Adhesion strength and viscoelastic properties of polydimethylsiloxane (PDMS) based elastomeric nanocomposites with embedded electrospun nanofibers.

In the present study, the adhesive and viscoelastic properties of polydimethylsiloxane (PDMS) based nanocomposite pressure sensitive adhesives (PSAs) with embedded electrospun polyacrylonitrile (PAN) and polyvinyl alcohol (PVA) nanofibers as fillers were investigated. PDMS nanocomposite adhesive films using PAN and PVA nanofibers were synthesized by dispersing fillers in the matrix by a solvent mixing process. The adhesion strength and reusability of the prepared nanocomposite PSA films were measured using peel tests as the fraction of nanofibers in the polymer matrix is increased. The variations of the adhesive properties of the PSAs as function of the type and loading of filler were related to their rheological properties in terms of shear and elastic moduli. Although 3-fold enhancement of the adhesion strength was achieved with 0.5 wt% loading for both types (PAN and PVA) of nanocomposites as compared to elastic PDMS, the composite adhesive with PAN nanofibers can provide a superior balance of rheological properties, resulting in improved reusability over other PSAs. The differences in the adhesion and viscoelastic properties of the composite PSAs are attributed to the polymer chemistry, processability, and architecture of the electrospun nanofibers in the soft PDMS matrix.

To study surface and sub-surface nanomechanical properties of electrospun polyacrylonitrile (PAN) nanofibers/polydimethylsiloxane (PDMS) composites.

We investigate surface and sub-surface nanomechanical properties of nanocomposites based on a crosslinked polydimethylsiloxane (PDMS) elastomer and electrospun polyacrylonitrile (PAN) nanofibers. Fabrication of PDMS substrates with anisotropy with respect to surface elasticity and their characterization in terms of local nanomechanical properties are important for many areas of adhesion applications. PDMS nanocomposite substrates with variations in surface elasticity over large areas are prepared by controllably embedding electrospun PAN nanofibers (∼600 nm) in a PDMS matrix using the solution casting technique. Variations of local surface stiffness properties of prepared composites are measured using force spectroscopy and force mapping modes of atomic force microscopy and compared with their macroscopic (bulk) mechanical properties. Since the surface of the prepared nanocomposite is elastically non-homogeneous, our studies are mainly focused on the investigation of the hysteresis (plasticity index) between loading and unloading curves which is a measure of energy dissipation in AFM indentation experiments. The distribution of the local plasticity index in the PAN/PDMS composites is related to the specific organization of electrospun nanofibers at the surface and sub-surface layers of the PDMS matrix. We observed that embedding 0.1-1% PAN nanofibers induces anti-plasticization effects for lower (0.1%) and higher (1%) concentrations of PAN nanofibers which represent the formation of interpenetrating networks and mat-like blended structures of PAN nanofibers within the PDMS matrix.

Making Nonsticky Surfaces of Sticky Materials: Self-Organized Microtexturing of Viscoelastic Elastomeric Layers by Tearing.

Fabrication of large area, multiscale microtextured surfaces engineered for antiadhesion properties remains a challenge. Compared to an elastic surface, viscoelastic solids show much higher surface stickiness, tack, and adhesion owing to the increased contact area and energy dissipation. Here, we show a simple, low cost, large-area and high throughput method with roll-to-roll compatibility to fabricate multiscale, rough microstructures resistant to adhesion in a viscoelastic layer by controlled tearing of viscous film. Even a high adhesive strength viscoelastic solid layer, such as partially cured PDMS, is made nonsticky simply by its controlled tearing. The torn surface shows a fracture induced, self-organized leaflike micropattern resistant to sticking. The topography and adhesion strength of these structures are readily tuned by changing the tearing speed and the film thickness. The microtexture displays a springlike recovery, low adhesive strength, and easy release properties even under the high applied loads.

Cavity shape transformation during peeling on elastic microchannel-patterned substrates filled with a viscous liquid.

Inspired by the detachment mechanics of natural adhesive pads, we studied the change in cavity shape during peel tests on a 10% cross-linked polydimethylsiloxane (PDMS) elastic microchannel filled with 1% cross-linked viscous PDMS liquid (patterned bilayer). During peeling, we explored cavity shape as a function of microchannel dimensions and correlated the dimensionless cavity shape factor (CSF) and characteristic stress decay length, K-1. The peel test on the liquid-filled elastic microchannel shows three distinct cavity-shape regimes, elliptical, circular, and binary, based on the values of CSF and K-1. Such cavity formation and shape regimes could be important for improving the design of pressure-sensitive adhesives.