We report here the swelling and relaxation properties of confined poly(n-butyl methacrylate) (PBMA) films having thicknesses of less than 70 nm under supercritical carbon dioxide (scCO2) using the X-ray reflectivity technique. Swellability is found to be dominant in thinner films compared to thicker ones as a consequence of the confinement-induced densification of the former. Swellability is proportionately increased with the density of the film. PBMA films exhibit a more significant swelling than do PS films, and their differences become more prominent with the increase in film thickness. A comparison between the results obtained for polystyrene (PS) and PBMA ultrathin films reveals that the swellability is dependent upon the specific intermolecular interaction between CO2 and the chemical groups available in the polymers. Owing to strong Lewis acid-base interactions with scCO2 and the lower glass-transition temperature (bulk Tg ≈ 29 °C), PBMA films exhibit a greater amount of swelling than do PS films (bulk Tg ≈ 100 °C). Though they reach to the different swollen state upon exposition, identical relaxation behavior as a function of aging time is evidenced. This unprecedented behavior can be ascribed to the strong bonding between trapped CO2 and PBMA that probably impedes the release of CO2 molecules from the swollen PBMA films manifested in suppressed relaxation. (Graph Presented). © 2016 American Chemical Society.

J K BAL

G VIGNAUD

I AHMED

Y GROHENS

University of Calcutta (informally known as Calcutta University or CU) is a collegiate public state university located in Kolkata, West Bengal, India.&nbsp;Within India it is recognized as a &quot;Five-Star University&quot; and accredited &quot;A&quot; Grade by National Assessment and Accreditation Council.

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The University of Calcutta has secured the Fifth position among the Universities of India in the prestigious &quot;Indian University Ranking 2019&quot; list, released by the NIRF of the Ministry of Human Resource Development, Government of India.

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It was established on 24 January 1857, and was one of the first institutions in Asia to be established as a multidisciplinary and Western-style university. Its alumni and faculty include several heads of state, heads of government, social reformers, prominent artists, only academy award winner and Dirac medal winner in India, many Fellows of the Royal Society and five Nobel laureates as of 2019 which is highest in South Asia.

University Of Calcutta

Langmuir

The influence of the sample preparation on the observed dependence of glass transition temperature (T-g) with thickness in polystyrene (PS) thin films has been investigated. Spectroscopic ellipsometry was used to measure the T-g of PS thin films ranging from 7 to 85 nm before and after exposition to supercritical carbon dioxide (sc-CO2). We take advantage of the sc-CO2 properties along with thermal annealing treatment to cancel out sample preparation history factor by removing the remnant solvent and residual stresses. The decrease in T-g shows exactly the same trend before and after sc-CO2 exposure plus thermal treatment. This result cancels out the influence of the spin coating process on the T-g depression in thin films. (C) 2017 Elsevier Ltd. All rights reserved.

Polymer

Reversibility in glass transition behavior after erasing stress induced by spin coating process

After more than 2 decades of intense research, the density variation in confined polymer films still remains a puzzling problem subject to controversy as the methods utilized to determine the density are often model dependent. Here, we propose a direct and model independent method to detect the density/refractive index variations in polymer thin films through the adsorption of ceria nanoparticles (NPs) onto their surface. The amount of adsorbed NP scales with the polymer film refractive index; hence, any increase/decrease in the NP surface coverage directly indicates an increase/decrease in the film refractive index and density. Experimenting our proposed novel approach on two well-studied polymers, we found that the density of polystyrene (PS) thin films deposited on oxide-free Si substrate increases with a reduction of the film thickness. On the contrary, poly(methyl methacrylate) (PMMA) films deposited on wafers with native silicon oxide show a decrease of their density when the film thickness is reduced. © 2017 American Chemical Society.

Macromolecules

Probing the Density Variation of Confined Polymer Thin Films via Simple Model-Independent Nanoparticle Adsorption

In polymer physics, the dewetting of spin-coated polystyrene ultrathin films on silicon remains mysterious. By adopting a simple top-down method based on good solvent rinsing, we are able to prepare flat polystyrene films with a controlled thickness ranging from 1.3 to 7.0 nm. Their stability was scrutinized after a classical annealing procedure above the glass transition temperature. Films were found to be stable on oxide-free silicon irrespective of film thickness, while they were unstable (<2.9 nm) and metastable (>2.9 nm) on 2 nm oxide-covered silicon substrates. The Lifshitz-van der Waals intermolecular theory that predicts the domains of stability as a function of the film thickness and of the substrate nature is now fully reconciled with our experimental observations. We surmise that this reconciliation is due to the good solvent rinsing procedure that removes the residual stress and/or the density variation of the polystyrene films inhibiting thermodynamically the dewetting on oxide-free silicon. (Graph Presented). © 2015 American Chemical Society.

ACS Nano

Stability of Polymer Ultrathin Films (<7 nm) Made by a Top-Down Approach

Ellipsometry and X-ray reflectivity were used to characterize the mass density and the glass transition temperature of supported polystyrene (PS) thin films as a function of their thickness. By measuring the critical wave vector (qc) on the plateau of total external reflection, we evidence that PS films get denser in a confined state when the film thickness is below 50 nm. Refractive indices (n) and electron density profiles measurements confirm this statement. The density of a 6 nm (0.4 gyration radius, Rg) thick film is 30% greater than that of a 150 nm (10Rg) film. A depression of 25 °C in glass transition temperature (Tg) was revealed as the film thickness is reduced. In the context of the free volume theory, this result seems to be in apparent contradiction with the fact that thinner films are denser. However, as the thermal expansion of thinner films is found to be greater than the one of thicker films, the increase in free volume is larger for thin films when temperature is raised. Therefore, the free volume reaches a critical value at a lower Tg for thinner films. This critical value corresponds to the onset of large cooperative movements of polymer chains. The link between the densification of ultrathin films and the drop in their Tg is thus reconciled. We finally show that at their respective Tg(h) all films exhibit a critical mass density of about 1.05 g/cm3 whatever their thickness. The thickness dependent thermal expansion related to the free volume is consequently a key factor to understand the drop in the Tg of ultrathin films. © 2014 American Chemical Society.

Densification and depression in glass transition temperature in polystyrene thin films

Relaxation of highly swollen (∼20-80%) ultrathin (≤21 nm) polystyrene films exposed to supercritical carbon dioxide (at 35 °C and 80 bar) is studied by monitoring the evolution of thickness as a function of time using X-ray reflectivity. Correlations between their swelling (that determines the departure from equilibrium), depressurization time, thickness relaxation rate and aging time are established. Swollen films that are depressurized very slowly (depressurization time ∼1 h) exhibit a small swelling and a very low relaxation rate at ambient condition, whereas fast depressurized films (depressurization time ∼2 min) show a large swelling and a high relaxation rate. The rate of depressurization not only impacts their out of equilibrium state but also their relaxation rate. The relaxation time is essentially governed by the temperature following an Arrhenius law below Tg and a strong temperature-dependent behavior above Tg and by the relative departure from equilibrium. © 2014 American Chemical Society.

Journal	Data powered by TypesetLangmuir
Publisher	Data powered by TypesetAmerican Chemical Society
ISSN	0743-7463