| 1. |
- Algers, John, et al.
(författare)
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Characterization of free volume and density gradients of polystyrene surfaces by low-energy positron lifetime measurements
- 2004
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Ingår i: Polymer. - : Elsevier BV. - 0032-3861. ; 45:13, s. 4533-4539
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Tidskriftsartikel (refereegranskat)abstract
- Free volume and density gradient widths of the free surface of high molecular mass polystyrene was characterized by use of a pulsed low-energy positron lifetime beam. A density gradient in terms of mass density (g/cm(3)) versus depth (nm) was obtained from the experimental beam data using a novel approach, yielding a width of the density gradient of approximately 3.5 nm. The procedure accounted for the broadening of the positron implantation probe as a function of energy as well as correcting for the increase in positron implantation depths due to the presence of a lower density at the surface region. Moreover, the spectra in the low implantation energy range were found to contain a long-lived lifetime that yielded very large estimates of the free volume effects at the surface. Accounting for this lifetime in the evaluation procedure, resulted in a much improved agreement with the results of other experimental and theoretical investigations in the literature. (C) 2004 Elsevier Ltd. All rights reserved.
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| 2. |
- Algers, John, et al.
(författare)
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Free volume and density gradients of amorphous polymer surfaces as determined by use of a pulsed low-energy positron lifetime beam and PVT data
- 2004
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Ingår i: Macromolecules. - : American Chemical Society (ACS). - 0024-9297 .- 1520-5835. ; 37:11, s. 4201-4210
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Tidskriftsartikel (refereegranskat)abstract
- Density gradient widths at the polymer-vacuum surfaces of poly(methyl methacrylate), styrene-co-acrylonitrile, and styrene-co-maleic anhydride were quantified to 2, 5, and 4 rim, respectively, by use of a pulsed low-energy positron lifetime beam and to approximately 1.5 nm for all three polymers on the basis of theoretical predictions from pressure-volume-temperature (PVT) data, making use of the Cahn-Hilliard theory of inhomogeneous systems in conjunction with the Sanchez-Lacombe lattice fluid theory. Excellent agreement between the two methods was found for the homopolymer, whereas for the copolymers, the former method gave larger density gradient widths, a result attributed to the surface orientation of the less polar polymer segments, which the theoretical predictions did not take into account. As has been previously proposed, the discrepancy between the depth ranges of the surface effects on density and the glass transition temperature (T-g) is suggested to result from a coupling between the dynamics of adjoining polymer segments, canceling a direct relationship between local density profile rho(z) and local T-g(z) as a function of distance z from the free surface.
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| 3. |
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| 4. |
- Algers, John
(författare)
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Free Volume Depth Profiling at Polymer Surfaces and Interfaces
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Pulsed low energy positron beams provide a unique and non-destructive tool to depth probe the free volume of polymers at surfaces and interfaces. Since the employment of low energy positron beams in polymer research is an emerging technique, some focus in this thesis has been on extracting methods and equations needed to interpret the experimental data. The polymers chosen for this study were selected on the basis of having previously well characterized free volume properties. This enabled interpretation of the positron beam data in more general quantities, thus permitting the results to be compared with other techniques and theoretical considerations of polymer surfaces and interfaces. The median positron implantation depth and the implantation depth distribution as a function of implantation energy have been studied in some detail. It was shown that the implantation characteristics are independent of the structure of the materials, and that the median implantation depth for the studied polymers, as well as those of some previously studied metals, can be described with the same equation. Furthermore, the free surfaces of polystyrene (PS), polymethyl methacrylate (PMMA), styrene-co-acrylonitrile (SAN) and styrene-co-maleic anhydride (SMA) were studied by measurements in the low implantation energy range. Linear density gradients could be fitted to the experimental data, yielding gradient widths in the order of a few nanometers. This was in good or reasonable agreement with theoretically derived density gradients. The discrepancy with the theory found for the copolymers SAN and SMA was suggested to arise from surface orientation driven by the difference in polarity between the molecular constituents. The free volume at a PMMA-PS interface was studied. By the preparation of samples with the interface located only 25 nm from the free surface, the sensitivity of the experimental data to properties of interface region was greatly increased in relation to previous investigations. A reduction of the free volume was found in the PMMA in the 20 nm region closest to the PS surface. This finding suggests that the PS surface constitutes a relatively attractive environment for the PMMA. By guiding UV-light into the positron beam, various measurements relevant for photo-responsive polymers were enabled. Free volume determination of an azobenzene-PMMA copolymer during isomerization was carried out at various temperatures, showing that the isomerization did not influence the free volume. The excited cis-azobenzene was however found to be an inhibitor for positronium annihilation, which enabled the isomerization kinetics of azobenzene to be probed with positrons.
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| 5. |
- Algers, John, et al.
(författare)
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Free volume determination of azobenzene-PMMA copolymer by a pulsed low-energy positron lifetime beam with in-situ UV illumination
- 2004
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Ingår i: Macromolecules. - : American Chemical Society (ACS). - 0024-9297 .- 1520-5835. ; 37:21, s. 8035-8042
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Tidskriftsartikel (refereegranskat)abstract
- The free volume properties of a poly(methyl methacrylate)-azobenzene copolymer were for the first time directly probed by use of a low-energy positron lifetime beam with in-situ excitation capabilities, showing that the free volume cavity size was not appreciably influenced by photoisomerization and thermal isomerization in the temperature range 34-180 degreesC. lsomerization is therefore suggested to occur without any molecular rearrangement of the glassy polymer matrix, which would also account for the lack of any shift to shorter wavelengths for the photoisomerization occurring in a glassy polymer in comparison to a chloroform solution. A decrease in the thermal isomerization rate at room temperature caused by the glassy polymer is explained in terms of a model in which only a fraction of the azobenzene is free to isomerize. The cis-azobenzene was found to be an efficient inhibitor of positronium formation, which enabled measurements of thermal isomerization rates and changes in the steady-state concentration of cis-azobenzene for an illuminated sample as a function of temperature.
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| 6. |
- Algers, John, et al.
(författare)
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Median Implantation Depth and Implantation Profile of 3-18 keV Positrons in Amorphous Polymers.
- 2003
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Ingår i: Physical Review B. - 1550-235X. ; 67:12, s. 1-125404
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Tidskriftsartikel (refereegranskat)abstract
- Most applications of positron beams require knowledge of the implantation characteristics for an appropriate interpretation of the experimental data. In this work, the median implantation depth as a function of implantation energy, z1/2(E), of 3–18 keV positrons and their implantation profile P(z,E) in a total of 13 thin films of atactic polystyrene, poly(styrene-co-acrylonitrile), and polymethylmethacrylate spin coated onto a silicon substrate were determined from positron lifetime measurements using a pulsed, low-energy positron beam. z1/2(E) and P(z,E) were determined from the measurement of the ortho-positronium yield obtained from the intensity I3 of the long lifetime. z1/2(E) was parametrized with the commonly used power-law fit z1/2(E) = (A/)En, with and E in units of g cm–3 and keV, respectively, yielding A = 2.81(±0.2) µg cm–2 and n = 1.71(±0.05). Excellent agreement between amorphous polymer and literature data on Al and Cu suggests that the median implantation depth of positrons for low- to medium-Z materials in the studied energy range is independent of structure and only a function of mass density. Fitting of the Makhovian implantation profile to the experimental data suggested that the value of the parameter m varies between 1.7 and 2.3, systematically increasing with z at constant implantation energy, but is independent of the implantation energy. Using an equation proposed by Baker et al., the experimental data of 12 of the 13 studied polymer films could be described with a slightly better agreement than the Makhovian equation. ©2003 The American Physical Society
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| 7. |
- Algers, John, et al.
(författare)
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Thermal isomerization of azo-benzene in PMMA probed by a pulsed low-energy positron beam
- 2004
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Ingår i: Materials Science Forum. - 0255-5476. ; 445-446, s. 262-264
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Konferensbidrag (refereegranskat)abstract
- Polymers containing the Azo-benzene group have attracted attention due to anticipated uses such as materials for data storage, membranes with controllable permeability and materials with changeable solubility [1,2]. These expectations are due to the change in shape and dipole moment of the Azo-compound when excited by photo isomerization from the trans-state to the cis-state by UV light of ca 360 nm. Films of copolymers of poly-methyl methacrylate (PMMA) containing 8 wt-% Azo-benzene were photo isomerized with a UV lamp and then measured as a function of time in a lowenergy positron beam. The observed changes in the lifetime parameters as a function of time were in agreement with the thermal isomerization (decay) of the cis-isomer as measured by conventional UV spectroscopy. The results show that positron lifetime spectra at low energy are sensitive for the cis-trans isomerization of the Azo-compound, suggesting trapping and annihilation of positrons at free volume sites around the Azo-benzene groups.
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| 8. |
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