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It is highly demanded to understand the confinement effect on nanoconfined polymers. Recent studies reported a strong perturbation of local dynamics and substantial alteration of glass transition temperature Tg at nanoscale. However, how confinement affects the mechanical properties of polymers is not fully understood. Here, we show that the modulus of thin polymer films could be remarkedly altered through a polymer–polymer interface. The modulus of a thin polystyrene (PS) film next to a polydimethylsiloxane (PDMS) was determined from the PS–PDMS bilayer bulging test. A series of experiments show that the modulus of PS can be increased up to 37%, when the modulus of the neighboring PDMS varies from 1.04 to 4.88 MPa. The results demonstrate a strong sensitivity of mechanical properties of thin polymers to the hard/soft environment, which we attribute to the change of high-mobility layer by the polymer–polymer interface. [ABSTRACT FROM AUTHOR]

The demand for applications, such as coatings, separation filters, and electronic packaging, has greatly driven the research of polymer films. At nanometer scale, mechanical properties of thin polymer films can significantly deviate from bulk. Despite outstanding progresses, there still lack deep discussions on nonlinear viscoelastic–viscoplastic response and their interactions under nanoconfinement. In this work, by conducting measurements via the bubble inflation method and modelling using Schapery and Perzyna equations, we demonstrate nonlinear viscoelastic–viscoplastic properties of freely standing thin polystyrene (PS) films. The results show the unchanged glassy compliance and the rubbery stiffening phenomenon for thin PS films, where the lower rubbery plateau in rubbery stiffening may originate from the induced molecular orientation by plastic deformation. With decreasing film thickness, viscosity and yield stress in viscoplasticity increase in an exponential and a linear trend, respectively, indicating the significant role of nanoconfinement effect on viscoplastic properties. These findings may reveal that there are many properties from linear viscoelasticity to nonlinear viscoelasticity–viscoplasticity that need to be explored and unveiled for sufficient understanding of the nanoconfinement effect on altering mechanical behavior of polymers. [ABSTRACT FROM AUTHOR]

Nanoscale polymeric thin films are widely used in diverse modern applications, where a satisfactory mechanical performance is a requirement to their full functionality. The mechanical response of polymer films is strongly affected by the size effects under nanoconfinement; however, the mechanism of such response in terms of molecular configurations and chain conformations has yet to be determined. In this work, we reveal the conformational origin of the stiffening behavior of crosslinked polymeric nanofilms via coarse-grained molecular dynamics and tailored experiments. We find that the biaxial modulus changes follow the alteration of polymer conformations, decoupled from size and thickness. We propose a theory to quantitatively link the elastic properties of the polymers to the distribution of their chain end-to-end distances, predicting a stiffening effect on uncoiled chains. Finally, we use such insight to obtain several PDMS nano-films of the same thickness but with a variability of two orders of magnitudes in their moduli. The mechanical response of polymer films is affected by size effects under nanoconfinement, but the mechanism of such response in terms of molecular configurations and chain conformations has proven elusive. The authors reveal the conformational origin of the stiffening behavior in crosslinked polymeric nanofilms via simulations and experiments. [ABSTRACT FROM AUTHOR]

We present the design and validation of an apparatus, which is developed based on micro-vibration in cyclic bulge and contraction, to determine dynamic mechanical properties of ultrathin polymer films. By controlling air pressure acting on a polymer membrane, the apparatus exerts simple harmonic or other type periodic stresses on the film, and the resultant real-time deformation of the freely standing film is recorded by using a high-speed CCD camera. From the image frame sequences and the gas pressure data, the real-time stress and strain of the polymer film are attained. Consequently, the dynamic mechanical properties, including biaxial storage and loss moduli, and loss factor can be determined for polymer films with thickness down to 20 nm. This apparatus could also be used to determine other mechanical properties such as fatigue and yield for nanoconfined films of soft matter. [ABSTRACT FROM AUTHOR]