Kari Dalnoki-Veress

Bio: Kari Dalnoki-Veress is an academic researcher from McMaster University. The author has contributed to research in topics: Glass transition & Capillary action. The author has an hindex of 38, co-authored 153 publications receiving 6563 citations. Previous affiliations of Kari Dalnoki-Veress include PSL Research University & Centre national de la recherche scientifique.

Effect of Free Surfaces on the Glass Transition Temperature of Thin Polymer Films.

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.

Interface and chain confinement effects on the glass transition temperature of thin polymer films

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 ${\mathrm{SiO}}_{x}.$

Molecular weight dependence of reductions in the glass transition temperature of thin, freely standing polymer films.

Abstract: We have used transmission ellipsometry to perform a comprehensive study of the glass transition temperature ${T}_{g}$ of freely standing polystyrene films. Six molecular weights ${M}_{w},$ ranging from $575\ifmmode\times\else\texttimes\fi{}{10}^{3}$ to $9100\ifmmode\times\else\texttimes\fi{}{10}^{3},$ were used in the study. For each ${M}_{w}$ value, large reductions in ${T}_{g}$ (as much as $80\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ below the bulk value) were observed as the film thickness h was decreased. We have studied in detail the dependence of the ${T}_{g}$ reductions on ${M}_{w}$ in a regime dominated by chain confinement effects. The empirical analysis presented is highly suggestive of the existence of a mechanism of mobility in thin freely standing films that is inhibited in the bulk and distinct from the usual cooperative motion associated with the glass transition.

Chain entanglement in thin freestanding polymer films.

Abstract: When a thin glassy film is strained uniaxially, a shear deformation zone (SDZ) can be observed. The ratio of the thickness of the SDZ to that of the undeformed film is related to the maximum extension ratio, lambda, which depends on the entanglement molecular weight, M(e). We have measured lambda as a function of film thickness in strained freestanding films of polystyrene as a probe of M(e) in confinement. It is found that thin films stretch further than thick films before failure, consistent with the interpretation that polymers in thin films are less entangled than bulk polymers, thus the effective value of M(e) in thin films is significantly larger than that of the bulk. Our results are well described by a conceptually simple model based on the probability of finding intermolecular entanglements near an interface.

Table Of Integrals Series And Products

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Linear systems

Abstract: Most of the signal processing that we will study in this course involves local operations on a signal, namely transforming the signal by applying linear combinations of values in the neighborhood of each sample point. You are familiar with such operations from Calculus, namely, taking derivatives and you are also familiar with this from optics namely blurring a signal. We will be looking at sampled signals only. Let's start with a few basic examples. Local difference Suppose we have a 1D image and we take the local difference of intensities, DI(x) = 1 2 (I(x + 1) − I(x − 1)) which give a discrete approximation to a partial derivative. (We compute this for each x in the image.) What is the effect of such a transformation? One key idea is that such a derivative would be useful for marking positions where the intensity changes. Such a change is called an edge. It is important to detect edges in images because they often mark locations at which object properties change. These can include changes in illumination along a surface due to a shadow boundary, or a material (pigment) change, or a change in depth as when one object ends and another begins. The computational problem of finding intensity edges in images is called edge detection. We could look for positions at which DI(x) has a large negative or positive value. Large positive values indicate an edge that goes from low to high intensity, and large negative values indicate an edge that goes from high to low intensity. Example Suppose the image consists of a single (slightly sloped) edge:

Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends.

Abstract: Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends