Elastic Properties and Glass Transition of Supported Polymer Thin Films

Unusual High-Frequency Mechanical Properties of Polymer-Grafted Nanoparticle Melts

Brillouin light spectroscopy is used to measure the elastic moduli of spherical polymer-grafted nanoparticle (GNP) melts as a function of chain length at fixed grafting density (0.47  chains/nm^{2}) and nanoparticle radius (8 nm). While the moduli follow a rule of mixtures (Wood's law) for long chains, they display enhanced elasticity and anomalous dissipation for graft chains <100  kDa. GNP melts with long polymers at high σ have a dry zone near the GNP core, surrounded by a region where the grafts can interpenetrate with chain fragments from adjacent GNPs. We propose that the departures from Wood's law for short chains are due to the effectively larger silica volume fraction in the region where sound propagates-this is caused by the short, interpenetrated chain fragments being pushed out of the way. We thus conclude that transport mechanisms (of gas, ions, sound, thermal phonons) in GNP melts are radically different if interpenetrated chain segments can be "pushed out of the way" or not. This provides a facile new means for manipulating the properties of these materials.

Glass transition of nanometric polymer films probed by picosecond ultrasonics

The possibility to measure the glass transition temperature in poly(methyl methacrylate) (PMMA) films by picosecond ultrasonics with thicknesses ranging from 458 nm to 32 nm is demonstrated. A shift of the longitudinal acoustic eigenmodes towards lower frequencies with temperature is observed accompanied by a change in the temperature-frequency slopes at the glass transition temperature. The contributions to the frequency shift from changes in film thickness and sound velocity are discussed and the latter is extracted below the glass transition temperature. Finally, the advantages and disadvantages of the current approach in a comparison to other methods based on acoustic measurements in the GHz regime are reviewed.

A micro-vibration apparatus for dynamic mechanical analysis of ultrathin polymer films

We present the design and validation of an apparatus, which is developed based on micro-vibration in cyclic bulge and contraction, to determine dynamic mechanical properties of ultrathin polymer films. By controlling air pressure acting on a polymer membrane, the apparatus exerts simple harmonic or other type periodic stresses on the film, and the resultant real-time deformation of the freely standing film is recorded by using a high-speed CCD camera. From the image frame sequences and the gas pressure data, the real-time stress and strain of the polymer film are attained. Consequently, the dynamic mechanical properties, including biaxial storage and loss moduli, and loss factor can be determined for polymer films with thickness down to 20 nm. This apparatus could also be used to determine other mechanical properties such as fatigue and yield for nanoconfined films of soft matter.

Flash Brillouin Scattering: A Confocal Technique for Measuring Glass Transitions at High Scan Rates

Glass transition temperatures T _g are most commonly measured by differential scanning calorimetry, a method that has been extended to the flash scanning calorimetry (FSC) regime by reducing sample volumes. However, significant manual preparation effort can render FSC impractical for, e.g., local probing of spatially heterogeneous specimens. Another strategy can be to select a small volume by focusing down a laser beam, where Brillouin Light Scattering (BLS) is a proven method for confocal T _g measurement. Here, we introduce Flash Brillouin Scattering, which extends BLS to fast scan rates, achieved by periodically heating the probed region with an infrared laser. For comparison with conventional BLS, we first characterize T _g of pure glycerol, and show how rapid quenching produces a less packed glass with downshifted sound velocity. We then turn toward its aqueous solutions, which crystallize too fast for a nonflash approach, and demonstrate scan rates in excess of 10⁵ K/s. These results are of interest not only because glycerol is a model system for hydrogen-bonded glass formers, but also because of its applications as a cryoprotectant for frozen biological samples. Light scattering studies of the latter, currently limited to cryo-Raman spectroscopy, are likely to be complemented by the technique introduced here.

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.

The Glass-Transition Temperature of Supported PMMA Thin Films with Hydrogen Bond/Plasmonic Interface

The interfacial effect is one of the significant factors in the glass-transition temperature (T_g) of the polymeric thin film system, competing against the free surface effect. Herein, the T_gs of poly (methyl methacrylate) (PMMA) films with different thicknesses and substrates are studied by fluorescence measurements, focusing on the influence of interfacial effects on the T_gs. The strong interaction between PMMA and quartz substrate leads to increased T_gs with the decreased thickness of the film. The plasmonic silver substrate causes enhanced fluorescence intensity near the interface, resulting in the delayed reduction of the T_gs with the increasing film thickness. Moreover, as a proof of the interface-dependent T_gs, hydrogen bonds of PMMA/quartz and molecules orientation of PMMA/silver are explored by the Raman spectroscopy, and the interfacial interaction energy is calculated by the molecular dynamics simulation. In this study, we probe the inter-relationship between the interfacial interactions arising from the different substrates and the T_g behavior of polymer thin films.

Elastic wave propagation in smooth and wrinkled stratified polymer films

Periodic materials with sub-micrometer characteristic length scale can provide means for control of propagation of hypersonic phonons. In addition to propagation stopbands for the acoustic phonons, distinct dispersive modes can reveal specific thermal and mechanical behavior under confinement. Here, we employ both experimental and theoretical methods to characterize the phonon dispersion relation (frequency versus wave vector). We employed Brillouin light scattering (BLS) spectroscopy to record the phonon dispersion in stratified multilayer polymer films. These films consist of 4-128 alternate polycarbonate (PC) and poly (methyl methacrylate) (PMMA) layers along and normal to the periodicity direction. The distinct direction-dependent phonon propagation was theoretically accounted for, by considering the polarization, frequency and intensity of the observed modes in the BLS spectra. Layer-guiding was also supported by the glass transition temperatures of the PC and PMMA layers. The number of phonon dispersion branches increased with the number of layers but only a few branches were observable by BLS. Introduction of an additional in-plane periodicity, through a permanent wrinkling of the smooth PC/PMMA films, had only subtle consequences in the phonon propagation. Using the frequencies of the periodicity induced modes and momentum conservation equation we were able to precisely back calculate the wrinkle periodicity. However, a wrinkling-induced acoustic stopband utilizing flexible layered materials is still a challenge.

Direct observation of polymer surface mobility via nanoparticle vibrations

Measuring polymer surface dynamics remains a formidable challenge of critical importance to applications ranging from pressure-sensitive adhesives to nanopatterning, where interfacial mobility is key to performance. Here, we introduce a methodology of Brillouin light spectroscopy to reveal polymer surface mobility via nanoparticle vibrations. By measuring the temperature-dependent vibrational modes of polystyrene nanoparticles, we identify the glass-transition temperature and calculate the elastic modulus of individual nanoparticles as a function of particle size and chemistry. Evidence of surface mobility is inferred from the first observation of a softening temperature, where the temperature dependence of the fundamental vibrational frequency of the nanoparticles reverses slope below the glass-transition temperature. Beyond the fundamental vibrational modes given by the shape and elasticity of the nanoparticles, another mode, termed the interaction-induced mode, was found to be related to the active particle-particle adhesion and dependent on the thermal behavior of nanoparticles.

Elastic Modulus and Thermal Conductivity of Thiolene/TiO2 Nanocomposites

Metal oxide based polymer nanocomposites find diverse applications as functional materials, and in particular thiol-ene/TiO2 nanocomposites are promising candidates for dental restorative materials. The important mechanical and thermal properties of the nanocomposites, however, are still not well understood. In this study, the elastic modulus and thermal conductivity of thiol-ene/TiO2 nanocomposite thin films with varying weight fractions of TiO2 nanoparticles are investigated by using Brillouin light scattering spectroscopy and 3ω measurements, respectively. As the TiO2 weight fraction increases from 0 to 90%, the effective elastic longitudinal modulus of the films increases from 6.2 to 37.5 GPa, and the effective thermal conductivity from 0.04 to 0.76 W/m K. The former increase could be attributed to the covalent cross-linking of the nanocomposite constituents. The latter one could be ascribed to the addition of high thermal conductivity TiO2 nanoparticles and the formation of possible conductive channels at high TiO2 weight fractions. The linear dependence of the thermal conductivity on the sound velocity, reported for amorphous polymers, is not observed in the present nanocomposite system.

Thermomechanical Properties and Glass Dynamics of Polymer-Tethered Colloidal Particles and Films

Polymer-tethered colloidal particles (aka "particle brush materials") have attracted interest as a platform for innovative material technologies and as a model system to elucidate glass formation in complex structured media. In this contribution, Brillouin light scattering is used to sequentially evaluate the role of brush architecture on the dynamical properties of brush particles in both the individual and assembled (film) state. In the former state, the analysis reveals that brush-brush interactions as well as global chain relaxation sensitively depend on grafting density; i.e., more polymer-like behavior is observed in sparse brush systems. This is interpreted to be a consequence of more extensive chain entanglement. In contrast, the local relaxation of films does not depend on grafting density. The results highlight that relaxation processes in particle brush-based materials span a wider range of time and length scales as compared to linear chain polymers. Differentiation between relaxation on local and global scale is necessary to reveal the influence of molecular structure and connectivity on the aging behavior of these complex systems.

Directional elastic wave propagation in high-aspect-ratio photoresist gratings: liquid infiltration and aging

Determination of the mechanical properties of nanostructured soft materials and their composites in a quantitative manner is of great importance to improve the fidelity in their fabrication and to enable the subsequent reliable utility. Here, we report on the characterization of the elastic and photoelastic parameters of a periodic array of nanowalls (grating) by the non-invasive Brillouin light scattering technique and finite element calculations. The resolved elastic vibrational modes in high and low aspect ratio nanowalls reveal quantitative and qualitative differences related to the two-beam interference lithography fabrication and subsequent aging under ambient conditions. The phononic properties, namely the dispersion relations, can be drastically altered by changing the surrounding material of the nanowalls. Here we demonstrate that liquid infiltration turns the phononic function from a single-direction phonon-guiding to an anisotropic propagation along the two orthogonal directions. The susceptibility of the phononic behavior to the infiltrating liquid can be of unusual benefits, such as sensing and alteration of the materials under confinement.

Phoxonic Hybrid Superlattice

We studied experimentally and theoretically the direction-dependent elastic and electromagnetic wave propagation in a supported film of hybrid PMMA (poly[methyl-methacrylate])-TiO2 superlattice (SL). In the direction normal to the layers, this one-dimensional periodic structure opens propagation band gaps for both hypersonic (GHz) phonons and near-UV photons. The high mismatch of elastic and optical impedance results in a large dual phoxonic band gap. The presence of defects inherent to the spin-coating fabrication technique is sensitively manifested in the band gap region. Utilizing Brillouin light scattering, phonon propagation along the layers was observed to be distinctly different from propagation normal to them and can, under certain conditions (SL thickness and substrate elasticity), reveal the nanomechanical properties of the constituent layers. Besides the first realization of unidirectional phoxonic behavior, hybrid (soft-hard) periodic materials are a promising simple platform for opto-acoustic interactions and applications such as filters and Bragg mirrors.

Densification and depression in glass transition temperature in polystyrene thin films

Ellipsometry and X-ray reflectivity were used to characterize the mass density and the glass transition temperature of supported polystyrene (PS) thin films as a function of their thickness. By measuring the critical wave vector (qc) on the plateau of total external reflection, we evidence that PS films get denser in a confined state when the film thickness is below 50 nm. Refractive indices (n) and electron density profiles measurements confirm this statement. The density of a 6 nm (0.4 gyration radius, Rg) thick film is 30% greater than that of a 150 nm (10Rg) film. A depression of 25 °C in glass transition temperature (Tg) was revealed as the film thickness is reduced. In the context of the free volume theory, this result seems to be in apparent contradiction with the fact that thinner films are denser. However, as the thermal expansion of thinner films is found to be greater than the one of thicker films, the increase in free volume is larger for thin films when temperature is raised. Therefore, the free volume reaches a critical value at a lower Tg for thinner films. This critical value corresponds to the onset of large cooperative movements of polymer chains. The link between the densification of ultrathin films and the drop in their Tg is thus reconciled. We finally show that at their respective Tg(h) all films exhibit a critical mass density of about 1.05 g/cm(3) whatever their thickness. The thickness dependent thermal expansion related to the free volume is consequently a key factor to understand the drop in the Tg of ultrathin films.

Controlling the thermomechanical behavior of nanoparticle/polymer films

We show that the mesoscale (∼200 nm) thermomechanical properties of polymer nanocomposites formed from silica nanoparticles (NPs) and poly(2-vinylpyridine) (P2VP) critically depend on their interfacial structure, which can be controlled by the casting solvent. The composite films are solvent cast from either pyridine (PYR) or methylethylketone (MEK), with uniform NP spatial distribution obtained in both cases. In the films cast from MEK, our previous work has shown that a bound layer of P2VP is formed at the NP surfaces, while no such bound layer is formed when PYR is used as the casting solvent. In PYR as-cast films, Brillouin light scattering reveals a single acoustic phonon with its longitudinal sound velocity increasing with NP loading. This implies a homogeneous mixture of the NP and the polymer on the mesoscopic scales for all compositions examined. However, in the MEK as-cast films, two longitudinal and two transverse acoustic phonons are observed at NP loadings above ∼20 wt % (or ∼11 vol %), reminiscent of two metastable microscopic phases. The dense microphase is attributed to the bridging of NPs by P2VP chains, whereas for the softer medium, we conjecture that there exists an interfacial lower density P2VP layer whose longitudinal sound velocity barely changes with NP loading. These solvent-induced differences in the (elastic) mechanical behavior disappear upon thermal annealing, suggesting that these nanocomposite interfacial structures in the as-cast state (far from equilibrium) locally approach equilibrium (i.e., near equilibrium after annealing). Consistent with these conclusions, the abrupt decrease of the longitudinal sound velocity with temperature occurs at a single glass transition temperature for the annealed nanocomposites irrespective of the casting solvent used, which assumes only a slightly higher (∼5 K at 45 wt % or ∼29 vol %) value than that in bulk P2VP. The results emphasize the important role of solvent in determining the interfacial structure of nanocomposites, which can be used to tailor their thermomechanical behavior.

Micro-/nanostructured mechanical metamaterials

Mechanical properties of materials have long been one of the most fundamental and studied areas of materials science for a myriad of applications. Recently, mechanical metamaterials have been shown to possess extraordinary effective properties, such as negative dynamic modulus and/or density, phononic bandgaps, superior thermoelectric properties, and high specific energy absorption. To obtain such materials on appropriate length scales to enable novel mechanical devices, it is often necessary to effectively design and fabricate micro-/nano- structured materials. In this Review, various aspects of the micro-/nano-structured materials as mechanical metamaterials, potential tools for their multidimensional fabrication, and selected methods for their structural and performance characterization are described, as well as some prospects for the future developments in this exciting and emerging field.

Engineering the hypersonic phononic band gap of hybrid Bragg stacks

We report on the full control of phononic band diagrams for periodic stacks of alternating layers of poly(methyl methacrylate) and porous silica combining Brillouin light scattering spectroscopy and theoretical calculations. These structures exhibit large and robust on-axis band gaps determined by the longitudinal sound velocities, densities, and spacing ratio. A facile tuning of the gap width is realized at oblique incidence utilizing the vector nature of the elastic wave propagation. Off-axis propagation involves sagittal waves in the individual layers, allowing access to shear moduli at nanoscale. The full theoretical description discerns the most important features of the hypersonic one-dimensional crystals forward to a detailed understanding, a precondition to engineer dispersion relations in such structures.

Ultrastable nanostructured polymer glasses

Owing to the kinetic nature of the glass transition, the ability to significantly alter the properties of amorphous solids by the typical routes to the vitreous state is restricted. For instance, an order of magnitude change in the cooling rate merely modifies the value of the glass transition temperature (T(g)) by a few degrees. Here we show that matrix-assisted pulsed laser evaporation (MAPLE) can be used to form ultrastable and nanostructured glassy polymer films which, relative to the standard poly(methyl methacrylate) glass formed on cooling at standard rates, are 40% less dense, have a 40 K higher T(g), and exhibit a two orders of magnitude enhancement in kinetic stability at high temperatures. The unique set of properties of MAPLE-deposited glasses may make them attractive in technologies where weight and stability are central design issues.

Structural relaxation of polymer nanospheres under soft and hard confinement: isobaric versus isochoric conditions

We have measured the glassy-state structural relaxation of aqueous suspended polystyrene (PS) nanoparticles (the case of soft confinement) and the corresponding silica-capped PS nanoparticles (the case of hard confinement) via differential scanning calorimetry. Suspended and capped PS nanoparticles undergo physical aging under isobaric and isochoric conditions, respectively. With decreasing diameter, suspended and capped PS nanoparticles exhibited reduced and bulk glass transition temperatures (T(g)), respectively. To account for T(g) changes with confinement, all physical aging measurements were performed at a constant value of T(g) - T(a), where T(a) is the aging temperature. With decreasing diameter, aqueous suspended PS nanoparticles exhibited enhanced physical aging rates in comparison to bulk PS. Due to differences in thermodynamic conditions during aging and interfacial effects from nanoconfinement, at all values of T(g) - T(a) investigated, capped PS nanoparticles aged at reduced rates compared to the corresponding aqueous suspended PS nanoparticles. We captured the physical aging behavior of all nanoparticles via the Tool, Narayanaswamy, and Moynihan model of structural relaxation.

Anisotropic elasticity of quasi-one-component polymer nanocomposites

The in-plane and out-of-plane elastic properties of thin films of "quasi-one-component" particle-brush-based nanocomposites are compared to those of "classical" binary particle-polymer nanocomposite systems with near identical overall composition using Brillouin light scattering. Whereas phonon propagation is found to be independent of the propagation direction for the binary particle/polymer blend systems, a pronounced splitting of the phonon propagation velocity along the in-plane and out-of-plane film direction is observed for particle-brush systems. The anisotropic elastic properties of quasi-one-component particle-brush systems are interpreted as a consequence of substrate-induced order formation into layer-type structures and the associated breaking of the symmetry of the film. The results highlight new opportunities to engineer quasi-one-component nanocomposites with advanced control of structural and physical property characteristics based on the assembly of particle-brush materials.

Precision and accuracy in film stiffness measurement by Brillouin spectroscopy

The interest in the measurement of the elastic properties of thin films is witnessed by a number of new techniques being proposed. However, the precision of results is seldom assessed in detail. Brillouin spectroscopy (BS) is an established optical, contactless, non-destructive technique, which provides a full elastic characterization of bulk materials and thin films. In the present work, the whole process of measurement of the elastic moduli by BS is critically analyzed: experimental setup, data recording, calibration, and calculation of the elastic moduli. It is shown that combining BS with ellipsometry a fully optical characterization can be obtained. The key factors affecting uncertainty of the results are identified and discussed. A procedure is proposed to discriminate factors affecting the precision from those affecting the accuracy. By the characterization of a model transparent material, silica in bulk and film form, it is demonstrated that both precision and accuracy of the elastic moduli measured by BS can reach 1% range, qualifying BS as a reference technique.

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.

Elastic constants and dimensions of imprinted polymeric nanolines determined from Brillouin light scattering

Elastic constants and cross-sectional dimensions of imprinted nanolines of poly(methyl methacrylate) (PMMA) on silicon substrates are determined nondestructively from finite-element inversion analysis of dispersion curves of hypersonic acoustic modes of these nanolines measured with Brillouin light scattering. The results for the cross-sectional dimensions, under the simplifying assumption of vertical sides and a semicircular top, are found to be consistent with dimensions determined from critical-dimension small-angle x-ray scattering measurements. The elastic constants C(11) and C(44) are found to be, respectively, 11.6% and 3.1% lower than their corresponding values for bulk PMMA. This result is consistent with the dimensional dependence of the quasi-static Young's modulus determined from buckling measurements on PMMA films with lower molecular weights. This study provides the first evidence of size-dependent effects on hypersonic elastic properties of polymers.