Author
James A. Forrest
Other affiliations: University of Waterloo, PSL Research University, University of Guelph ...read more
Bio: James A. Forrest is an academic researcher from Perimeter Institute for Theoretical Physics. The author has contributed to research in topics: Glass transition & Thin film. The author has an hindex of 40, co-authored 110 publications receiving 7958 citations. Previous affiliations of James A. Forrest include University of Waterloo & PSL Research University.
Topics: Glass transition, Thin film, Polystyrene, Polymer, Contact lens
Papers published on a yearly basis
Papers
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TL;DR: The concept of cooperative rearrangement was introduced in an attempt to unify these two views of the glass transition by demonstrating that such cooperativity, coupled with a thermodynamic glass transition, resulted naturally insystem dynamics such as those described by the WLFequation for temperatures near freezing.
Abstract: We report the first measurements of the glass transition temperature ${T}_{g}$ for thin freely standing polystyrene (PS) films. We have used Brillouin light scattering to measure ${T}_{g}$ for freely standing films of different thicknesses. We find that ${T}_{g}$ decreases linearly with film thickness $h$ for $h\ensuremath{\le}700\AA{}$, with a reduction of 70 K for a film with $h\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}290\AA{}$. These measurements characterize unambiguously the effects of the free surface on ${T}_{g}$ of thin polymer films. Results are compared to similar results for supported PS films [Keddie et al., Europhys. Lett. 27, 59 (1994)], and we find that their measured values are influenced strongly by the substrate.
1,052 citations
TL;DR: In this article, a detailed account of important recent developments in the rapidly evolving area of glass transitions in thin polymer films is presented. But the case of polymer films supported by substrates, and a definite experimental consensus exists.
747 citations
TL;DR: In this paper, Brillouin light scattering and ellipsometry were used to measure the glass transition temperature of thin polystyrene (PS) films as a function of the film thickness for two different molecular weights.
Abstract: We have used Brillouin light scattering and ellipsometry to measure the glass transition temperature ${T}_{g}$ of thin polystyrene (PS) films as a function of the film thickness $h$ for two different molecular weights ${M}_{w}.$ Three different film geometries were studied: freely standing films, films supported on a ${\mathrm{SiO}}_{x}$ surface with the other film surface free (uncapped supported), and films supported on a ${\mathrm{SiO}}_{x}$ surface and covered with a ${\mathrm{SiO}}_{x}$ layer (capped supported). For freely standing films ${T}_{g}$ is reduced dramatically from the bulk value by an amount that depends on both $h$ and ${M}_{w}.$ For $h\ensuremath{\lesssim}{R}_{\mathrm{EE}}$ (the average end-to-end distance of the unperturbed polymer molecules), ${T}_{g}$ decreases linearly with decreasing $h$ with reductions as large as 60 K for both ${M}_{w}$ values. We observe a large ${M}_{w}$ dependence of the ${T}_{g}$ reductions for freely standing films which provides the first strong evidence of the importance of chain confinement effects on the glass transition temperature of thin polymer films. For both the uncapped and capped supported films, ${T}_{g}$ is reduced only slightly $(l10\mathrm{K})$ from the bulk value, with only small differences in ${T}_{g}$ $(l4\mathrm{K})$ observed between uncapped and capped supported films of the same thickness. The results of our experiments demonstrate that the polymer-substrate interaction is the dominant effect in determining the glass transition temperature of PS films supported on ${\mathrm{SiO}}_{x}.$
745 citations
TL;DR: By partially embedding and then removing gold nanospheres, a high surface coverage of well-defined nanodeformations on a polystyrene surface is produced, providing strong direct evidence for enhanced surface mobility relative to the bulk.
Abstract: The motion of polymer chain segments cooled below the glass transition temperature slows markedly; with sufficient cooling, segmental motion becomes completely arrested. There is debate as to whether the chain segments near the free surface, or in thin films, are affected in the same way as the bulk material. By partially embedding and then removing gold nanospheres, we produced a high surface coverage of well-defined nanodeformations on a polystyrene surface; to probe the surface dynamics, we measured the time-dependent relaxation of these surface deformations as a function of temperature from 277 to 369 kelvin. Surface relaxation was observed at all temperatures, providing strong direct evidence for enhanced surface mobility relative to the bulk. The deviation from bulk α relaxation became more pronounced as the temperature was decreased below the bulk glass transition temperature. The temperature dependence of the relaxation time was much weaker than that of the bulk α relaxation of polystyrene, and the process exhibited no discernible temperature dependence between 277 and 307 kelvin.
586 citations
TL;DR: In this article, the authors discuss recent experiments that directly measure mobility at or near the surface of glassy polymers and indicate that enhanced mobility near the free surface can exceed bulk mobility by several orders of magnitude and extend for several nanometers into the bulk polymer.
Abstract: The past 20 years have seen a substantial effort to understand dynamics and the glass transition in thin polymer films. In this Perspective, we consider developments in this field and offer a consistent interpretation of some major findings. We discuss recent experiments that directly measure mobility at or near the surface of glassy polymers. These experiments indicate that enhanced mobility near the free surface can exceed bulk mobility by several orders of magnitude and extend for several nanometers into the bulk polymer. Enhanced mobility near the free surface allows a qualitative understanding of many of the observations of a reduced glass transition temperature Tg in thin films. For thin films, knowledge of Tg by itself is less useful than for bulk materials. Because of this, new experimental methods that directly measure important material properties are being developed.
437 citations
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TL;DR: Polylactic acid is proving to be a viable alternative to petrochemical-based plastics for many applications It is produced from renewable resources and is biodegradable, decomposing to give H2O, CO2, and humus, the black material in soil as mentioned in this paper.
Abstract: Polylactic acid is proving to be a viable alternative to petrochemical-based plastics for many applications It is produced from renewable resources and is biodegradable, decomposing to give H2O, CO2, and humus, the black material in soil In addition, it has unique physical properties that make it useful in diverse applications including paper coating, fibers, films, and packaging (see Figure)
2,537 citations
TL;DR: Recent research on nanoparticle physicochemical properties important for protein binding, techniques for isolation and identification of nanoparticle-bound proteins, and how these proteins can influence particle biodistribution and biocompatibility are reviewed.
1,535 citations
TL;DR: This paper proposes a morphology evolution that consists of an initial crystallization of P3HT chains, followed by diffusion of PCBM molecules to nucleation sites, at which aggregates ofPCBM then grow.
Abstract: Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends
1,438 citations
TL;DR: In this review recent achievements and new perspectives on protein adsorption processes are comprehensively discussed and the main focus is put on commonly postulated mechanistic aspects and their translation into mathematical concepts and model descriptions.
1,328 citations
TL;DR: Protein Nanoparticle Interactions: Opportunities and Challenges
Abstract: Protein Nanoparticle Interactions: Opportunities and Challenges Morteza Mahmoudi,* Iseult Lynch, Mohammad Reza Ejtehadi, Marco P. Monopoli, Francesca Baldelli Bombelli, and Sophie Laurent National Cell Bank, Pasteur Institute of Iran, Tehran, Iran Nanotechnology Research Center, Faculty of Pharmacy, Tehran University of Medical Sciences, Tehran, Iran School of Chemistry and Chemical Biology & Conway Institute for Biomolecular and Biomedical Research, University College Dublin, Belfield, Dublin 4, Ireland Department of Physics, Sharif University of Technology, Tehran, Iran School of Pharmacy, UEA, Norwich Research Park, Norwich,U.K. Department of General, Organic, and Biomedical Chemistry, NMR and Molecular Imaging Laboratory, University of Mons, Avenue Maistriau 19, B-7000 Mons, Belgium
1,214 citations