Elastic Properties and Glass Transition of Supported Polymer Thin Films

Abstract

The present work demonstrates the first application of Brillouin light scattering (BLS) to probe film-guided elastic waves in transparent-substrate supported polymer thin films. In comparison with earlier BLS studies that were restricted to films either free-standing or supported on opaque substrates, the progress made in this work substantially extends the applicability of BLS and permits direct access to the elastic properties of thin films lying on transparent substrates, which is of important practical relevance. A series of thin supported polystyrene and poly(methyl methacrylate) films with thickness in the range of 40-500 nm were explored, and no noticeable trend in elastic properties with thickness has been found, in conformity with earlier BLS results. The first measurement of glass transition temperature, T g , of supported polymer thin films by BLS is also reported. We observed that the ultrathin (40 nm) films for both polymers exhibit a clear reduction in T g .

High Frequency Acoustics in Colloid-Based Meso- and Nanostructures by Spontaneous Brillouin Light Scattering

“…Assuming a constant density, this f (1,2) value corresponds to a shear modulus c 44 ≈1.45 GPa in the confined polymer melt at about T g +60 K. For comparison, a corresponding decrease in the transverse sound velocity of PS films is about 40% (i.e., c 44 <1 GPa). [49] This remarkable small temperature effect on the rigidity of the present threedimensionally confined PS-SiO 2 particles as a result of the increased pressure in the core is unprecedented.…”

“…2(b) in Ref. [49]). The sample was heated from room temperature to 165 • C in steps between 30 K (low T ) and 10 K (high T ) within a few minutes for each step.…”

“…10 • C and 200 • C. The oven is a metal cylinder with filament and coolant tubes in the wall. [49] A small slit at the hight of the laser and a cylindrical quartz glass insert in order to avoid heat transfer through the slit allow the light to reach the sample and the detector afterwards. The temperature is controlled electronically (built in-house) by two Pt-100 temperature sensors inside the wall and inside the heat chamber, close to the sam-ple.…”

“…Both sound velocities (c l , c t ) are sensitive index of T g , [36,49,110,111] due to the rapid increase of compressibility (C −1 11 ) and decrease of shear rigidity (C 44 ). Hence, tracking the frequency of a distinct vibration mode (Eq.…”

“…The elastic constants are frequency dependent, and BLS specifically yields their limiting high frequency values, which relate to local packing and interactions, as well as, the glass transition temperature. [49] The detection of more than two particle elastic excitations in the experimental (BLS) spectra of Fig. 4.18 allows for an unambiguous determination of the elastic moduli, shear modulus G = ̺c 2 t and Young modulus E = ̺c 2 l (1 + σ)(1 − 2σ)/(1 − σ), with σ = (c 2 l − 2c 2 t )/[2(c 2 l − c 2 t )] being the Poisson ratio and ̺ being the mass density.…”