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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]

Optimizing mechanical performance is crucial for the practical utilization of stimuli-responsive polymers, while complex viscous and elastic behavior hinders a deep understanding of functional polymers under external field excitation. Here, we demonstrate the in situ dynamic and static mechanical responses under electric stimuli of poly(methyl methacrylate) (PMMA) above glass transition temperature (Tg) by applying a direct current electric field vertically to the mechanical loading. The results show that the electro-mechanical response of PMMA is directly correlated to chain relaxation modes with different length scales: for local segments, polarization provides resistance for molecular motion, manifested by enhanced moduli, increased transient viscosity, and a wider linear viscoelastic range, whereas in a larger spatial range, polarization-induced conformation change causes faster relaxation, reduced elastic modulus, and a lowered modulus plateau. Moreover, flow viscosity is reduced because of weaker friction between chain segments under polarization. Our results suggest effective strategies for precisely tuning the viscoelastic behavior of polymers above Tg through electric stimuli. [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]