Size-dependent mechanical behavior of free-standing glassy polymer thin films
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The mechanical properties of nanoscale free-standing polymer thin films exhibit size dependence due to surface effects. However, it remains a challenge to determine the length scales at which differences are exhibited between film and bulk polymer properties. Here we use molecular dynamics simulations to uncover the dependence of elastic modulus (E) of free-standing films on film thickness and bulk properties. Comparison of the glass transition temperature (Tg) and E indicates that Tg converges to the bulk value slightly faster as the film thickness increases. The free-surface effects that give rise to a depression in E and Tg are observed to be stronger for polymers with weaker intermolecular interactions. The most intriguing aspect of our study is the finding that despite the observed decrease in the modulus of the film up to a thickness of over 100 nm, the local stress distribution reveals that the preserved length scale of perturbation of the free surface is only several nanometers.
I. INTRODUCTION
Ultra-thin polymer films with thicknesses in the range of tens of nanometers are increasing in relevance for a wide array of applications in the field of nanotechnology and bioengineering, such as nanoelectronics,1 nanocomposites,2 coatings,3 polymeric membranes, 4 and biosensors. 5 For instance, in nanoelectronics, the thermal and mechanical responses of nanopatterned polymeric structures obtained from lithography are important in determining the stability and reliability of these systems.6,7 In thin-supported and free-standing films, the effects of the free surfaces on the molecular structure and dynamics of polymer chains fundamentally change the behavior of these systems compared to the bulk, and these effects become significantly more pronounced as the film thickness is reached below 100 nm. The freesurface effects are complex and depend on many factors such as molecular weight of polymers, thickness of film, and polymer chemistry, but a common observation is that the presence of surfaces in polymer thin films may result in a notable reduction of thermomechanical properties by shifting the relaxation time and enhancing the molecular mobility.8–12 Understanding the fundamental mechanisms pertaining to the thermomechanical properties, in particular, the glass transition temperature (Tg) and elastic modulus (E) of sub-100 nm polymer thin films are important for numerous technological applications of soft nanomaterials, and are being studied through a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2014.289 J. Mater. Res., 2014
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novel experimental and computational techniques suited for the length scales of interest. Previous studies have shown that intermolecular noncovalent weak interactions between polymer chains are crucial in determining the thermomechanical responses of amorphous polymers. Experimental and theoretical studies have consistently shown that intermolecular forces between polymer chains can be tailored to tune t
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