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John M Torkelson - One of the best experts on this subject based on the ideXlab platform.
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suppression of the fragility Confinement Effect via low molecular weight cyclic or ring polymer topology
Macromolecules, 2017Co-Authors: Lanhe Zhang, Ravinder Elupula, Scott M Grayson, John M TorkelsonAbstract:We used differential scanning calorimetry and spectroscopic ellipsometry to measure the molecular weight (MW) dependence of bulk fragility (mbulk) and spectroscopic ellipsometry to measure the thickness dependences of the glass transition temperature (Tg) and fragility (m) in supported thin films of low MW cyclic or ring polymer. The Effects of Confinement on Tg and m of thin polymer films are important in a range of advanced technology applications, including nanoimprinting. It has previously been shown that nanoconfined films of high MW linear polystyrene (PS) exhibit major Tg- and m-Confinement Effects whereas films of low MW cyclic PS (c-PS) show at most a very weak Tg-Confinement Effect. In the absence of chain ends, c-PS exhibits very weak Tg,bulk– and mbulk–MW dependences compared to linear PS. Despite low MW c-PS having mbulk values similar to that of high MW linear PS, we found that low MW c-PS films show a very weak m-Confinement Effect because of a weak free-surface Effect; e.g., m for a 27 nm ...
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enhanced tg Confinement Effect in cross linked polystyrene compared to its linear precursor roles of fragility and chain architecture
Macromolecules, 2016Co-Authors: John M TorkelsonAbstract:The glass transition temperature (Tg) of cross-linked polystyrene (PS) is directly compared to that of its linear precursor polystyrene-co-vinylbenzocyclobutene (PS–VBCB) in bulk and confined states. The VBCB units incorporated into a linear PS precursor cross-link with one another upon annealing at 250 °C. Bulk Tg (Tg,bulk) is characterized by differential scanning calorimetry (DSC) and ellipsometry, with Tg,bulk and Tg breadth increasing with increased cross-linking. The Tg-Confinement Effect is characterized by ellipsometry in supported PS–VBCB films before and after cross-linking; Tg decreases with decreasing nanoscale thickness in both supported linear and cross-linked polymer films. The magnitude of the Confinement Effect is greater in cross-linked PS compared to linear precursors; e.g., with PS-VBCB containing 8.5 mol % VBCB, Tg – Tg,bulk = −2 to −3 °C for a 23 nm thick film of linear polymer whereas Tg – Tg,bulk = −8 to −9 °C after cross-linking. The larger Tg reduction upon Confinement in cross-l...
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tuning the tg Confinement Effect in thin polymer films via minute levels of residual surfactant which cap the free surface
Polymer, 2016Co-Authors: Lawrence Chen, John M TorkelsonAbstract:Abstract We compare the Effect of Confinement on the glass transition temperature ( T g ) of ultrathin supported films of polystyrene produced via emulsion polymerization (E-PS) and anionic polymerization (A-PS). Using spectroscopic ellipsometry and fluorescence to characterize T g , we find that E-PS films supported on silica exhibit T g -Confinement Effects that are suppressed and even eliminated when compared to A-PS films on silica. We prepared E-PS samples with varying dodecyl sulfate surfactant concentration made via sequential purification. Using Epton’s Method, we determined the amount of surfactant present in bulk unwashed E-PS sample and in several washed samples. Films made from E-PS containing as little as 0.023 wt% surfactant show elimination of the T g -Confinement Effect within error down to a thickness of 14 nm. Films made from E-PS containing 0.0058 wt% surfactant show suppression of the T g -Confinement Effect relative to that observed by both ellipsometry and fluorescence in A-PS films. Using X-ray photoelectron spectroscopy, we observe that the sulfur atom in the surfactant is localized at the top 2–4 nm of the film next to the polymer-air interface. The residual surfactant left from emulsion polymerization at or very near the free surface restricts the perturbation of T g by the free surface, thereby suppressing and even eliminating the T g -Confinement Effect in ultrathin E-PS films relative to that in A-PS films.
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fragility Confinement Effects apparent universality as a function of scaled thickness in films of freely deposited linear polymer and its absence in densely grafted brushes
Macromolecules, 2016Co-Authors: Tian Lan, John M TorkelsonAbstract:Ellipsometry measurements as a function of cooling rate are used to study nanoscale Confinement Effects on dynamic fragility (kinetic fragility), m, in supported films of freely deposited, linear polymer. Polymers include neat polystyrene (PS), neat polycarbonate (PC), and PS + 2 wt % 1,10-bis(1-pyrene)decane (BPD) as small-molecule diluent; in each case, the substrate/polymer interface lacks significant attractive interactions. In terms of both the length scale at which Confinement Effects become evident and the percentage reduction in m from its bulk value, the magnitude of the m-Confinement Effect increases with increasing bulk polymer system m. Additionally, for films of linear polymer lacking significant attractive interactions with the substrate surface, m-Confinement Effects are evident at larger onset thicknesses than those commonly reported in the literature for the glass transition temperature (Tg)-Confinement Effect. Evans et al. [Macromolecules 2013, 46, 6091] found that the Tg-Confinement eff...
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Enhanced Tg‑Confinement Effect in Cross-Linked Polystyrene Compared to Its Linear Precursor: Roles of Fragility and Chain Architecture
2016Co-Authors: Kailong Jin, John M TorkelsonAbstract:The glass transition temperature (Tg) of cross-linked polystyrene (PS) is directly compared to that of its linear precursor polystyrene-co-vinylbenzocyclobutene (PS–VBCB) in bulk and confined states. The VBCB units incorporated into a linear PS precursor cross-link with one another upon annealing at 250 °C. Bulk Tg (Tg,bulk) is characterized by differential scanning calorimetry (DSC) and ellipsometry, with Tg,bulk and Tg breadth increasing with increased cross-linking. The Tg-Confinement Effect is characterized by ellipsometry in supported PS–VBCB films before and after cross-linking; Tg decreases with decreasing nanoscale thickness in both supported linear and cross-linked polymer films. The magnitude of the Confinement Effect is greater in cross-linked PS compared to linear precursors; e.g., with PS-VBCB containing 8.5 mol % VBCB, Tg – Tg,bulk = −2 to −3 °C for a 23 nm thick film of linear polymer whereas Tg – Tg,bulk = −8 to −9 °C after cross-linking. The larger Tg reduction upon Confinement in cross-linked PS is correlated with increased bulk fragility after cross-linking as measured by DSC and ellipsometry. Neat linear PS–VBCB copolymers provide an extension to lower fragility of the correlation between the Tg-Confinement Effect and bulk fragility observed previously [Evans Macromolecules 2013, 46, 6091] for neat linear polymers lacking attractive polymer–substrate interactions; i.e., the strength of the Tg-Confinement Effect increases with increasing bulk fragility. Both cross-linked PS and doped PS films deviate from the relationship for neat linear polymers, the former being weaker and the latter stronger. These results indicate that chain architecture and dopant content modify the relationship between the strength of the Tg-Confinement Effect and bulk fragility observed in many neat, linear polymers
Alberto Anedda - One of the best experts on this subject based on the ideXlab platform.
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phonon Confinement Effect in mixed sn ir oxide nanocrystals
Chemical Physics Letters, 2010Co-Authors: M Salis, Pier Carlo Ricci, G Cappelletti, Alberto AneddaAbstract:Abstract Raman spectra in Sn 1− x Ir x O 2 ( x ⩽ 0.15) nanocrystals were investigated to estimate and separate the contributions of phonon Confinement and composition Effects. To this end, the Raman frequency of the A 1 g bulk-mode of pure SnO 2 and Ir-mixed nanocrystals has been studied in details as a function of the size and the Iridium concentration. As an initial working hypothesis it was assumed a linear dependence of the Raman shift as a function of the Iridium molar concentration, while a quadratic expansion of the dispersion curve has been utilised in the phonon Confinement Effect. Comparison of predicted and experimental results is reported.
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phonon Confinement Effect in calcium fluoride nanoparticles
Chemical Physics Letters, 2007Co-Authors: Pier Carlo Ricci, Alberto Casu, Giovanni De Giudici, P Scardi, Alberto AneddaAbstract:Abstract Nanocrystalline CaF 2 were obtained by ball-milled synthetic fluorite powders. Raman spectra of nanocrystals with average sizes of 10–25 nm are reported and the correlation between the band shape of the only allowed Raman mode at 322 cm −1 and the crystals dimensions is discussed. While in such systems local temperature and pressure Effects play a minor role, variations in the shape of the Raman band can be correctly reproduced by a model based on the phonon Confinement that Effectively takes into account the anisotropy of the phonon dispersion curve.
Shun Ito - One of the best experts on this subject based on the ideXlab platform.
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phonon Confinement in silicon nanowires synthesized by laser ablation
Physica B-condensed Matter, 2006Co-Authors: Naoki Fukata, T Oshima, Naoya Okada, Tokushi Kizuka, Takao Tsurui, Shun Ito, Kouichi MurakamiAbstract:Abstract The phonon Confinement Effect was investigated by Raman measurements for Si nanowires (SiNWs) synthesized by laser ablation of a Si target with nickel (Ni) catalyst and for SiNWs thermally oxidized at 700–1000 °C. The Si optical phonon peak for SiNWs, unlike that for bulk Si, showed a downshift and an asymmetric broadening. Thermal oxidation caused a further downshift and broadening. These phenomena can be explained by the phonon Confinement Effect due to the decrease in the diameter of the Si core of the SiNWs. It was additionally found that excess oxidation caused an upshift of the optical phonon peak due to compressive stress.
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phonon Confinement Effect of silicon nanowires synthesized by laser ablation
Applied Physics Letters, 2005Co-Authors: Naoki Fukata, T Oshima, Tokushi Kizuka, Takao Tsurui, Kouichi Murakami, Shun ItoAbstract:A gradual downshift and asymmetric broadening of the Si optical phonon peak were observed by Raman scattering measurements of continuously thermally oxidized silicon nanowires (SiNWs) synthesized by laser ablation. This downshift and broadening can be interpreted by the phonon Confinement Effect. Further thermal oxidation produced a reverse change; namely, an upshift of the optical phonon peak. This is considered to be due to compressive stress since this stress was relieved by removing the oxide layers formed around the SiNW cores, resulting in a downshift of the optical phonon peak.
Cheol Jong Choi - One of the best experts on this subject based on the ideXlab platform.
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quantum Confinement Effect of silicon nanocrystals in situ grown in silicon nitride films
Applied Physics Letters, 2004Co-Authors: Tae Youb Kim, Tae Yeo Seong, Nae M Park, Kyunghyu Kim, Gun Yong Sung, Cheol Jong ChoiAbstract:Silicon nanocrystals were in situ grown in a silicon nitride film by plasma-enhanced chemical vapor deposition. The size and structure of silicon nanocrystals were confirmed by high-resolution transmission electron microscopy. Depending on the size, the photoluminescence of silicon nanocrystals can be tuned from the near infrared (1.38eV) to the ultraviolet (3.02eV). The fitted photoluminescence peak energy as E(eV)=1.16+11.8∕d2 is evidence for the quantum Confinement Effect in silicon nanocrystals. The results demonstrate that the band gap of silicon nanocrystals embedded in silicon nitride matrix was more Effectively controlled for a wide range of luminescent wavelengths.
Naoki Fukata - One of the best experts on this subject based on the ideXlab platform.
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phonon Confinement in silicon nanowires synthesized by laser ablation
Physica B-condensed Matter, 2006Co-Authors: Naoki Fukata, T Oshima, Naoya Okada, Tokushi Kizuka, Takao Tsurui, Shun Ito, Kouichi MurakamiAbstract:Abstract The phonon Confinement Effect was investigated by Raman measurements for Si nanowires (SiNWs) synthesized by laser ablation of a Si target with nickel (Ni) catalyst and for SiNWs thermally oxidized at 700–1000 °C. The Si optical phonon peak for SiNWs, unlike that for bulk Si, showed a downshift and an asymmetric broadening. Thermal oxidation caused a further downshift and broadening. These phenomena can be explained by the phonon Confinement Effect due to the decrease in the diameter of the Si core of the SiNWs. It was additionally found that excess oxidation caused an upshift of the optical phonon peak due to compressive stress.
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phonon Confinement Effect of silicon nanowires synthesized by laser ablation
Applied Physics Letters, 2005Co-Authors: Naoki Fukata, T Oshima, Tokushi Kizuka, Takao Tsurui, Kouichi Murakami, Shun ItoAbstract:A gradual downshift and asymmetric broadening of the Si optical phonon peak were observed by Raman scattering measurements of continuously thermally oxidized silicon nanowires (SiNWs) synthesized by laser ablation. This downshift and broadening can be interpreted by the phonon Confinement Effect. Further thermal oxidation produced a reverse change; namely, an upshift of the optical phonon peak. This is considered to be due to compressive stress since this stress was relieved by removing the oxide layers formed around the SiNW cores, resulting in a downshift of the optical phonon peak.