The plastic in microplastics: A review.

Despite the decade-long study of the effect of nanoconfinement on the glass-transition temperature (Tg) of amorphous materials, the quest to probe the distribution of Tgs in nanoconfined glass formers has remained unfulfilled. Here the distribution of Tgs across polystyrene films has been obtained by a fluorescence/multilayer method, revealing that the enhancement of dynamics at a surface affects Tg several tens of nanometres into the film. The extent to which dynamics smoothly transition from enhanced to bulk states depends strongly on nanoconfinement. When polymer films are sufficiently thin that a reduction in thickness leads to a reduction in overall Tg, the surface-layer Tg actually increases with a reduction in overall thickness, whereas the substrate-layer Tg decreases. These results indicate that the gradient in Tg dynamics is not abrupt, and that the size of a cooperatively rearranging region is much smaller than the distance over which interfacial effects propagate.

The distribution of glass-transition temperatures in nanoscopically confined glass formers

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 main objective of this review is to describe some of the important topics related to the use of marine and protective coatings for anticorrosive purposes. In this context, “protective” refers to coatings for containers, offshore constructions, wind turbines, storage tanks, bridges, rail cars, and petrochemical plants while “marine” refers to coatings for ballast tanks, cargo holds and cargo tanks, decks, and engine rooms on ships. The review aims at providing a thorough picture of state-of-the-art in anticorrosive coatings systems. International and national legislation aiming at reducing the emission of volatile organic compounds (VOCs) have caused significant changes in the anticorrosive coating industry. The requirement for new VOC-compliant coating technologies means that coating manufacturers can no longer rely on the extensive track record of their time-served products to convince consumers of their suitability for use. An important aspect in the development of new VOC-compliant, high-performance anticorrosive coating systems is a thorough knowledge of the components in anticorrosive coatings, their interactions, their advantages and limitations, as well as a detailed knowledge on the failure modes of anticorrosive coatings. This review, which mainly deals with European experience and practice, includes a description of the different environments an anticorrosive coating system may encounter during service. In addition, examples of test methods and standards for determination of the performance and durability of anticorrosive coatings have been included. The different types of anticorrosive coatings are presented, and the most widely applied generic types of binders and pigments in anticorrosive coatings are listed and described. Furthermore, the protective mechanisms of barrier, sacrificial, and inhibitive coatings are outlined. In the past decades, several alternatives to organic solvent-borne coatings have reached the commercial market. This review also presents some of these technologies and discusses some of their advantages and limitations. Finally, some of the mechanisms leading to degradation and failure of organic coating systems are described, and the reported types of adhesion loss are discussed.

Anticorrosive coatings: a review

The thermomechanical responses of polymers, which provide limitations to their practical use, are favourably altered by the addition of trace amounts of a nanofiller. However, the resulting changes in polymer properties are poorly understood, primarily due to the non-uniform spatial distribution of nanoparticles. Here we show that the thermomechanical properties of ‘polymer nanocomposites’ are quantitatively equivalent to the well-documented case of planar polymer films. We quantify this equivalence by drawing a direct analogy between film thickness and an appropriate experimental interparticle spacing. We show that the changes in glass-transition temperature with decreasing interparticle spacing for two filler surface treatments are quantitatively equivalent to the corresponding thin-film data with a non-wetting and a wetting polymer–particle interface. Our results offer new insights into the role of confinement on the glass transition, and we conclude that the mere presence of regions of modified mobility in the vicinity of the particle surfaces, that is, a simple two-layer model, is insufficient to explain our results. Rather, we conjecture that the glass-transition process requires that the interphase regions surrounding different particles interact.

/pdf/quantitative-equivalence-between-polymer-nanocomposites-and-45k6t8s4rs.pdf

Quantitative equivalence between polymer nanocomposites and thin polymer films

We have measured the Doppler broadening energy spectra of positron annihilation using a variable monoenergetic positron beam as a function of positron energy and of temperature in polystyrene. The $S$ parameters from the energy spectra versus temperature show that onset temperatures significantly decrease as the positron energy decreases. This result gives direct evidence that the glass-transition temperature is suppressed at a magnitude of 57 \ifmmode^\circ\else\textdegree\fi{}C at the depth of 50 \AA{} from the surface.

Glass transition of polystyrene near the surface studied by slow-positron-annihilation spectroscopy

Positron annihilation lifetime and Doppler broadening of annihilation radiation experiments are performed in a polyurethane film using the mono-energetic slow positron probe as a function of positron energies. Significant variations of positron annihilation signals are observed at a short distance from the surface . The ortho-positronium lifetime in the polymer increases near the surface, while its intensity decreases. The intensity results are consistent with the description of a free-volume hole model for positronium formation proposed by Brandt, Berko and Walker. This study further confirms that positron annihilation spectroscopy is a sensitive probe for the characterization of physical properties of sub-nanometre defects, such as free volumes and holes for polymeric materials.

http://iopscience.iop.org/article/10.1088/0953-8984/10/46/011/pdf

Free-volume hole model for positronium formation in polymers: surface studies

The degradation of polymer coating systems due to UV irradiation is studied using positron annihilation spectroscopy. Doppler broadened spectra of positron annihilation are measured as a function of slow positron implantation energy in a series of polymer coating systems which were exposed to UV irradiation for up to 2000 h. The S parameters from the Doppler broadened energy spectra vs positron energy show an increase at very low positron energy (<575 eV) and then a decrease to about 8 keV, followed by a slight increase up to 50 keV. The UV irradiation systematically decreases the S parameter as a function of exposure duration. The significant S parameter decrease is interpreted as the degradation of polymers and the change of the sub-nanometer defect profiles due to UV irradiation. These interpretations are supported by the data from the atomic force microscope, UV absorption, Fourier transform infrared, and profilometry measurements.

Degradation of Polymer Coating Systems Studied by Positron Annihilation Spectroscopy. 1. UV Irradiation Effect

Development of positron annihilation spectroscopy to test accelerated weathering of protective polymer coatings

Photodegradation of a polyurethane coating induced by accelerated UV irradiation is studied using three different light sources:  340 nm UVA, 313 nm UVB, and Xe lamps. Doppler-broadened energy spectra (DBES) and positron annihilation lifetime (PAL) are measured as a function of incident positron energy (0−30 keV) and of irradiation time (0−800 h). The photodegradation of the coating is characterized in terms of subnanometer defect changes. Significant variations of S parameter, orthopositronium lifetime, intensity, and lifetime distribution are observed as a function of depth and of exposure time. These results indicate a decrease of free volume and holes at the atomic level during the degradation process. The UV irradiation at 313 nm shows a significant effect compared with that at 340 nm on the photodegradation: (1) the magnitude of S reduction increases a factor of 2 near the surface; (2) the photodegradation rate increases ∼25 times near the surface; (3) the irradiation effect attenuates with depth, a...

R. Zhang

Papers

Glass transition of polystyrene near the surface studied by slow-positron-annihilation spectroscopy

Free-volume hole model for positronium formation in polymers: surface studies

Degradation of Polymer Coating Systems Studied by Positron Annihilation Spectroscopy. 1. UV Irradiation Effect

Development of positron annihilation spectroscopy to test accelerated weathering of protective polymer coatings

Degradation of Polymer Coating Systems Studied by Positron Annihilation Spectroscopy. 3. Wavelength Dependence of UV Irradiation Effect