Saswati Pujari

Alignment of perpendicular lamellae in block copolymer thin films by shearing

Lamellar block copolymers, with the lamellae standing perpendicular to the substrate, are attractive candidates as templates for nanostructure array fabrication. However, no process currently exists to impose long-range in-plane alignment of such perpendicular lamellae on simple unpatterned substrates—to align the lamellar normal over macroscopic distances. Here, we have generated such aligned films of perpendicular lamellae in a polystyrene-poly(methylmethacrylate) diblock, PS-PMMA, by neutralizing the substrate with a random terpolymer brush and shearing the film with a moving polydimethylsiloxane (PDMS) pad in contact with the film surface. At sufficiently high shear stresses, the lamellae align over the entire (cm2) area of the pad; the perpendicular orientation of the lamellae is preserved, although for films thicker than the lamellar spacing, a “capping layer” of PS forms in contact with the PDMS pad. However, when compared with typical shear-aligned block copolymers having a morphology of in-plane cylinders, a significantly higher stress is required to align the lamellar PS-PMMA, and the orientational order is poorer and the dislocation density higher; a limiting order parameter ψ2 ≈ 0.8 is achieved at high stresses.

Shear-induced anisotropy of concentrated multiwalled carbon nanotube suspensions using x-ray scattering

X-ray scattering is used to measure particle orientation in concentrated multiwalled carbon nanotube (MWNT) suspensions under shear flow. MWNTs were dispersed in a Newtonian suspending fluid (uncured epoxy). The dispersions exhibit shear thinning, approaching the matrix viscosity at high shear rates. This is accompanied by progressive development of MWNT orientation along the flow direction with increasing shear rate. The impact of MWNT aspect ratio and concentration on steady-state orientation is explored. In one sample (2 wt. % dispersion of short MWNTs), orientation was measured in both the flow-gradient (1-2) and flow-vorticity (1-3) planes of shear flow to provide a more complete picture of the three-dimensional orientation state. Also in this sample, 1-3 plane measurements were conducted using both small- and wide-angle x-ray scattering (SAXS and WAXS). While the two methods produce qualitatively similar results, WAXS-derived measures of flow-induced anisotropy are consistently larger than SAXS data...

Shear Induced Alignment of Multi‐Walled Carbon Nanotube Dispersions via Small Angle X‐Ray Scattering

We report small‐angle x‐ray scattering studies of shear‐induced alignment of multi‐walled carbon nanotube (MWCNT) dispersions. Uncured epoxy was used as a viscous, Newtonian suspending medium, and samples were prepared from ‘aligned’ MWCNTs using methods previously reported (Rahatekar et al. J Rheol 40:599, 2006); here we emphasize measurements on rather dilute dispersions. Flow‐induced alignment was studied in both the flow‐gradient (1–2) plane, and the flow‐vorticity (1–3) plane using, respectively, annular cone and plate and rotating disk x‐ray shear cells. Small‐angle x‐ray scattering patterns were rendered anisotropic under application of shear flow. Measurements in the 1–2 plane indicate that the average MWCNT orientation direction is intermediate between the flow and gradient directions. Transient measurements of structure evolution enabled by high flux synchrotron radiation allowed study of time‐dependent behavior following flow reversal and flow cessation.