While the density is a central property of a polymer film, it can be difficult to measure in films with a thickness of ∼100 nm or less, where the structure of the interfaces and the confinement of the polymer chains may perturb the packing and dynamics of the polymers relative to the bulk. This Article demonstrates the use of magneto-Archimedes levitation (MagLev) to estimate the density of thin films of hydrophobic polymers ranging from ∼10 to 1000 nm in thickness by employing a substrate with a water-soluble sacrificial release layer to delaminate the films. We validate the performance of MagLev for this application in the ∼1 μm thickness range by comparing measurements of the densities of several different films of amorphous hydrophobic polymers with their bulk values of density. We apply the technique to films < 100 nm and observe that, in several polymers, there are substantial changes in the levitation height, corresponding to both increases and decreases in the apparent density of the film. These apparent changes in density are verified with a buoyancy control experiment in the absence of paramagnetic ions and magnetic fields. We measure the dependence of density upon thickness for two model polymeric films: poly(styrene) (PS) and poly(methyl methacrylate) (PMMA). We observe that, as the films are made thinner, PS increases in density while PMMA decreases in density and that both exhibit a sigmoidal dependence of density with thickness. Such changes in density with thickness of PS have been previously observed with reflectometric measurements (e.g., ellipsometry, X-ray reflectivity). The interpretation of these measurements, however, has been the subject of an ongoing debate. MagLev is also compatible with nontransparent, rough, heterogeneous polymeric films, which are extremely difficult to measure by alternative means. This technique could be useful to investigate the properties of thin films for coatings, electronic devices, and membrane-based separations and other uses of polymer films.
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Polymers under nanoconfinement: where are we now in understanding local property changes?
Polymers are increasingly being used in applications with nanostructured morphologies where almost all polymer molecules are within a few tens to hundreds of nanometers from some interface. From nearly three decades of study on polymers in simplified nanoconfined systems such as thin films, we have come to understand property changes in these systems as arising from interfacial effects where local dynamical perturbations are propagated deeper into the material. This review provides a summary of local glass transition temperature T g changes near interfaces, comparing across different types of interfaces: free surface, substrate, liquid, and polymer–polymer. Local versus film-average properties in thin films are discussed, making connections to other related property changes, while highlighting several historically important studies. By experimental necessity, most studies are on high enough molecule weight chains to be well entangled, although aspects that connect to lower molecule weight materials are described. Emphasis is made to identify observations and open questions that have yet to be fully understood such as the evidence of long-ranged interfacial effects, finite domain size, interfacial breadth, and chain connectivity.
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- NSF-PAR ID:
- 10313842
- Date Published:
- Journal Name:
- Chemical Society Reviews
- Volume:
- 50
- Issue:
- 14
- ISSN:
- 0306-0012
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1039/D1CS00054C
