In this article, the effects of size and confinement at the nanometre size scale on both the melting temperature, Tm, and the glass transition temperature, Tg, are reviewed. Although there is an accepted thermodynamic model (the Gibbs–Thomson equation) for explaining the shift in the first-order transition, Tm, for confined materials, the depression of the melting point is still not fully understood and clearly requires further investigation. However, the main thrust of the work is a review of the field of confinement and size effects on the glass transition temperature. We present in detail the dynamic, thermodynamic and pseudo-thermodynamic measurements reported for the glass transition in confined geometries for both small molecules confined in nanopores and for ultrathin polymer films. We survey the observations that show that the glass transition temperature decreases, increases, remains the same or even disappears depending upon details of the experimental (or molecular simulation) conditions. Indeed, different behaviours have been observed for the same material depending on the experimental methods used. It seems that the existing theories of Tg are unable to explain the range of behaviours seen at the nanometre size scale, in part because the glass transition phenomenon itself is not fully understood. Importantly, here we conclude that the vast majority of the experiments have been carried out carefully and the results are reproducible. What is currently lacking appears to be an overall view, which accounts for the range of observations. The field seems to be experimentally and empirically driven rather than responding to major theoretical developments.

https://iopscience.iop.org/article/10.1088/0953-8984/17/15/R01/pdf

Effects of confinement on material behaviour at the nanometre size scale

The glass transition in thin polymer films

Spin-coating is widely employed for the highly reproducible fabrication of thin film coatings over large areas with high structural uniformity. Research in recent years has extended the scope of spin-coating by chemically engineering the interface of support and solution to obtain specific structural order in the resulting supported thin films. This review will discuss both the fundamental physical and chemical processes governing the conventional spin-coating process and describe methodologies for the preparation of spin-coated polymer thin films. Furthermore, a range of advanced applications and recent developments within this field will be reviewed with focus on engineering chemical and topological structure during the coating process.

6 Studies of spin-coated polymer films

Secondary structure and elevated temperature crystallite morphology of nylon-6/layered silicate nanocomposites

In this Letter we propose the use of hypersonic phononic crystals to control the emission and propagation of high frequency phonons. We report the fabrication of high quality, single crystalline hypersonic crystals using interference lithography and show that direct measurement of their phononic band structure is possible with Brillouin light scattering. Numerical calculations are employed to explain the nature of the observed propagation modes. This work lays the foundation for experimental studies of hypersonic crystals and, more generally, phonon-dependent processes in nanostructures.

/pdf/hypersonic-phononic-crystals-p7jphcieat.pdf

Hypersonic phononic crystals.

Neutron reflectivity measurements on polystyrene thin films (6.5–79.0 nm thick) supported on silicon substrates indicate that the mass density is near the bulk value regardless of film thickness. To account for possible inaccuracies arising from sample misalignment, reflectivity measurements were made both from the free-surface and silicon-substrate sides of the thin film, a method termed twin reflectivity. For films spin coated on the hydrogen-terminated silicon surface the relative uncertainty in the density measurement was on the order of 1%, but for films spin coated onto the silicon native-oxide surface the analysis was more difficult because of subtleties in data fitting due to the oxide layer. Nevertheless, within the limits of greater uncertainty, these films also showed no systematic change in density with thickness.

Mass density of polystyrene thin films measured by twin neutron reflectivity

Water absorption in thin films (<1000 A) of a commercial polyimide was evaluated by monitoring dimensional changes induced by a humid environment. Film thickness was measured using x-ray reflectivity, which is a nondestructive technique offering angstrom resolution in the measurements of thin film or multilayer thickness. The effect of several variables on the absorption of moisture were monitored in polyimide films adhered to polished silicon substrates, including total dry film thickness, exposure time, and the contribution ofa coupling agent. The percentage increase in film thickness due to moisture uptake is found to be a weak function of dry film thickness, decreasing as dry film thickness increases, and to be somewhat affected by the use of an interfacial coupling agent. The observed behavior points to the polymer/substrate interface as a strong factor controlling the absorption of moisture in the polyimide/ silicon system, and is believed to reflect the presence of a highly moisture-saturated interfacial layer. A bilayer model is proposed, and the feasibility of using this model to describe the observed behavior is considered.

Interface effects on moisture absorption in ultrathin polyimide films

Phonon spectra of polymeric linear nanostructures have been characterized using Brillouin light scattering. In addition to phonon modes similar to those present in uniform thin films, the phonon spectra of the nanolines reveal a new mode with a lower frequency that depends on the width of the nanolines. Classic wave theory and finite element analysis were combined to identify this new mode as a flexural vibration of the nanolines. Analysis of the phonon spectra gave estimates of elastic constants in the nanostructures and indicated that there is no significant deviation from bulk mechanical properties and no mechanical anisotropy in structures as small as 88nm.

http://absimage.aps.org/image/MAR06/MWS_MAR06-2005-001900.pdf

Acoustic modes and elastic properties of polymeric nanostructures

For a range of applications, polymers are now being patterned into nanometer-sized features. In these applications, the robust mechanical properties of the nanostructures are critical for performance and stability. Brillouin light scattering is presented as a nondestructive, noncontact tool used to quantify the elastic constants in such nanostructures. We demonstrate this through a series of thin films and parallel ridges and spacings (gratings) with ridge widths ranging from 180 to 80 nm. For the set of films and structures presented here, the room-temperature elastic moduli did not change with decreasing film thickness or grating ridge width, and this implied that one-dimensional and two-dimensional confinement-induced changes of the mechanical properties were not significant down to feature sizes of 80 nm. Additionally, Brillouin spectra of submicrometer gratings revealed new modes not present in the spectra of thin films. The origin of these new modes remains unclear. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1106–1113, 2004

/pdf/brillouin-scattering-studies-of-polymeric-nanostructures-1b9rythu49.pdf

Brillouin scattering studies of polymeric nanostructures

We used x-ray reflectivity in combination with optical ellipsometry to measure the optical index of refraction, n, in thin spun cast polystyrene films. We have found that n is independent of the molecular weight, but is a sharp function of the film thickness for films less than 100 nm. In all cases the deviation from the bulk, Δn, is negative and varies linearly with wavelength in the visible region. The magnitude of Δn, was found to be as large as 0.25 for films 7 nm thick. The bulk index of refraction was recovered in all films after annealing for2habove Tg at 160 °C. X-ray reflectivity measurements of the scattering critical angle show minimal density deviations from the bulk (less than 0.5%) between the annealed and unannealed films. Consequently the large molecular-weight-independent value of Δn is interpreted as being due to a radially symmetric segmental orientation induced by the spinning process.

Wen-Li Wu

Papers

Mass density of polystyrene thin films measured by twin neutron reflectivity

Interface effects on moisture absorption in ultrathin polyimide films

Acoustic modes and elastic properties of polymeric nanostructures

Brillouin scattering studies of polymeric nanostructures

Anomalies in the Optical Index of Refraction of Spun Cast Polystyrene Thin Films