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Deformation Mechanisms in Ultra-thin Polymer Glasses
The properties of polymer glasses have been studied classically due to their importance for a wide range of technologies ; however, there is increasing need and demand to use these materials in geometries where dimensions approach the length scale of an individual polymer chain. Although significant efforts over the past few decades have provided some insight into how physical properties, especially the glass transition temperature, can change as a function of dimensional confinement, there has been significantly less development with regards to other properties, including mechanical properties, which are critical for their performance. One reason for this dearth of understanding is the lack of quantitative measurement methods. Here,
we present a new experimental method, which allows for the direct measurement of the complete stress-strain relationship for uniaxial stretching of ultra-thin polymer films. We demonstrate this method with measurements on polystyrene films with thickness as small as 15nm. We provide quantitative confirmation of changes in the low strain properties, e.g. elastic modulus, as a function of thickness as well as new measurements of the yielding and fracture processes as molecules within the polymer glass are dimensionally confined. In particular, our results reveal a new, sharp transition from craze deformation processes to shear deformation zones for films that are less than 30nm in thickness. We relate this deformation mechanism transition to other property changes in bulk and thin film polymer glasses. This method and these results not only provide fundamental insight into the physics of polymer
materials but also new opportunities for designing ultra-thin film materials with enhanced performance.