51 research outputs found
Submicrometer Pattern Fabrication by Intensification of Instability in Ultrathin Polymer Films under a Water-Solvent Mix
Dewetting of ultrathin (< 100 nm) polymer films, by heating above the glass
transition, produces droplets of sizes of the order of microns and mean
separations between droplets of the order of tens of microns. These relatively
large length scales are because of the weak destabilizing van der Waals forces
and the high surface energy penalty required for deformations on small scales.
We show a simple, one-step versatile method to fabricate sub-micron (>~100 nm)
droplets and their ordered arrays by room temperature dewetting of ultrathin
polystyrene (PS) films by minimizing these limitations. This is achieved by
controlled room temperature dewetting under an optimal mixture of water,
acetone and methyl-ethyl ketone (MEK). Diffusion of organic solvents in the
film greatly reduces its glass transition temperature and the interfacial
tension, but enhances the destabilizing field by introduction of electrostatic
force. The latter is reflected in a change in the exponent, n of the
instability length scale, {\lambda} ~h^n, where h is the film thickness and n =
1.51 \pm 0.06 in the case of water-solvent mix, as opposed to its value of 2.19
\pm 0.07 for dewetting in air. The net outcome is more than one order of
magnitude reduction in the droplet size as well as their mean separation and
also a much faster dynamics of dewetting. We also demonstrate the use of this
technique for controlled dewetting on topographically patterned substrates with
submicrometer features where dewetting in air is either arrested, incomplete or
unable to produce ordered patterns
Thermoresponsive Reversible Behavior of Multistimuli Pluronic-Based Pentablock Copolymer at the Air−Water Interface
Micromechanical properties of glassy and rubbery polymer brush layers as probed by atomic force microscopy
Abstract Carboxylic acid terminated polystyrene and polybutylacrylate were grafted from melt onto a silicon substrate modified with the epoxysilane monolayer. The tethered layers fabricated from polymers of different molecular weights are smooth, uniform, mechanically stable, and cover homogeneously the modified silicon surface. Micromechanical properties of the dry glassy and rubbery brush layers were measured with atomic force microscope. We observed that for the PS layers with the thickness higher than 7 nm, the average value of the elastic moduli reached 1.1 GPa, which is close, but still lower than the expected for bulk polymer. The elastic modulus of PS polymer brush layers dramatically depends upon molecular weight and follows the inverse law with segment molecular weight, M c of 18,000 known for bulk PS. This result indicates that the process of the formation of the physical network within polymer melt of chains tethered to a solid substrate is similar to that occurring in unconstrained polymer melt. Under these conditions, three PS brush layers studied in this work represent different cases of chains without stable entanglements for M , M c as well as chains with stable entanglements for brushes with M , M c : This transition shows itself in significant reduction of the compliance reflected in twofold increase in elastic modulus. Our estimation predicts that modest lowering of 'limiting' elastic modulus of 1.4 GPa can be expected for thicker polymer brushes.
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