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J A Forrest - One of the best experts on this subject based on the ideXlab platform.
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free surfaces cause reductions in the Glass Transition Temperature of thin polystyrene films
Physical Review Letters, 2003Co-Authors: James S. Sharp, J A ForrestAbstract:The effect of free surfaces on the Glass Transition Temperature (T g ) of thin polystyrene films was studied. Measurements were performed on films (8 nm < h < 290 nm) with one free surface and on films capped with a 5 nm thick metal layer (no free surfaces). Potential problems with evaporative deposition were eliminated by studying samples made of two supported films placed with their free surfaces in contact and annealed. Uncapped films displayed reduced T g values for h ≤ 40 nm while all "properly" capped films exhibited a T g value the same as that of the bulk polymer (370 ′ 1 K). When the free surface was restored, the measured T g values the same as those of uncapped films of the same thickness. These results show that free surfaces are crucial for observing T g reductions in thin polymer films and address the role of the sample preparation history.
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molecular weight dependence of reductions in the Glass Transition Temperature of thin freely standing polymer films
Physical Review E, 2001Co-Authors: J A Forrest, Kari Dalnokiveress, Chris A Murray, Christian Gigault, John DutcherAbstract: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.
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interface and chain confinement effects on the Glass Transition Temperature of thin polymer films
Physical Review E, 1997Co-Authors: J A Forrest, Kari Dalnokiveress, John DutcherAbstract:We have used Brillouin light scattering and ellipsometry to measure the Glass Transition Temperature Tg 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 SiOx surface with the other film surface free ~uncapped supported!, and films supported on a SiOx surface and covered with a SiOx layer ~capped supported!. For freely standing films Tg is reduced dramatically from the bulk value by an amount that depends on both h and M w . For h&REE ~the average end-to-end distance of the unperturbed polymer molecules!, Tg 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 Tg 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, Tg is reduced only slightly (,10 K) from the bulk value, with only small differences in Tg (,4 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 SiOx . @S1063-651X~97!00711-3#
J A Forres - One of the best experts on this subject based on the ideXlab platform.
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free surfaces cause reductions in the Glass Transition Temperature of thin polystyrene films
Physical Review Letters, 2003Co-Authors: James S. Sharp, J A ForresAbstract:The effect of free surfaces on the Glass Transition Temperature (T(g)) of thin polystyrene films was studied. Measurements were performed on films (8 nm
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reductions of the Glass Transition Temperature in thin polymer films probing the length scale of cooperative dynamics
Physical Review E, 2000Co-Authors: J A Forres, Joha MattssoAbstract:We report measurements of the Glass Transition Temperature, T(g), in free standing polymer films in a low M(n) limit where chain confinement effects are not observed. The measured T(g) values are accurately described by a layer model incorporating a mobile surface layer with a size determined by the length scale of cooperative dynamics. The analysis leads to a surface T(g) value and length scale of cooperative motion near bulk T(g) which quantitatively agree with independently determined values. The model and parameters provide a framework within which all previous measurements of T(g) values in thin supported films may be understood and provides values for the length scale of cooperative motion over an extended range of Temperatures below the bulk T(g) value.
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interface and chain confinement effects on the Glass Transition Temperature of thin polymer films
Physical Review E, 1997Co-Authors: J A Forres, Kari Dalnokiveress, Joh DutcheAbstract:We have used Brillouin light scattering and ellipsometry to measure the Glass Transition Temperature Tg 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 SiOx surface with the other film surface free ~uncapped supported!, and films supported on a SiOx surface and covered with a SiOx layer ~capped supported!. For freely standing films Tg is reduced dramatically from the bulk value by an amount that depends on both h and M w . For h&REE ~the average end-to-end distance of the unperturbed polymer molecules!, Tg 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 Tg 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, Tg is reduced only slightly (,10 K) from the bulk value, with only small differences in Tg (,4 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 SiOx . @S1063-651X~97!00711-3#
James S. Sharp - One of the best experts on this subject based on the ideXlab platform.
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free surfaces cause reductions in the Glass Transition Temperature of thin polystyrene films
Physical Review Letters, 2003Co-Authors: James S. Sharp, J A ForrestAbstract:The effect of free surfaces on the Glass Transition Temperature (T g ) of thin polystyrene films was studied. Measurements were performed on films (8 nm < h < 290 nm) with one free surface and on films capped with a 5 nm thick metal layer (no free surfaces). Potential problems with evaporative deposition were eliminated by studying samples made of two supported films placed with their free surfaces in contact and annealed. Uncapped films displayed reduced T g values for h ≤ 40 nm while all "properly" capped films exhibited a T g value the same as that of the bulk polymer (370 ′ 1 K). When the free surface was restored, the measured T g values the same as those of uncapped films of the same thickness. These results show that free surfaces are crucial for observing T g reductions in thin polymer films and address the role of the sample preparation history.
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free surfaces cause reductions in the Glass Transition Temperature of thin polystyrene films
Physical Review Letters, 2003Co-Authors: James S. Sharp, J A ForresAbstract:The effect of free surfaces on the Glass Transition Temperature (T(g)) of thin polystyrene films was studied. Measurements were performed on films (8 nm
Bhesh Bhandari - One of the best experts on this subject based on the ideXlab platform.
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effect of protein concentration on the surface composition water sorption and Glass Transition Temperature of spray dried skim milk powders
Food Chemistry, 2007Co-Authors: Ashok K Shrestha, Barry J Wood, T Howes, Benu Adhikari, Bhesh BhandariAbstract:The effects of dilution of protein content in skim milk (34-8.5% protein content), by lactose addition. on the surface composition, water sorption property and Glass Transition Temperatures of spray-dried powders were investigated. The X-ray photoelectron spectroscopy (XPS) study of spray-dried powders showed preferential migration of proteins toward the surface of the milk particles whereas the lactose remained in the bulk. Sorption studies showed that the lower protein concentration in milk powders is linked to an increased water adsorption property and lowering of water activity (a(w)) for lactose crystallization. Analysis of Glass Transition Temperature (T-g) of the powders sorbed at different humidities showed no distinct change in T-g values, indicating the dominant effect of lactose on the Glass Transition Temperature of all the powders. (c) 2007 Elsevier Ltd. All rights reserved.
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optimization of co current spray drying process of sugar rich foods part i moisture and Glass Transition Temperature profile during drying
Journal of Food Engineering, 2005Co-Authors: Vinh Truong, Bhesh Bhandari, T HowesAbstract:A steady state mathematical model for co-current spray drying was developed for sugar-rich foods with the application of the Glass Transition Temperature concept. Maltodextrin-sucrose solution was used as a sugar-rich food model. The model included mass, heat and momentum balances for a single droplet drying as well as Temperature and humidity profile of the drying medium. A log-normal volume distribution of the droplets was generated at the exit of the rotary atomizer. This generation created a certain number of bins to form a system of non-linear first-order differential equations as a function of the axial distance of the drying chamber. The model was used to calculate the changes of droplet diameter, density, Temperature, moisture content and velocity in association with the change of air properties along the axial distance. The difference between the outlet air Temperature and the Glass Transition Temperature of the final products (AT) was considered as an indicator of stickiness of the particles in spray drying process. The calculated and experimental AT values were close, indicating successful validation of the model. (c) 2004 Elsevier Ltd. All rights reserved.
T Howes - One of the best experts on this subject based on the ideXlab platform.
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effect of protein concentration on the surface composition water sorption and Glass Transition Temperature of spray dried skim milk powders
Food Chemistry, 2007Co-Authors: Ashok K Shrestha, Barry J Wood, T Howes, Benu Adhikari, Bhesh BhandariAbstract:The effects of dilution of protein content in skim milk (34-8.5% protein content), by lactose addition. on the surface composition, water sorption property and Glass Transition Temperatures of spray-dried powders were investigated. The X-ray photoelectron spectroscopy (XPS) study of spray-dried powders showed preferential migration of proteins toward the surface of the milk particles whereas the lactose remained in the bulk. Sorption studies showed that the lower protein concentration in milk powders is linked to an increased water adsorption property and lowering of water activity (a(w)) for lactose crystallization. Analysis of Glass Transition Temperature (T-g) of the powders sorbed at different humidities showed no distinct change in T-g values, indicating the dominant effect of lactose on the Glass Transition Temperature of all the powders. (c) 2007 Elsevier Ltd. All rights reserved.
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optimization of co current spray drying process of sugar rich foods part i moisture and Glass Transition Temperature profile during drying
Journal of Food Engineering, 2005Co-Authors: Vinh Truong, Bhesh Bhandari, T HowesAbstract:A steady state mathematical model for co-current spray drying was developed for sugar-rich foods with the application of the Glass Transition Temperature concept. Maltodextrin-sucrose solution was used as a sugar-rich food model. The model included mass, heat and momentum balances for a single droplet drying as well as Temperature and humidity profile of the drying medium. A log-normal volume distribution of the droplets was generated at the exit of the rotary atomizer. This generation created a certain number of bins to form a system of non-linear first-order differential equations as a function of the axial distance of the drying chamber. The model was used to calculate the changes of droplet diameter, density, Temperature, moisture content and velocity in association with the change of air properties along the axial distance. The difference between the outlet air Temperature and the Glass Transition Temperature of the final products (AT) was considered as an indicator of stickiness of the particles in spray drying process. The calculated and experimental AT values were close, indicating successful validation of the model. (c) 2004 Elsevier Ltd. All rights reserved.