigation of the rheological properties of IL films. Furthermore, IL-VLS (vapor-liquid-solid) growth, a vacuum deposition via IL, has been found successful, enhancing the crystallinity of vacuum-deposited crystals and films, and sometimes controlling their surface morphology and polymorphs. Among recent applications of ILs are the use of metal ions-containing IL and thin film nano IL gel. The former is proposed as a low temperature evaporation source of metals, such as Ta, in vacuum deposition, while the latter is demonstrated to work as a gate electrolyte in an electric double layer organic transistor.

Adsorption Kinetics of Polystyrene and Poly(9-anthracenyl methyl methacrylate) onto SiO2 Surface Measured by Chip Nano-Calorimetry

The alternating current (AC) chip nano-calorimetry is a powerful tool to investigate the physical properties of polymer thin films. In this paper, we report on the adsorption kinetics of polymers in which an AC chip nano-calorimetry was used for the first time. This technique allows for the real-time measurement of the adsorption kinetics of polymer chains onto the SiO2 surface. We used polystyrene (PS) and poly(9-anthracenyl methyl methacrylate) (PAMMA), which have different chemical natures and side group sizes. It was confirmed that the observed adsorption kinetics for PS were consistent with previously reported results obtained by dielectric spectroscopy. For PAMMA, we found characteristic adsorption kinetics, which shows a clear kink at the crossover between the early and later stages, while PS exhibits a lesser tendency of showing the kink as demonstrated by previously reported results.

Progress towards a phenomenological picture and theoretical understanding of glassy dynamics and vitrification near interfaces and under nanoconfinement

The nature of alterations to dynamics and vitrification in the nanoscale vicinity of interfaces-commonly referred to as "nanoconfinement" effects on the glass transition-has been an open question for a quarter century. We first analyze experimental and simulation results over the last decade to construct an overall phenomenological picture. Key features include the following: after a metrology- and chemistry-dependent onset, near-interface relaxation times obey a fractional power law decoupling relation with bulk relaxation; relaxation times vary in a double-exponential manner with distance from the interface, with an intrinsic dynamical length scale appearing to saturate at low temperatures; the activation barrier and vitrification temperature T_g approach bulk behavior in a spatially exponential manner; and all these behaviors depend quantitatively on the nature of the interface. We demonstrate that the thickness dependence of film-averaged T_g for individual systems provides a poor basis for discrimination between different theories, and thus we assess their merits based on the above dynamical gradient properties. Entropy-based theories appear to exhibit significant inconsistencies with the phenomenology. Diverse free-volume-motivated theories vary in their agreement with observations, with approaches invoking cooperative motion exhibiting the most promise. The elastically cooperative nonlinear Langevin equation theory appears to capture the largest portion of the phenomenology, although important aspects remain to be addressed. A full theoretical understanding requires improved confrontation with simulations and experiments that probe spatially heterogeneous dynamics within the accessible 1-ps to 1-year time window, minimal use of adjustable parameters, and recognition of the rich quantitative dependence on chemistry and interface.

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.

Correlating fragility and heterogeneous dynamics in polystyrene through single molecule studies

Many macroscopic properties of polymers depend on their molecular weight, with one notable example being glass transition temperature: polymers with higher molecular weights typically have higher glass transition temperatures than their lower molecular weight polymeric and oligomeric counterparts. Polymeric systems close to their glass transition temperatures also exhibit interesting properties, showing both high (and molecular weight dependent) fragility and strong evidence of dynamic heterogeneity. While studies have detailed the correlations between molecular weight and fragility, studies clearly detailing correlations between molecular weight and degree of heterogeneous dynamics are lacking. In this study, we use single molecule rotational measurements to investigate the impact of molecular weight on polystyrene's degree of heterogeneity near its glass transition temperature. To this end, two types of fluorescent probes are embedded in films composed of polystyrene ranging from 0.6 to 1364.0 kg mol^-1. We find correlation between polystyrene molecular weight, fragility, and degree of dynamic heterogeneity as reported by single molecule stretching exponents but do not find clear correlation between these quantities and time scales associated with dynamic exchange.

Temperature-Controlled Solvent Vapor Annealing of Thin Block Copolymer Films

Solvent vapor annealing is as an effective and versatile alternative to thermal annealing to equilibrate and control the assembly of polymer chains in thin films. Here, we present scientific and practical aspects of the solvent vapor annealing method, including the discussion of such factors as non-equilibrium conformational states and chain dynamics in thin films in the presence of solvent. Homopolymer and block copolymer films have been used in model studies to evaluate the robustness and the reproducibility of the solvent vapor processing, as well as to assess polymer-solvent interactions under confinement. Advantages of utilizing a well-controlled solvent vapor environment, including practically interesting regimes of weakly saturated vapor leading to poorly swollen states, are discussed. Special focus is given to dual temperature control over the set-up instrumentation and to the potential of solvo-thermal annealing. The evaluated insights into annealing dynamics derived from the studies on block copolymer films can be applied to improve the processing of thin films of crystalline and conjugated polymers as well as polymer composite in confined geometries.

Shell Architecture Strongly Influences the Glass Transition, Surface Mobility, and Elasticity of Polymer Core-Shell Nanoparticles

Despite the growing application of nanostructured polymeric materials, there still remains a large gap in our understanding of polymer mechanics and thermal stability under confinement and near polymer-polymer interfaces. In particular, the knowledge of polymer nanoparticle thermal stability and mechanics is of great importance for their application in drug delivery, phononics, and photonics. Here, we quantified the effects of a polymer shell layer on the modulus and glass-transition temperature (T g) of polymer core-shell nanoparticles via Brillouin light spectroscopy and modulated differential scanning calorimetry, respectively. Nanoparticles consisting of a polystyrene (PS) core and shell layers of poly(n-butyl methacrylate) (PBMA) were characterized as model systems. We found that the high T g of the PS core was largely unaffected by the presence of an outer polymer shell, whereas the lower T g of the PBMA shell layer decreased with increasing PBMA thickness. The surface mobility was revealed at a temperature about 15 K lower than the T g of the PBMA shell layer. Overall, the modulus of the core-shell nanoparticles decreased with increasing PBMA shell layer thickness. These results suggest that the nanoparticle modulus and T g can be tuned independently through the control of nanoparticle composition and architecture.

Metallic glass ultrathin films with hierarchical structure and their dynamic and thermodynamic behavior

Metallic glass ultrathin films with hierarchical structure have been achieved which exhibit relatively high mobility and a large supercooled liquid region.

Effect of substrate interactions on the glass transition and length-scale of correlated dynamics in ultra-thin molecular glass films

Interfacial interactions can play an important role in the glass transition temperature (T _g ) and relaxation dynamics of ultra-thin glass polymer films. We have recently shown that similar to the polymeric systems in ultra-thin molecular glass films of N, N'-Bis(3-methylphenyl)-N, N'-diphenylbenzidine (TPD), the T _g is reduced and dynamics are enhanced. Furthermore, in molecular glass systems, as the film thickness is reduced below ∼30 nm, the dynamics at the two interfaces correlate such that the range of the gradients in the dynamics induced by the free surface narrows compared to thicker films. These observations indicate that the dynamics of the glassy thin films are strongly correlated and cannot be explained by a simple two-layer model consisting of a bulk and a thin interfacial layer with a constant thickness and constant range of dynamical gradients. Here, we investigate the effect of film/substrate interactions on the film dynamics by varying the TPD/substrate interfacial interactions. We show that thin TPD films with thicknesses below ∼60 nm show a smaller extent of T _g reduction and enhanced dynamics when supported on a near-neutral substrate (wetting) compared to a weakly interacting (dewetting) substrate. However, the ∼30 nm length scale, where the activation energy significantly reduces from its bulk value as measured by the onset of the glass transition remains unchanged. Coarse-grained molecular dynamics simulation also shows a narrowing in the range of relaxation times once the thickness is sufficiently reduced for the two interfaces to dynamically correlate, consistent with previous work. These results suggest that the length-scale for the correlated dynamics is independent of interfacial interactions and the polymeric nature of the film and may originate from the bulk glass properties.

Synergic Swelling of Interactive Network Support and Block Copolymer Films during Solvent Vapor Annealing

We report the effect of "interactive" polymer network (PN) supports on the solvent-vapor processing of thin polymer films. Densely cross-linked surface-attached network exhibits under experimental time scale a glassy swelling behavior with the conformational states and solvent-uptake clearly sensitive to the degree of solvent vapor saturation in the atmosphere. Pretreatment of the thermally cured PN films by complete immersion or by swelling in saturated chloroform vapors facilitates relaxation of the residual stresses and induces irreversible changes to the network structure as revealed by the swelling/deswelling tests. The presence of a polymer film on top of the PN support results in a mutual influence of the layers on the respective swelling kinetics, steady-state solvent uptake, and chain dynamics. Using UV-vis ellipsometry, we revealed a significantly faster swelling and higher solvent uptake of glassy PN layer below a polymer film as compared to a single PN layer on silicon substrate. Remarkably, the swelling of the network support continues to increase even when the overall swelling of the bilayer is in a steady-state regime. Block copolymer films on PN supports exhibit a faster ordering dynamics and exceptional stability toward dewetting as compared to similar films on silicon wafers. The mechanical stress produced by continuously swelling PN is suggested to account for the enhanced segmental dynamics even at low solvent concentration in the block copolymer film. Apart from novel insights into dynamics of solvent uptake by heterogeneous polymer films, these results might be useful in developing novel approaches toward fast-processing/annealing of functional polymer films and fibers.

Unexpected behavior of ultra-thin films of blends of polystyrene/poly(vinyl methyl ether) studied by specific heat spectroscopy

Specific heat spectroscopy (SHS) employing AC nanochip calorimetry was used to investigate the glassy dynamics of ultra-thin films (thicknesses: 10 nm-340 nm) of a polymer blend, which is miscible in the bulk. In detail, a Poly(vinyl methyl ether) (PVME)/Polystyrene (PS) blend with the composition of 25/75 wt. % was studied. The film thickness was controlled by ellipsometry while the film topography was checked by atomic force microscopy. The results are discussed in the framework of the balance between an adsorbed and a free surface layer on the glassy dynamics. By a self-assembling process, a layer with a reduced mobility is irreversibly adsorbed at the polymer/substrate interface. This layer is discussed employing two different scenarios. In the first approach, it is assumed that a PS-rich layer is adsorbed at the substrate. Whereas in the second approach, a PVME-rich layer is suggested to be formed at the SiO₂ substrate. Further, due to the lower surface tension of PVME, with respect to air, a nanometer thick PVME-rich surface layer, with higher molecular mobility, is formed at the polymer/air interface. By measuring the glassy dynamics of the thin films of PVME/PS in dependence on the film thickness, it was shown that down to 30 nm thicknesses, the dynamic T_g of the whole film was strongly influenced by the adsorbed layer yielding a systematic increase in the dynamic T_g with decreasing the film thickness. However, at a thickness of ca. 30 nm, the influence of the mobile surface layer becomes more pronounced. This results in a systematic decrease in T_g with the further decrease of the film thickness, below 30 nm. These results were discussed with respect to thin films of PVME/PS blend with a composition of 50/50 wt. % as well as literature results.

Tuning relaxation dynamics and mechanical properties of polymer films of identical thickness

Using dewetting as a characterization tool, we demonstrate that physical properties of thin polymer films can be regulated and tuned by employing variable processing conditions. For different molecular weights, the variable behavior of polystyrene films of identical thickness, prepared along systematically altered pathways, became predictable through a single parameter P, defined as the ratio of time required over time available for the equilibration of polymers. In particular, preparation-induced residual stresses, the corresponding relaxation times as well as the rupture probability of such films (of identical thickness) varied by orders of magnitude following scaling relations with P. Our experimental findings suggest that we can predictably enhance properties and hence maximize the performance of thin polymer films via appropriately chosen processing conditions.

Molecular Probe Diffusion in Thin Polymer Films: Evidence for a Layer with Enhanced Mobility Far above the Glass Temperature

We studied experimentally the influence of interfaces on the dynamics in thin polymer films at temperatures far above the glass temperature (T_g + 80 °C). Polyisoprene (PI) was employed as a model system. We examined glass substrate supported films with thicknesses (d) spanning the range from 10 μm to 10 nm that correspond to d/R_g from 400 to 1, where R_g is the polymer radius of gyration. We employed fluorescence correlation spectroscopy (FCS) to monitor the translational diffusion of small fluorescent tracer molecules, dispersed at nanomolar concentrations in the PI matrix. In thick films, a single diffusion process correlated to the bulk segmental dynamics of the matrix polymer was present. However, when the film thickness was smaller than the normal dimension of the FCS observation volume, a second, faster diffusion process appeared, reflecting enhanced segmental dynamics near the free surface. Our results provide direct experimental evidence for the existence of a layer with enhanced mobility near the free surface of supported PI films at temperatures as high as 80 °C above the bulk T_g.

Experimental investigation of the glass transition of polystyrene thin films in a broad frequency range

In this study, we investigate the α process of a polystyrene thin film using inelastic neutron scattering (INS), dielectric relaxation spectroscopy (DRS), and thermal expansion spectroscopy (TES). The DRS and TES measurements exhibited a decrease in glass transition temperature (T_{g}) with film thickness. On the other hand, an increase in T_{g} was observed in INS studies. In order to interpret this contradiction, we investigated the temperature dependence of the peak frequency (f_{m}) of the α process probed by DRS and TES. The experiments revealed an increase in the peak frequency (f_{m}) with decreasing film thickness in the frequency region. This observation is consistent with the observed decrease in T_{g} with thickness. Interestingly, the increase in T_{g} with film thickness was confirmed by fitting the temperature dependence measurements of the peak frequency with the Vogel-Fulcher-Tammann equation, within the frequency region probed by INS. The discrepancy between INS and DRS or TES descriptions of the α process is likely to be attributed to a decrease in the apparent activation energy with film thickness and reduced mobility, due to the impenetrable wall effect.

Unraveling the Dynamics of Nanoscopically Confined PVME in Thin Films of a Miscible PVME/PS Blend

Broadband dielectric spectroscopy (BDS) was employed to investigate the glassy dynamics of thin films (7-200 nm) of a poly(vinyl methyl ether) (PVME)/polystyrene (PS) blend (50:50 wt %). For BDS measurements, nanostructured capacitors (NSCs) were employed, where films are allowed a free surface. This method was applied for film thicknesses up to 36 nm. For thicker films, samples were prepared between crossed electrode capacitors (CECs). The relaxation spectra of the films showed multiple processes. The first process was assigned to the α-relaxation of a bulklike layer. For films measured by NSCs, the rates of α-relaxation were higher compared to those of the bulk blend. This behavior was related to the PVME-rich free surface layer at the polymer/air interface. The second process was observed for all films measured by CECs (process X) and the 36 nm film measured by NSCs (process X2). This process was assigned to fluctuations of constraint PVME segments by PS. Its activation energy was found to be thickness-dependent because of the evidenced thickness dependency of the compositional heterogeneity. Finally, a third process with an activated temperature dependence was observed for all films measured by NSCs (process X1). It resembled the molecular fluctuations in an adsorbed layer found for thin films of pure PVME, and thus, it is assigned accordingly. This process undergoes an extra confinement because of frozen adsorbed PS segments at the polymer/substrate interface. To our knowledge, this is the first example where confinement-induced changes were observed by BDS for blend thin films.

Decoupling of Dynamic and Thermal Glass Transition in Thin Films of a PVME/PS Blend

The discussions on the nanoconfinement effect on the glass transition and glassy dynamics phenomena have yielded many open questions. Here, the thickness dependence of the thermal glass transition temperature T_g^therm of thin films of a PVME/PS blend is investigated by ellipsometry. Its thickness dependence was compared to that of the dynamic glass transition (measured by specific heat spectroscopy) and the deduced Vogel temperature (T₀). While T_g^therm and T₀ showed a monotonous increase, with decreasing film thickness, the dynamic glass transition temperature (T_g^dyn) measured at a finite frequency showed a nonmonotonous dependence that peaks at 30 nm. This was discussed by assuming different cooperativity length scales at these temperatures, which have different sensitivities to composition and thickness. This nonmonotonous thickness dependence of T_g^dyn disappears for frequencies characteristic for T₀. Further analysis of the fragility parameter showed a change in the glassy dynamics from strong to fragile, with decreasing film thickness.

Irreversible Adsorption Governs the Equilibration of Thin Polymer Films

We demonstrate that the enhanced segmental motion commonly observed in spin cast thin polymer films is a nonequilibrium phenomenon. In the presence of nonrepulsive interfaces, prolonged annealing in the liquid state allows, in fact, recovering bulk segmental mobility. Our measurements prove that, while the fraction of unrelaxed chains increases upon nanoconfinement, the dynamics of equilibration is almost unaffected by the film thickness. We show that the rate of equilibration of nanoconfined chains does not depend on the structural relaxation process but on the feasibility to form an adsorbed layer. We propose that the equilibration of the thin polymer melts is driven by the slow relaxation of interfacial chains upon irreversible adsorption on the confining walls.

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.

Grafting density dominant glass transition of dry polystyrene brushes

The effects of the grafting densities (σ_p), molecular weights (M_n) and thicknesses of dry polystyrene (PS) brushes on their glass transition temperature (T) were investigated by ellipsometry. The results show that T strongly depends on the grafting density of the PS brushes. The T of the PS brushes with σ_p > 0.30 increases with decreasing M_n (or brush thickness) and is mainly dominated by entropic effects, in which the grafted chains are highly extended along the film thickness direction resulting in a sharp reduction in configurational entropy. The T of PS brushes with σ_p < 0.30 decreases with decreasing M_n (or brush thickness) which is mainly dominated by surface effects. For intermediate-density brushes (σ_p = 0.30), T becomes independent of M_n or brush thickness. The reason for this grafting density dependence of T is attributed to the transition of the PS brush conformation from mushroom-to-brush.

Unveiling the Dynamics of Self-Assembled Layers of Thin Films of Poly(vinyl methyl ether) (PVME) by Nanosized Relaxation Spectroscopy

A combination of nanosized dielectric relaxation (BDS) and thermal spectroscopy (SHS) was utilized to characterize the dynamics of thin films of poly(vinyl methyl ether) (PVME) (thicknesses: 7-160 nm). For the BDS measurements, a recently designed nanostructured electrode system is employed. A thin film is spin-coated on an ultraflat highly conductive silicon wafer serving as the bottom electrode. As top electrode, a highly conductive wafer with nonconducting nanostructured SiO₂ nanospacers with heights of 35 or 70 nm is assembled on the bottom electrode. This procedure results in thin supported films with a free polymer/air interface. The BDS measurements show two relaxation processes, which are analyzed unambiguously for thicknesses smaller than 50 nm. The relaxation rates of both processes have different temperature dependencies. One process coincides in its position and temperature dependence with the glassy dynamics of bulk PVME and is ascribed to the dynamic glass transition of a bulk-like layer in the middle of the film. The relaxation rates were found to be thickness independent as confirmed by SHS. Unexpectedly, the relaxation rates of the second process obey an Arrhenius-like temperature dependence. This process was not observed by SHS and was related to the constrained fluctuations in a layer, which is irreversibly adsorbed at the substrate with a heterogeneous structure. Its molecular fluctuations undergo a confinement effect resulting in the localization of the segmental dynamics. To our knowledge, this is the first report on the molecular dynamics of an adsorbed layer in thin films.

Glass transition of polymers in bulk, confined geometries, and near interfaces

When cooled or pressurized, polymer melts exhibit a tremendous reduction in molecular mobility. If the process is performed at a constant rate, the structural relaxation time of the liquid eventually exceeds the time allowed for equilibration. This brings the system out of equilibrium, and the liquid is operationally defined as a glass-a solid lacking long-range order. Despite almost 100 years of research on the (liquid/)glass transition, it is not yet clear which molecular mechanisms are responsible for the unique slow-down in molecular dynamics. In this review, we first introduce the reader to experimental methodologies, theories, and simulations of glassy polymer dynamics and vitrification. We then analyse the impact of connectivity, structure, and chain environment on molecular motion at the length scale of a few monomers, as well as how macromolecular architecture affects the glass transition of non-linear polymers. We then discuss a revised picture of nanoconfinement, going beyond a simple picture based on interfacial interactions and surface/volume ratio. Analysis of a large body of experimental evidence, results from molecular simulations, and predictions from theory supports, instead, a more complex framework where other parameters are relevant. We focus discussion specifically on local order, free volume, irreversible chain adsorption, the Debye-Waller factor of confined and confining media, chain rigidity, and the absolute value of the vitrification temperature. We end by highlighting the molecular origin of distributions in relaxation times and glass transition temperatures which exceed, by far, the size of a chain. Fast relaxation modes, almost universally present at the free surface between polymer and air, are also remarked upon. These modes relax at rates far larger than those characteristic of glassy dynamics in bulk. We speculate on how these may be a signature of unique relaxation processes occurring in confined or heterogeneous polymeric systems.

Examining dynamics in a polymer matrix by single molecule fluorescence probes of different sizes

Rotational motion of single fluorescence probes with different sizes doped inside films of polyvinylacetate is investigated by defocused single molecule fluorescence microscopy. Discrete vibration modes in the rotational motion of individual probes are clearly exposed in the power spectra of the rotation trajectories, reflecting multiple relaxation dynamics and also the difference in dynamics among separate locations. The power spectra show a strong dependence on the probe size and temperature. By sampling the rotation of a large number of probes, the averaged power spectra show that the activated rotation of the probes falls into the frequency range of the α-relaxation and data analysis shows that activation of the vibration modes with all probes corresponds to the maximum population of rotating probes, and this made the comparison between single molecule data and the ensemble data meaningful (differential scanning calorimetry, as an example). Such an analysis shows a coincidence between the temperature of a significant occurrence of glass transition and the activation of rotational motion of all probes with one specific size, indicating that such a dimension correlates with the size of the cooperative rearrangement region - a volume of approximately 1.0 nm(3).