Glass Transition Measurements of Ultrathin Polystyrene Films

Abstract

Data from various experiments suggest that the mechanical properties of thin polymer films depend, in a complex manner, on the film preparation process, in conjunction with interfacial interactions. In this paper, we discuss how and why interfacial interactions can affect the shear mechanical properties of spincoated polymer films. For example, confinement effects due to the spin coating process have been observed to exceed the nearest neighbor interaction distances by one order of magnitude. We introduce shear modulation scanning probe microscopy (SPM) as a technique to measure local shear property changes in thin polymer films as a function of temperature. Comparisons with bulk and lateral force measurements demonstrate the reliability of the technique. Using the shear modulation SPM, we observe a molecular weight dependence on the bulk glass transition. Measurements taken on polystyrene surfaces of various film thickness provide insight into how the glass transition of films are affected by substrate confinement.2

Probing Macromolecular Dynamics and the Influence of Finite Size Effects

“…The slow creeping process above T g is documented elsewhere [45]. While the accuracy of SM-FM T g measurements compares well with other techniques [46], Fig. 5(b), SM-FM also offers the versatility for probing rheological properties in confined sample geometries.…”

“…Other methods involve dilatometric measurements for the determination of the specific volume, mechanical property measurements (thermomechanical analysis, TMA, and dynamic mechanical analysis, DMA), and dielectric measurements [4]. Near-edge X-ray absorption fine structure (NEXAFS) spectroscopy [12], x-rays diffraction [13], slow-positron-annihilation spectroscopy (SPAP) [14,15], brillouin light scattering (BLS) [16,17], photon correlation spectroscopy and quartz crystal microbalance techniques [18], spectroscopic ellipsometry (SE) [19], attenuated total reflection (ATR) [20], and scanning probe microscopy (SPM) [21][22][23][24][25][26][27] have all been employed to determine T g for thin polymer films.…”

“…18. For film thicknesses, δ > 200 nm, the T g values correspond to the bulk T g of 95 o C [24]. A two phase boundary layer is encountered within ~200 nm of the substrate: (a) T g values are depressed relative to the bulk in a sublayer with a thickness on the order of R G, i.e.…”

“…Relating the stresses from the yield criteria to the mean contact pressure under the indenter, cY p m = (46) where, for the onset of constrained plastic yield, c has a value of about 0.5 for conical indenters, and may vary depending on the indenter geometry and the friction at the interface [38].…”

Molecular Mobility and Confined Plasticity in NEMS Applications

“…Particularly for ultrathin films, the SFM based shear modulation force microscopy SM-FM [12] has proven itself to be a very reliable method, in excellent agreement on the macroscale with differential scanning calorimetry and electron spin resonance over a wide range of molecular weight, and applicable to many polymer systems [13][14][15][16][17]. The method, depicted in Figure 1, is briefly described as follows: A nanometer sharp SFM cantilever tip is brought into contact with the sample surface.…”