Elastic moduli, E, of free-standing polystyrene (PS) single-layers and polystyrene–polydimethylsiloxane (PS-PDMS) bilayers are measured by uniaxial tensile testing at room temperature under different strain rates, γ̇, and for PS thicknesses, h, from 8 to 130 nm. As γ̇ increases, E increases initially, then approaches the bulk value, E bulk, when γ̇ exceeds a characteristic value (≡ τ–1) that decreases with increasing h. The noted variation of E with γ̇ shows that stress relaxation occurs in the films during measurement when γ̇τ ≪ 1, while the noted variation of τ–1 with h shows that thinner films relax faster. Consequently, E decreases with decreasing h if γ̇ is small, but displays independence of h if γ̇ is large. Visually, the crossover takes place at around γ̇ = 0.0015 s–1, where at γ̇τ > 1 for all films.

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The bending-induced micro-crack leads to the incomplete rupture of the conducting filaments and reduces the local depletion-region width.

No abstract

Surface glass-transition temperature (T g surf) and transition width (W) within 1 nm of the surface were measured by monitoring a qualitative change in the contact angle or density of end-groups (by time-of-flight secondary ion mass spectrometry) with temperature. Polystyrene (PS) films with various thicknesses (h) and molecular weights were studied. For unannealed PS supported on oxide-coated silicon or poly(dimethyl siloxane) with h > ∼60 nm, T g surf approached a plateau value of ∼25 K below the bulk T g; below 60 nm, T g surf decreased with decreasing h. Separately, W exhibited a stepwise increase when h was decreased below the radius of gyration of the polymer. Upon thermal preannealing or deposition on a PS brush or adsorbed layer, the films ceased to exhibit T g surf reductions or stepwise change in W. We discuss how an out-of-equilibrium density profile with a deficit near the substrate and de Gennes’ sliding mode may explain these observations.

This study examines the origin of the widely different length scales, h t —nanometers to micrometers—that have been observed for the propagation of the near-surface enhanced mobility in glassy polymers. Mechanical relaxations of polystyrene films with thicknesses, h , from 5 nm to 186 μm have been studied. For h < ~1 μm, the films relaxed faster than the bulk and the relaxation time decreased with decreasing h below ~100 nm, consistent with the enhanced dynamics originating from a near-surface nanolayer. For h > ~1 μm, a bulk-like relaxation mode emerged, while the fast mode changed to one that extended over ~1 μm from the free surface. These findings evidence that the mobile surface region is inhomogeneous, comprising a nanoscale outer layer and a slower microscale sublayer that relax by different mechanisms. Consequently, measurements probing the enhanced mobility of different mechanisms may find vastly different h t ’s as shown by the literature.

No abstract

Undoped and Cu doped SnO 2 thin films were fabricated by sol-gel method. The influence of annealing temperature, Cu doping and the addition of polyethyleneglycol (PEG)/glycerine on the film crystallinity, morphology and electrical properties were investigated. It is shown that annealing temperature can affect the film crystallization by controlling the crystallinity of the film and adhesion between the substrate and the films. Besides, both the Cu doping and PEG/glycerine addition can reduce the cracks quantity on the film. Cu doping can form Cu: SnO 2 solid solution which depress the onset of cracks. PEG/glycerine can increase the viscosity of solution which reduces the film drying speed, and thus provides more time for grain recombination and stress release. The conductivity of both the Cu: SnO 2 film and films added with PEG/glycerine improved comparing with pure SnO 2 film, which can be explained by the formation of oxygen vacancies induced by the Cu substitutional doping and the reduced amounts of cracks due to the additive of PEG/glycerine, respectively. This work may provide some useful information to optimize the SnO 2 gas-sensing devices.